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Video Threshold Circuit Disclosure Number: IPCOM000052864D
Original Publication Date: 1981-Jul-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 3 page(s) / 65K

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Gillingham, RD: AUTHOR


A threshold circuit is shown for converting an analog video signal to a binary output (which represents black or white on a document being scanned).

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Video Threshold Circuit

A threshold circuit is shown for converting an analog video signal to a binary output (which represents black or white on a document being scanned).

As shown in Fig. 1, the input video signal 10 is filtered (11) and amplified
(12) to the signal shown in Fig. 2. The white follower 13 follows the most positive peaks, and the black follower 14 follows the least positive peaks. The threshold voltage is between the white and black followers. Voltages above the threshold voltage give an up-level output (representing white). Voltages below the threshold give a down-level output voltage (representing black).

The threshold voltage is approximately half-way between the white and black follower voltages, regardless of absolute voltage magnitudes. This is necessary to prevent the threshold from being affected by the reflectance of the document being scanned.

Variations in the photodiode, optical system, and document reflectance can all cause variations in the magnitude of the white and black follower voltages. The ability of the threshold to be independent of the magnitude is important. However, for large variations in the video input signal, the linear operating range of the white and black followers can be exceeded.

To keep the circuits in the linear range, an automatic gain control (AGC) 15 is used. The output of the AGC is controlled by the white follower voltage 16. When the white follower voltage lowers (due to aging of the light bulb or scan of a low reflectance document), the AGC gain increases. The AGC output signal changes only slightly for a large change in input signal. This keeps the signal to the white and black followers nearly constant regardless of long-term system degradations or type of document scanned.

The signal is only meaningful when the light falls on the document. Thus, the SCAN signal 17 is active during the scan of the document.

The threshold must also be independent of signal variations due to the mechanical and optical system components. There are essentially two kinds of mechanical and optical variations: long term and short term. The long term variations are caused by light-bulb deterioration, dirt on lenses and lens misalignment. The long term variations may be compensated for by the AGC, as described above.

The most notable short term variation is due to the document glass. The document glass is what the document lies upon. As the scanner...