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








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






• 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







• 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






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









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






PCM – Signal encoding and

decoding principles






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








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.






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.







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.






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.






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






• 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.






• 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.







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






• 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.




Audio (multimedia components unit 1)

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