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					School of Computer Science &
           Information Technology
               G6DPMM



           Digital Audio
School of Computer Science &
           Information Technology


     G6DPMM - Lecture 14


           Digital Audio
Properties of Sound
   Acoustics
   Pressure waves in air
   Pitch - frequency - time (kHz)
   Amplitude - level - energy (dB)
   Subjective Volume - frequency and energy
Analogue vs Digital
   Analogue technologies
       Physical
       Magnetic
   Digital technologies
       Sample sound
       Approximation of analogue
       Must be captured (a2d conversion)
Sound in Multimedia
   MIDI
       Defines the synthesis of sound
       Analogous to vector graphics

   Digital Audio
       Captured waveform
       Analogous to bitmap graphics
MIDI

 Musical Instrument Digital Interface.
 Standard for synthesisers and electronic
  instruments and devices.
 MIDI “score” describes time stamped sequence
  of notes.
 Reproduction dependent upon hardware (MIDI
  device).
Advantages of MIDI

   Very compact (up to 1,000 times smaller than
    CD audio)
   Sounds produced by hardware - therefore low
    system overhead.
   Low bandwidth requirements.
   High quality - dependent upon hardware.
   Scaleable, Editable & cross platform.
Disadvantages of MIDI
  Hardware dependent - only a faithful
   reproduction if the equipment is constant.
  Cannot record “real world” sounds.
  Difficult to represent speech.

     Creation of MIDI
        Via editing software (sequencer)
        Via instruments / peripherals
        Usually requires musicians or sound effects
         specialists.
Digital Audio
   Sampled Sound
       At regular time intervals a sample is taken, and the
        information is stored digitally.

   Applications
       CD
       Digital tape (DAT)
       Digital broadcast (radio / TV)
       Most multimedia sound
Sampling (capturing)
    Sampling hardware - from any analogue source
     (usually line in).

    Sampling rate
        frequency of samples
        often called “frequency”

    Sample size
        amount of information stored.
        often called “resolution”
Sampling Rate
Sample Size
Sampling Parameters

   Common Sample Rate (frequency)
       44.1 kHz (CD audio)
       22.05 kHz
       11.025 kHz

   Common Sample Sizes (resolution)
       8 bit (256 amplitude states)
       16 bit (65,536 amplitude states)
Examples

  Frequency Resolution Tracks Storage
     (kHz)    (bits)          (1 min)

    44.1       16        2    10.5 Mb “CD quality”
    44.1       16        1    5.25 Mb High quality mono
     22        16        1    2.5 Mb Reasonable speech
     22         8        2    2.6 Mb Medium quality
                                       stereo
     22         8        1    1.3 Mb “TV quality”
     11         8        1    650 Kb “Telephone quality”
Distortion

    Distortion is caused when the reconstruction
     of the waveform is unacceptable.

    Usually arises from incorrect settings of
     disparate equipment (amplitude / levels).

    Also caused by insufficient sampling
     frequency (Nyquist theory).
The Nyquist Rule
Implications of Nyquist

  Each half of the waveform must be recorded
   there must be 2 samples per period
   the sampling frequency must be at least
   twice the highest signal frequency.

        For example:
         if the highest frequency is 14,080 Hz, then at least 2x14.08
         = 28.16 kHz must be used. Thus 22.05 would not suffice -
         44.1 would be needed.
Editing Digital Sound
   Wide range of software
       Commercial (eg Sonic Foundry Sound Forge)
       Shareware/Freeware
       Bundled with hardware
   Common Operations
     Trimming
     Splicing and assembly
     Volume adjustments
     Downsampling
     Fades & other effects
Sound Compression

   In principle very similar to image or movies

   Usually (not always) lossy

   Requires codec for compression and playback

   Streaming is commonplace and well developed (eg
    RealAudio, QuickTime, MS Media Player).
Common Codecs
  GSM - mostly for voice (3.5-28 Mb per hour)
  ADPCM (Microsoft or IMA) - high quality, but low
   compression (14-152 Mb per hour)
  Lernout & Hauspie - voice only (3.5-7 Mb per hour)
  CCIT A-Law - European TAPI devices, very high quality
   (24-302 Mb per hour)
  CCIT -Law - American TAPI devices, very high quality
   (24-302 Mb per hour)
Common File Formats
    Very large number
        Audio IFF (AIFF) - developed by Apple for Macintosh, also used by
         SGI and various software.
        AU Audio - uncompressed format developed by Sun, widely used on
         the Internet.
        WAV - developed by Microsoft for Windows.
        MP3 - highly compressed, high quality spin-off of the MPEG project
         RealAudio RAM - developed for streaming by RealMedia.
    Differ in terms of:
        Compression algorithms
        Metadata
        Security & Encryption

				
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