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In a digital system, the information to be transmitted is converted into binary data (ones and zeros). In the case of text, characters are converted into ASCII format and transmitted. In audio or video, the analog signal is converted into a digital format and then transmitted.
To make best use of a communication channel, the data rate has to be reduced to the extent possible without compromising quality. All information (text, graphics, voice, or video) contains redundancy, and this redundancy can be removed using compression techniques. Use of low bit rate coding techniques (also called source coding techniques) is very important to use the bandwidth efficiently. Particularly in radio systems, where radio bandwidth has a price, low-bit rate coding is used extensively.
When designing a communication system, the designer has to consider the following issues related to information data rates:
What are the information sources to the communication system: data, voice, fax, video, or a combination of these?
How many information sources are there, and is there a need for multiplexing them before transmitting on the channel?
How many information sources need to use the communication channel simultaneously? This determines whether the channel bandwidth is enough, whether multiple access needs to be used, etc.
If the channel bandwidth is not sufficient to cater to the user requirements, the designer has to consider using data compression techniques. A trade-off is possible between quality and bandwidth requirement. For instance, voice signals can be coded at 4.8kbps, which allows many more voice channels to be accommodated on a given communication channel, but quality would not be as good as 64kbps PCM-coded voice.
The services to be supported by the communication system—data, voice, fax, and video services—decide the data rate requirement. Based on the available communication bandwidth, the designer has to consider low bit rate coding of the various information sources.
Note | Compression techniques can be divided into two categories: (a) lossless compression techniques; and (b) lossy compression techniques. The file compression techniques such as Winzip are lossless because the original data is obtained by unzipping the file. Compression techniques for voice, image, and video are lossy techniques because compression causes degradation of the quality. |
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