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.

Abstract: The present invention is directed to the implementation of a reliable data communications system using the horizontal scan lines of video signals, and is particularly concerned with the minimization of problems which are caused as a result of false acquisition of horizontal scan lines and missed or dropped lines. In accordance with the present invention, a two-tier approach to receiving data transmitted via television signals is used. The two-tier approach includes a search mode and a lock mode. The present system operates on the principle that if a horizontal scan line (or lines) has been perfectly identified to contain data for successive fields of a video signal, then this scan line (or lines) can be presumed to be a valid data bearing scan line (or lines) in fields of the video signal yet to be received. The lock mode is then utilized to favor the retrieval of what would otherwise be missed or dropped horizontal scan lines were the system operating in the more restrictive search mode.