Q. Describe about digital communication systemsfl

A digital signal can be deflned as having any one of a finite number of discrete amplitudes at any given time. The signal could be a voltage or current, or just a number such as 0 or 1. A signal for which only two amplitudes are possible is known as a binary digital signal, the type of which is commonly used in computers andmost digital communication systems. A communication system that is designed to process only digital signals (or messages) to convey information is said to be digital. The recent trend is to make as much of the system digital as possible, because:

• Discrete data are efflciently processed.

• Analog messages can also be converted to digital form.

• Digital systems interface well with computers.

• Digital systems offer great reliability and yield high performance at low cost.

• Being flexible, digital systems can accommodate a variety of messages with ease.

• Security techniques are available to offer message privacy to users.

• Advanced signal-processing techniques can be added on.

However, the most serious disadvantages are the increased complexity needed for system synchronization and the need for larger bandwidths than in an equivalent analog system. A digital system can directly interface with a source having only discrete messages, because of the inherent characteristic of the digital system. With suitable conversion methods, however, systems currently exist that can simultaneously transmit audio, television, and digital data over the same channel.

Before we begin discussing digital systems, it is helpful to talk about the methods by which analog messages are converted into digital form. Sampling, quantization, and coding are the three operations needed for the transmission of an analog signal over a digital system.