AT&T MERLIN LEGEND Installation & Maintenance Manual page 79

System Description
The digitally encoded signals are routed from one interface port
to another interface port by assigning source and destination to
specific time slots on the TDM bus. In this way, signals can be
transmitted to one or several destinations and reconstructed at
the original amplitude. The result is no signal loss during
switching and transmission from one point to another.
Quantizing
The process of converting PAM samples into discrete PCM
values is called quantizing. In the case of voice-grade signals,
eight bits are commonly used to encode one PAM sample.
Quantizing with eight bits allows 256 unique values to represent
the range of amplitudes in the analog signal being sampled.
Quantizing causes distortion due in part to the "rounding off" of
PAM samples into discrete PCM values. This distortion can be
minimized by increasing the number of bits used to encode each
PAM sample or by increasing the sampling rate. Using 8-bit
coded samples taken at an 8-kHz rate results in toll-grade
transmission quality.
Companding
Distortion also occurs in quantizing because equal consideration
is given to all amplitude levels. Linear quantizing divides the
amplitude range into equal segments, which results in
unnecessary quality at levels where voice signals are unlikely to
occur and not enough quality where they do occur. Commanding
eliminates this distortion by using a nonlinear scale that has
smaller divisions and more accuracy in areas of the scale where
voice signals are most likely to occur.
1-48 Signal Processing