Adaptive data compression method and apparatus

Coded data generation or conversion – Digital code to digital code converters – To or from code based on probability

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341106, H03M 740

Patent

active

048621676

ABSTRACT:
A method and apparatus for compressing data, particularly useful in a modem. The preferred method is implemented in a microprocessor within a modem, and dynamically adapts to changing data statistics. Parallel encoding and decoding tables are provided at the encoder and the decoder, and are updated for each character processed. Each table has a plurality of digital compression codes associated with characters of an alphabet. In response to an item of data presented for encoding, a compression code which corresponds to the character presented for encoding is selected using the encoding table. The selected compression code is provided as an output. Periodically, the association between the codes and the characters of the alphabet in the table is adjusted as a function of the frequency of occurrence of characters of the alphabet, over a plurality of characters. As the frequency of occurrence of characters presented for encoding changes, the more frequently occurring characters become associated with the shorter codes in the encoding table. By performing the same steps in the decoding table, the compression codes are used at the decoder to select a character of the alphabet, which is provided as a decoded output.

REFERENCES:
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patent: 4612532 (1986-09-01), Bacon et al.
patent: 4748638 (1988-05-01), Friedman et al.
Data Compression, Table of Contents, 1983, Gilbert Held.
Data Compression, Table of Contents, pp. 63-67, pp. 302-305, 1985, Thomas J. Lynch.
Bentley et al., "A Locally Adaptive Data Compression Scheme", Communications of the ACM, vol. 29, No. 4 (Apr. 1986), pp. 320-330.

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