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616 Chapter 12 APPLICATIONS IN COMMUNICATIONS
FIGURE 12.5 Example of a quantizer with an adaptive step size ([10])
12.3.1 ADPCM STANDARD
Figure 12.6 illustrates, in block diagram form, a 32, 000 bps ADPCM
encoder and decoder that has been adopted as an international (CCITT)
standard for speech transmission over telephone channels. The ADPCM
encoder is designed to accept 8-bit PCM compressed signal samples at
64,000 bps, and by means of adaptive prediction and adaptive 4-bit
quantization to reduce the bit rate over the channel to 32,000 bps. The
ADPCM decoder accepts the 32,000 bps data stream and reconstructs
the signal in the form of an 8-bit compressed PCM at 64,000 bps. Thus we
have a configuration shown in Figure 12.7, where the ADPCM encoder/
decoder is embedded into a PCM system. Although the ADPCM encoder/
TABLE 12.1 Multiplication factors for adaptive step size adjustment ([9])
PCM
DPCM
2 3 4 2 3 4
M(1) 0.60 0.85 0.80 0.80 0.90 0.90
M(2) 2.20 1.00 0.80 1.60 0.90 0.90
M(3) 1.00 0.80 1.25 0.90
M(4) 1.50 0.80 1.70 0.90
M(5) 0.80 1.20
M(6) 0.80 1.60
M(7) 0.80 2.00
M(8) 0.80 2.40
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