(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-ROMsSony 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). SamplingAudio Coding Rate Bit RatesFormat 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 18.9 151.2ADPCM for Speech CodecsThe 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 SamplingG. Coding Rate Bit RatesNumber 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A-Law PCM 8 64 G.721 ADPCM 8 32 G.722 ASPCM 7 48/56/64 G.722.1 MLT 16 24/32 G.722.2 ACELP 16 from 6.6 to 23.85 G.723 (withdrawn) G.723.1 MP-MLQ 8 6.3 G.723.1 ACELP 8 5.3 G.726 ADPCM 8 16/24/32/40/64 G.727 ADPCM 8 16/24/32/40/64 G.728 LD-CELP 8 16 G.729 CS-ACELP 8 8
A voice encoding technique used to convert analog signals to digital format. ADPCM improves the quality of DCPM by adapting to the incoming signal, without increasing the bit requirement. ADPCM increases the range of signal changes that can be represented by a 4-bit value, thereby adapting to provide higher quality for voice transmission. ADPCM also overcomes the deficiency of DPCM with respect to support of modem transmissions. A modem transmission is characterized by abrupt shifts in amplitude and frequency levels that DPCM cannot accommodate. ADPCM can distinguish the presence of a modem tone, and can adapt by reverting to a channel width of 64 kbps, or by forcing the modem to adapt to a lower speed. ADPCM operates at 64, 56, 48, 40, 32, 24, and 16 kbps, with 32 kbps being the most commonly used. As central office (CO) exchanges are based on PCM rather than ADPCM, it is necessary to use a bit compression multiplexer (BCM) be used to insert two 32-kbps compressed voice conversations into a single PCM channel. See also analog, BCM, digital, DPCM, encode, modem, and PCM. OC-3 OC-3 OC-1 TM DS-3 ADM.