Why DPCM is used over PCM?

Answer 1

Let (M) and (D) be the peak amplitudes of signals m(t) and d(t) respectively. If the same quantization level (L), is used to sample both signals. The quantization step (Dv) in DPCM is reduced by a factor of (D/M). The quantization noise power is [(Dv)^2]/2.

So we can say, that in DPCM the quatization noise reduces by the factor (M/D)^2. As the SNR is inversely proportional to Noise, this parameter (SNR) increases by the same factor.

In Practice the SNR improvement may be as high as 25 dB in such cases as short-term voiced speech spectra. Alternatively for the same SNR, the bit rate for DPCM could be lower than for PCM by 3 to 4 bits per sample.

Thus, telephone systems using DPCM can often operate at 32 kbits/s (4 bits * 8000 saples/s) or even 24 kbit/s (3 bits * 8000 saples/s).

So DPCM could improve the SNR of the system, using less bandwidth.