Noise reducing apparatus and methods for television receivers

Abstract

A high-performance television receiver synchronously detects the picture IF signal. A synchronous impulse noise in the IF signal generates either positive-going noise or negative-going noise in the video signal supplied from a synchronous video detector, which noise is subsequently replaced by a value of video signal ocurring previous to the impulse noise in the following manner. The video signal contaminated with impulse noise is supplied as input signal both to an impulse noise detector and to a delay line. The output signal from the delay line, delayed by about 240 ns, is supplied as the signal for tracking by a track-and-hold circuit. The detected impulse noise signal is used to actuate the hold condition in the track-and-hold circuit. By stretching the pulse from the impulse noise detector for the 600-800 ns duration overlapping the duration of most impulse noise, the track-and-hold circuit replaces most impulse noise with previously stored values of the delayed video signal. This not only generally avoids the appearance of white impulse noise and ensuing black impulse noise in the televised picture, but avoids the application of impulse noise to the chroma channel receiving synchronously detected video signal as its input signal. Shock excitation of the chroma channel is accordingly avoided, so chroma 'twinkle' does not appear in the televised picture.