Audio compression zAlgorithms. zStandards.

Coding gain zRatio of uncompressed size to compressed size. zSources: yLossless coding. yLossy perceptual coding.

Companding zChange signal level to lift it above noise. * levelinvert *

Word length zCan use fewer bits to encode signal. yAdds 6 dB/bit distortion. zNon-uniform encoding: yStep size depends on magnitude. yHigher distortion at higher signal levels.

Subband coding zDivide signal into bands. yHigher correlation within a smaller band. ySome bands are less significant. ySimplifies masking. f1() f2() f3() coder

Transform coding zPerform transform at coder, inverse transform at decoder. zOperates on finite-sized blocks. yMust window data to avoid truncation artifacts. f

MPEG audio standards zLayer 1: yLossless compression of subbands + optional simple masking model zLayer 2: yMore advanced masking model. zLayer 3: yAdditional processing for lower bit rates.

MPEG audio rates zInput sampling rates: y32, 44.1, 48 kHz. zOutput bit rates: y23, 48, 64, 96, 112, 128, 192, 256, 384 kbits/sec. zOutput can be mono, dual-channel (bilingual, etc.), stereo.

Other standards zDolby Digital (AC-3): yUses modified discrete cosine transform. zATRAC (MiniDisc): yUses subband + modified DCT. zMPEG-2 AAC.

MPEG Layer 1 z384 samples/block at all frequencies. yEquals 8 ms at 48 kHz. zOptional masking model. yDriven by separate FFT for better accuracy.

MPEG Layer 1 data frame zBit allocation codes specify word length in each subband. zScale factors give gain for each band. headerCRC bit allocation scale factors subband samples aux data

MPEG Layer 1 encoder Filter bank Choose Scale factor Masking model requantize * FFT mux

MPEG Layer 1 decoder demux Scale factor ** Step size Inverse filter bank inverse quantize expand