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# Video Threshold Circuit with Low Contrast Clamp and Absolute Black Clamp

IP.com Disclosure Number: IPCOM000052690D
Original Publication Date: 1981-Jun-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 2 page(s) / 33K

IBM

## Related People

Gillingham, RD: AUTHOR

## Abstract

Shown above is a threshold circuit for digitizing analog video into black and white data. Threshold V(T) is generated by voltage divider action between resistors 10 and 12. Resistor 10 is connected to voltage V(W), which is a white follower voltage. Voltage V(W) follows the white peaks in the analog video. Resistor 12 is connected to black following voltage V(B). Voltage V(B) follows the black peaks in the analog video signal. If resistors 10 and 12 are equal, the threshold V(T) will be half way between V(W) and V(B). This is, in effect, a contrast threshold that is dependent upon the dynamically varying white and black peaks in the analog signal.

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Video Threshold Circuit with Low Contrast Clamp and Absolute Black Clamp

Shown above is a threshold circuit for digitizing analog video into black and white data. Threshold V(T) is generated by voltage divider action between resistors 10 and 12. Resistor 10 is connected to voltage V(W), which is a white follower voltage. Voltage V(W) follows the white peaks in the analog video. Resistor 12 is connected to black following voltage V(B). Voltage V(B) follows the black peaks in the analog video signal. If resistors 10 and 12 are equal, the threshold V(T) will be half way between V(W) and V(B). This is, in effect, a contrast threshold that is dependent upon the dynamically varying white and black peaks in the analog signal.

In the event of a lightly printed document with low contrast, the threshold voltage V(T) could go so close to white as to cause comparator 14 to digitize noise as black data. To prevent this, a low contrast clamp, consisting of operational amplifier 16 and diode 18, is provided. If video threshold V(T) tries to go above low contrast voltage V(C), diode 18 becomes conductive and clamps the video threshold V(T) to voltage V(C). This places a maximum limit on voltage V(T) so that it will not go too close to the white voltage. For many threshold circuits it is advantageous to make voltage V(C) a function of V(W). For example, if V(C) = 0.8 V(W), the threshold will clamp at 80% of the white level. This will allow a 2O% noise margin.

On the other hand, i...