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Automatic-color-level-circuit-with-peak-detector

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Automatic color level circuit with peak detector

Details

Inventors: Jirka, Howard F.;
Assignee: Zenith Radio Corporation (Chicago, IL)
Primary Examiner: Griffin; Robert L.
Assistant Examiner: Saffian; Mitchell
Attorney, Agent or Firm: Camasto; Nicholas A.

A color television receiver includes an automatic chroma control (ACC) system and a switch selectable manual color level potentiometer and a closed loop automatic color level limiter (ACL) with a preset color level potentiometer. The ACL loop is of the sample-and-hold type and includes a peak detector for detecting chroma signals in excess of a predetermined threshold level and a transistor for controlling the preset color level potentiometer for adjustment of the chroma amplifier gain. A relatively small capacitor is connected across the collector and base of the transistor and, in conjunction with a bias resistor, presents a relatively long time constant by reflecting a magnified input capacitance between its base and emitter electrodes due to the Miller effect. In the event the transistor is driven to cutoff during periods when the switch is in the manual position, the input capacitance is very small enabling the ACL circuit to recover quickly.

DETAILED DESCRIPTION The invention includes the combination of means for amplifying signals subject to substantial amplitude variations and a closed loop sample-and-hold control system including peak detector means for detecting portions of the signals in excess of a predetermined threshold level for producing a compensatory control potential to adjust the gain of the amplifying means.
The control means include a transistor normally operated near saturation but which is subject to cutoff, and means whereby the transistor presents a relatively long time constant during conduction and a relatively short time constant during cutoff.
OBJECTS OF THE INVENTION It is, accordingly, a general object of the present invention to provide an improved color television receiver.
Another object of the present invention is to provide an improved automatic chroma level limiter in a color television receiver.
A further object of this invention is to provide a novel sample-and-hold control system.

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