Pulse Code Modulation (PCM) Encoding & Decoding of Continuous Signal

Objective

To study the Pulse Code Modulation (PCM)Encoding & Decoding of Continuous Signal.

Introduction

In the classical communications model, intelligence (the message) moves from a transmitter to a receiver over a channel. A number of transmission media can be used for the channel including: metal conductors (such as twisted-pair or coaxial cable), optical fibre and free-space (what people generally call the "airwaves").

Regardless of the medium used, all channels have a bandwidth. That is, the medium lets a range of signal frequencies pass relatively unaffected while frequencies outside the range are made smaller (or attenuated). In this way, the channel acts like a filter.

This issue has important implications. Recall that the modulated signal in analog modulation schemes (such as AM) consists of many sinewaves. If the medium's bandwidth isn't wide enough, some of the sinewaves are attenuated and others can be removed altogether. In both cases, this causes the demodulated signal (the recovered message) to no-longer be a faithful reproduction of the original.

Similarly, recall that digital signals are also made up of many sinewaves (called the fundamental and harmonics). Again, if the medium's bandwidth isn't wide enough, some of them are attenuated and/or removed and this can change the signal's shape.

To illustrate this last point, Fig.1 below shows what happens when all but the first two of a squarewave's sinewaves are removed. As you can see, the signal is distorted.

Fig.1 Distorted sine waves.
Making matters worse, the channel is like a filter in that it shifts the phase of sinewaves by different amounts. Again, to illustrate, Fig. 2 below shows the signal in Fig. 1 but with one of its two sinewaves phase shifted by 40°. Fig.2 Phase shifted with Fig.1 wave.
Imagine the difficulty a digital receiver circuit such as a PCM decoder would have trying to interpret the logic level of a signal like Fig. 2. Some, and possibly many, of the codes would be misinterpreted and incorrect voltages generated. The makes the recovered message "noisy" which is obviously a problem.