Abstract: A signal voltage level dual clamping circuit is disclosed for use in a receiving circuit for extraction of timing information from a signal. A first, start-up voltage level clamp is provided, the operation of which is independent of the signal timing information. A second, gated voltage level clamp is provided, the operation of which is dependent on the signal timing information. A switching circuit operates to switch the first clamp out of operation and switch the second clamp into operation once sufficient timing information has been extracted from the signal to permit operation of the second clamp.

Abstract: A data and synchronization extraction circuit for processing composite video signals containing closed captioning data is disclosed. A dual mode voltage clamp is realized in CMOS technology which includes temperature compensated current sources in the form of complementary current mirrors. A modified version of such current sources is also disclosed which permits trimming of the current after manufacture and packaging. Sync pulses are separated by doubling the amplitude of a composite video signal with an amplifier and comparing the amplified signal with a back porch level derived by a sample-and-hold device. Frequency and phase synchronization is accomplished by a combination of a frequency lock loop and a phase lock loop working in concert to generate a control voltage for a voltage controlled oscillator in a flywheel mode. The voltage controlled oscillator provides a clean source of timing information for the circuit.