Facsimile and static presentation processing – Facsimile – Specific signal processing circuitry
Patent
1991-07-29
1993-12-14
Kostak, Victor R.
Facsimile and static presentation processing
Facsimile
Specific signal processing circuitry
358174, 341110, H04N 518
Patent
active
052708195
ABSTRACT:
A single loop analog-to-digital conversion and video clamping circuit includes a low-pass filter and analog amplifier that present the incoming analog video signal to an analog-to-digital converter and to the input of a limit detector and protection circuit. The output of the analog-to-digital converter is sampled an integer multiple of four times during the black interval, with the results being summed and averaged. The resulting value is then scaled and limited, and compared to the desired level for a representation of black. Any resulting error quantity is converted from digital back to, analog and modified if necessary by the output of the limit detector and protection circuit. The signal that results from that modification is low-pass filtered and converted to a current by a transconductance amplifier. The resulting feedback current is then applied to a capacitor in the input circuitry, thereby clamping the input and resulting digital output of the overall circuit to the desired level. This circuit is also compatible with automated digital diagnostics, in that changing the desired black level by a small but detectable amount and verifying correct operation can he accomplished even in the presence of a video signal. And, in the absence of a video signal, the reference level can be changed between black and white and the digital output video value can be read back by a microprocessor to verify correct operation.
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Griffith Boulden G.
Kostak Victor R.
The Grass Valley Group Inc.
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