PCM Encoding and Decoding

Aim:
Introduction to PCM encoding and decoding.
Introduction:
PCM Encoding:
The input to the PCM ENCODER module is an analog message. This must be constrained to a
defined bandwidth and amplitude range.
The maximum allowable message bandwidth will depend upon the sampling rate to be used. The
Nyquist criterion must be observed.
The amplitude range must be held within the ± 2.0 volts range of the TIMS ANALOG REFERENCE
LEVEL. This is in keeping with the input amplitude limits set for all analog modules.
A step-by-step description of the operation of the module follows:
1. the module is driven by an external TTL clock.
2. the input analog message is sampled periodically. The sample rate is determined by the
external clock.
3. the sampling is a sample-and-hold operation. It is internal to the module, and cannot be viewed
by the user . What is held is the amplitude of the analog message at the sampling instant.
4. each sample amplitude is compared with a finite set of amplitude levels. These are distributed
(uniformly, for linear sampling) within the range ± 2.0 volts (the TIMS ANALOG REFERENCE
LEVEL). These are the system quantizing levels.
5. each quantizing level is assigned a number, starting from zero for the lowest (most negative)
level, with the highest number being (L-1), where L is the available number of levels.
6. each sample is assigned a digital (binary) code word representing the number associated with
the quantizing level which is closest to the sample amplitude. The number of bits ‘n’ in the digital
code word will depend upon the number of quantizing levels. In fact, n = log2(L).
7. the code word is assembled into a time frame together with other bits as may be required
(described below). In the TIMS PCM ENCODER (and many commercial systems) a single extra
bit is added, in the least significant bit position. This is alternately a one or a zero. These bits are
used by subsequent
decoders for frame synchronization.
8. the frames are transmitted serially. They are transmitted at the same rate as the samples are
taken. The serial bit stream appears at the output of the module.