Audio (multimedia components unit 1)
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Transcript of Audio (multimedia components unit 1)
7/31/2019 Audio (multimedia components unit 1)
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Audio
Two types of audio signal
• Speech signal as used in a variety of interpersonal
applications including telephony and video telephony
• Music-quality audio as used in applications such
as CD-on-demand and broadcast television
• Audio can be produced either
naturally by means of a microphone or
electronically using some form of
synthesizer
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Sampling
• To include sound in a multimedia application, the
sound waves must be converted from analog to digital
form
• This conversion is called sampling – every fraction of a
second a sample the of sound is recorded in digital bits
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• The bandwidth of a typical speech signal varies from
50Hz -10 kHz and that of a music signal from 15Hz – 20
kHz
• Hence sampling rate must be in excess of nyquist rate
which is 20 Kbps for speech and 40 Kbps for music.
• No of bits per sample must be chosen so that thequantization noise generated by the sampling process is at
acceptable level relative to minimum signal level
• Tests have recommended the use of aminimum of 12 bits per sample for
speech and 16 bits for music
• For stereo sound bit rate must be double
that of a mono signal
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• In practice, both the sampling rate used and the number
of bits per sample are often less than these values
• In speech, BW of network used is often much less than
the BW of the source signal thus dictating a lower
sampling rate with fewer bits per sample.
• In music, sampling rate is lowered in order to reduce
the amount of memory that is required to store a music
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Problem:
• Assuming the BW of a speech signal is from 50Hz to 10KHz and
that of a music signal is from 15Hz to 20KHz, derive the bit rate
that is generated by the digitization procedure in each case
assuming the Nyquist sampling rate is used with 12bps for the
speech signal and 16bps for the music signal. Derive the memory
required to store a 10 minute passage of stereophonic music.
• Ans: Bit rates: Nyquist rate = 2f max
• Speech: 2x10 = 20KHz, 12bps
20x12 = 240kbps
• Music: 2x20 = 40KHz, 16bps
40x16 = 640Kbps (mono), 2x640 = 1280Kbps
(stereo)
• Memory req = 1280x103
x600/8 = 96Mbytes
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PCM (Pulse Code Modulation) speech
• Initially a PSTN operated with analogue signals throughout,
the source speech signal being transmitted and switched inits original form
• However, today these have been replaced with digitalcircuits
• In order to support interworking of the analogue and digitalcircuits the design of the digital equipment is based on theanalogue network operating parameters
• The BW of a speech circuit was limited to 200 Hz to 3.4
kHz, also nyquist rate is 6.8KHz• Sampling rate of 8KHz is required to
avoid aliasing
• The digitization procedure is known as
pulse code modulation
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PCM – Signal encoding and
decoding principles
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PCM Speech – Compressor/ExpanderCharacteristics
• In linear quantization irrespective of the signal amplitudesame level of quantization noise is produced ( noise level issame for the quiet signals and loud signals)
• Pulse Code Modulation consists of twoadditional circuits: Compressor (encoder)
and Expander (decoder) to help reduce the
effect of quantization noise with just 8 bits
per sample, making the intervals non-linear with narrower intervals for small
amplitude signals than larger amplitude
signals. This is achieved by the means of
the compressor circuit
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PCM Speech – Compressor/ExpanderCharacteristics
• The analogue output from the DAC is passed to theexpander circuit which performs the reverse operationof the compressor circuit
• The overall operation is known as companding
• The compression and expansioncharacteristics is known as A-law in
Europe, µ-law in north America and
Japan.
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CD- quality audio
• The discs used in CD players and CD-ROM’s are digital storage
devices for stereophonic music and more general multimediainformation streams.
• Standard associated with these devices known as CD-digital audio
(CD-DA) standard.
• Music: BW- 15Hz to 20KHz, sampling rate – 40Kbps.• Actual rate used is higher than this rate, to allow imperfections in
the band limiting filter used and the bit rate is compatible with one
of the higher transmission channe3l bitrates available.
• Sampling rate used is 44.1 ksps which meansthe signal is sampled at 23µs intervals.
• Since the BW of a recording channel on a CD
is large, high number of bits per sample can be
used.
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CD- quality audio
• Bit rate per channel = sampling rate x bits per sample
= 44.1 x 103 x 16 = 705.6 Kbps
= 2 x 705.6 = 1.411Mbps
• This is the bit rate used with CD-ROM’s which are widely usedfor multimedia titles.
• In computer, to reduce the access delay, multiples of this rate
are used.
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Synthesized audio
• The amount of memory required to store a digitizedaudio can be very large, even for short passages
• Synthesized audio is used in multimedia applications,
since the amount of memory required is 2 to 3 times
less than that required for equivalent digitized form.
• It is much easier to edit synthesizedaudio and to mix several passages.
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Synthesized audio
• The 3 main components are thecomputer, keyboard, sound
generators.
• The computer takes inputcommands from the keyboard and
outputs these to the sound
generators which produce the
corresponding sound waveform –
via DAC - to drive the speakers
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• Pressing a key has similar effects to pressing a keyboard of a
computer. For each key press a different codeword (message –
indicating the key pressed and the pressure applied ) is
generated
• The control panel contains range of different switches and
sliders that collectively allow the user to indicate to theprogram information such as the volume of the generated output
and selected sound effects to be associated with each key
• Secondary storage interface allows the sequenceof messages, including control panel – relating
to a particular piece of audio to be saved on
secondary storage like floppy disk.
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• To discriminate between the inputs from differentpossible sources, a standard known set of
messages have been defined for both i/pt & o/pt
to the corresponding set of sound generators:
Mu sic Instrument Digital Interface (MIDI)
• In addition there are programs to allow user to edit or mix several
passages.
• The sequence program associated with the synthesizer then ensuresthat the resulting integrated sequence are synchronized and output
to sound generators to create merged passage.
• As well as a (piano) keyboard, there is other inputs frominstruments such as electric guitar, all of which generate messages
similar to those produced by the keyboard.
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MIDI This also defines the type of connectors, cables, & electrical
signals that are used to connect any type of device to thesynthesizer.
• The format of a MIDI message consist of :
• Status byte - This defines the particular event that has caused themessage to be generated
• Data bytes – Which collectively define a set of parameters(pressure applied, identity of the key) associated with the event
• Event – A key being pressed
• It is important to identify the different types of
instruments that generated the events
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• Each instrument has a MIDI code associated with it – e.g Piano has a code of 0 and violin 40
• Some codes have been assigned for special effects such as sound from cannon and applause
• Since the music is in the form of MIDI messages it is vital tohave a sound card in the client computer to interpret thesequence
• A passage of audio produced by a synthesizer
consist of a very compact sequence of messages,each comprising a string of bytes, which can
either be played out by the sequencer program
directly and hence heard by the composer or
saved in a file.