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Hybrid Data Compression Technique for Change Block Skipping in an APA Printer

IP.com Disclosure Number: IPCOM000052221D
Original Publication Date: 1981-May-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 2 page(s) / 13K

Publishing Venue

IBM

Related People

Spivey, PR: AUTHOR

Abstract

The data compression technique described in the previous article and referred to as the "Change Block Skipping" (CBS) code has a very fast execution time implemented in software, but its compression ratio is not as good as some other techniques. Another technique, the run length method, has just the opposite problem. A hybrid technique takes advantage of the processing speed of the CBS code and the compression ratio of the run length code. In effect, the hybrid technique is a compromise approach.

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Hybrid Data Compression Technique for Change Block Skipping in an APA Printer

The data compression technique described in the previous article and referred to as the "Change Block Skipping" (CBS) code has a very fast execution time implemented in software, but its compression ratio is not as good as some other techniques. Another technique, the run length method, has just the opposite problem. A hybrid technique takes advantage of the processing speed of the CBS code and the compression ratio of the run length code. In effect, the hybrid technique is a compromise approach.

As described in the previous article, the CBS method reduces the raw input data into a control bank and a data bank. The data bank is simply a table of all the changing byte data in the input character. The control bank indicates which bytes change, that is, how to assemble the information in the data bank to reconstruct the input character.

The run length method described in detail in [*] uses variable length symbols to represent certain white or black (0 or 1) run lengths. The shortest symbols are assigned to the most probable run lengths, and the longest symbols are assigned to the least probable run lengths.

The hybrid technique disclosed below is simply an application of the run length method to encode the control bank of the CBS code. The following is a simple example: A. Construct a table or array of bits which indicate which

bytes in a scan are different from the corresponding

bytes in the previous scan.

CHANGING BYTES

SCAN 1 2 3 4 5

1 0 0 1 1 0

2 0 1 1 0 0

3 0 0 0 1 0

4 0 0 0 0 0

B. Form a serial string of data from the array by wrapping

the end of a scan to the beginning of the next scan.

0 0 1 1 0 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 C. Construct a symbol table of white/black run lengths. 0 0 1 1 0 0 1 1 0 0 0 0...