What is PCM?

PCM is a Time-Domain Waveform coding method and is defined within CCITT G.711, and AT&T 43801. Basically, an analog signal is sampled at a rate of 8000 times per second. In each sample, the amplitude of the signal is assigned (quantized) a digital value. 

For a true linear system, the value of each "step" is uniform, requiring anywhere from 11 to 14 bits to transmit. However, PCM, as defined and standardized, utilizes a logarithmic scale in the weighting of each step. 

There are two PCM algorithms defined within CCITT G.711, called "A-Law" and "Mu-Law". Mu-Law PCM is used in North America and Japan, and A-Law used in most other countries. In both A-Law and Mu-Law PCM, the values used to represent the amplitude is a number between 0 and +/- 127; therefore, 8 bits are required to represent each sample (2 to the eight power = 256). 

It can be seen then that PCM operates at a rate of: 8 bits/sample * 8000 samples/sec = 64000