Coding theory: Difference between revisions

Revision as of 17:05, 7 October 2005

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# Channel Coding [Error Control Encoding]

The first is Source Encoding which attempts to compress the data from a source in order to transmit it more efficiently. We see this in practice every day on the Internet where the commom "Zip" data compression is used to reduce the network load and make files smaller. The second is Channel Encoding. This technique adds extra data bits, commonly called parity bits, to make the tranmission of data more robust to disturbances present on the transmission channel. There are many application that the ordinary user is not aware of that utilize channel encoding. A typical music CD has powerful BCH block codes to correct for scratches and dust. In this application the transmission channel is the CD itself. Cell phones also use powerful coding techniquie to correct for the fading and noise of high frequency radio transmission. Data modems, telephone transmission and of course NASA all employ powerful channel coding to get the bits through.

== Source Encoding ==

The aim of source encoding is to take the source data and make it smaller. FAX transmission which has been around for many years uses a simple run length code. The principle is to recognize that most documents are white space with brief interruptions for the black typing. So FAX compresses a document by adding a repeat count to the next transisition. It may tell the receiver that 100 of the next pixels are white. Another common encoding technique is string compression. This is used for data files. The encoder has a dictionary of strings. It matches the incoming text to the strings in the dictionary and when found, it will send a single number to the receiver which is the index to the string. All know implementations of string compression are adaptive in nature and allow the encoder to create new strings and transmit them to the decoder so the two dictionaries remain the same.