Abstract: 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.

Abstract: Query signals are transmitted by wireless means from the television station to the television set. Response units at receiver locations have a radio frequency transmitter send a response to the station inquiry sent on horizontal lines of the T.V. signal at controlled times identifying each station. This response is a simple r-f beep synchronized with the T.V. signal, in such a way that each set transmitter emits the beep only in its unique time slot. Simplified response units are tuned to the standard intermediate frequency of the T.V. sets and thus need not be wired in and need not have a channel selector.