6.05 Chapter 3 Compression
- Lossless or reversible compression
- Lossy or irreversible compression
Variable_length Encoding
Morse
Run Length Encoding
For example: a B B B B a a B B equires to a 1 B 4 a 2 B 2
It is not convenient to deliver information which does not have much repeated information
Static Dictionary
unused code be set to frequently occurring sequences of symbols
However, there is an inherent risk: the effect of errors is apt to be much greater.
rise the account of the single code
This technique works well for sets of messages that are quite similar
Semi-adaptive Dictionary
The static-dictionary approach requires one dictionary, defined in advance, that applies to all messages. If a new dictionary could be defined for each message, the compression could be greater because the particular sequences of symbols found in the message could be made into dictionary entries
The dictionary must compress with message, and it is necessary to analyze the information before compression. The PC need to have enough space to store
Dynamic Dictionary
动态字典，最好的解决办法是消息传入后即时进行计算解包，[[LZW Algorithm]]