Adpcm Definition


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.

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