Mode recognition for vertical countdown

摘要

An improved vertical countdown system for a television receiver which recognizes weak signal conditions in the received signal. Upon recognition of weak signal conditions or a noise indication from a noise dectector, circuitry provides a signal which is used to drive the receiver circuitry into a forced-standard operation mode for internally generating the vertical drive pulse. Circuitry is employed to monitor the received signal to determine whether sufficient vertical sync information is being received thus detecting whether the received signal is too weak. Also, compatible logic circuitry is monitored to determine whether the received signal has extraneous pulses normally characteristic of a noisy signal. Also, the circuit may be modified to increase or decrease its sensitivity to signal strength fluctuation.