Recovery of time base for teletext system - uses PLL at output of bistable circuit, with counter to reset bistable after time period

摘要

The time base is recovered from a composite video synchronisation signal to produce line synchronisation pulses for use in a teletext system. The output of the composite sync. circuit drives a phase locked loop to avoid disturbance to the image quality. A D-type bistable (1) receives the composite video sync. signal (V) at its clock input. The Q-output is applied to a phase locked loop (2) and to an integrator (3) which produces a sawtooth waveform which is compared with a threshold signal (5) by a comparator (7). The comparator output presets the bistable after a time period. The Q-bar output is connected to a similar integrator (10) and comparator (14) to reset the bistable after a time period. The time periods produced by the comparators provide a window for the line synchronisation signal.