Pulse Code Modulation Decoding

Objective

To study the Pulse Code Modulation (PCM) Decoding with respect to Ground.

Introduction

Pulse code modulation (PCM) is a system for converting message signals to a continuous serial stream of binary numbers (encoding). Recovering the message from the serial stream of binary numbers is called decoding.
At its simplest, decoding involves:

• Identifying each new frame in the data stream.
• Extracting the binary numbers from each frame.
• Generating a voltage that is proportional to the binary number.
• Holding the voltage on the output until the next frame has been decoded (forming a pulse amplitude modulation (PAM) version of the original message signal).
• Reconstructing the message by passing the PAM signal through a low-pass filter.

The PCM decoder's clock frequency is crucial to the correct operation of simple decoding systems. If it's not the same frequency as the encoder's clock, some of the transmitted bits are read twice while others are completely missed. This results in some of the transmitted numbers being incorrectly interpreted, which in turn causes the PCM decoder to output an incorrect voltage. The error is audible if it occurs often enough. Some decoders manage this issue by being able to "self-clock".

There is another issue crucial to PCM decoding. The decoder must be able to detect the beginning of each frame. If this isn't done correctly, every number is incorrectly interpreted. The synchronising of the frames can be managed in one of two ways. The PCM encoder can generate a special frame synchronisation signal that can be used by the decoder though this has the disadvantage of needing an additional signal to be sent. Alternatively, a frame synchronisation code can be embedded in the serial data stream that is used by the decoder to work out when the frame starts.