SECTioN 9.Wl-ly EARS DON'T ALwAYS CORRELATE
WiTI-I SPECS
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T hi s section of the manual examines a few of the many reasons
why it is seldom possible to know whatsomething will sound like merely byreading its specifications, no matterhow much you know about specifications. The two opposite, extreme pointsof view are:
a) "If it sounds good, the heck with thespecs," or
b) "If the specs are good, then it mustsound good."
Ideally, equipment ought to soundgood and exhibit good specs. In reality,this is not always the case. We believethat specifications are important, andshould be considered in any decision tobuy, rent, use, or recommend soundequipment. But the equipment mustsound good when properly installed inan appropriate sound system so onemust not disregard the "golden ears"approach, either.
Note that in this context, when weuse the term ears or by ear we understand that this includes our brain forprocessing and evaluating the raw datafrom the ears.
9.1 DiffERENT POiNTSOf ViEW
Any performance specification,whether written or graphic, is merely arepresentation of some physical behavior. It is not the behavior itself.
When evaluating how a soundsystem may actually sound, those whowould rely entirely upon measurements and specifications are makingjudgements based on a representation.One may indeed learn a lot about theperformance of a system from carefullyreviewing the specs... especially withthe insight gained from studyingSection 8 of this handbook. The finalcriterion, however, is "Does it soundgood?" or, in more practical terms,"Does it sound good enough for theintended application?" Ultimately, ifthe experience is listening, thenlistening should be the preferred, firsthand means to evaluate the sound.
9.1.1 Calibrated Mics vsEars
When acoustic measurements aremade of a sound system, a calibratedmicrophone is typically utilized. Thisis nothing more than a conventionallymanufactured mic that is made to verystrict tolerances, and then calibrated toa standard so any frequency responsedeviations can be compensated by theassociated test equipment.
Our ears work differently. First, wenormally work with two at all times.They are non-linear with respect toamplitude (reference the FletcherMunson equal loudness contours).They do, however, measure time ofarrival of reflected sounds, and therebyprovide phase information to ourbrains instead ofthe amplitude variation logged by the calibrated mic(which will result from phase additionsand cancellations between reflectedand direct (or multiple reflected) soundwaves arriving at a single mic diaphragm.
Then, too, the microphone is connected to test equipment, and thatequipment generally will examine onlyone or two parameters of the sound ata time: for example, distortion or amplitude response or phase response.Our ears listen to and evaluate allthese factors at once, which can make amajor difference.
9.1.2 Average Ears vs."Golden Ears"
Just as the average man on thestreet could not be expected to walkinto an athletic stadium and instantlybecome a referee qualified to makecritical calls for a professional sportscontest, neither can the average personbe expected to make critical auralevaluations of a professional soundsystem. Most mixing engineers, as wellas many musicians and producers,spend a lot of years fine-tuning theirperceptions of sound. Ifyou're notamong these people, you may find thishard to believe, but many audio professionals really do hear things that theaverage person does not hear. Theireardrums may not vibrate any differently than the next person's, but theyhave acquired a heightened nervebrain sensitivity and a greater abilityto carefully interpret those physicalvibrations. AB a result, they oftendemand sonic improvements for defectsthat the average person may notperceive at all, or may discount asbeing unimportant. These audio professionals, along with many amateuraudiophiles, should be treated withrespect because they generally do havethe so-called "golden ears" that haveled to many ofthe refinements and improvements in sound equipment. Fromour experience, we'd give the goldenear the benefit ofthe doubt.Tf a qualified person claims to hear somethingsignificant in a sound system, and youdon't hear it and can't seem to measureit, then you're probably not performingthe appropriate tests.
There are, of course, wide areas ofdisagreement as to what sounds goodand what sounds better. If someonejust loves heavy bass, a flat amplituderesponse sound systemwill seem thin
and lacking guts to that person. That'swhy it is important to find outfirstwhat people want, and to have themdemonstrate what they do and do notlike, in order to interpret their criticisms or requests for a given system orpiece of sound equipment.
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9.2 TEST EQuipMENTMEASUREMENTS VS.
LisTENiNG TESTS
It is never really possible to listen toa single component. There is alwayssome kind of signal generator or signalsource, some sort of amplifier, possiblya mixer or signal processor, and anoutput transducer (loudspeaker orheadphone). In order to hear anyone ofthese components, you will always behearing the result as colored by theother components, as well as by theacoustic environment. On the otherhand, you can test a given componentby feeding a calibrated test signal tothe input, and measuring the outputdirectly, with no other equipment andfew environmental factors to color themeasurements.
9.2.1 Test Signals vs.Program Material
Consistent with this approach, manyprofessionals will calibrate loudspeakers using single-frequency test tones orband-limited noise. Tones or noise areuseful for balancing the output levels,and for adjusting equalization, butthey are not representative of actualprogram material.
As an example, let's consider astereo pair of full range loudspeakers,
each channel driven by one channel ofa power amplifier. The amplifier, inturn, is preceeded by a pair of 113-octavegraphic equalizers. (Refer to Figure9-1). Due to minor manufacturingvariations in the loudspeakers, and astage layout that is not entirely symmetrical (few are once equipment andprops are in place), the listener perceives some difference between the leftand right channels during the soundcheck. While repositioning the loudspeakers might improve the balance,this is not practical because they arehanging above the stage in semipermanent fixtures. Therefore, thesound engineer (or contractor, or producer... whomever is responsible)reasons, the graphic EQ should beadjusted to obtain a better balancebetween the two loudspeakers.
The engineer wants as close a matchas possible over a broad range of frequencies, so he drives each channel ofthe EQ/amp/speaker system with thesame pink noise source (one at a time),measures the results on a real-timeaudio spectrum analyzer, and movesgraphic EQ filter sliders up and downto get the best possible match betweenthe two channels. This is a simplification, of course, but let's assume thatthe procedure works to the extent thatthe spectra for the two loudspeakerchannels are within a dB of one another using the pink noise. Now whatdoes it sound like?
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CALIBRATED MIC "<,
SPECTRUM """ANALYZER :;'"
EQUALlZER~
Figure 9-1. Simplified illustration of loudspeaker system beingfine-tuned to the environment
Running typical stereo programmaterial through the system providesinconclusive results... the image maybe wandering somewhat, and certaintransients sound different in the twochannels. Feeding the identical (mono)program to both channels yields evenless pleasing results, with an obviousimbalance in sound quality betweenthe two loudspeakers. What hasoccurred?
The carefully measured resultsusing expensive test equipment, whichindicated virtually identical outputfrom each channel, are not verified bythe listening tests.
There are probably several reasonswhy this hypothetical example oftenoccurs in reality. For one thing, thespectrum analyzer was measuring onlythe amplitude of the signals, onechannel at a time, not the phase shiftwithin a channel or delay between thetwo. When program material is pipedinto the system, and the human earsare employed as the analyzer, then theother factors are measured. Equalization not only adjusts the amplitude in agiven band (which is all the analyzerwas evaluating), it also creates phaseshift, and possibly other changes as
SECTiON 9
o
-10
~
rn~ -20..Jw>~ -30w>~..J -40wa::
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1I V \/ ; /i \A,
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/.V "RAW" ROOM RESPONSE ~,-
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\
If 'y'"
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-50
-60
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20 50 100 200 500 1K 2KFREQUENCY (Hz)
5K 10K 20K
o+15
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• 11 I 1
-15
o50 80 125 200 315 500 800 1.25K 2K 3.15K 5K 8K 12.5K
o1/3 OCTAVE GRAPHIC EQUALIZER
Figure 9-2. Comparison of uncorrected and corrected spectrum analysisfor a sound system, along with graphic EQ settings
needed to achieve the corrections.
NOTE: Phase shift, background noise, and changes in distortion and group delayare not directly indicated by the spectrum analyzer. Therefore, EQ corrections willnot always produce the expected change in the curve.
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WiTk SPECS
well, including various forms of distortion, and group delay (which maychange the directional characteristicsof the system, hence the reverberantfield and certain comb filter effects dueto interaction of direct and reflectedsound waves). Since the EQ settingsare not identical in each channel, theperceived sound quality changes, eventhough the amplitude response may bemade nearly identical.
This illustrates one reason why instrumentation and ears don't alwayscorrespond. There is yet another factorinvolved, as detailed in Section 9.2.2.
9.2.2 Location and Numberof Test Microphones
Normally, human hearing utilizes apair of mics that we call ears, and theirphysical location in space is constantlychanging to some extent (unless one'shead is in a dental examination chair,
well braced against motion). The twoears receive slightly different versionsof whatever sound source is excitingthe environment... versions that differin time of arrival and frequency balance. When sounds reflect from roomboundaries, they often reach the twoears a split second apart, with the headshadowing (partially blocking andreducing the amplitude 00 the higherfrequencies at the ear opposite thesound source or the reflecting surface.
Reflections cancel or reinforce oneanother at different points in space,and at different frequencies at thesame point. At the highest frequencies,the increased or decreased acousticlevels change as one moves just a fewtenths of an inch. However, with theslight rocking and twisting motion ofthe head, one's ears and brain are ableto construct an average of the soundfield in the vicinity of the listener, andthe result is what we perceive to be thesound of the system and the environment.
PERCEIVEDSPECTRUM(EARS!BRAIN)
MEASUREDSPECTRUM(TEST MIC)
A
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"The test mic receives an yequal blend of sound fromlocations 2, 3 & 4, and agreater proportion of directsound (1) which together createcomb filtering (phase cancellation!reinforcement). The listener's left earobtains a mix (in decreasing intensity)from sources 1, 3 & 4, and his right earfrom sources 1, 2 & 4 ... with phasedifferences resolved by the brain. Theear's sensitivity is less at the ends of thespectrum. This is a gross oversimplification.
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FREQUENCY
Figure 9-3. Sound field comparison:(A) Sound field at human ears and at a test mic
(B) Representative frequency response at the mic or ears
When a calibrated microphone hearssound for a spectrum analyzer, it hearsit at only a single point. That pointdoes not vary, assuming a fixed mic.Reflections that reinforce or cancel at agiven frequency will tend to do so in afixed, unvarying relationship, so thatone can often observe deep notches andpeaks (comb filtering) in the measuredfrequency response of the soundsystem. If you were to move the micjust a few inches to one side, the entirespectrum would probably change.Moving a few feet would, once again,cause major changes in the measuredresponse. Therefore, it is misleading tomake EQ or other adjustments merelyby measuring the results at one point.
In fact, at least one test equipmentmanufacturer has offered a multiplexing system that averages three mics together to display a closer facsimile ofthe average sound field. However, it isimportant to note that the brain independently processes two differentacoustic measurements, whereasmixing mic inputs together electricallywill cause signal cancellation andreinforcement that do not yield thesame kind of results as independentprocessing. For this reason, someanalyzer manufacturers providememories so that the response can bemeasured in several locations, andthen the measured graphs can beoverlaid on the analyzer display forhuman visual interpretation. This is animprovement, but it's not the samething as our constantly shifting ears.
9.2.3 Dynamic Range
We often think of dynamic range interms of the difference between themaximum level the sound system iscapable of producing, and the background noise of the environment.Indeed, this is what we hear at concerts, speeches or shows. But testequipment also has a dynamic range,and often that range is considerablyless than the dynamic range of thesound system. Until recently, forexample, most spectrum analyzerscould display only 30 dB on screen.Now better units generally display a60 dB range, and some can do better. Agood sound system, though, may becapable of 80 dB or, in some cases,100 dB of useable dynamic range, andour ears have a range of some 120 dBor better. It's obvious that the spectrum analyzer cannot measure thingsthat we may be hearing. Of course theanalyzer may have attenuation pads orrange switches so that it can ultimately measure all the sound, but itcan't do it simultaneously the way ourears do.
Spectrum analyzers are not alone inthis regard. Most audio test equipment- even the voltmeter - is limited inthe scope of values it can measurewithout range switching. If an IMdistortion product is 65 dB below thefundamental frequencies, it may notshow up on the test equipment,but itmay be very audible to the casuallistener. This is yet another factor toconsider when pondering why we mayhear things that differ from what wemeasure.
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DON'T ALwAYS
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WiTI1 SPECS
9.~ STATic vs DyNAMicTESTS
As we've hinted earlier, sometimesthe ears don't match the tests simplybecause the right tests are not made.This is particularly evident when oneconsiders the nature of static testsignals (i.e., a sine wave tone or asteady noise signal) versus the typicalprogram material (sharp attacks, widedynamic changes, and sudden jumps infrequency).
Many tests are now made using tonebursts, which come closer to a programsignal in this regard, but few printedspecifications indicate the results ofsuch tests.
When a circuit, or even a transformer or transducer, is subjected to atone burst (or a loud percussive note),the component can ring. That is, thevoltage in the circuit can overshoot theintended value, then resonate up anddown until it settles. (Some circuitsdon't settle down, but instead go intooscillation; they are said to be unstable.) The same circuit may notexhibit any such behavior if the inputsignal is gradually increased in level,or is a steady state signal.
lA.yr
.','Square Wave Output of
Circuit Which is "Ringing"
Ringing is often caused by improperloading, not only in terms of the loadimpedance itself, but in terms of howmuch capacitive or inductive reactanceis part of that impedance. A power amplifier, for example, may performflawlessly when bench tested with an8 ohm, non-inductive resistor. Even themost stringent tone bursts may causeno aberrations. Connect that sameamplifier to an 8 ohm electrostaticloudspeaker, which is mostly capacitivereactance, and it may sound awful. Itmay ring to the point of incipientoscillation, and its protection circuitrymay trip so often as to chatter. Theringing may occur because a resonantcircuit has been created. The protectionproblems may occur because reactiveloads can force the output stage of theamplifier to dissipate up to twice thepower of an equivalent (in ohms)purely resistive load.
The difference between static anddynamic testing is recognized in atleast one specification, Transient Intermodulation Distortion (or TIM). In theearly 1970s this measurement wasfirst proposed as a way to quantify theaudible distortion that occurred with
Tone Burst Test of SameCircuit Which is "Ringing"
Figure 9-4. A Square wave which shows "ringing" anda tone burst test of that same circuit
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We can hear ringing as a form of distortion. Ideally there should be noovershoot or ringing in the circuitry ofsound reinforcement or recordingequipment. Equipment that is nottested with pulse or burst signals maylook great on paper, but may performdisappointingly due to this type ofcircuit problem.
certain transient signals, even thoughthe steady-state I.M. measurementwould not indicate a serious distortionproblem. It is interesting to note thatthis specification was long consideredsomewhat controversial, and not easilymeasured.
9.4 MASkiNG EffECTSANd EQuipMENTINTERACTioN
It is easy to make assumptionsabout specifications that do not reflectactual, perceived sound. Let's take aspecific example: harmonic distortion.If a loudspeaker is rated at 1% T.R.D.,and a power amplifier at 0.1% T.R.D.,would you expect to hear the amplifier's distortion through the loudspeaker? Most people would answer,"no." In fact, many loudspeakers havemeasurable distortion of greater than1%, and if one were to perform an A-Bcomparison to different power ampseach having 0.1 % T.H.D. ratingsthrough such a speaker, the differencesin sound quality would very possibly beaudible. Why?
Well, the very term T~R.D. implies asum-a total of various distortioncomponents. As discussed earlier, theear finds some harmonics more objectionable than others. Specifically, oddorder harmonics (3r d
, 5th, 7th , etc.) aremore harsh, and even-order harmonics(2nd
, 4th, 6th
, ete.) are more musical tothe average listener. Also, higher orderharmonics (6th
, ~, 8t h, 9th
, etc.) aremore noticeable and objectionable thanlower order harmonics (2nd
, 3r d, 4th
) .
If the loudspeaker with 1% T.R.D.generates mostly 2nd and 4th harmonics, and the power amplifier with0.1 % T.R.D. happens to generate an inordinate amount of 5th and 7th harmonics, it may well be that the predominant audible problem is caused by thepower amp. In this case, the muchhigher total percentage distortion inthe loudspeaker does not mask thehigh order odd harmonics generated bythe amp.
Granted, this is an extreme example, but it makes our point. Don'tassume that a high noise level ordistortion level in one piece of equipment makes it impossible to hearlesser noise or distortion in othercomponents. A digital delay line'squantizing noise, for example, may beaudible even though it is several dBbelow the noise floor of the analogequalizer to which it is connected. Theorganized digital hash can be recognized through the random analog hiss,even sometimes when it is below the
room's noise level. (In fact, digital noisecan be a real problem.) Similarly, someprogram signals can be heard belowthe noise floor, which is why theeffective dynamic range may notexactly match the mathematical sum ofthe measured signal-to-noise ratio plusheadroom.
Then, too, there are other psychoacoustic factors that may affect ourperception of measurable performancedefects. For example, tape recordingsand certain synthesizers will exhibit atype of noise known as modulationnoise. This consists of noise sidebandsthat accompany a given note. Fortunately, the ear tends not to hearsidebands within an octave or so oneither side of the actual note, a socalled masking effect, and residualwideband noise tends to obscure therelatively low modulation noise energybeyond that 2-octave wide maskingwindow. An exception can occur with arecording of a strong, low frequencynote on a very quiet recorder. Say thenote is at 82 Hz. It will have modulation noise sidebands at lower andhigher frequencies. The lower frequency sidebands tend not to beaudible because the ear is even lesssensitive at the lowest frequencies, andbecause they are within the maskingwindow. The higher frequency sidebands, however, occur in a frequencyrange where the ear is more sensitive,and may not be entirely masked by thenote. What may be heard, then, is anincrease in hiss when the note isplayed. Ifyou listen to certain taperecordings, particularly of classicalpiano or guitar on an analog recorderin which noise reduction is used, youmay be aware of this modulation noise.In most cases the residual backgroundnoise of the tape, along with theinherent masking of nearby frequencies by the very program signal thatcauses the modulation noise (or byother notes) will make the modulationnoise inaudible. What can be measuredmay not be heard.
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10.1.1 Dynamic
Figure 10-1. Construction ofa dynamic microphone
AIR GAP (C)
SINTERED DISC (D)(ACOUSTIC RESISTANCE)
DIAPHRAGM (A)
Next to the dynamic, the mostcommon microphone type is the condenser. Figure 10-2 illustrates theconstruction of a condenser element.
A gold-coated plastic diaphragm,Figure 10-2 (a), is mounted above aconductive back plate (b), which isoften made of gold-plated ceramic. Thediaphragm and back plate, separatedby a small volume of air (c), form anelectrical component called a capacitor(or condenser).
Figure 10-2. Construction ofa condenser microphone
10.1.2 Condenser
When sound strikes the diaphragm,the diaphragm surface vibrates inresponse. The motion of the diaphragmcouples directly to the coil, whichmoves back and forth in the field of themagnet. As the coil cuts through thelines of magnetic force in the gap, asmall electrical current is induced inthe wire. The magnitude and directionof that current is directly related to themotion ofthe coil, and the current thusis an electrical representation of theincident sound wave.
Dynamic microphones are highly dependable, rugged and reliable. For thisreason, they are extremely common instage use, where physical strength isvery important. They are also reasonably insensitive to environmentalfactors, and thus find extensive use inoutdoor paging applications. Finally,because moving-coil technology is fairlyrefined and is capable of very goodsonic characteristics, dynamic microphones also are widely used in recording studios.
MAGNETICRETURNCIRCUIT (E)
MAGNETIC GAP (C)DIAPHRAGM (A)<;
Microphone is a generic term that isused to refer to any element whichtransforms acoustic energy (sound) intoelectrical energy (the audio signal). Amicrophone is therefore one type froma larger class of elements calledtransducers - devices which translateenergy of one form into energy ofanother form.
The fidelity with which a microphone generates an electrical representation of a sound depends, in part, onthe method by which it performs theenergy conversion. Historically, anumber of different methods have beendeveloped for varying purposes, andtoday a wide variety of microphonetypes may be found in everyday use.
10.1 Mrrhods ofTRANSduCTioN
By far the most common type ofmicrophone in contemporary soundwork is the dynamic. The dynamicmicrophone is like a miniature loudspeaker - in fact, some dynamicelements serve dual functions as bothloudspeaker and microphone (forexample, in intercoms).
Figure 10-1 illustrates the basic construction of a dynamic microphone.
A flexibly-mounted diaphragm,Figure 10-1 (a), is coupled to a coil offine wire (b). The coil is mounted in theair gap of a magnet (c) such that it isfree to move back and forth within thegap.
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SECTiON 10
10.1.4 Ribbon
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AIR GAP (B)
CORRUGATEDRIBBON
DIAPHRAGM (A)]IMPEDANCEMATCHING PERMANENT
TRANSFORMER MAGNET (D)(E)
Figure 10-3. Construction ofa ribbon microphone
Ribbon microphones employ atransduction method that is similar tothat of dynamics. Figure 10-3 illustrates the construction of a typicalribbon element.
A very light, thin, corrugated metalribbon, Figure 10-3 (a), is stretchedwithin the air gap of a powerful magnet (b). The ribbon is clamped at theends, but is free to move throughout itslength.
When sound strikes the ribbon, theribbon vibrates in response. As is thecase with the dynamic coil element, themoving ribbon cuts the magnetic linesof force in the air gap, and a voltage isthereby induced in the ribbon. The
MAGNETICEND CAP (C)
amplifier and battery are housed in asmall case that is connected to theelement by a cable. Increasingly,phantom power is being used instead ofa built-in battery on electret condensermodels.) The purpose ofthe amplifierhere is primarily to buffer the highimpedance condenser capsule outputfrom the relatively lower impedance ofthe mic input.
Electrets are increasingly commonin both recording and reinforcement.Because they may be made very small,electrets make possible some uniqueclose-miking techniques. The technology is also relatively inexpensive, soelectret elements are often used inconsumer products. Electrets can be ofhigh quality, and some very fineelectret microphones are available forprofessional recording and laboratoryapplications.
10.1.3 Electret Condenser
The electret is a special class of condenser microphone. Electrets incorporate diaphragms made of a uniqueplastic material that retains a staticcharge indefinitely. The manufacturercharges the diaphragm when the element is made (usually by irradiating itwith an electron beam), and no external polarizing voltage is required.
Electrets still require a built-inamplifier, however, and this is normally a transistor unit. The amplifieroften is powered with a battery - between 1.5 and 9 volts - housed in themicrophone case. (In some designs, the
A polarizing voltage of between 9and 48 volts is applied to the diaphragm by an external power supply,charging it with a fixed, static voltage.When the diaphragm vibrates inresponse to a sound, it moves closer toand farther from the back plate. As itdoes so, the electrical charge that itinduces in the back plate changesproportionally. The fluctuating voltageon the back plate is therefore anelectrical representation of the diaphragm motion.
Condenser microphone elementsproduce a signal voltage with almostno power. Thus they present a veryhigh impedance. For these reasons, allcondenser microphones incorporate anamplifier, which drives the microphoneline. Its function is both to boost thesignal level and to isolate the elementfrom the lower impedance of the inputto which the microphone is connected.Early condenser microphones employedtube amplifiers and thus were physically quite large. Modern condensersuse transistor amplifiers, and can bemade very small.
Because the diaphragm ofa condenser is not loaded down with themass of a coil, it can respond veryquickly and accurately to an incidentsound. Condensers therefore generallyhave excellent sonic characteristics,and are widely used in recording.Being somewhat more sensitive tophysical shocks and environmentalfactors (humidity), however, classiccondensers are less often used in soundreinforcement.
MicROpl-lONES
Figure 10-4. Construction ofa carbon microphone
11f:~ALe p UT
\ 'STEP-UP
TRANSFORMER(E)
CARBONGRANULES (F)
BRASSCUP (A)
CARBONDISC
/RHEOSTAT (H)
BRASSBUTTON (B)
~[ZjRUBBER
MOUNT (G)
METAL _DIAPHRAGM (C)
RUBBERMOUNT (G)
~I'--FZJ-J\fINlr-l11\
6V BATTERY (D)
impedance of the element from that ofthe input to which it is connected, andto block the battery DC from the input.
Carbon microphones are not knownfor excellent sonic characteristics, butthey are quite inexpensive, and rugged.For this reason, they are still widelyused in utility sound applications. (Thestandard telephone mic element haslong been a carbon type, althoughdynamic mics are used in many newerphones.) Carbon microphones can losesome efficiency and become noisy if thegranules in the button become compacted, but simply tapping the elementagainst a hard surface usually curesthe problem.
10.1.5 Carbon
voltage is very small and the ribbonimpedance very low, so all ribbonmicrophones incorporate a built-intransformer. The transformer servesthe dual functions of boosting thesignal voltage and isolating the ribbonimpedance from the load presented bythe input to which the microphone isconnected.
Early ribbon microphones were extremely fragile. The ribbon could bedamaged simply by blowing or coughing into the microphone! Not manymicrophone manufacturers now makeribbon units, but those that are available are much more rugged than olderunits. All but a few modern ribbonmics remain more fragile than dynamicor condenser units, so they are usedprimarily in recording (a couple ofnotable exceptions are used for reinforcment).
Ribbon microphones usually haveexcellent sonic characteristics, withgreat warmth and gentle high-frequency response. They also haveexcellent transient response and verylow self-noise. For these reasons, someribbon mics are prized as vocal microphones, and are also very effective withacoustic instruments.
The carbon type is among the earliest microphone elements ever developed. Figure 10-4 illustrates the construction of a typical carbon element.
A small cup, Figure 10-4 (a), ispacked with pulverized carbon andenclosed at one end by a brass diskcalled a button (b), which is coupled toa circular metal diaphragm Cc). Thebutton and a back plate at the rear ofthe cylinder form the connectionterminals. A battery (d) provides anactivating voltage across the carbon.
When sound strikes the diaphragm,the carbon granules in the buttonvibrate, becoming alternately more andless dense as the diaphragm moves.The electrical resistance of the carbonthereby fluctuates, and converts thebattery voltage into a correspondingfluctuating current that is an electricalrepresentation of the sound. Thecurrent is stepped up by a transformer(e), which also serves to isolate the low
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10.1.6 Piezoelectric
Another very early microphone typeis the piezoelectric. Figure 10-5 illustrates the principle of piezoelectricmicrophones.
A flexible diaphragm, Figure 10-5 (a)is coupled to a crystal element (b) by adrive pin (c). The crystal element is of amaterial that exhibits the piezoelectric(pressure-electric) effect. When it isphysically deformed by pressure ortorsion, the crystal generates an electrical voltage (potential) across itsfaces.
When sound strikes the diaphragmit vibrates, and the crystal is therebydeformed slightly. The crystal generates a voltage in response to thisbending, and this varying voltage is anelectrical representation of the sound.
Piezoelectric microphones (sometimes called crystal or ceramic types),like carbon types, are not generallyknown for their sound quality, but arequite inexpensive. Properly implemented, a crystal element can performvery well, and the principle is oftenused for contact-type pickups.
Piezo elements are high-impedancedevices, and they produce substantialoutput levels. They can be damagedirreparably by physical abuse, and aresusceptible to both heat and humidity.
In addition to the method of
DRIVE PIN (C)
PIEZOELECTRICGENERATINGELEMENT (B)
t OUTPUT____ • SIGNAL
Figure 10-5. Construction ofa piezoelectric microphone
10.2 FUNCTioNAL DESiGN
transduction and pickup pattern,microphones are further classifiedaccording to their functional design.Many different microphone designs areavailable, and each is optimized for aspecific range of applications.
10.2.1 Hand-Held
By far the most prominent microphone design is the hand-held type.Figure 10-6 shows a few typical handheld microphones.
As the name implies, this micro-
Figure 10-6. A few typical hand-held microphones
phone is designed so that it may beheld in hand by a lecturer or singer. Ofcourse, such microphones also are veryoften mounted on a stand using athreaded mounting clip.
The most common pattern in handheld microphones is the cardioid,although other patterns are available.Whatever the pickup pattern or type ofcapsule (sound generating element), ifit's in a hand-held mic, it must be wellisolated from physical vibration toprevent handling noise, and thecapsule must be protected from beingdropped. Rubber shock mounts andprotective screens are standard features of most hand-held mies.
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MicROpltONES 10.2.2 Stand-Mounting
Some microphones are designedspecifically for stand (or boom) mounting only; Figure 10-7 shows examplesof such microphones.
10.2.3 Lavalier
Lavalier microphones are very smallelements that are designed to pindirectly to clothing or to be hung on alanyard around the neck. Figure 10-8(next page) shows a typicallavaliermicrophone.
Figure 10-7. Stand mount microphones
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Microphones like this are most commonly made for recording. Typically,older tube-type condensers were madefor stand-mounting only, being toolarge for convenient hand-held use.Even the most modern mics, however,may be designed for stand mount inthe studio because more elaborateexternal shock and vibration isolationis then possible. Video and motionpicture production mics, mounted onbooms, are often elaborately shockmounted to keep vibration out of themic. Smaller, very unobtrusive modernstand-mounting microphones areusually electret types, and are designed specifically for reinforcementand broadcasting applications whereappearance is a primary consideration.
It used to be that lavaliers werenearly always dynamics, since theywere much less costly to build in therequired small package. Modernlavalier microphones are almostalways electret condenser types, sinceelectret elements now can be madevery small in size, offering excellenttop-end response and sensitivity for areasonable cost. The most common pattern for lavaliers is omnidirectional, although recently some cardioid andhypercardioid types have been introduced. The omni pattern has severaladvantages in this application. It doesnot emphasize the already resonantchest cavity because it does not haveproximity effect, and it can be clippedin different orientations without itssound quality changing. This is crucialifthe sound is to remain consistent.
Lavalier microphones are widelyused in television broadcasting, sincethey can be made very unobtrusive.For the same reason, they are alsooften used in theatre (coupled with awireless transmission system).
Figure 10-8. A typicallavaliermicrophone
A major advantage of lavalierelements is that, since they are affixedto the speaker's person, the distancebetween source and microphone isconstant and sound quality therefore ismore consistent. Lavaliers must bemounted with care to avoid extraneousnoise from clothing.
10.2.4 Contact Pickup
Contact pickups are microphone elements that are designed to detectsound waves in a solid medium, ratherthan in air. Figure 10-9 shows one typeof contact pickup.
Contact pickups are most commonlypiezoelectric devices, although thedynamic principle has been used forthis application, also. A recent type ofcontact transducer that has garneredconsiderable interest in sound reinforcement circles uses the condenserprinciple, and comes in the form of aflexible strip.
Contact pickups are used almost exclusively for instruments (the exception is throat microphones, which aresometimes used in communications),and their placement is extremely critical. The complex resonant characteris-
tics of instrument bodies result inradically different sound qualities indifferent locations, and considerableexperimentation is necessary toachieve satisfactory results. The meansby which they are affixed to the instrument can affect the sound quality andthe instrument. A sticky wax is oftenused, since it can be removed withoutdamaging the instrument.
Figure 10-9. A typical contactmicrophone
Because contact pickups rarely yielda true sound quality, they are not oftenused in recording, except for specialeffects. In reinforcement, however,they offer exceptional resistance tofeedback, but they can be very susceptible to handling noise.
10.2.5 Pressure Response
The so-called pressure responsemicrophone is a fairly recent development, and is subject to patent andtrademark restrictions. The commercial implementation of the principle iscommonly called the PZMTM, and ismanufactured under a licensingagreement by Crown International ofElkhart, Indiana. Figure 10-10 showsone ofthe Crown PZMTM units.
The microphone element is placedextremely close to and facing a flatplate. In theory, the microphonesamples pressure variations in the tinyair gap between the element and theplate, rather than responding to airvelocity.
Figure 10-10. A Crown PressureZone Microphone (Courtesy of
Crown International Corp.)
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MicROplioNE5 10.2.7 Parabolic
The parabolic microphone is actuallya conventional mic element coupledwith a reflector that concentratessound on the element. Figure 1-12 (a)shows the principle of operation, andFigure 10-12 (b) a polar pattern for thistype of mic.
The parabolic reflector is a cupshaped surface whose cross section is acurve that is called a parabola. Mathematically, the parabola is built arounda focal point and a plane surface.Acoustically, the parabolic reflectorconcentrates all sound that arrivesalong the primary axis to a pointlocated at the mathematical focus. It isthus highly directional, and serves toincrease the sensitivity of the mic element dramatically.
Parabolic microphones are usedwidely in nature recording. Since theirlow-frequency response is directlyrelated to their size, practical handheld units are limited to frequenciesabove about 1 kHz and thus are mostuseful for bird and insect songs. Foryears they have been seen on the sidelines at football games so broadcastaudiences can listen to on-field sounds,such as bodies colliding on a footballfield. Parabolic microphones are neverused in reinforcement.
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Originally developed for recordingand implemented using condenser instrumentation elements, the pressurezone principle offers certain benefits.Among these are good imaging qualities and, if the element is mounted on afloor or wall, freedom from path-lengthcancellations. Low-frequency responseof the pressure-zone microphone isdirectly related to the size of theboundary plate. The larger the plate,the better the pickup oflows.
Pressure-zone microphones aresometimes used in sound reinforcement, but since they are inherentlyomnidirectional, they offer little helpwith feedback. Recently, directionalunits have been developed to deal withthis problem, and are finding some application in conferencing situations aswell as instrument amplification.
10.2.6 Shotgun
The shotgun microphone is a highlydirectional unit. Figure 1-11 shows atypical shotgun microphone.
Shotgun microphones are most oftenused in broadcasting, and they are particularly popular in film work, whereisolating actors' dialog from ambientnoise is a constant concern. They arealso sometimes employed creatively forspecial effects in the studio. Shotgunmics are also used for long distancepickup in some sports events. Successful use of shotgun microphones in reinforcement is rare.
Figure 10-11. A typical shotgunmicrophone
Figure to-12. A Parabolic mic:(a) Principle of operation
(b) Polar response
A
B
/" PARABOLIC REFLECTOR
SOURCEOF
---- MICROPHONE SOUND
0°
1800
200Hz --6CXlHz --
1000Hz ----4000 Hz ---- - ---8000Hz ---
10.2.8 Multi-Element Arrays
A few special microphones havebeen constructed using two or moretransducer elements. Such units normally require auxilliary networks tocontrol the combining of signals fromthe elements.
One such unit is called a two-waycardioid microphone. This device usestwo elements - one for high frequencies and another for lows - much likea two-way loudspeaker. A crossovernetwork combines the signals from thetwo elements, crossing over at about500 Hz. Advantages of the techniqueare wide and flat frequency responseboth on and off axis, and absence ofproximity effect (see Sections 10.3.2and 10.3.3).*
Another multi-element system is thestereo recording microphone, whichincorporates two identical elements ina single body. Several manufacturersproduce such units, usually employingcondenser elements. Some advantagesin recording are relative freedom fromphase discrepancies between channels,ease of use, and unobtrusive appearance (which is of particular benefit inlive recording).
An unusual application of multielement technology is the CalrecSoundfield'P' microphone, designed byCalrec of England for stereo recording.Four condenser elements are mountedin a tetrahedral arrangement andconnected with a special active combining network. The unit produces a set ofsignals which, when recorded on amultitrack recorder, can be reprocessedin the studio to steer the stereo imageas desired. Both imaging and soundquality are said to be excellent, and theunit is finding increasing use in professional recording.
* Originally, all cardioid mics were dualelement mics, though the two elementswere used to achieve direetionality... alarge, expensive method to build a mic.Other cardioid mics used complex plumbing(ducts and ports) to achieve directionalitywith a single element, though these werevery large and expensive, too. In 1940, thefirst single-element self-patterning cardioidmic was developed, ultimately leading tosmaller cardioid mics.
10.2.9 Noise-CancellingMicrophones
Noise-cancelling (or differential)microphones employ either two miccapsules wired in reverse polarity, or asingle diaphragm that is open on bothsides to sound pickup. Such mics tendto discriminate for close on-axis soundsources, which produce higher pressureon one side ofthe diaphragm (or oneone ofthe capsules) than on the other.More distant and off-axis sounds tendto produce equal pressures on bothsides ofthe single diaphragm (or onboth capsules) and thus are cancelled.
Differential microphones are usedfor speech communication only, andare most beneficial in noisy environments like factories or airplane cockpits. (If you are using one, be sureyou're speaking directly into it at adistance ofless than 2 inches - or itwill treat your voice, however loud, likenoise and reject it.)
~--_ The two capsules are joinedin reversed polarity.
--Figure 10-13. Cross-sectional view
of a dual-element differentialmicrophone.
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MicROpl10NES 10.~ ACOlJSTiCAL ANdELECTRiCALCl-tARAcTERisTics useful in recording, and virtually
every studio owns at least a few ofthem.
10.3.1.2 Cardioid
Figure 10-14. Polar pattern ofa cardioid microphone
1,000 Hz
4,000 Hz
125Hz 500 Hz
-dB
The cardioid is unreservedly themost popular of all microphone pickuppatterns. Figure 10-14 shows a typicalcardioid polar response pattern.
Note that the pattern is heartshaped - hence the name "cardioid."As Figure 10-14 clearly shows, the cardioid microphone is most sensitive tosounds coming in on the primary axis,and rejects sounds from the sides andrear of the microphone.
The directional qualities of thecardioid make it a natural choice forsound reinforcement, since they help inreducing feedback and increasingsystem gain (see Section 5.3, "FeedbackControl"). This effect is overrated, andomnidirectional mics are often a betterchoice for close work than is a cardioid.Cardioids tend to have more colorationwhen sound does not arrive on axisbecause their directional qualities varywith frequency.
Cardioids are quite common in recording, since they can be used todiminish unwanted sounds arrivingfrom off-axis. Their frequency responseis usually rougher than that of an omniand they are somewhat more sensitiveto wind noise and breath popping.
10.3.1.1 Omnidirectional
Microphones are classified not onlyby the method of transduction but alsoby their pickup pattern. The pickuppattern is the way in which the element responds to sounds coming infrom different directions, and there areseveral different standard patterns.(This is akin to the polar response of aloudspeaker... in reverse.)
The acoustical and electrical characteristics of a microphone together determine both the quality of its performance and its suitability for a particularapplication or system. No single factorpredominates; all work together, and itis important to understand the rangeof qualities that may be expected intypical professional and semi-professional equipment. After all, even amediocre system has a better chance ofperforming well if the sound source isclean, and even the best sound systemcan't make a poor quality mic soundgood.
10.3.1 Pickup Patterns
Omnidirectional elements, as theirname implies, pick up sound more-orless equally from all directions. Figure10-15 (next page) shows a set of polarresponse patterns for a typical omnimicrophone.
One might think that omnidirectional microphones are never used insound reinforcement, since they offerno protection from feedback. This isgenerally the case, but not entirely so.There is a myth that cardioids arebetter, but omnis have better lowfrequency response, and less susceptibility to breath noise and wind noise.Because omnidirectional mics tend tohave much smoother frequency response than directional mics, there arefewer peaks to trigger feedback, sosometimes a good omni is as useful (ormore so) as a mediocre directional mic.Lavalier mics (mics worn on a lanyardaround the neck, or clipped to a shirt)are often omnis. Omni mics are quite
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Figure 10-15. Polar response of an omnidirectional microphone
125 Hz
500 Hz
2,000 Hz
8,000 Hz
250 Hz
1,000 Hz
4,000 Hz
16,000 Hz
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10.3.1.3 Bidirectional orFigure-8
A somewhat more unusual but veryuseful pickup pattern is the so-calledfigure-8 or bidirectional. Figure 10-16shows a typical polar response plot of abidirectional element.
The derivation of the name for thispattern is obvious from Figure 10-16.Bidirectional elements are mostsensitive to sounds coming in from thefront or rear of the microphone, andreject sounds from the sides.
Figure-S microphones are veryuseful in circumstances where pickupof two separate voices is desired - forexample, in an interview situation or abarbershop quartet (where opposingsingers can see each other and bepicked up, but the audience is off axisand is not picked up). In recording andreinforcement, the figure-8 may beused to pick up two adjacent instruments when separate control is notdesired. For example, it may be placedbetween two tom-toms in a drum set.
10.3.1.4 Supercardioid
The supercardioid is a highly directional microphone element. Figure10-17 shows a polar response plot of atypical supercardioid microphone.
Note that, in contrast to the cardioid, the supercardioid does exhibitmore of a rear pickup lobe, thoughsmall. It thus supplies far less rejectionof sounds coming in directly from therear than does the cardioid. Theforward pickup lobe is far more concentrated and the supercardioid offerssuperior rejection of sounds coming infrom the sides.
Supercardioids are used in specialsituations where greater side rejectionis desired, but some rear pickup maybe tolerated. Because of the concentrated forward lobe, they also may"reach" farther than a typical cardioid,and are sometimes used for pickup ofdistant sources.
Incidentally, the supercardioid issimilar to, but not identical to anothervery directional microphone, the hypercardioid.
125 Hz
~- 500Hz
:::::",<:>:~~~:::>::>::::;.::: 1,000 Hz
4,000 Hz
125 Hz.._m_ 500 Hz:::;:::::~:::::»»::::::;::::: 1,000 Hz
4,000 Hz
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-dB
°i~··.....10··· .., .....·~ ..
Figure 10-16. Polar pattern ofa figure 8 microphone
Figure 10-17. Polar pattern ofa supercardioid microphone
10.3.2 Frequency Response
The frequency response of a microphone is a measure of the consistencywith which it translates a given soundpressure level into a given audio signallevel at different frequencies.
We could say that an ideal microphone would translate a given pressurelevel to the same signal level no matterwhat the frequency (within the limitsofthe audio band, or 20 Hz to 20 kHz).Such a microphone would be said tohave flat frequency response.
While some recording microphonesand many instrumentation microphones closely approach this ideal,most of the units used in professionalsound work deviate from flat response- sometimes quite significantly. Butfrequency response variations are notnecessarily bad.They are often introduced intentionally in order to producespecific performance advantages inpractical applications. Ifyou know theresponse of a particular microphone,you may be able to use that response tocompensate for deficiencies in thequality of a sound source.
At the low end, it is not uncommonfor a microphone's response to fall offbelow about 100 Hz, particularly in thecase of vocal microphones. Since thehuman voice is generally incapable ofproducing energy that low in frequency, the effect of this limitation isto discriminate for voice frequencies,and simultaneously help to eliminateextraneous noise. For instrumentamplification and recording, responseto 50 Hz or below is preferred.
Many microphones exhibit a response peak in the upper frequencies.This is called a presence peak, andagain is characteristic of vocal microphones. A presence peak can help toincrease the intelligibility of words, soit may be a desirable characteristic.But it can also increase the possibilityof feedback in sound reinforcementand is generally to be avoided in micsused for recording.
It is most important, in the case ofdirectional microphones, that the frequency response remain reasonablyflat off-axis, although the sensitivitydrops. Otherwise there will be achange in tonality if the person orinstrument being picked up by the micshifts off axis. Hand-held mics aremost susceptible, since a slight changein the angle on which they are held
will change the tonal color unless themic has uniform frequency response offaxis. Even if a mic remains in a stand,the response should be uniform offaxis, or any reverberant energy willhave a distorted tonal color.
Uniform frequency response off axisis characteristic of good quality mics,and is one performance aspect to lookfor in selecting a microphone. It isprobably more important than theabsolute sensitivity or the actualresponse on axis. Remember, if the frequency response gets rough off-axis,then the quality of a voice will changeas the performer moves around in frontof the mic. This is hardly desirable!
Frequency response variations are amajor factor governing a microphone'scharacteristic sound. It is importantthat the sound of a mic be matched tothe application and the sound source,and this is best done by ear. You canget a clue from the spec sheet. Forinstance, a rising response with a peakin the 5 to 8 kHz range indicates themic is probably optimized for leadvocals, some solo instruments, and forordinary speech reinforcement. A micwith extended low frequency responseand not particularly extended highfrequency response would be useful fordrums or other low frequency instruments. A mic with very flat responsemay be useful in recording, as well asin reinforcement. Since transientquality, coloration, and other factorsaffecting sound quality are difficult tomeasure with any single specification,it is difficult to provide useful guidelines for selection of a mic simply byreading the spec sheet. When in doubt,listen.
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MicROpkoNES 10.3.3 Proximity Effect 10.3.4 Transient Response
Proximity effect is an increase inlow-frequency response when a microphone is very close to the sound source,and is an inherent characteristic of directional microphones (omnis do notexhibit the effect). Figure 10-18 illustrates proximity effect.
Proximity effect increases dramatically when the microphone is less than2 feet away from the source, and canproduce 16 dB or more of bass boost.(Actually, the overall sound levelincreases due to the closer proximity ofthe sound source to the mic, but higherfrequencies are cancelled more thanlower ones by the mic at this distance,so there is the equivalent of bassboost.) This can sometimes causepreamplifier overload, resulting ingross distortion. Announcers andvocalists often use proximity effect toadd fullness to the sound of the voiceand an experienced performer incorporates it as part of his or her mic technique. Public speakers, by contrast, areoften naive about the effect, and itoften destroys intelligibility in publicaddress applications (a low cut filter isusually an effective cure in this case).
Transient response is a measure of amicrophone's ability to render verysharp, fast musical attacks and signalpeaks. The main limitation on transient response is diaphragm mass, socondensers and ribbon mics generallyexhibit better transient response thaneven the best dynamics. Figure 10-19shows a transient response comparisonbetween a typical dynamic (top trace)and a condenser (bottom trace),
Transient response is not very important in vocal reproduction, but itattains great importance with percussive sources such as drums, piano andplucked string instruments. Thetransient nature of such sources is anintegral part of their musical personality, so the ability to render transientsaccurately is highly desirable whenworking with them - either in reinforcement or recording. A ribbon orcondenser is the best choice here.
Generally speaking, the smaller amicrophone, the better its transientresponse will be. This is because asmaller diaphragm has less mass andthus responds more quickly. The trendin recent years has been towardsmaller microphones - this is due inpart to the development of betterelectret elements - so transientresponse has been getting better. As arule, you can expect modern electretunits to have excellent transientcharacteristics.
p . 'ty Effect (B B 1)/' roxnru ass oos
1/4;'~- .. ,(, " -..... \1\- ~
/':W
+10iD~Qj
3 0Cl>>~
~ -10
CondenserMic
DynamicMic
r\OJlV J \ ~ h+ .J V
r\ v
t \f'1 \ f\ h
/ J VV I
IV
50 Jls-I I- ...Note the steep wavefront of the initial transient ismuch more accurately traced by the condenser mic,The rest of the test signal is lower in frequencyand amplitude, and is reproduced about the sameby either mic.
50 100 200 500 1K 2K 5K 10KFrequency Response (Hz)
Figure 10-18. Proximity Effect
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Figure 10-19. Comparison oftransient response in
typical condenserand dynamic microphones
10.3.5 Output Level orSensitivity
Since the transduction element invariably is very small, microphonesgenerate small signal levels whencompared with line-level devices suchas mixing consoles or tape machines.For this reason, a microphone requiresa preamplifier to bring its outputsignal up to line level (see Section 11).This function is normally included inthe mixing console or recorder to whichthe microphone is connected. (Theimpedance converter contained in condenser-type mics should not be confused with a preamplifier. Condensermics still require a preamplifier to beconsistent with line level.)
Microphone output level is alwaysspecified with reference to a specificinput sound pressure level, usually at1000 Hz. The specification is thus anindicator ofthe sensitivity ofthe unit.A more sensitive microphone will produce more output level at a given SPL.
Two reference SPL levels are commonly used for microphone ouput levelspecifications. These are 74 dB SPL(which is the level of an average speaking voice at a distance of 3 feet) and 94dB SPL (corresponding to a relativelyloud speaking voice at 1 foot). Theselevels may also be expressed as:
74 dB SPL == 1 microbar or 1 dyne/cm/
94 dB SPL == 10 microbar or 10 dyne/cmf
Microbars and dynes-per-squarecentimeter are both units of pressure.
The microphone output signal levelis given in dB, with either of twodifferent references: dBV (dB re 1 volt)and dBm (dB re 1 milliwatt). The firstis a voltage reference and the second isa power reference, so the two units arenot directly comparable withoutknowing the specific load impedance(see Sections 13 and 14).
A typical microphone sensitivityspecification, then, might read:
SENSITIVITY:-74 dBm re ImW/microbar
Translated, this means that themicrophone will deliver a signal at1 microbar whose power is 74 dB belowone milliwatt. To determine the signalvoltage that this power level corresponds to, we need to know the loadimpedance.
A more useful form of sensitivityspecification is:
SENSITIVITY:Output level of -47 dBVat94dBSPL
This specification needs no translation, and allows direct and simple calculation of the output signal voltage atvarious sound pressure levels.
10.3.6 Overload
Distortion in a sound system is oftenblamed on microphone overload. Infact, it is rarely the mic that is overloading, but usually the preamplifierstage to which it is connected. A goodquality professional microphone shouldbe able to withstand sound pressurelevels of 140 dB SPL or more withoutoverloading. This is 10 dB beyond thethreshold of pain!
The overload point of a microphonecan be very important in some reinforcement applications, however. Whilethe peak level seen by a mic from arock vocalist who swallows the microphone may be 130 dB SPL or a bitmore, the peak levels encounteredwhen close-miking drums can easily be140 dB SPL. For such applications,then, we should look for a unit with anoverload point approaching 150 dBSPL. With condenser mics, the overload point will be reduced if a lowerbattery (or phantom power) voltage isapplied. Ifyour battery-powered micbecomes distorted for no apparentreason, replace the battery. If a phantom powered mic develops distortion athigh sound levels (high, but still withinthe realm the mic should be able tohandle), you can try using a higherphantom voltage supply, if permittedby the mic manufacturer.
It is important to relate the overloadpoint of a microphone to its sensitivity.Consider, for example, a mic with asensitivity rating of -47 dBV at 94 dBSPL. Ifwe use this mic in close proximity to a drum, it may see peak levels of140 dB SPL. The output level thenwould be:
-47 + (140 - 94) == -1 dBV
or very close to 1 volt! This will certainly overload the mixer's
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preamplifier stage unless a pad is usedto.drop t~e signallevel, (Most qualityIIDXersWIll provide such a pad eitherswitch-activated or in the fo~ of acontinuous rotary control.)
10.3.7 Impedance
The source impedance of a microphone is the equivalent total ACresistance to current flow that wouldbe seen looking into the microphone'soutput (see Section 8). Source impedance determines the size of the loadth?t the microphone can comfortablydnve. Ideally, a microphone should beconnected to a load whose inputimpedance is roughly ten times themic's. source, or output, impedance.
Microphones are usually dividedinto two basic classes: high impedanceand low impedance. Most professionalmicrophones are low impedancedevices, meaning that their sourceimpedance is below 150 ohms (so theyshould be terminated by a 1,000 to1,500 ohm input). Piezoelectric contactp~ckup.s, guitar pickups and inexpensrve microphones are usually high impedance, meaning that their source impedance is 25 kohms or greater (theybenefit from 50 kohm to 250 kohmactual load impedances).
Low impedance microphones arepreferred in sound reinforcement andrecording since, properly connected,they are far less susceptible to extraneous ~oise pickup in the cable (they arese~smgmore current than voltage, so?-Olse mu~t have more energy to getmto the circuit). Such devices usuallyrequire a transformer, when they areconnected to a high impedance input,~o preserve their noise immunity. MoreImportant, low impedance mics candrive cabl~s hundreds of feet long,whereas high impedance mics arelimited to about 20 foot long cables.
High impedance microphones andpickups require a transformer or bufferamplifier when they are used with lowimpedance inputs and/or long miccables. In this case, the transformerconverts the devices' high impedance toa low impedance suitable for drivingthe connection. High impedance micsand pickups usually produce a larger
output signal voltage,which may bewhy they are often used in inexpensiveequipment. Another reason is thatwithout the cost of the transformer orbuffer amplifier, or the 3-pin XLR typeconnector, such mics are usually lessexpensive to manufacture than a lowimpedance mic.
Microphone impedance bears no consistent relationship to price andquality. It is a design factor that isweighed like any other in optimizing amic for a given application. The important point is simply that we must knowthe source impedance of the microphone, and provide the appropriatecircuit or matching transformer tomate it with the input to which it willbe connected.
10.3.8 Balanced andUnbalanced Connections
An unbalanced connection is a twowire system. One wire carries theaudio signal, and the other (called theshield) is connected to ground, or theelectrical reference point. Another termfor unbalanced circuits is single-ended,a.lthough we don't feel the term is particularly precise since it is also used todescribe the operation of certain noisereduction systems.
A balanced connection is a threewire system. Two separate wires carrythe signal - one inverted in polaritywith respect to the other - and thethird is the shield, which again isconnected to ground.
Balanced connections are almostalways used for low impedance microphones. The balanced system is moreimmune to noise, and is by far thepreferred method in professional audio.The most common balanced connectoris the three-pin XLR-type, which ischosen for several reasons. It has 3conductors, it is shielded, it locks inplace, and the ground pin makescontact first to bleed static from thecable and avoid pops.
Unbalanced connections are used forhigh-impedance microphones andpickups, and sometimes for low impedance mics in consumer equipment. Theunbalanced system is susceptible tonoise pickup, and is generally notpreferred in professional work. The
most common unbalanced microphoneconnector is the %-inch phone connector.
Particularly for microphones,balanced connections should be usedwherever possible. Sometimes this willrequire the addition of an externaltransformer at the mixer input, but theadvantages in noise immunity andreliability more than justify the addedexpense.
From BalancedProfessional Microphone
2
BalancedMic Connections
SECTioN 10
3
Standard Mic Cable
Balanced MicrophonePreamp
r---------------------------------------------------
! Transformer Balanced: Input
3:
INPUT(Female XLR)
From UnbalancedMicrophone Unbalanced
Mic Connections
Unbalanced Mic Preamp
jlNPUTj(Female 1/4"iTip/Sleeve Phone Jack),,1 -------------------------------
Figure 10-20. Balanced and unbalanced microphone connections
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10.4 AppllcvrtorsINFoRMATioN
10.4.1 Windscreens andPop Filters
Every microphone is, to some extent,susceptible to extraneous noise frombreath pops or, outdoors, from wind. Insevere cases, these noises can destroyintelligibility or damage particularlysensitive loudspeakers. For the mostpart, the effect of air noise is morepronounced with directional microphones, such as those used in both recording and reinforcement.
Every microphone manufacturerprovides some sort of windscreen,which is basically an air velocity filterthat protects the diaphragm elementfrom air noise. The most commonmodern type is made of highly porousacoustical foam. Typical foam andmetal mesh covered foam windscreensare shown in Figure 10-21.
Figure 10-21.Commonwindscreens
The foam is essentially transparentto sound pressure waves, but acts likea kind oflabyrinth for high velocitywind or breath gusts. The air gustslose their energy as they travelthrough the channels in the foam, sothat they dissipate before reaching thediaphragm.
In recording studios, breath pops areoccasionally a problem while recordingvocalists. A simple pop filter consistingof nylon mesh stretched on a smallhoop can help. The hoop is mounteddirectly to the microphone, about 3 to 6inches away from the microphone body.The vocalist sings at the microphonethrough the nylon mesh, which stopsthe gusts produced by explosive consonants such as p's and t's.
In an emergency, a makeshift windscreen can be made from a whiteathletic sock slipped over the microphone. While this is hardly the mostattractive solution from a visualstandpoint, it can save the day inoutdoor paging applications. Then, too,you might just try switching from acardioid to an omnidirectional mic,which may sufficiently reduce windnoise or vocal pops to eliminate theneed for any sort of wind sock or filter.
10.4.2 Shock Mounts
Stand mounted microphones aresometimes subject to physicalvibrations coupled from the stand tothe mic housing through the mountingclip. To diminish this source ofextraneous noise, some microphonemanufacturers provide shock absorbingmounts. One typical such mount isshown in Figure 10-22.
Here, the mic is suspended within aframe by a kind of eat's cradle of elasticbands, which effectively absorbs noisesfrom jarring of the stand.
In an emergency, a makeshift shockmount may be improvised with a pieceof acoustical foam or kitchen spongeand some duct tape. Wrap the foam orsponge around the mic housing andsecure it with a bit of tape, then :fix themic to the stand with more tape.Again, this may look a bit scruffy, butit can greatly improve shock isolation.
Figure 10-22. A typical shockmounted microphone on a stand
10.4.3 Phantom Power 10.4.4 Effect of the Numberof Open Microphones (NOM)
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Figure 10-23. How phantom power and audio share the same cable.
Condenser microphones require apolarizing voltage and power for theirbuilt-in amplifiers. Sometimes provision is made to supply this voltagedirectly through the microphone cable.The procedure is called phantompowering, and the most commonphantom supply voltage available inmixing consoles is 48 VDC, although24 V supplies are widely used. Mostphantom powered mics can operate ona wide range of supply voltages from aslittle as 1.5 or 9 volts up to 50 volts.
In a phantom power system, the polarizing supply voltage is placed onboth of the signal lines in a balancedconnection, with the same polarity oneach line. Dynamic microphonesconnected in a balanced system with aphantom power input are then protected from damage, theoretically,since the system results in a net zeroDC potential across the coil. A dynamicmic connected unbalanced to a phantom power input may be destroyed,however!
It is therefore very important to beaware of whether a mixing consoleinput is wired for phantom power.Most such inputs provide a switch todisable the phantom power when it isnot needed. Always be sure that thisswitch is set to off when dynamics, orelectret condensers with internalbatteries, are connected to the input.
Figure 10-23 illustrates how phantom power is delivered to the impedance converter in a mic along the sameconductors that carry audio from themic to the console preamplifier.
Phantom Power isdelivered to acondenser mic, andaudio signal to aconsole via the same2-conductor sheldedmicrophone cable.
+48V
BLOCKINGCAPS
NOTE: DC cancelsat transformerbut AC signalgoes through
(similar withactive, balanced
input)
As we learned in Sections 5 and 6, aprimary concern in sound reinforcement is maximizing a system's acousticgain. To do so, we must:
a) keep the distance between the micand loudspeaker as large as ispractical;
b) keep the distance between the micand the source as small as is practical; and
c) use directional mics and loudspeakers, placed so that their interactionis minimized.
All the calculations in our discussionof system gain and feedback controlassume a single mic. It is intuitivelyobvious that as we add mics to asystem, the potential acoustic gainmust decrease and feedback potentialrise. In actual fact, every time youdouble the number of open mics (micsthat are turned on or whose level isbrought up on the mixing console), thesystem gain must be reduced by 3 dBto avoid feedback.
In operating a sound system, then,we need to be aware that it is best atany given time to turn on only thosemics that are required. So long as agiven microphone is not used at anytime, it should be muted, or its faderon the mixer should be brought down.There is something to be said for usingthe least number of mics that you canget away with. It's fine to try to duplicate studio quality in stage work, butusing six or eight mics on the drum setmight be self-defeating.
All of this means more work and agreater demand for creativity from thesound engineer, but it can result inhigher system gain, better sound
quality, and less potential forfeedback.
J1~
~~oSignal
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CONSOLEINPUT
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10.4.5 Gain andMicrophone Placement
System gain can also be enhanced bygood mic placement techniques. Ingeneral terms, the closer that we canmic sound sources in a reinforcementsystem, the less electrical gain we willneed. This means both cleaner soundand less potential for feedback.
It can also mean greater potentialfor overloading the preamplifier input(see Section 1.3.5, "Overload"). In closemiking situations, if gross distortion isencountered, it is most likely to be dueto the preamp - not the mic - and itmay be necessary to use a pad at thepreamp input. The mics used shouldhave high SPL capability so that micoverload is avoided.
Public speakers can present problems ifthey are unused to workingwith a microphone. A lecturer whowanders around in front ofthe mic willproduce a widely varying signal level,particularly if the mic is a cardioid.The best solution with such a source isto use a lavalier. The sound qualityand level will be much more consistent,making the engineer's job much easier,and the result better.
Figure 10-24. Spaced pair stereoplacement, with a
separate soloist mic
10.4.6 Stereo Recording
Ironically, one of the subjects mostopen to debate in professional sound issimple, two-microphone stereo recording. A number of different techniqueshave been proposed and used - eachrequiring particular pickup patternsand placements - with varyingdegrees of success. Since sound is ahighly subjective matter, of course,each method has its avid proponents.
A full discussion of all the variousmethods of stereo recording is beyondthe scope of this handbook, and we willconfine our discussion to the threemost common techniques: the spacedpair, X-Y, and M-S methods.
The spaced pair is an extremelysimple and successful technique. Twomicrophones are used, and these areplaced on stands spaced 6 to 8 feetapart and 6 feet or so above theground. Either omnidirectional orcardioid units may be used. Omnis canoffer slightly better quality, but lessrejection of audience noise and reverberation. A third mic may be used tofeature a soloist, if desired. Figure10-24 illustrates the technique.
While the spaced pair can yield acceptable results, it is susceptible todelay problems (or localization problems) associated with the differentpath lengths from sources to the twomicrophones. In an attempt to improvestereo imaging, some recordists use atechnique called X-Y.
Figure 10-25. Stereo microphonesin an X-Y configuration
The technique requires two cardioidunits, preferrably with matchedcharacteristcs. The two are mounted onthe same stand with a special mounting bar and angled at about 45 to 60degrees, with the diaphragms as closetogether as possible. Stereo imagingmay be compromised (there is disagreement here), but is generally quite
good when heard with headphones andacceptable over loudspeakers. Figure10-25 illustrates the technique.
A third technique that is often usedfor recording for broadcast is the M-Smethod (an abbreviation for Mid-SideStereophony). M-S recording requiresone cardioid and one figure-8 element,placed on the same stand with theirpatterns oriented as shown in Figure10-26.
Stereo information is extracted fromthe mic signals by a matrix whichproduces a sum channel (the two addedtogether) and a difference channel (thefigure-8 signal subtracted from thecardioid signal). The technique isvalued for broadcast because it retainsmono compatibility. The sum ofthe twosignals cancels the figure-8 signal,leaving only the cardioid signal.
ISOURCE
IMI.____0__
to:\ 4~
Figure 10-26. Sensitivity patternsof cardioid & figure-8 mics in an
M-S stereo configuration
10.5 WiRELESS
INTERCOM SYSTEMS
Some information in the wirelessintercom and wireless mic sections ofthe handbook was originally written byGary Davis and Associates in cooperation with Bill Swintek ofSwintekEnterprises, Inc., ofSunnyvale, CA (awireless microphone and intercomsystem manufacturer) for a 1983magazine article. That copy has beenedited and expanded here, and is usedwith the permission ofSwintek Enterprises. The majority of the wireless micand intercom information here wasprovided by HME Electronics, Inc. ofSan Diego, CA, also a manufacturer ofwireless mic and intercom equipment.We are appreciative of the assistanceprovided by both of these companies.
10.5.1 What is a WirelessIntercom?
A wireless intercom is a system withwhich two or more people can communicate from reasonable distances apart.Its equipment is miniaturized, and isnot connected by wires or cables, thusproviding maximum mobility to itsusers.
There are two basic types of professional, wireless intercom systems. Onetype is operated with a console basestation and one or more wireless,remote units. The other type is operated with a battery powered, wirelesstransceiver and one or more wireless,remote units. The remote units consistof small belt pack transceivers, andheadsets with attached microphones.The base station operator can communicate with all crew or team membersusing the belt packs, simultaneously.
10.5.2 Who Uses WirelessIntercoms?
Wireless intercom systems havebecome a necessary part of manycommunication networks in recentyears. The flexibility provided throughtheir use is indispensable in manyproduction, training, security andindustrial applications. Rapid growthin the videotape production industry
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has created a need for wireless intercoms to be used as extensions of wiredsystems already in place. They areinvaluable as communication aidsbetween directors, stage managers andcamera, lighting and sound crews intheater and film productions. In sportsevents, wireless intercoms are not onlyused by coaches, spotters and players,but also by sportscasters and newsproduction crews. In activities such asstunt filming, circus acts and gymnastics, in which cues and timing arecrucial to safety and successful performance, the wireless intercom hasbecome a critical asset. The applications of wireless intercom systems arelimited only by the imagination of theirusers.
(FCC) allocation of specified frequencybands for wireless intercoms haseliminated radio interference fromother services.
Today's wireless intercoms performas well as conventional, wired intercoms. In the 1980s they are beingmanufactured with improved dynamicrange and smaller transponders, aresult of better compandor integratedcircuitry and advanced circuit designtechniques. A variety of wirelessintercom equipment is presentlyavailable in various configurations.
10.5.4 Types of WirelessIntercoms
TRANSMITIER
Figure 10-27. Simplex wirelessintercom system
There are three basic types ofwireless intercom systems: simplex,half duplex and full duplex. A simplexsystem permits one-way communication only, such as ordinary radio broadcasting in which the listener can hearthe announcer but cannot respond. Ahalf duplex system operates like awalkie talkie, allowing the users tocommunicate one at a time, only whilepressing a button. A full duplex system, however, provides continuous twoway communication without pressing abutton. This is the most desirable typeof system, since it provides completehands-free mobility to the user withthe advantages of normal uninterrupted conversation. Brief descriptionsof each type of system are given below.
• SimplexSince only one-way communication
is possible in a simplex system, it isuseful only in dispersing information
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10.5.3 What Is TheBackground of WirelessIntercoms?
Early intercom systems generallyconsisted of fixed units, which werewired in place. Any mobility dependedon the length of cable connecting theirheadsets to base stations. As the usersmoved around, the cables had to bedragged with them and lifted overobstacles. Outside interference noisewas also a problem.
Walkie talkies were the earliestform of wireless intercoms used. Theywere heavy and cumbersome and hadto be connected to large batteries fromwhich they obtained their power. Theirreception was easily distorted andnoisy. Wireless communication hascome a long way since the walkietalkie.
Technological advances since thelate 1960s have tremendously affectedboth the size and performance ofwireless intercoms. The development ofsemiconductor technology improvedtheir dynamic range and audio qualitysignificantly.
Technology in the early 1970s introduced the integrated circuit compandor, which was incorporated intowireless intercoms to reduce noise.Later, the application of diversityreception minimized the problem ofdropouts (transmission losses), greatlyimproving system reliability. TheFederal Communications Commission
IRECEIVER I
",--?IRECEIVER I
IRECEIVER I/,,--IRECEIVER I
when no reply is necessary. Pagingsystems at airports or in departmentstores and hospitals are simplex systems. Because of its simple circuitry,this is the least expensive type of intercom system. (Refer to Figure 10-27.)
• Half-duplexBecause of its affordable price range
and the fact that it provides two-waycommunication capabilities, this is themost popular type of wireless communication system. (Refer to Figure 1028.) Half-duplex systems consist of oneunit which serves as the base stationand several remote units. The basestation may either be a console whichplugs into an AC outlet or is poweredby a 12 volt battery, or it may be amobile belt-pack unit with a miniaturized transceiver. The base stationoperator, usually a director or crewsupervisor, can communicate freelywith all crew members, transmittingand receiving simultaneously. Hisinstructions can be heard by all crewmembers at once. Thus, prioritymessages from the director reach allcrew members without delay. The basestation also simultaneously rebroadcasts all incoming messages. Each crewmember's communication with thebase station can thereby be heard byall fellow crew members. Althoughcrew members cannot communicatedirectly with each other, they cancommunicate via their supervisor ordirector. Crew members can hearincoming messages at all times. Inorder to transmit from a remote unit,crew member must press a button ontheir belt packs. Only one member isable to transmit at a time. The halfduplex is cost effective and efficient for
mostope/~~VC'
~CPRESS-1O-TALK VC'~I. REMOTE
~~~~'~vO~. REMOTE
rii
Figure 10-28. Half-duplex wirelessintercom system
• Full-duplexThis is the ideal form of wireless
intercom system since it provides theonly truly hands-free operation. With afull duplex intercom, uninterrruptedcommunication is possible, as in anormal telephone conversation. Themajor difference in this system and thehalf duplex is that a full duplex intercom is capable of continuous transmission in both directions. It is not necessary to press a button to transmit. (SeeFigure 10-29.) The discrete full-duplexsystem, operates with only two units: abase station and one remote unit.Transmitting and receiving by theseunits is done on two different frequencies. A message is transmitted on onefrequency by one of the units andreceived on that same frequency by theother and visa versa. If a larger communication network is required withmore than two wireless belt packs, thesystem becomes complex. In order toaccomplish this, a base station thatwill transmit to all the wireless beltpacks on a single frequency is needed.At the same time, separate receiversfor each wireless belt pac are necessaryat the base station. The base stationthen functions as a repeater, receivingmessages from each remote unit andretransmitting them back to all theremote receivers at once on a singlefrequency. The complexity ofthis extended full duplex system increases itscost significantly and is therefore notcost effective for all operations. Currently available systems permit use offour to six full duplex, wireless remotebelt pack units in this fashion, providing full hands-free communication toall users at once.
/1 HANDs-FREE VC'REMOTE
~ VC'HANDs-FREEREMOTE
~I VC'HANDs-FREEREMOTE
Figure 10-29. Full-duplex wirelessintercom system
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• Integrated SystemsThere are as many varied
configuration requirements for wirelessintercom systems as there are users.Systems may be integrated in almostany imaginable combination. One usermay need to link a PA system (simplex)to a full duplex system. Another usermay want to hook up several halfduplex wireless belt pack units to anexisting cabled intercom system. Atypical remote belt pack transceiverhas provisions for either half duplex(push-to-talk) operation or may beswitched to full duplex, which providescontinuous hands-free transmissionand reception. With nine volt alkalinebatteries, the belt packs may operatecontinuously for eight to ten hours.More than four wireless belt packs mayeasily be accomodated by adding basestation for additional channels, or byletting several belt packs use the sametransmit frequency. In this case, pushto-talk is mandatory because only onesignal can be transmitted withoutinterference at a given time. Wiredstations, generally used at fixedpositions for cameras and lights, arethe most cost effective. But the directoror crew supervisor may prefer wirelessstations for mobility. Wireless systemsare also needed for positions that arenot practical to wire. Whatever its application, the wireless intercom provides greater mobility than its cabledcounterpart.
SERVICEAREAS
10.5.5 Frequencies Used
Audio bandwidth is not a criticalfactor with wireless intercom systems.Some frequency bands are moresubject to interference from adjacentfrequencies than other. For example,the 400 to 470 MHz, ultra-highfrequency band, of which wirelessintercoms and microphones utilize the450 to 451 and 455 to 456 MHz frequencies, is also used by police, fireand public health service radios. Anytime nearby frequencies such as theseare in intermittent use, there is a riskof random interference that was notdetected during equipment setup.
The most practical and commonlyused frequency band for wirelessintercom systems in the U.S.A. is theVHF band, from 26 to 27, 35 to 43 and154 to 174 MHz. Different manufacturers use different frequencies, and theirsystems are necessarily preset to thosegiven frequencies. The buyer must beaware of these factors in choosing themost appropriate system in order toavoid interference from local broadcast.Interference from harmonics of scheduled broadcast must also be considered. That is, if an FM radio programis being broadcast at 88 MHz, it willalso appear at 176 MHz, and othermultiples ofthe primary broadcastfrequency.
Some manufacturers utilize a "splitband" system. In this type of system,the base station may transmit on theVHF high band, while the remote unitstransmit at VHF low band. In splitband operation, coordination with theFCC is important to be sure that bothfrequencies are in compliance with thesame section under FCC regulations.
To operate a wireless intercomlegally in the United States, a FederalCommunication Commission stationlicense is required. The type of licensedepends on the use to which yourintercom will be put. Local FCC officesor the equipment manufacturer should
A
B
C
~...............................................................................................................................................................................~ ~ ~ ~
FREQUENCY (MHz)
Figure 10-30. Wireless intercom frequency bands
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o 100 200 300 400 500 600 700 800 900
be contacted for further information.Figure 10-30 indicates the radio frequency allocations for wireless intercoms in the United States. In othercountries different frequency allocations as well as operating regulationsmay apply. Local authorities should beconsulted prior to system selection oroperation.
10.5.6 Improved Range andNoise Reduction
There are now numerous systemsdesigned for improved audio range andnoise reduction. Wireless intercomsystems do not need to have high leveldynamic range since they are usedprimarily for speaking and do not haveto produce a natural or enhancedmusical quality. However, it has beendemonstrated that the natural voicequality of today's systems is lessfatiguing over a long period of timethan the highly compressed audiosound of a few years ago. This is abenefit of the improved range andsignal-to-noise ratio of state-of-the-artwireless intercom technology.
Prior to the late 1970s, most wireless intercoms could not efficientlyreduce unwanted noise. Today, manysystems include cornpander circuitry,the most advanced noise reductiontechnology. In a compander circuit, afull-range compressor is built into theintercom transmitter and an audioexpander into the receiver. When thesignal is compressed, the audio levelremains well above the residual noisefloor. When it is expanded again, thenoise is reduced and the signal is muchcleaner and relatively noise-free. Lowlevel hiss and static are virtuallyeliminated. Companding the audiosignal also provides improved dynamicrange over a straight transmission. Insome cases the wireless intercom mayactually be quieter than its cabledcounterpart.
10.5.7 Evaluating andSelecting a System
There are a number of criteria thatmust be considered in evaluating andselecting a wireless intercom systemsuitable for professional use. Ideallysuch a system must work perfectly andreliably in a variety of tough environments with good intelligibility andmust be useable near strong RF fields,lighting dimmers and other sources ofelectromagnetic interference.
• Operating FrequencyIf a wireless intercom is going to be
used effectively at frequencies adjacentto other strong signals which mightinterfere with the clarity of its
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reception,the extra expense of a morecomplex receiver will be necessary.Thus the operating frequency of awireless intercom system is a factor tobe considered in selecting your equipment.
• DiplexerWireless intercom and microphone
systems normally operate on like frequency bands, thereby oftenbenefitting by combining systems,enabling a director or crew supervisorto have closed and open communication. Up to 24 discrete VHF high-bandmicrophones and intercoms can beoperated in the space of a single TVchannel. However, such complexsystems often experience desensing,the muting of a receiver becauseanother mic or intercom is transmitting in close proximity, thus limitingits effective range. Some systems haveantenna diplexers and may thereforebe immune to the desensing problem.
• Side ToneAn important feature to look for in
an intercom is side tone which confirmsthat communication is actually takingplace. Side tone simply means that theuser hears his voice as he talks, butonly after it has been retransmitted tohim. Non-duplex systems cannot offerside tone or have local side tone inwhich the voice is fed directly into theearpiece through a preamp and thusdoes not confirm two-way communication.• Headsets
Some wireless intercoms are builtentirely into headsets. While theseunits are very compact, they are oftenheavy and uncomfortable as well aspoor in serviceability and soundquality. In other intercoms the transceiver is packaged separately and isdesigned to work with a variety ofheadsets. This will usually be most costeffective for the buyer who already hasheadsets. To assure compatibility,investigation should be made regarding which kind of headsets are bestsuited to the particular transceiverunder consideration.
• BatteriesThe type of batteries used in a
wireless intercom must also be considered. A rechargeable system can be
economical over a long period of time.On the other hand, fresh throw-awaybatteries before each show provideconfidence that a wireless intercom willlast to the end ofthe show. The systemshould be capable of operating at least4 to 6 hours on one set of batteries.Rechargeable nickel-cadmium batteries are more economical in the longrun, but they are also more difficult tomaintain. Ifnot deep-cycled (fullydischarged and recharged), they willnot yield nearly as long an operatinglife between charges as a set of fresh,non-rechargeable alkaline batteries.
• Future NeedsOne of the most important consid
erations to be made in wireless intercom selection is future needs. A systemshould be compatible with other typesof systems and equipment to allow thegreatest possible adaptability to futureneeds. Perhaps one system may besomewhat more expensive than another, but it may be much more economical in the long run in maximizingfuture operational capabilities.
10.5.8 Conclusions
Today's wireless intercoms are agreat improvement over the cabledsystems of just a few years ago. Themobility they provide is an invaluableasset to nearly any industry. Theirversatility, through integration withexisting cabled intercom systems, aswell as with wireless or cabled microphone systems, is another advantage.Their audio bandwidth and signalclarity far exceeds the requirements ofmost users. They excel in sound quality, and in their ability to solve manytypes of production communicationproblems.
10.5.9Glossary of WirelessIntercom Terms
Bandwidth - The range of frequencies within which performancefalls, with respect to some characteristic - usually the -3 dB points.
Belt pack - Communication equipment worn on a belt. In a wirelessintercom system, the belt pack (or beltpac) usually includes a transmitter, areceiver and a headset with built-inmicrophone.
Compander - A combination of acompressor at one point in a communication path for reducing the amplituderange of signals, followed by an expander at another point for a complementary increase in the amplituderange.
Compressor - A signal processorthat, for a given input amplituderange, produces a smaller outputamplitude range.
Diplexer - An electronic apparatus which allows a single antenna toconnect to a transmitter and receiversimultaneously.
Diversity Reception - Where asignal is obtained by combination orselection or both, of two or moresources of received-signal energy thatcarry the same modulation or intelligence, but that may differ in strengthor signal-to-noise ratio at any giveninstant minimizes the effects of fading.
Dynamic Range - The difference,in decibels, between the overload (ormaximum) level and the minimumuseable signal level in a system.
Expander - A signal processorthat, for a given amplitude range ofinput voltages, produces a larger rangeof output voltages.
Frequency Band - A continuousrange of frequencies extending betweengiven low and high frequency limits.
Integrated Circuit - A combination of electronic circuits, each of whichhas an independent function, but arelinked together as an interdependentnetwork; usually packaged in a single,small chip or semiconductor-basedmicrocircuit.
Preamplifier - An amplifier whichboosts a low-level signal, providingenough level for the signal to befurther processed.
Repeater - An electronic devicewhich receives a signal and thentransmits the same signal on a different frequency.
RF Field - An energy field (a definable area) where radio frequencysignals are prevalent.
Semiconductor - An electronicconductor, with resistivity in the rangebetween metals and insulators, inwhich the electric charge-carrierconcentration increases with increasing temperature (over some temperature range).
Signal-to-Noise Ratio - The ratioofthe level ofthe signal to that ofthenoise level (usually expressed in dB).
Split Band - A communicationsystem in which signals are transmitted in opposite directions on differentfrequency bands.
State-of-the-Art - The mostmodem technology available to solve aproblem in a unique way for the firsttime.
Transceiver - The combination ofradio transmitting and receiving equipment in a common housing, usually forportable or mobile use, and employingcommon circuit components for bothtransmitting and receiving.
Transponder - A receiver-transmitter combination that receives asignal and retransmits it at a differentcarrier frequency.
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10.6 WiRELESSMicROpl-tONE SySTEMs
10.6.1 What is a WirelessMic?
A wireless microphone system is asmall scale version of a typical commercial FM broadcasting system. In acommercial broadcasting system, aradio announcer speaks into a microphone that is connected to a highpower transmitter in a fixed location.The transmitted voice is picked up byan FM receiver and heard through aspeaker or headset.
In a wireless microphone system,the components are miniaturized butthe same principles apply. The transmitter is small enough to fit into themicrophone handle or into a smallpocket sized case. Since the microphone and transmitter are batterypowered, the user is free to movearound while speaking or singing intothe mic. The transmitted voice ispicked up by a receiver that is wired toa speaker.
~otypesofmicrophonesare
available with wireless mic systems:the handheld mic, with a transmitterin its handle; and the lavalier mic,which is small enough to be concealedas a lapel pin or hung around the neck.Lavalier mics are wired to miniaturebody-pack transmitters, which fit intoa pocket or clip onto a belt.
10.6.2 Who Uses WirelessMics?
Wireless microphones are widelyused today in television and videotapeproduction. They eliminate the needfor stage personnel to feed cablesaround cameras, props, etc. For location film production, as well as ENG(Electronic News Gathering) and EFP(Electronic Field Production), wirelessmics make it possible to obtain usablefirst take sound tracks in situationsthat previously required post-production dialogue looping. The cost savingcan be significant.
Handheld mics are used by performers on camera where they provide thefreedom needed to move around thestage and gesture spontaneously. Theyare used by speakers and entertainers
who need to pass the mic from oneperson to another. In concerts, handheld wireless mics permit vocalists towalk and dance around the stage andeven into the audience without restriction and with no chance of shock in theevent of rain.
Lavalier mics are used in gameshows, soap operas and dance routines.They eliminate the need for boom micsand help to alleviate visual clutter.Lavalier mics are used by MCs, panelists, lecturers, clergy, stage actors, anddancers because they can be concealedeasily and provide hands-free mobility.Some lavalier transmitter models havehigh impedance line inputs that acceptcords to create wireless electric guitars.
10.6.3 What is theBackground of WirelessMics?
Technological advances since thelate 1960s have tremendously affectedboth the size and performance ofwireless mics. Until that time wirelessmics were large and used miniaturevacuum tubes, offering limited dynamic range and poor audio quality.The development of semiconductortechnology in the late 1960s reducedthese problems significantly.
Technology in the early 1970s introduced the integrated circuit compandorwhich was incorporated into wirelessmics to reduce noise. At about thesame time, the FCC authorized the useof frequencies in TV channels 7-13 forwireless mics. Thus the wirelessmicrophone's most serious problem,radio interference from other services,was virtually eliminated. Later, theapplication of diversity receptionminimized the problem of dropouts(transmission losses due to cancellation of radio waves), greatly improvingsystem reliability.
Today's wireless mics perform aswell as conventional wired mics. In the1980s, wireless mics are being manufactured with improved dynamic rangeand smaller transmitters, a result ofbetter compandor integrated circuitryand advanced circuit design techniques. A variety of standard microphones with different sound characteristics is available.
10.6.4. Radio FrequenciesUsed
There are no international standards for wireless mic radio frequencyallocations. Performance is not controlled for transmitter power limits,frequency stability, or RF bandwidthoccupancy. Wireless mics could therefore, theoretically, operate at any:frequency. Certain frequency bands,are more commonly used. (Refer toFigure 10-31.)
taneously without RF intermodulationproducts causing interference.
• Wireless mics are permitted tooperate in the commercial FM broadcasting band, providing their power isnot greater than 50 JiVIM radiation at15 meters. With this power restriction,it is not practical to use this band forprofessional applications in which
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0 0 0
~r-,
0 0<0 "<t
~~ ~~ Jr--a!. 0 ~'" <0 ~ "<t
..Jl.llJlJ LD
g
i_________IJ.
FREQUENCY (MHz)
Figure 10-31. Commonly used international frequency bands
In the United States, the FCC regulates the operation of wireless mics atspecified frequency bands. Frequencybands typically used are shown inFigure 10-32. The applicable FederalCommunications Commission regulations are as follows:
SERVICEAREAS
reliable transmission performance isexpected.
• Wireless mics may operated on ashared basis with business radioservice. Continuous radio transmissionis authorized if the transmitter poweris limited to 120 mW. The businessradio service frequencies for wireless
······························R·················..····· .
A
B
C
D
"".·-·-R-···---·_··'-·-···-·-·_··-·-·-·-·-H--'-·--···-·-·- - --y.-.-.-.-.-.-.-.-.-.- ~~.-.- -. ----.-.-~-.- -.- -.. ·__ .. ········ ....-·~·········_·--R·_· .. ·_···-·· ..·_---_·
o 100 200 300 400 500 600 700 800 900
FREQUENCY (MHz)Figure 10-32. Wireless microphone frequency bands used
in the United States
• Low power communication devicesmay operate in the 49.81 MHz to49.90 MHz band (Figure 10-32 (a)).Power is limited to 10,000 JiVIM radiation at a 3 meter distance (approximately 1 to 5 mW) and with a 5 kHzaudio frequency limit. This segment ofthe RF spectrum is susceptible to manmade noise generated by auto ignition,flourescent lights, dimmers, etc. Therestriction imposed by the FCC on lowpower equipment aggravates theproblem of signal-to-noise ratio in thisband. Because these frequencies areevenly spaced (15 kHz apart), onlythree wireless mics can operate simul-
mics are: 30.76 MHz to 43 MHz, VHFlow-band; 150 MHz to 173.4 MHz, VHFhigh-band: 457MHz to 470 MHz, UHFlow-band; and 806 MHz to 866 MHz,UHF high-band (Figure 10-32 (b)).At 150 MHz and higher, man-madenoise decreases significantly. With thehigher power and larger transmissionbandwidth allowed by the FCe, alongwith many more available frequenciesand the shorter antenna requirement,operation in the VHF high-band andhigher is desirable. The major disadvantage is interference from otherbusiness radio services. An operatingstation license is required and the
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transmitter must be type acceptedunder FCC regulations. Contact yourlocal FCC office for aForm 25 if youwant to obtain a license. Anyone canoperate a wireless mic system at thesefrequencies. Specific frequenciesdesignated in this part ofthe regulation for wireless mics are known as "Bfrequencies." The bands are from169.445 MHz to 170.245 MHz, from171.045 MHz to 171.845 MHz , from169.505 MHz to 170.305 MHz and from171.105 MHz to 171.905 MHz (Figure10-32 (c». "B" frequency transmissionsmust not exceed a bandwidth of 54 kHzand output power of 50 mW.
• For broadcasting, video production,and filmmaking applications, wirelessmics may operate in the 174 MHz to216 MHz range (TV channels 7 thru13), on a non-interference basis.(Figure 10-32(d». This means that fora given location, wireless mics mayoperate on unused TV channels.Transmitter power is limited to50 mW. An operating station license isrequired for broadcasters andfilmmakers and the transmitter usedmust be type accepted under FCCregulations. This VHF high-band offersthe best operating area for wirelessmics. It is free of citizens band (CB)and business radio interference, andany commercial broadcast stations thatmight cause interference operate on aschedule, and thus can be avoidedeasily.
10.6.5 Technical Problems
• Transmission LossThere is a calculated transmission
loss between transmitter and receiverthrough use of an isotropic antenna.Less transmitter power is required foran equivalent signal strength at thereceiver as frequency is lowered. Oneproblem with wireless microphones isthe difficulty in designing antennasthat are small but efficient in the VHFlow-band area. However, for the VHFhigh-band, small and efficient antennas are practical.
Interference from other radioservices is the major problem at bothVHF and UHF. The only clear channels available are the unused TV
channels in a given location and the"B" channels. For touring groups theTV channels become a problem, as aclear TV channel in one city may not beclear in another. Therefore, the "B"channels are recommeded for thispurpose.
• DropoutLoss of reception at the receiver of a
wireless mic due to radio wave cancellation called multipath reflections isusually referred to as dropout. Thisproblem has several possible sources.Dropout characteristics are differentbetween VHF and UHF frequencybands. The dropout zones are muchshorter at UHF where rapid flutter isoften heard.
Loss of reception may also be causedby a transmitter being too far from thereceiver. This may be corrected byrelocating either the transmitter andreceiver antennas closer to each other.
The power of a signal received by anantenna is a critical factor in causingdropout. When examining practicalsolutions and limitations in alleviatingdropout, it is important to considerthat not all of the power transmittedwill reach the receiver. A wireless mictransmitter radiates power in manydirections simultaneously, dependingon the specific mechanical configuration of the antenna system. This makesthe transmission vulnerable to manytypes of interference.
System performance is degraded bypath losses due to interfering objectsbetween the transmitter and receiver,such as other equipment or people, aswell as by the position of the transmitter antenna and interfering signals dueto multipath reflections.
Several paths can occur when theenvironment in which the wirelessmicrophone is operating containsobjects such as cameras, lightingequipment, or stage props made ofmetal or other materials that reflectradio signals. Due to phase differentialof the arriving signal, the resultantsignal can be enhanced or totallycancelled, creating multipath dropouts.These path losses affect the total powerreceived at the antenna. Multipathcancellation is the most common causeof dropout.
10.6.6 Solutions
• Use a High Gain ReceivingAntenna at the Mix PositonHigh gain antennas can improve the
SIN ratio, and may thus reduce fadesand dropouts if they are due to weaksignals. Signal cancellations will not beaided. High gain receiving antennasare generally also a bad idea because:(a) the transmitter is constantlymoving around with the performer sothe antenna would have to be continuously re-aimed, and (b) much of thereceived radio signal is actually caughton the bounce from walls, props, etc.,so even if one stood offstage and aimeda beam antenna at the performer, itcould be aiming at the wrong target.
• Place the Receiving Antenna(s)and Receiver(s) Near the Mic(s)and Run Audio Signals Back tothe Mix Positon
With wireless mics, an alternative isto place the receiving antenna(s) on orabove the stage, run a moderate lengthantenna cable to a nearby wireless micreceiver, and then run as long a standard audio cable as necessary betweenthe receiver's audio output and themixing console's input. Most receiversprovide line level outputs that are idealin this situation. This keeps the mictransmitting antenna(s) and thereceiving antenna(s) reasonably close,which optimizes the RF SIN ratio.
• Diversity ReceptionIn some wireless microphone instal
lations, it may be impossible to locate asingle antenna to eliminate multipathdropout or signal fading. The techniquethat has been adapted for wirelessmicrophones to minimize multipathdropouts is called diversity reception.This is the application of two or morereceiving antennas to receive signalsthat have been diverted into more thanone path (multipath). The idea, ingeneral, is that if the signal is weak atone antenna, it will probably bestronger at the other, at any giveninstant. Diversity reception enhancesthe performance of a wireless micsystem. It is usually effective, althoughnothing will guarantee a total absenceof dead spots. There are a number ofdifferent ways to accomplish diversityreception, and each manufacturer ofwireless microphones tends to favor
one approach or another. The conditions required to achieve this receptionare:
• a single transmitter source• uncorrelated, statistically
independent signals• multiple receiving antenna systems
The success of any diversity reception system depends on the degree towhich the independently receivedsignals are uncorrelated. Ifa diversityreception system cannot produceuncorrelated, statistically independentsignals, then diversity reception doesnot exist. A diagram of a basic diversity reception system is shown inFigure 10-33.
-----"s~~~~~~~~~~-"'7
UNCORAELATED INDEPENDENT SIGNALS
MULTIPLERECEIVINGANTENNASYSTEMS
Figure 10-33. Diversity receptionsystem
Implementation of a diversityreception system can be accomplishedin several ways, but all system implementations have the need to combinethe received, independent signals insome method. The major drawbackwith any multiple reception diversitysystem is cost. Combining techniquesare chosen based on cost and thedegree of improvement required. Theless predictable or less closely relatedthe signals, the more significant thebenefits of the diversity system.
There are various techniques ofdiversity reception based on the exactmethod for processing and extractingthe transmitted signals. Space diversity is the technique most commonlyused for wireless mics, Space diversitycan be implemented in many differentways, but the three basic requirementsof diversity reception mentioned earliermust be satisfied. Two or more receiving antennas are required and must beat least one half wavelength apart(typically 3 feet). The amount of
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separation determines the degree ofthe uncorrelated signals. Polarizationdiversity is a method of space diversityin which the antennas on the receivingsystem are placed at angles to eachother in order to capture the uncorrelated, independent signal. Each antenna provides an independent paththat is selected or combined to producethe desired signal improvement. Theseselecting and combining methods ofprocessing the independent signals areshown in Table 10-1.
COMBINING METHOD TECHNIQUE
Selection (Also refer- Switches to optimumred to as 'switching' inputor 'optimal swltchinq'
Maximal ratio (also Adds signals with vert-referred to as 'vorl- able gain amplifiersable gain')
Equal gain (also refer- Adds signals linearlyred to as 'linear ad-der')
Another method of signal improvement is that of combining the incomingindependent signals. The two methodsof doing this are called maximal ratiocombining and equal gain combining.The techniques for maximal ratio andequal gain combining are illustrated inFigure 10-35. In maximal ratio combining, independent signals are combined in order to derive the maximumsignal voltage/noise power ratios fromeach of them. A modification of thisapproach is equal gain combining inwhich all incoming signals are set to anaverage constant value. A comparisonof the signal selection and combiningmethods is shown in Figure 10-36 andTable 10-2.
GAIN SET &SUMMING
AMPLIFIERS
Table 10-1. Combining methodsfor diversity reception
In space diversity the incomingsignal with the best signal-to-noiseratio is selected from the two or moreantennas used. This signal selection,illustrated in Figure 10-34, can beaccomplished either prior to or afteraudio detection.
*n = numberof antennas
PRE-DETECnON R.F.OR
POST-DETECTION AUDIO
Figure 10-35. Maximal ratiocombining and
equal gain combining
PRE-DETECTION R.F.OR
POST-DETECnON AUDIO
Figure 10-34. Space diversitysignal selection
IMAXIMAL RATIO ~;
~P" l....ooo"~
IEQUALGAIN -L ~~
v:/'~v:
hv ~
.-loo'-
'I ~",,-
~
I / lSELECTIONj
11r
2
4
6
dB
0 1 2 3 4 5 6 7 8 9 10NUMBER OF ANTENNA (n)
Figure 10-36. Comparison ofsignal selection andcombining methods
8
10
*n = numberof antennas
INCOMING I IINCOMING ISIGNAL #2 SIGNAL ... n
I
,----1-----.~ ~ ~ /
'----&-.--0 0---;.----'/ // /,--_._---
IBEST
SIGNAL-TO-NOISERATIO
I
INCOMING ISIGNAL #1
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Clearly, the maximal ratio combining method offers the best possibilityfor improvement over a non-diversitysystem, although it is the most difficultto implement. Wireless mics typicallyuse selection or equal gain combiningdiversity. The choice is based ongreatest reduction of the probability ofdropouts. Any of the selection/combining techniques can be implemented inthe pre-detection or post-detectionstage of the receiver.
mic's receiver. The net result is flataudio response through the transmitter/receiver chain, but any hiss thatenters the system as a result of theradio transmission is also cut by thede-emphasis.
Audio cornpanders are availablewith variable gain amplifiers, whichrespond to changing input levels.Without a cornpandor, a wireless micwould be more subject to noise from itstransmission medium and would be
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Method Advantages Disadvantages
Selection No no-phasing Switching transientsrequired
Maximal ratio Best improvement Cost and complexitycombining in SIN ratio
Equal gain Improvement in Co-phasing for optimumcombining SIN ratio, performance
Low cost
Table 10-2. Comparison of space diversitysignal reception methods
• CompandersThe compander was originally used
to reduce static and increase thedynamic range on telephone lines. Acompander is a two part system consisting of a compressor, which reducesthe audio range by providing more gainto weak signals and an expander,which restores the signal to its originaldynamic range ratio. The degree towhich the audio energy is compressed(and subsequently expanded) is referred to as compression ratio. Typically, a wireless microphone uses aratio of 2:1. This compression keepsloud sounds from overmodulating thetransmitter and keeps quiet soundsabove the hiss and static. Expansionrestores the loud sounds after reception and further reduces any low-levelhiss or static.
Almost all FM wireless mic systemsuse some form of pre-emphaais and deemphasis to reduce hiss or highfrequency noise. Basically, the highfrequencies are boosted (pre-emphasized) at the mic's transmitter and,conversely, cut (de-emphasized) at the
unacceptable for most professionalapplications. Compander systems aresubject to phenomena known asbreathing or pumping. This softhissing is most noticeable during lowinput levels. Pre-emphasis networks(similar to those used in FM) are usedto further improve the transmittedsignal. Although the audio companderis required for professional applications, an alternate system, called alimiter, is also available. The limitercan only prevent peak levels frombecoming distorted. Hence the dynamicrange, without distortion, is increasedat the input by up to 40 dB, but theoutput remains constant with noimprovement in dynamic range.
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10.6.7 Multiple MicrophoneSystems
A wireless microphone requiressystem design and analysis consistentwith the channels and particulardesign being used. When using multiple wireless mics, the followinginterference sources must be considered:• transmitter spurious emissions• transmitter and receiver inter
modulation• Splatter
Spurious signals are generatedwithin the transmitter due to mixingproducts created in multiplying thecrystal oscillator to the carrier frequency. These mixing products, if theyfall within the bandwidth of thereceiver, will be heard as squeals orchirping sounds. The spurious outputsof the transmitter are discrete spectralsignals (splatter), and typically cannotbe removed easily once a transmitter isdesigned.
Transmitter intermodulation (IM)occurs when a carrier frequency fromanother source is coupled into theoutput stage of a transmitter andbecomes a second signal source. Thetransmitted IM products will overwhelm the receiver and will be recognized as acceptable signals, thuscreating the chirping and squeals andoverall sensitivity degradation.
10.6.8 Compatibility ofWireless Mic Systems
Wireless mics from one manufacturer will not usually be compatiblewith equipment from another manufacturer (in fact, sometimes different linesmade by the same manufacturer maynot be compatible). This is because ofthe different frequencies and differentnoise reduction circuits used by thevarious manufacturers, which canresult in signal distortion. To assurecompatibility, check with your factoryrepresentative before adding othermanufacturers' equipment to yourexisting wireless microphone system.What follow are descriptions of a fewareas that breed incompatibility:
• FM deviationDeviation (the amount the carrier
frequency changes for a given amount
of audio input) may vary with differentmanufacturers'equipment.~
(narrow band FM) transmitters willnot yield good results with widebandFM receivers, and a wideband transmitter will sound even worse with anNFM receiver. The deviation set by onemaker of wireless mics simply may notbe appropriate for that set by anothermaker.
• CompandingThere are no legal or even industry
wide standards regarding the use ofcompanding. Some manufacturers usenone, some use it only on certainmodels, and others offer it on theirentire line. Expanding an uncompressed signal, or vice-versa, willcreate horrible sounding audio. Butjust because both mic.transmitt~randreceiver use companding, there IS noguarantee of success. If one manufacturer relies on 1.5:1 compression and1:1.5 expansion, while another sets hisequipment for 2:1 compression and 1:~
expansion, then using one system's IDlC
with the other system's receiver willresult in dynamic errors. Incorrectdecoding of the companded signal cancause surging (with too much expansion) or too little dynamic range (withtoo little expansion). Beyond that, themethod oflevel detection (peak, average or rms) may vary, so even with thesame compression/expansion ratios,the equipment may still be incompatible.
NOTE: An ordinary audio tapenoise reduction system is notsuitable for wireless mic signalprocessing because the noise spectrum of tape and that ofFM radiobroadcast differ, and hence different pre-emphasis curves are re- .quired for optimum results. s~ tfyou have a compander-type mic andan ordinary receiver, your dbx orDolby tape decoder will not helpyou out, even though the noiseavoidance principle may be aboutthe same.
• Pre- and De-EmphasisA similar problem exists with pre
emphasis and de-emphasis. EveJ."Yoneuses different turnover frequencies,and different amounts of boost andcomplementary cut. Intermix one type
of mic transmitter with another type ofreceiver, and the frequency response ofthe overall wireless mic system may beseriously degraded.
10.6.9 Antenna Cables
Antenna cables for wireless microphone systems should always becoaxial type as these are not prone tointerference. Proper connectors shouldalso be used. Use of improper connectors or cable can cause signal loss andperformance degradation.
Be aware that not all coaxial cable isthe same. Polyfoam cable (foam centerdielectric) has lower losses and holdsup better in portable use than standard cable. Moreover, there are genuine differences in the shielding on somebrands of cable.
In cable runs of over 100 feet, it isgenerally advantageous to install anRF preamplifier. Placing the preamp ator near the antenna raises the signalwell above the noise, improving thesignal-to-noise ratio before cableattenuation takes its toll. The preamp,if adjustable, should be set for onlyenough gain to compensate for cablelosses. Excess gain can overload thereceiver, defeat squelch circuits, orincrease intermodulation distortion.
Table 10-3 (next page) shows thetwo types of cable most commonly usedin wireless microphone systems andindicates whether or not preamps arenecessary at various cable lengths.
10.6.10 Evaluating WirelessMicrophone Systems
There are a number of criteria thatmust be considered in selecting awireless microphone system suitablefor professional use. It must be reliablein a variety of tough environmentswith good intelligibility and must beusable near strong RF fields, lightdimmers and other sources of electromagnetic interference. This relatesdirectly to the type of modulation(standard or narrow-band FM), theoperating frequency (HF, VHF, orUHF), the receiver selectivity, and soforth. Ideally the system should becapable of operating at least 4 to 6
hours on one set of batteries. Rechargeable nickel-cadmium batteries, asdescribed in Section 10.5.7, offer atrade off of long-term economy againstthe need for increased maintenance.
Published specifications are oflittleuse in evaluating wireless microphoneperformance. Depending on the manufacturer, specifications may be exaggerated or have qualified conditions thatmay not be applicable in actual use.Because RF power output is limited bythe FCC, most systems operati~ginthe same band are comparable III
transmission distance, but otherparameters more critical to performance and reliability should be carefullyexamined. The following considerationsshould be evaluated prior to selectionof a wireless microphone system:
• ConstructionFor reliable operation, the equip
ment should meet accepted designstandards, such as spacing betweenlines on the printed circuit boards,stress reliefs on wire bends, clearancebetween components, and manufacturing tolerance. In evaluating assemblytechniques, one should consider soldering (avoid solder bridges, splashes,. coldjoints) and the overall workmanship(there should be no flux on printedcircuit boards). The battery compartment should be readily accessible andmechanically durable.
• ComparisonThe following comparison tests can
be made by getting a wired mic whichuses the same element as your wirelessmic and feeding both outputs into amixer or an A-B switch.
Frequency response: Both micsshould sound identical; one shouldbe no more brilliant than the other.
Gain: Output levels should benearly identical.
Phase: With both mics placednear each other, a properly phasedwireless mic will not show anycancellation vs. the wired version.
Dynamic range: Shout into themic. Listen for distortion at highlevels. Note any pumping action orother cornpandor characteristics.
Noise floor: With gain level onthe mixer set about equal, listen foroverall noise floor differences.
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• Radio FrequencySet the receiver squelch for normal
quieting. Remove the transmitterantenna, if possible, to induce dropout.Listen for squelch action when dropoutoccurs. A good design will minimize theannoying sound of dropout.
Ultimately, if a handheld wirelessmic is being used for TV production, ora concert that is being videotaped, orwith live projection TV effects, themic's looks can be as important as itsperformance, especially when the audiocan be dubbed in post production.Remember that the performer cares alot about visual aesthetics.
10.6.11 Conclusions
Today better professional wirelessmicrophone equipment is as good as, orsuperior to many of the tape machinesor sound systems to which the mics areconnected. A wireless mic can never bebetter than, only as good as, a wiredversion of the same type. Becausewireless mics are sophisticated radiosystems as well as audio systems, caremust be taken in setup, and a thoroughunderstanding of the system's parameters is advisable. Wireless systemsoffer many practical advantages to theuser.
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COAXIAL TYPICAL RF POLYFOAM RFCABLE RG-58/U PREAMP RG-8/U PREAMPLENGTH (dB loss NEEDED? (dB loss NEEDED?(FEEl) @ 150 MHz)* @ 150 MHz)*
50 3.0 No 1.0 No75 4.5 Optional 1.5 No
100 6.0 Yes 2.0 No125 7.5 Yes 2.5 No150 9.0 Yes 3.0 Optional175 10.5 Yes 3.5 Yes200 12.0 Yes 4.0 Yes250 13.5 Yes 5.0 Yes300 15.0 Yes 6.0 Yes400 16.5 Yes 8.0
Table 10-3. Cable/preamp recommendations forwireless mic systems
(*There is a wide variation between similar cables, depending on how they aremanufactured. Also, loss is logarithmically less at lower frequencies)
10.6.12 Glossary ofWireless MicrophoneTerms
Compander - A combination of acompressor at one point in a communication path for reducing the amplituderange of signals, followed by an expander at another point for a complementary increase in the amplituderange.
Compressor - A signal processorthat for a given input amplitude rangeproduces a smaller output range.
Cordless Mic - An older term forwireless mic.
Dead Spot - Those locationscompletely within the coverage areawhere the signal strength is below thelevel needed for reliable communication.
De-emphasis - The use of anamplitude-frequency characteristiccomplementary to that used for preemphasis earlier in the system. (Seepre-emphasis)
Diversity Reception - (Defined inSection 10.5.9.)
Dynamic Range - The difference,in decibels, between the overload (ormaximuim) level and the minimumacceptable signal level in a system orsignal processor.
Expander - A signal processorthat for a given amplitude range ofinput voltages produces a larger rangeof output voltages.
Intermodulation - The modulation of the components of a complexwave by each other, as a result ofwhich waves are produced that havefrequencies equal to the sums anddifferences of integral multiples of thefrequency components of the originalcomplex wave.
Isotropic Radiator - A hypothetical antenna having equal radiationintensity in all directions.
Limiter - A signal processor inwhich some characteristic of the outputis automatically prevented fromexceeding a predetermined value.
Multipath Transmission - Thepropagation phenomenon that resultsin signals reaching the radio receivingantenna by two or more paths.
Polarization Diversity Reception - That form of diversity reception that utilizes separate verticallyand horizontally polarized receivingantennas,
Pre-emphasis - A process in asystem designed to emphasize themagnitude of some frequency components with respect to the magnitude ofothers, to reduce adverse effects, suchas noise, in subsequent parts of thesystem. A specific type of equalization.
Space Diversity Reception That form of diversity reception thatutilizes receiving antennas placed indifferent locations.
Squelch Circuit - A circuit forpreventing a radio receiver fromproducing audio-frequency output inthe absence of a signal having predetermined characteristics.
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