Keying signal generator with input control for false output immunity

Abstract

A source of keying signals for use by keyed circuits in a television receiver comprises a keyed transistor, an input coupling circuit including a timing control circuit responsive to video sync signals for timing the operation of the keyed transistor to normally produce keying signals during blanking intervals of the television signal, and a switching network coupled to the input circuit. The switching network responds to switching signals for enabling the sync signals to be coupled to the keyed transistor via the input coupling circuit during the blanking intervals. During picture intervals of the television signal, the switching network disables the input coupling circuit to prevent the transistor from generating false keying signals in response to spurious input signals occurring during the picture intervals. In addition, the timing control circuit and transistor are isolated from the switching signal during picture intervals so that the switching signal does not upset the normally expected input timing bias of the transistor during picture intervals.