Control of rf answer pulses in a TV answer back system

摘要

In a TV answer back system where many receiver stations communicate with a TV transmitter station by means of rf pulses generated by rf oscillators at the receiver station, the requirement for keeping all oscillators on frequency is achieved without crystal temperature control by automatically controlling the rf oscillator frequency using synchronization signals carried in the video signal received from the TV studio. By this invention it has been possible to control the frequency of multiple megahertz oscillator as accurately as desired from relatively low frequency synchronizing signals such as the vertical and horizontal video sync pulses. Thus an oscillations counter is gated for a very accurate time period by the sync pulses, and the oscillation count is compared with a datum count representing the desired oscillation frequency. The requirement to prevent receiver stations from answering with rf pulses in response to queries contained in home video recordings of previously broadcast signals is achieved by encoding broadcast time on all query containing TV transmissions, and by comparing at receiver stations broadcast time information from received signal with time from a local battery backed clock contained at each receiver station, thus discrimination of video recordings is possible by comparing received video broadcast time with local time.