Simple Audio Compression Methods

Traditional lossless compression methods (Huffman, LZW, etc.) usually don't work well on audio compression (the same reason as in image compression).

The following are some of the Lossy methods applied to audio compression:

• Silence Compression - detect the "silence", similar to run-length coding

• Adaptive Differential Pulse Code Modulation (ADPCM)

  e.g., in CCITT G.721 - 16 or 32 Kbits/sec.

  (a) encodes the difference between two consecutive signals,

  (b) adapts at quantization so fewer bits are used when the value is smaller.

  • It is necessary to predict where the waveform is headed -> difficult

  • Apple has proprietary scheme called ACE/MACE. Lossy scheme that tries to predict where wave will go in next sample. About 2:1 compression.

• Linear Predictive Coding (LPC) fits signal to speech model and then transmits parameters of model. Sounds like a computer talking, 2.4 kbits/sec.

• Code Excited Linear Predictor (CELP) does LPC, but also transmits error term - audio conferencing quality at 4.8 kbits/sec.