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A NOVEL AUTOMATIC GAIN CONTROL METHOD FOR IMAGE INPUT TERMINALS

IP.com Disclosure Number: IPCOM000024794D
Original Publication Date: 1982-Feb-28
Included in the Prior Art Database: 2004-Apr-04
Document File: 2 page(s) / 90K

Publishing Venue

Xerox Disclosure Journal

Abstract

Image Input Terminals (IITs) using Charged Coupled Devices (CCDs) typically employ some method of gain control to compensate for varying illumination, video amplifier drift, and unit-to-unit variations. In addition, the documents to be scanned may exhibit a high degree of variation in the effective background optical density, (e.g., colored paper). There is described herein an approach which will enable an IIT to automatically adapt to these variables in order to accurately scan an input document.

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XEROX DISCLOSURE JOURNAL

A NOVEL AUTOMATIC GAIN CONTROL METHOD FOR IMAGE INPUT TERMINALS Glenn M. Mehalek

Proposed Classification
U.S. Cl. 358/284 Int. C1. H04n 1/40

Image Input Terminals (IITs) using Charged Coupled Devices (CCDs) typically employ some method of gain control to compensate for varying illumination, video amplifier drift, and unit-to-unit variations. In addition, the documents to be scanned may exhibit a high degree of variation in the effective background optical density, (e.g., colored paper). There is described herein an approach which will enable an IIT to automatically adapt to these variables in order to accurately scan an input document.

The CCD scans an input document one raster line at a time (each line consisting of many picture elements or pixels) and outputs the scan line in a serial fashion, bit by bit, to a video amplifier. The video amplifier increases the magnitude of the signal and performs a DC restoration on the signal. This restoration clamps the level of the video corresponding to "blacktf to a fixed voltage, typically 0 volts (this black video level may be obtained by scanning a black target located at the edge of the scan line). The level of the video corresponding to white may now be positive, typically 1 volt.

However, as the illumination, electronics performance and document background vary, the maximum "white" level measured at the amplifier output may vary considerably. If the video signal for one scan line were peak detected, the resulting voltage level could be considered "white", and the gain of the video amplifier could be automatically adjusted to bring th...