PCM

• The fundamental and most important pulse digital modulation technique is the pulse code modulation (PCM).This technique is the breakthrough for moving from analog to digital communication.

• PCM technique is essentially the result of the thought process to represent message signal in digital form rather than the original analog form.

• The motivation is the merit of digital signal over analog signal for communication, namely, noise robustness.

• PCM may be treated as an extension of PAM.

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.

   1. Also available from the module is a synchronizing signal FS (‘frame synch’). This signals the end of each data frame.

PCM Decoding:

• The PCM DECODER module is driven by an external clock. This clock signal is synchronized to that of the transmitter.

• For this a ‘stolen’ clock will be used. Upon reception, the PCM DECODER:

1. Extracts a frame synchronization signal FS from the data itself (from the embedded alternate ones and zeros in the LSB position), or uses an FS signal stolen from the transmitter.

2. Extracts the binary number, which is the coded (and quantized) amplitude of the sample from which it was derived, from the frame.

3. Identifies the quantization level which this number represents.

4. Generates a voltage proportional to this amplitude level.

5. Presents this voltage to the output Vout. The voltage appears at Vout for the duration of the frame under examination.

6. Message reconstruction can be achieved, albeit with some distortion, by lowpass filtering. A built-in reconstruction filter is provided in the module. For the PCM decoding, you will use the PCM decoder module.