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Compression Algorithm for Display Data Using Character-Encoding Tables

IP.com Disclosure Number: IPCOM000036484D
Original Publication Date: 1989-Oct-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 2 page(s) / 13K

Publishing Venue

IBM

Related People

Callaway, JR: AUTHOR [+4]

Abstract

Disclosed is an algorithm for compressing ASCII data using a character- encoding scheme. Display data is to be transmitted over phone lines at relatively slow speeds for real-time updates of a remote display. Minimizing the amount of transmitted data by using the described compression algorithm is the key to the performance of the application.

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Compression Algorithm for Display Data Using Character-Encoding Tables

Disclosed is an algorithm for compressing ASCII data using a character- encoding scheme. Display data is to be transmitted over phone lines at relatively slow speeds for real-time updates of a remote display. Minimizing the amount of transmitted data by using the described compression algorithm is the key to the performance of the application.

Small amounts of data (usually less than 100 bytes), representing changes in a remote computer's display screen, are to be compressed and transmitted. A table showing which bit strings the ASCII characters are encoded to is used in both the compression of the data and in the decompression on the receiving end. In this type of compression it is important to build the table to fit the data to be compressed, so that the shortest bit strings are used to represent the most frequently used characters. However, dynamically building a table based on the data and then transmitting a 256-word table to use in decompressing 50 bytes of data would be severely detrimental to performance. Instead, a set of tables are built to fit different types of data, such as numerical data, alphabetic text, and graphics characters.

The compression of the data is a nibble-based encoding scheme similar to Huffman bit encoding which uses short bit strings to represent more frequently used characters and longer bit strings for less frequently used characters. For this application, compression is based on using a single nibble to represent the most frequently used characters in the data. Two, three, or even four nibbles are used for the less frequently used characters in the data.

The encoding of the data allows some codes at the upper end of the comp...