ual sample voltage and the predicted voltage. This technique is called differential PCM and can provide comparable speech quality with only 4 bits per sample, 32,000 bits per second. The common differential speech coder, adaptive differential pulse code modulation (ADPCM), converts linear 13-bit PCM to 15 levels, 4 bits per symbol. If the source is analog voice, then it first must be converted to linear PCM. u-Law and A-law PCM streams are converted from their logarithmic steps to linear PCM with equal steps be- tween levels. It will help us understand the ADPCM encoding process if we look at the decoder first. As each ADPCM 4-bit sample A comes into the decoder, it goes through an inverse adaptive 15-level quantizer, as shown in Fig. 18.7. The output of the inverse adaptive 15-level quantizer is a difference calculation D. The sequence of D samples is fed into an adaptive predictor whose output is a prediction P. When the prediction and differ- ence terms are added, P + D becomes the linear PCM output. This linear PCM stream can be converted to u-law or A-law PCM for transmission, or it can be converted to ana- log audio for somebody to hear. Now let us turn our attention to the ADPCM encoding process shown in Fig. 18.8. As each linear PCM sample L comes into the encoder, it is compared with the adaptive pre- diction P for that sample. The difference L - P goes into the adaptive 15-level quan- tizer and becomes the ADPCM output sample A. This same output sample goes into an inverse adaptive 15-level quantizer and an adaptive predictor exactly like the ADPCM decoding process to form the prediction for the next sample. In this way, the prediction samples P used in the encoding difference calculation are the exact same sequence of digital bits as the prediction samples P used in the decoding process. This feedback process continuously steers the differential calculations so that the output of the ADPCM decoder D + P is as close as possible to the linear PCM stream L that went into the encoder. How is ADPCM able to 