Introduction to Pulse Code Modulation (PCM)

Pulse code modulation (to be discussed in depth later) is the heart of technology in communications in today’s digital world. It’s a process in which analog signals are converted to digital form. The analog signal is represented by a series of pulses and non-pulses (1 or 0 respectively). At this stage to fully understand we can refer back to the notes on signals.

Why PCM?

The stream of pulses and non-pulse streams of 1’s and 0’s are not easily affected by interference and noise. Even in the presence of noise, the presence or absence of a pulse can be easily determined. Since PCM is digital, a more general reason would be that digital signals are easy to process by cheap standard techniques. This makes it easier to implement complicated communication systems such as telephone networks (covered later in this course).

The practical implementation of PCM makes use of other processes. The processes are carried out in the order in which they appear below:

• Filtering
  
• Sampling
• Quantizing
  
• Encoding
  

The filtering stage removes frequencies above the highest signal frequency. These frequencies if not removed, may cause problems when the signal is going through the stage of sampling. Sampling of a waveform means determining instantaneous amplitudes of a signal at fixed intervals. You may have a problem in understanding what this sentence means, but if you take time to look at Figure 7 on the following Slide, you should be able to understand.

Figures 7a, b, c: The Action of Sampling

Figure 7a shows the determination of instantaneous amplitude at uniform intervals. 7b shows the samples corresponding to instantaneous amplitudes of the input signal, and 7c shows the output which represents the reconstructed input signal. Earlier we talked about PCM as the process of changing/converting signals from one form (analog) to the other (digital). Sampling is the first part of the answer as to how the signal changes from one form to the other. Quantization is the process of allocating levels to the infinite range of amplitudes of sample values of the analog signal. This may not be clear now, but it should be after looking at the example below.

In this example the maximum amplitude value is +8 and the minimum is –8. The amplitude values are quantized into four levels. The full range (from –8 to +8) of values is 16. Therefore the width of each level is 16 divided by 4 which gives 4 volts. Quantization is done such that half the steps are at the top and half at the bottom.

We now move onto the last process which is encoding. In this process each step level is assigned a number. The numbers start with zero at the lowest level. These assigned numbers are then expressed in binary form (in terms of 0’s and 1’s). This will be the last part of the conversion and the PCM signal will be transmitted/sent.

One disadvantage of PCM is that the signal accuracy is reduced because of the quantizing of the samples.