Audio Compression Techniques

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Audio Compression Techniques

• MUMT 611, January 2005
• Assignment 2
• Paul Kolesnik

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Introduction

• Digital Audio Compression
• Removal of redundant or otherwise irrelevant
  information from audio signal
• Audio compression algorithms are often referred
  to as audio encoders
• Applications
• Reduces required storage space
• Reduces required transmission bandwidth

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Audio Compression

• Audio signal overview
• Sampling rate ( of samples per second)
• Bit rate ( of bits per second). Typically,
  uncompressed stereo 16-bit 44.1KHz signal has a
  1.4MBps bit rate
• Number of channels (mono / stereo / multichannel)
• Reduction by lowering those values or by data
  compression / encoding

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Audio Data Compression

• Redundant information
• Implicit in the remaining information
• Ex. oversampled audio signal
• Irrelevant information
• Perceptually insignificant
• Cannot be recovered from remaining information

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Audio Data Compression

• Lossless Audio Compression
• Removes redundant data
• Resulting signal is same as original perfect
  reconstruction
• Lossy Audio Encoding
• Removes irrelevant data
• Resulting signal is similar to original

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Audio Data Compression

• Audio vs. Speech Compression Techniques
• Speech Compression uses a human vocal tract model
  to compress signals
• Audio Compression does not use this technique due
  to larger variety of possible signal variations

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Generic Audio Encoder
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Generic Audio Encoder

• Psychoacoustic Model
• Psychoacoustics study of how sounds are
  perceived by humans
• Uses perceptual coding
• eliminate information from audio signal that is
  inaudible to the ear
• Detects conditions under which different audio
  signal components mask each other

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Psychoacoustic Model

• Signal Masking
• Threshold cut-off
• Spectral (Frequency / Simultaneous) Masking
• Temporal Masking
• Threshold cut-off and spectral masking occur in
  frequency domain, temporal masking occurs in time
  domain

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Signal Masking

• Threshold cut-off
• Hearing threshold level a function of frequency
• Any frequency components below the threshold will
  not be perceived by human ear

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Signal Masking

• Spectral Masking
• A frequency component can be partly or fully
  masked by another component that is close to it
  in frequency
• This shifts the hearing threshold

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Signal Masking

• Temporal Masking
• A quieter sound can be masked by a louder sound
  if they are temporally close
• Sounds that occur both (shortly) before and after
  volume increase can be masked

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Spectral Analysis

• Tasks of Spectral Analysis
• To derive masking thresholds to determine which
  signal components can be eliminated
• To generate a representation of the signal to
  which masking thresholds can be applied
• Spectral Analysis is done through transforms or
  filter banks

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Spectral Analysis

• Transforms
• Fast Fourier Transform (FFT)
• Discrete Cosine Transform (DCT) - similar to FFT
  but uses cosine values only
• Modified Discrete Cosine Transform (MDCT) used
  by MPEG-1 Layer-III, MPEG-2 AAC, Dolby AC-3
  overlapped and windowed version of DCT

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Spectral Analysis

• Filter Banks
• Time sample blocks are passed through a set of
  bandpass filters
• Masking thresholds are applied to resulting
  frequency subband signals
• Poly-phase and wavelet banks are most popular
  filter structures

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Filter Bank Structures

• Polyphase Filter Bank
  used in all of the MPEG-1 encoders
• Signal is separated into subbands, the widths of
  which are equal over the entire frequency range
• The resulting subband signals are downsampled to
  create shorter signals (which are later
  reconstructed during decoding process)

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Filter Bank Structures

• Wavelet Filter Bank
  used by Enhanced Perceptual Audio Coder (EPAC)
  by Lucent
• Unlike polyphase filter, the widths of the
  subbands are not evenly spaced (narrower for
  higher frequencies)
• This allows for better time resolution (ex. short
  attacks), but at expense of frequency resolution

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Noise Allocation

• System Task derive and apply shifted hearing
  threshold to the input signal
• Anything below the threshold doesnt need to be
  transmitted
• Any noise below the threshold is irrelevant
• Frequency component quantization
• Tradeoff between space and noise
• Encoder saves on space by using just enough bits
  for each frequency component to keep noise under
  the threshold - this is known as noise allocation

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Noise Allocation

• Pre-echo
• In case a single audio block contains silence
  followed by a loud attack, pre-echo error occurs
  - there will be audible noise in the silent part
  of the block after decoding
• This is avoided by pre-monitoring audio data at
  encoding stage and separating audio into shorter
  blocks in potential pre-echo case
• This does not completely eliminate pre-echo, but
  can make it short enough to be masked by the
  attack (temporal masking)

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Pre-echo Effect
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Additional Encoding Techniques

• Other encoding techniques techniques are
  available (alternative or in combination)
• Predictive Coding
• Coupling / Delta Encoding
• Huffman Encoding

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Additional Encoding Techniques

• Predictive Coding
• Often used in speech and image compression
• Estimates the expected value for each sample
  based on previous sample values
• Transmits/stores the difference between the
  expected and received value
• Generates an estimate for the next sample and
  then adjusts it by the difference stored for the
  current sample
• Used for additional compression in MPEG2 AAC

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Additional Encoding Techniques

• Coupling / Delta encoding
• Used in cases where audio signal consists of two
  or more channels (stereo or surround sound)
• Similarities between channels are used for
  compression
• A sum and difference between two channels are
  derived difference is usually some value close
  to zero and therefore requires less space to
  encode
• This is a case of lossless encoding process

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Additional Encoding Techniques

• Huffman Coding
• Information-theory-based technique
• An element of a signal that often reoccurs in the
  signal is represented by a simpler symbol, and
  its value is stored in a look-up table
• Implemented using a look-up tables in encoder and
  in decoder
• Provides substantial lossless compression, but
  requires high computational power and therefore
  is not very popular
• Used by MPEG1 and MPEG2 AAC

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Encoding - Final Stages

• Audio data packed into frames
• Frames stored or transmitted

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Conclusion

• HTML Bibliography
• http//www.music.mcgill.ca/pkoles
• Questions