ADPCM

(communications) A method of compressing speech and music signals in which the transmitted signals represent differences between input signals and predicted signals, and these predicted signals are synthesized by predictors with response functions representative of the short- and long-term correlation inherent in the signal. Abbreviated ADPCM.

(Adaptive Differential PCM) A widely used variation of PCM that codes the difference between sample points like differential PCM (DPCM), but can also dynamically switch the coding scale to compensate for variations in amplitude and frequency. See PCM, DPCM and sampling.

ADPCM for CD-ROMs

Sony and Philips set the encoding standards for the CD (Red Book) and CD-ROM (Yellow Book), and several ADPCM codecs were defined for mixing audio and data on a CD-ROM. Because of the larger sampling rates and two channels (left and right stereo), the following bit rates are much larger by comparison to the subsequent list of ADPCM codecs used for speech (the G. standards).

                         Sampling
 Audio          Coding   Rate
 Format         Method   (kHz)   Bit Rates (kBps)

 CD-DA (music CD)  PCM    44.1   1411.2
 CD-ROM Level A  ADPCM    37.8    604.8
 CD-ROM Level B  ADPCM    37.8    302.4
 CD-ROM Level C  ADPCM    18.9    151.2

ADPCM for Speech Codecs

The ITU has defined numerous speech codecs, some of which use ADPCM. Note the lower bit rates for these applications because of the smaller sampling rates.

 Codec              Sampling
 G.       Coding    Rate
 Number   Method    (kHz)   Bit Rates (kBps)

 G.711    Mu-Law PCM   8    64
 G.711    A-Law PCM    8    64
 G.721    ADPCM        8    32
 G.722    ASPCM        7    48, 56, 64
 G.722.1  MLT         16    24, 32
 G.722.2  ACELP       16    9 bit rates (6.6-23.85)
 G.723    (withdrawn)
 G.723.1  MP-MLQ       8    6.3
 G.723.1  ACELP        8    5.3
 G.726    ADPCM        8    16, 24, 32, 40, 64
 G.727    ADPCM        8    16, 24, 32, 40, 64
 G.728    LD-CELP      8    16
 G.729    CS-ACELP     8    8

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