Digital Audio Compression

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Digital Audio Compression
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Formats

• There are many different formats for storing and
  communicating digital audio
• CD audio
• Wav
• Aiff
• Au
• MP3

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The Storage Problem

• CD quality recording
• 44100Hz sampling rate
• 16 bit quantization
• 2 channels (stereo)
• 176.4 Kbytes per second
• 1 minute is 10.5 Mbytes
• 74 minutes is 780 Mbytes

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Psychoacoustics

• The study of the psychological and physiological
  principles of sound perception
• CDs try to accurately reproduce the original
  audio signal
• But we do not hear all of this signal
• The parts that we dont hear are redundant
• If we remove these parts we can store the signal
  using less data but without effecting the
  perceived sound

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Threshold of Hearing Masking

• The threshold of hearing curve describes the
  minimum level at which the ear can detect a tone
  at a given frequency

Fletcher-Munson curves
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Amplitude Masking

• Amplitude masking occurs when a tone shifts the
  threshold curve upwards in the frequency region
  that surrounds it

0.
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Critical Band

• Hair cells on the Basilar membrane respond to the
  strongest stimulation in their local region
• This local region is called the critical band
• Critical bands are smaller for low frequency
  signals than they are for high frequency signals

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Critical Bands
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Amplitude Masking Thresholds
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Temporal Masking

• Masking can also occur when tones are sounded at
  slightly different times
• Premasking signal A is masked by signal B which
  occurs later
• Postmaking signal A is masked by signal B which
  ends before signal A has started
• Temporal masking increases as time differences
  reduce

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Temporal Masking
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Masking

• Amplitude and temporal masking form a masking
  area in the time-frequency domain

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Perceptual Coding

• Perceptual coders analyse the frequency and
  amplitude content of the input signal and compare
  it to a model of human auditory perception
• Parts of the input signal which are inaudible are
  removed

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Perceptual Coding

• A perceptual coder uses a digital filter bank to
  split a short duration of audio signal into
  multiple frequency bands

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Perceptual Coding

• The coder analyses the energy in each of these
  subbands to determine which subbands contain
  audible information

• Subbands which are not audible are not coded

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Perceptual Coding

• Quantization bits are assigned according to
  signal strength above the audibility curve

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Perceptual Coding

• The purpose of perceptual coding is to reduce the
  data rate
• Perceptual coders maintain sampling frequency,
  selectively decrease word length
• Coders reduction ratio is the ratio of input bit
  rate to output bit rate
• Ratios of up to 61 are often transparent

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Perceptual Coding

• Because the inaudible content of the signal is
  removed the playback systems ability to convey
  audible music should improve
• In theory it is possible to get better
  reproduction after perceptual coding than the
  original! (In theory)
• Perceptual coders more properly code an audio
  signal for passage through an audio system

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MP3

• Mpeg 1 Audio Layer 3
• Developed to support audio coding for playback
  with video
• Uses
• A filterbank producing 32 subbands from 24ms of
  audio data
• Perceptual coder originally produced by the
  Fraunhofer Institut Integrierte Schaltungen
• Lossless Huffman coding

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MP3
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MP3

• Sound quality is highly dependent on the
  performance of the encoder
• Most encoders use constant-bitrate (CBR)
  encoding. In this mode you choose a target
  bitrate (e.g. 128kBit/s)
• Codecs
• Fraunhofer
• Xing MP3 encoder
• Etc

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Joint Stereo Coding

• Takes advantage of interchannel redundancy
  between stereo channels
• Some sounds and some components are equal in both
  channels
• Low frequencies Bass instruments, strings, low
  components of drums
• Centrally placed signals typically vocals
• Removing duplication reduces data without
  effecting perceived sound

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MIDI

• Musical Instrument Digital Interface
• Serial interface standard for interfacing between
  electronic instruments (31.25Kbits
  uni-directional)
• Involves communicating sound events rather than
  sampled audio

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MIDI

• The information transmitted between MIDI devices
  is in a form called a MIDI message, which encodes
  aspects of sound such as
• note played, note number, velocity
• sustain pedal, breath controllers, aftertouch,
  pitch bend, modulation

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MIDI Message Format

• Control messages are sent as groups of bytes,
  preceded by one start bit and followed by a stop
  bit

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MIDI Message Format

• There are two basic types of MIDI
  message byte
• Status byte (starts with a 1)
• Data byte (starts with a 0)

8 bits
1sssnnnn 0xxxxxxx 0yyyyyyy
Status
Data 1
Data 2
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MIDI Message Format
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MIDI

• There are sixteen basic MIDI channels and
  instruments can usually be set to receive on one
  (omni off) or all (omni on)
• General MIDI sets the standard for the ordering
  and naming of sounds
• Group 1-8 Piano 65-72 Reed
• 9-16 Chromatic Percussion 73-80 Pipe 17-24
  Organ 81-88 Synth Lead 25-32 Guitar 89-96
  Synth Pad 33-40 Bass 97-104 Synth FX 41-48
  Strings 105-112 Ethnic Instruments 49-56
  Ensemble 113-120 Percussive Instrms 57-67
  Brass 121-128 Sound effects
• Plus 47 percussive sounds

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MIDI

• Using MIDI for transferring music can be
  problematic
• Even with general MIDI the output depends on the
  quality and type of synthesiser used
• Not so good for Hi-Fi
• Very good for multimedia or games sound effects
  and music

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MIDI Sequencers

• Capable of storing a number of tracks of MIDI
  information and manipulating them

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MIDI Sequencers
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MIDI Sequencers
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Fin

• Fin