G.726 - ADPCM

ADPCM, Adaptive Differential Pulse Code Modulation, is a way an analogue signal can be converted to a digital signal. In the 1980s that was first standardized by the CCITT in G.721 for 32 kbps. Later came the standards G.726 and G.727 for 40, 32, 24 and 16 kbps. G.726 replaces G.721 and G.723.

G.726 specifies how a 64 kbps A-law or µ-law PCM signal can be converted to 40, 32, 24 or 16 kbps ADPCM channels where the 24 and 16 kbps channels are used for voice in Digital Circuit Multiplication Equipment (DCME) and the 40 kbps is for data modem signals (especially modems doing 4800 kbps or higher) in DCME. [13]

Adaptive Differential Pulse Code Modulation (ADPCM) codecs are waveform codecs, which instead of quantizing the speech signal directly, like PCM codecs, quantize the difference between the speech signal and a prediction that has been made of the speech signal. If the prediction is accurate then the difference between the real and predicted speech samples will have a lower variance than the real speech samples, and will be accurately quantized with fewer bits than would be needed to quantize the original speech samples. At the decoder the quantized difference signal is added to the predicted signal to give the reconstructed speech signal. The performance of the codec is aided by using adaptive prediction and quantization, so that the predictor and difference quantizer adapt to the changing characteristics of the speech being coded. [12]

Subsequent of the A-law or µ-law PCM input signal to uniform PCM, a difference signal is obtained, by subtracting an estimate of the input signal form the input signal itself. An adaptive 31-, 15-, 7-, or 4-level quantizer is used to assign five, four, three or two binary digits, respectively, to the value of the difference signal for transmission to the decoder. An inverse quantizer produces a quantized difference signal from these same five, four, three or two binary digits, respectively. The signal estimate is added to this quantized difference signal are operated reconstructed version of the input signal. Both the reconstructed signal and the quantized difference signal are operated upon by an adaptive predictor which produces the estimate of the input signal, thereby completing the feedback loop. Illustration of the encoding process is presented in the Figure below. The encoding process is further explained in the G.726 recommendation from CCITT (ITU). [25]

fig. ADPCM Encoder

The decoder includes a structure identical to the feedback portion of the encoder, together with a uniform PCM to A-law or µ-law conversion and a synchronous coding adjustment.

The synchronous coding adjustment prevents cumulative distortion occurring on synchronous tandem coding (ADPCM-PCM-ADPCM, etc., digital connections) under certain conditions [see 25 §3.7]. The synchronous coding adjustment is achieved by adjusting the PCM output codes in a manner which attempts to eliminate quantizing distortion in the next ADPCM encoding stage. Illustration of the decoding process is presented in the Figure below. The decoding process is further explained in the G.726 recommendation from CCITT (ITU) [25].

fig. ADPCM decoder