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Definition: 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.

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).

 Audio          Coding   Rate    Bit Rates
 Format         Method   (kHz)   (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   Bit Rates
 Number   Method   (kHz)  (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    from 6.6
                            to 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 G.723    (withdrawn)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