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Abstract

A composite synchronization extraction circuit is particularly suited for receiving composite video signals containing closed captioning data in raster scan line 21 by means of a signal CMOS integrated circuit device. A dual mode voltage clamp is realized in CMOS technology. The clamp includes temperature compensated current sources in the form of complementary current mirrors through which a clamped composite synchronization node of is charged and discharged, the output of which controls a transistor for charging the composite synchronization node. Detected pulse amplitude is set by slicing the incoming pulse at the back porch level and then doubling the amplitude with an amplifier and comparing that level with the back porch level as derived from a sample-and-hold device. The slice voltage level is maintained without an off-chip capacitor by an analog-digital-analog conversion process. Frequency and phase synchronization is accomplished by a combination of frequency lock loop and phase lock loop working in concert to generate a control voltage for a voltage controlled oscillator in a flywheel mode. The voltage controlled oscillator is not subject to noise in the incoming signal and provides a clean source of timing information for the circuit. The effects of impulse noise in the detection of the vertical retrace pulse are eliminated by the use of digital counting circuits to count the requisite number of horizontal synchronization pulses which occur between valid retrace pulses and to block pulses which appear at other times.

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