Home Theater Audio Systems

3Home Theater Audio Systems

HOME THEATER AUDIO SY STEMS

In the early days of the movies there was just the visualimage to enjoy. Motion picture technology had not yetadvanced to the point where synchronized sound tracks

were possible. This detail, however, did little to dampenthe enthusiasm for films produced during this “silent era”,movies were big stuff, but it is interesting to note that in anattempt to compensate for this sound deficiency manystudios released features with sheet music scores so thatpiano players could play along with the film. As we knownow, sound is a critically important part of the motionpicture experience, but in the early days, this was justbecoming known.

The Silent Era lasted until the late 1920s. By this time,several studios had developed synchronized motionpicture sound tracks systems and began showing themtheaters. By today’s standards, these early “talkies” filmswere pretty crude, but the concept of sound to accompanythe on-screen picture was rapidly embraced by theviewing public.

Just a decade later, in a continuing quest for technicalachievement, the Hollywood studios began to experimentwith multi-channel cinema sound. Interestingly, one of the

first multichannel films was Walt Disney’s well knownFantasia, which was released in 1941 in “Fantasound.”This 6 discrete channel (!), high-fidelity release causedquite a stir and soon Hollywood became awash in a sea ofincompatible, multichannel formats issued by many of themajor studios.

Standardization on a common multichannel format did notoccur until the 1960s when Ray Dolby, a young engineerwith Ampex corporation, developed a process forrecording multichannel sound optically onto movie film.This was considered a major advance in the motionpicture industry because the “Dolby-encoded” opticalsound tracks could be played in both the old mono/stereomovie theaters, and in the new “Dolby Stereo” fourchannel surround theaters. Almost overnight it became thestandard for cinema sound.

Although Dolby Stereo, and it’s home version, DolbySurround, were the best and only systems for a long time,the advent of digital technology in the 1980s advancedsound technology even further. Today most major moviesfor theatrical viewing are released in one or more of thefollowing digital formats: Dolby Digital, DTS, or SDDS,

Home Theater Audio Systems4

(More on these later).

How The Original Dolby Stereo System Works

In the original commercial, analog Dolby Stereo cinemasound systems, the left and right audio tracks are readoptically off the film strip and fed to a Dolby StereoCinema Processor (see the diagram below.) Thisprocessor decodes the Dolby Stereo signal into fourdifferent channels, in particular:

The Center Channel. Here the filmmakers place most ofthe on-screen dialogue spoken by the actors.

The Left and Right Front Channels. These channels carrymore traditional stereo sounds, such as music and soundeffects, but they are also used for dialogue and effectswhen actors move off to either side of the screen.(Example: When you see a shot of a train coming onto thescreen on the right and going across to the left, thedirector will do the same to the train sound. It will progressfrom right speaker to the center speaker and then on tothe left speaker.)

The Surround Channel. First, it is important to note thatthere is only one surround channel. In many theaters thereis an array of speakers at the rear but with the DolbyStereo system, only one channel from the Dolby Stereodecoder drives them. The surround channel is for twothings. The first and foremost is for ambience. In other

words, in order to give a feeling of “being there”, directorsoften mix the sounds that lead to spaciousness, like thesound of rain, for example, into the surround tracks.Another thing surround channels are often used for is fordirectional sound effects. (Example: as helicopters roaroverhead, directors often mix the sound tracks so that thechoppers pass from the front speakers to the rearspeakers in a thundering roar.

Since it’s release in the late 1960s, over 6000 movieshave been recorded with Dolby Stereo-encoded soundtracks. Whether the movie is on videocassette, laserdisc,from your cable system or over the air, chances are it is inDolby Stereo.

The New Theatrical Digital Formats

Today most movies for commercial theater viewing arereleased in one or more of the following digital formats:Dolby Digital, DTS, or SDDS. These new digital soundsystems are incompatible but share the followingcharacteristics: 1) they are pure digital; 2) they usecompression algorithms to fit multichannels either on thefilm (Dolby Digital and SDDS), or on a separate CD (DTS);and 3) they have six discrete channels, left front, center,right front, left surround, right surround, and low frequencyeffect channel(LFE), referred to as 5.1 with the .1 for theLFE. (Note: SDDS provides 7.1 channels, with twoadditional front channels. At this point, Sony, the developerof the system have any plans to develop a consumer


SDDS in the future. So, at least at this point, we only haveto deal with DD and DTS in the following discussion.)

First, DD and DTS are incompatible systems. If a movietheater wants to show both DD and DTS movies, it musthave two separate decoders. Similarly, if you want to useboth DD and DTS encoded software in your home, youmust have the capability to decode both.

What comes off any digital software is a digital bit stream,a series of ones and zeros. It’s exactly what comes offyour stereo CDs, but there’s a difference between CDsand DD or DTS. A CD has just two discrete channels andthe bit stream contains all of the music (i.e. nocompression). But DD and DTS has six channels or threetimes the information. And while CDs are typically onehour, movies are typically two hours. So now, there’stwelve times the information. Imagine the size of a CD thatholds 12 times what a current CD holds!

To get around the massive information storage problem,engineers have developed perceptual coding algorithmswhich radically reduce the size of the bit stream, but insuch a way, that ideally it sounds no different from a full bitstream. This is done by “exploiting” commonly knownproperties of human hearing. Here is a simple example:When you are listening to a symphony orchestra, someinstruments or sections of the orchestra are playing loudly,and others softly. To your ears, the loud sectionsovercome the soft ones, so you do not hear them. Hard tobelieve, but true. So the engineers at DD and DTS use

perceptual (what you perceive) coding which eliminatesthe played, but unheard, sounds and thereby reduces thebit stream. (The original term for DD, AC-3, is, in fact, thedesignation for DD’s perceptual coding system)

A DD or DTS decoder takes the bit stream, using thealgorithm, divides it into 5.1 channels, converts it into 5.1or 6, analog channels and it’s ready to be played by yoursix channel audio system. But here’s the problem. Theperceptual coding algorithms developed by DD and DTSare entirely different. So if you want to listen to bothformats, you need outboard DD and DTS processors, or apreamp/processor or receiver which decodes both. Thiswas prohibitively expensive, but with the development ofone solution, IC chips that perform both DD and DTS,prices are coming down.

Whats Needed In Home Theaters

So which do you need in your home theater, DD, DTS, orboth? Honestly, this question is more easily asked thenanswered, but we will review software that is and willbecome available in future CDs and this may help withyour decision.

Compact Discs: There are a few CDs recorded in DD.There are many more DTS CDs, but still fewer than 100.Many more are planned. But, to confuse the situation,audio-only DVDs will soon be released which will, ofcourse, require new audio DVD players, which may are


may not incorporate the video DVD function. Furthercomplicating the matter is that two separate DVD audio-only discs will be released. One, released by Sony/Philips,has two layers-one compatible with CD players, andanother multichannel layer. The DVD consortium hasanother DVD audio disc which also be multichannel, butnot compatible with CD players. The two systems willencode their bit streams in entirely different ways. Further,neither uses DD or DTS. The multichannel decoding willbe executed inside the player and output as 6 channels.As you can see, the situation with CDs and DD/DTSencoding is messy and evolving as we speak.

LaserDiscs: There are quite a few LDs in the DD formatand an increasing number in DTS. The point of confusionhere is how the digital bit streams are placed on the LDs.

In the days of old, the audio track of LDs was an analogsignal similar to an FM radio signal, with or without CXnoise reduction. With the advent of CDs, LDs werereleased with the Dolby Surround (analog in theaters)track placed on the LD as a CD PCM (pulse CodeModulation) system. The old analog tracks were used forthe directors commentaries and the like.

With the advent of digital movie theater sound, DD andDTS proposed two different systems of encoding thedigital soundtrack. DTS proposed replacing the PCMdigital signal with a DTS signal which would require littlecompression. DD proposed replacing the left analog signalwith it’s bit stream modulated as an FM carrier in order topreserve the PCM/Dolby surround signal for those withoutDD decoders. And that’s how they’re released today. ForDTS LDs, you just go from the digital output of thelaserdisc player into the digital input of the DTS processoror receiver. For DD LS, you must first pass the signalthrough an RF demodulator which strips the FM carrier offthe digital bit stream. The output of the demodulator is

then input into the digital input of the DD processor orreceiver.

But this discussion may be academic. With the release ofDVDs, sales of LD software have plummeted. For anyonestarting out new, an LD is not recommended. Buy a DVDplayer. But thousands have LD players and LD collections.Unless they plan on replacing their LDs with DVDs as theycome out (and many old movies may not be released onDVD for years), this discussion is still relevant.

Digital Video Discs: The DVD standard requires that allDVD releases have a PCM track or a DD track. It’s apractical matter, all but a handful have been released withDD. Great, you think. All movies are 5.1 channels on DVDbecause they’re DD. Unfortunately, no. While all DDtheatrical releases and nearly all DD and LDs are 5.1channels, not all DD DVDs are. Here’s why. For DVDs,DD only refers to the perceptual coding used. DD DVDscan be mono, stereo, 4 channel (dolby surround), 5.0channel )No LFE) or 5.1 channel (!). The reason for this issimple. DD DVDs can only encode and play the originalsound track. Since nearly all pre star wars movies weremono, the DD DVD will also be DD mono. Operas andconcerts recorded in Stereo will be in DD stereo. DolbySurround movies will be 4 channel. And digital 5.1 movieswill be 5.1. (Here another confusing thing-DTS theatricalreleases are released as DD DVDs). The only exceptionto this is where studios have remixed the Dolby Surroundmovies to be 5.1 releases.

DTS has announced that it will be releasing DTS encodedDVDs. These DVDs have a DTS “flag” on the DVD whichthe DVD must be able to read. Unfortunately, no firstgeneration, and only a few second generation DVDplayers can read the DTS flag. However, the cost to themanufacturer of making a DVD player DTS compatible isminimal. If DTS establishes a foot hold in the DVD market,

Filmstrip showing optical stereo and Dolby Digital sound tracks

Dolby Laboratories developed Dolby Digital forcompatibility with existing optically based film-soundsystems. The challenge was to fit both the opticaland Dolby Digital sound tracks onto one film stock.To do this, the engineers at Dolby placed the AC-3digital data between the film sprocket holes.

On the decode end, during projection, Dolby Digitaldecoders take the digital bit stream and decode itinto 5 discrete full-bandwidth channels and onesubwoofer bandwidth-limited channel (.1). These 6channels are then fed to amplifiers, and then to thetheater’s speaker system.

How Dolby Digital works In Commercial Theaters


all players will be DTS compatible. Because all DVDsmust contain a DD track, all DTS DVDs will contain a DTStrack. But here’s the trick. Remember, for DVD, DD doesnot equal automatic 5.1. So a DTS DVD may have a 5.1DTS track, but only a 2 channel DD track, most likelymatrixed analog Dolby Surround. But few decoders onsensing a stereo digital signal will automatically switch toDolby Pro Logic (the home analog Dolby surround), andmost cannot be forced into that mode, so you’ll just have astereo signal. Steven Spielberg is a part owner of DTS,and rumor has it that he will only allow his movies to be

released in 5.1 DTs, not 5.1 DD. So, for DVD, DD vs DTSmay not be merely a choice of which 5.1 format you preferwith duplicate releases.

Digital Television: At least for DTV, the situation issimple. DD is the mandatory standard, with noalternatives. Thank God, this one is simple, at least fornow.

Honestly, this is a fight we are reluctant to take sides on, there simply are a lot of good arguments illustratingthe claims of both systems. We urge you to do some reading, and listening, and form your own conclusions!However, as you collect information, here are some of the issues you will find.

The DTS system uses significantly less compression, and it’s proponents claim it sounds closer to the originalsound tracks. Dolby responds that it’s DD system in A/B comparisons is indistinguishable from the master tapes,so who cares about compression amount. DTS also claims it has a more robust bass and distinct surroundchannels. DD responds that DTS artificially boosts those channels(!) so that is why some listeners form thoseconclusions. If you are thinking of doing your own comparison, keep this in mind. Rarely, even on the same moviereleased in DD and DTS consumer versions, is the same film “mix” used, so A/B tests of the same release willoften sound different. Finally, three of the leading consumer home theater magazines performed rigorous, blindA/B comparisons using DTS/DD LDs with the exact same mix. The result ? One said there was no detectabledifference; the second said there were “some” differences and the third said DTS sounded better than DD.Warning: your results may differ!

DTS vs DD - Is There A Real Difference Sonically?


The THX Story...

In the late 1970s, blockbusters such as STAR WARS andothers drew the public’s attention to the potential of highquality sound in the cinema sound. However, GeorgeLucas suspected that all the attention that he and otherfilmmakers put into the sound tracks of their movies wasbeing sabotaged by inferior sound systems in theatersaround the world. So Lucas hired a young engineer by thename of Tomlinson Holman to look into the matter anddubbed the project THX, for “Tom Holman’s Experiment,”and, not so coincidently, the name of Lucas’s firstcommercial film: THX 1138.

The results? Holman’s theater survey confirmed theirworst fears: The sound systems in theaters around thiscountry and the rest of the world ranged from acceptableto down right terrible. Upon receipt of this conclusion,Lucas immediately set Holman to the task of writing a setof standards for the entire movie sound “chain” that wouldhelp.

In specific, he wanted theaters to reproduce the audioquality achieved on the finest Hollywood dubbingstages. (Note: The dubbing stage is a highlyspecialized, theater-sized sound track mixingenvironment. The various sound elements of a film,such as dialogue, sound effects, music scores, etc., aremixed by skilled sound engineers in this special studio.The dubbing stage is the reference standard for a film’saudio tracks.)

Thus began the highly successful Lucasfilm Ltd. THX®

program. Today, the THX name has becomesynonymous with the highest level of theater soundsystem reproduction. Arguably the most important

theater in the world, the 1000 seat Samuel GoldwynTheater of the Motion Picture Arts and Sciences (it is herethat Academy Award nomination screenings are held), is aTHX theater. Currently, there are over 950 theaters anddubbing stages around the world that have been designedor re-engineered for THX reproduction.

After the massive success of the commercial THXbusiness, the next step, naturally, was to design a systemthat would reproduce the dubbing stage experience in thehome. In order to achieve this, Holman identified six goalsthat all “Home THX” systems must achieve:

1) Accurate frequency response. Dubbing stages areequalized with the industry standard “X” curve. Thisstandard is defined by the International StandardsOrganization (ISO) in their specification ISO 2969-1977and is designed to compensate for the acoustics of largemovie theaters. Lucasfilm realized that this equalizationdoes not apply to a home system, so a proprietary “Re-equalization circuit” is specified for home use. The

UnderstandingHome THX


diagram approximates the Home THX re-equalizationcurves.

2) Intelligible dialogue. With rare exception, every wordspoken in a movie is intended to be heard clearly by theaudience. THX systems assure that this is the case byspecifying a center channel speaker with a narrow verticaldispersion pattern and driven by an electronic processorutilizing a Lucasfilm THX® proprietary dialogueequalization circuit.

3) Full dynamic range. Filmmakers spend a great deal oftime adjusting the components of their sound tracks toplay at specific levels (see the diagram at the right). THXspecifications require that a Home THX system becalibrated so that the standard 70 dB THX reference toneis played back at exactly 70 dB Sound Pressure Level(SPL.)

4) Accurate on-screen localization. The location of thesound field coming from an on-screen image, such as aglass breaking, should appear in the same place as thevisual image. In order to localize on-screen sources,Lucasfilm specifies that front speakers also be designed

with narrow vertical dispersion patterns, and be preciselylocated in reference to the screen.

5) Smooth panning. This requires a set of well matchedspeakers, properly placed for minimum room reflections.Speakers that are mismatched and misplaced, can causea panned or moving sound source (like a actor’s dialogue)to jump about or change in tone. This can ruin the effectdesired by the director.

6) An enveloping sound field. The sound coming fromthe surround speakers is for “diffuse ambiance”. Itshouldn't be “localized” (the source of the sound located),and THX systems use a special decorrelation circuit anddipole speaker radiation pattern to assure that this is thecase.

How To Get THX Sound In a Home Theater

For you to achieve what the Lucasfilm creative forceshave designed, the cinema dubbing stage experience inyour own home, several items are required:

A Home THX controller. This proprietary circuitry is

THX specifications require that dipolespeakers be used in THX home theaters .Dipole speakers create “acoustic nulls” inthe audience seating area. This preventslocalization of the speaker and a mor erealistic feeling to the surround field.


licensed from Lucasfilm and is built into many A/Vreceivers and stand alone processors. The diagram aboveillustrates the signal processing that a THX licensed unitperforms. THX licensed A/V receivers and processors areavailable from a number of manufacturers. With theadvent of DD and DTS, there are now two THX standardsfor a controller, THX 4.0 and THX 5.1. THX 4.0 is used foranalog Dolby Surround and THX 5.1 isused for DD and DTS. Most THXstandards (subwoofer crossover, re-equalization) are the same. But in 5.1 thedecorrelation circuits for the surroundsshuts off when the surround channels arecarrying discrete information, and kicksback in when the surround channelinformation is the same for the l and Rsurround, maintaining a diffuse soundfield.

A Home THX speaker system. The loudspeakerrequirements of the home THX system are tightly specifiedand include requirements for loudspeaker distortion,directional properties, frequency response, sensitivity,impedance and power handling. For example:

Front Speakers: The three front speakers (left, center andright) must be identically matched, with a flat responseover 80hz to 20kHz. They also should have identicallyfocused dispersion patterns and be magnetically shieldedto prevent distortion in nearby televisions.

Surround Speakers: Home THX systems are required toutilize two dipole design surround speakers to preventlocalization of the surround information. See the diagramabove.

Subwoofers: THX systems are required to have at leastone subwoofer to reproduce the bass range (20hz to80Hz) of the three front channels. The crossover of thesubwoofer and the front speakers must be electronicallymatched with 24 dB/octave slopes. THX subwoofers mustbe able of sustaining a 105 dB SPL, in a 3000 cubic foot

room, in the farthest seating position.Note: as the diagram below illustrates, forbest sub-woofer performance, place thesubwoofers in the corner of the room.

Home THX amplifiers. Home THXamplifiers must pass minimum electricalspecifications and be capable ofproducing a constant sound pressure levelwithout overload. For example, THXamplifiers must provide at least 100 wattsin order to drive the speakers to the upper

limits of a film’s sound tracks (105 dB). Furthermore, at alllevels of amplification, the THX amplifiers must notproduce audible distortion. Further, they must be able todeliver their rated power into a 3.2 Ohm load, all channelsdriven, on a continuos basis.

Home THX Receivers. THX also certifies receivers forhome use, both with Dolby Surround and Dolby Digital. Nodoubt, DTS/DD/THX receivers are in the offing. Receiversmust have THX controller circuitry (crossover, re-equalization, timbre matching, decorrelation). THXreceivers also have stringent receiver power amplifierrequirements which far exceed the typical receiver. Theyare, however, as stringent as the requirements forseparate power amplifiers (eg no 3.2 Ohm powerrequirement).


In a home theater, speakers need to be placed in specificlocations. Frequency response and audio localizationare greatly affected by where you place them. There is a

great deal of academic research documenting this, soconsider it a truth. Location, location, location is not just anold saw for real estate professionals.

Let’s consider the front speakers first. Ideally, it is best tohave left, center, and right speakers of the same type andaligned in a horizontal line through the middle of the videoimage. Unfortunately, this is not possible in most hometheaters because the front speakers need to go rightsmack through the middle of the picture. The next bestposition is to flank the television, or video screen, with theleft and right speakers, and install the center channeleither above or below the picture. (Note: for those that arebuilding high-end home theaters with front projectors,several screen manufacturers offer perforated acousticallytransparent screens so that speakers can actually beplaced directly behind the screen. However the centerspeaker must match the voice characteristics of the l&Rspeakers, typically by using the same tweeter and mid-range.)

There are several schools of thought about the type andplacement of surround speakers, but the most popular isto place the speakers so they are flanking the audienceseating area, and somewhat above the listener's earlevel. In the days of analog Dolby Surround, there was ageneral consensus that di-polar (multi-driver, wired out-of-phase) were best for creating a diffuse sound field. In fact,that was, and is, a THX requirement. With the advent ofDD and DTS, with discrete L&R surround channels, manybegan to advocate front radiating point source speakersand surrounds. The problem with this approach is thatwhen mounting as shown in the diagram, the speakers,though excellent at directional special effects, did not dowell at creating a diffuse surround sound field, and wereidentifiable as point sources. Several A/B tests by hometheater experts confirmed this. They found that di-polarspeakers did a more than adequate job at localized,directional sound effects while otherwise maintaining adiffuse sound field, even with stereo music in the surroundchannels. THX still recommends dipolar speakers in THX5.1. The only possible exception would be in a large roomwhere direct radiating fronts could be placed substantially

HOME THEATER SPEAKER SYSTEMS


behind the listening position (at least 1/2 the front seatingdistance.) Finally, one or two subwoofers are a must withtodays home theater systems. The LFE (low frequencyeffects) .1 channel of DD and DTS are simply beyond thecapabilities of most woofers in full range front speakers.Even Dolby Surround has bass-laden sound tracks whichdemand a separate subwoofer. Further, the use ofsubwoofer(s) can [permit you to buy smaller L&Rspeakers saving money and enhancing ease ofplacement.

To finish our thoughts about placement, let’s take a look atsome common home theater speaker configurations.

In Home Theater Room Number One. In this case wehave mounted the di-polar surround channels up on abeam that traversesthe room and added asecond subwoofer.Alternatively, twospeakers matching thefronts can be placedon stands behind thecouch.

Home Theater RoomNumber Two shows apicture tube typetelevision mounted inan A/V furniturecabinet. The centerchannel speaker isplaced on top of thetelevision and the leftand right frontspeakers flank it onshelves. A subwooferis placed below thetelevision. This maybe less optimum forbass reinforcement,

but it conceals the subwoofer enclosure and makes thesystem less intrusive. In this particular room, placingdipole speakers on the walls was not an option because ofthe window locations, so circular surround speakers wereplaced in the ceiling.

The last illustration, Home Theater Room NumberThree, we show high-end home theater with a large, front-projected video image. Here we have used in-wallspeakers around the screen to give the room a clean,theater-like look. There are many manufacturers thatmake in-wall speakers specifically for home theaterapplications. We recommend you look at some of theenthusiasts magazines for specific brands.

The rear surround channels are also in-wall speakers.

The Dolby Digital speaker systemlayout is similar to pro-logiclayout. The main difference

between the formats is that thesurround channels in Dolby Digital

are full bandwidth

CENTER

RIGHT

LEFT

SUBWOOFER

RIGHTSURROUND

LEFTSURROUND

Home TheaterRoom

Number OneSUB

WOOFER


Home Theater Room Number Three

CENTER

RIGHT

LEFT

SUBWOOFER

RIGHTSURROUND

LEFTSURROUND

SUBWOOFER

CENTER

RIGHT

LEFT

SUBWOOFER

RIGHTSURROUND

LEFTSURROUNDHome Theater

Room Number Two

Because this is a high-end enthusiast’s hometheater room, we put twosubwoofers in the frontcorners for full lowfrequency capability.Another option is to builtthe subwoofer’s into thewalls or into the floor, ifyou want to hide them.

Home Theater Audio Systems - Welcome To CinemaSource

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Transcript of Home Theater Audio Systems - Welcome To CinemaSource