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Facsimile Threshold with Full Background Control

IP.com Disclosure Number: IPCOM000079872D
Original Publication Date: 1973-Sep-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 48K

Publishing Venue

IBM

Related People

Cassada, TE: AUTHOR [+2]

Abstract

Many facsimile schemes exist utilizing a fixed level as a threshold for digitizing data. With most of these schemes, a change in background level picked up by the scanner will cause a shift of the relative "0" of the signal being digitized. This shift in absolute 0 will result in a smaller or greater percentage of the signal being thresholded at a "1" level. A method is described for eliminating this 0 absolute shift.

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Facsimile Threshold with Full Background Control

Many facsimile schemes exist utilizing a fixed level as a threshold for digitizing data. With most of these schemes, a change in background level picked up by the scanner will cause a shift of the relative "0" of the signal being digitized. This shift in absolute 0 will result in a smaller or greater percentage of the signal being thresholded at a "1" level. A method is described for eliminating this 0 absolute shift.

Some schemes exist for taking into consideration this change in background level. The least complicated of these requires some memory to remember what the average background level was with some given time constant and an adjustment of the scanner signal reference to this level. The present system requires no memory and is a truly differential scheme. In Fig. 1, the scanner output is first passed through a low-pass filter to eliminate any high-frequency noise that might be present. This filter signal is then passed through a differentiator. The differentiation process makes the frequencies is the scanner output be the determining factors for the digitization process, rather than the absolute levels of voltage which could be changed if background or reflectivity of paper changes.

If now, this differentiated signal is thresholded with both a high and a low threshold, quantization can then be performed. In Fig. 2, a 0 or low level of quantization is assumed until the differentiated signal exceeds some pre...