Co-Channel Television Interference and its Reduction

The undesired effects of co-channel television interference vary both with the intensity and the frequency of the interfering signal. It is found that there are three types of frequency-variation of objectionability, namely: (1) A progressive decrease of objectionability as the frequency of the undesired signal is shifted away from the carrier frequency of the desired signal, which we have called the Beat Size Effect; (2) superimposed on the Beat Size Effect there is a cyclic variation in objectionability which repeats at intervals corresponding to the horizontal scanning frequency; and (3) superimposed on each of these variations there is a ``finegrain´´ cyclic variation which repeats at intervals corresponding to the vertical scanning frequency. We have called the cyclic variation (2) and (3) Beat Pattern Effects. Advantage may be taken of the Beat Size Effect and the Beat Pattern Effects to reduce co-channel television interference by appropriate specification of the operating frequencies and frequency tolerances of the desired and undesired stations. It is shown that it is also possible to reduce the objectionable effects of co-channel television interference by the use of certain post-detection devices in the television receiver. Narrow-band video filters, high-pass video filters, and diode clamp video correction circuits have given promising results.