Coded data generation or conversion – Digital code to digital code converters – To or from code based on probability
Patent
1987-02-24
1989-08-29
Shoop, Jr., William M.
Coded data generation or conversion
Digital code to digital code converters
To or from code based on probability
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.
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Bentley et al., "A Locally Adaptive Data Compression Scheme", Communications of the ACM, vol. 29, No. 4 (Apr. 1986), pp. 320-330.
Hayes Microcomputer Products, Inc.
Hoff Marc S.
Shoop Jr. William M.
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