A subtractive synchronous color-killer circuit for color television receivers

Every color television receiver requires a color-killer circuit in order to eliminate spurious color patterns during the absence of the color burst signal and to prevent the admission of noise through the color channel to the picture tube. This color-killer circuit has to be arranged so that it functions properly in the presence of heavy noise since color information is also useful at low signal-to-noise ratios. Since the color burst signal is only transmitted during a colorcast, it is chosen as a reliable reference signal which actuates the killer tube. This tube, in turn, disables the color bandpass during a monochrome (no burst) transmission and turns it on again when color is transmitted.