Reduction of NTSC co-channel interference by referencing carrier frequencies to the LORAN-C signal

Co-channel interference may be noticed when the desired signal at the receiver is 60 dB stronger than the undesired signal. The spacing distances between transmitters which would be required to avoid this interference are prohibitive. Because FCC rules require much smaller minimum spacings, frequency offset is specified for each station to reduce co-channel interference. The offsets used in the U.S. are /spl plusmn/10 kHz while the carrier frequency tolerance is /spl plusmn/1 KHz. Differences between carriers can be 8-12 kHz. Interference minima occur every 59.94 Hz within this range with interference maxima in between these minima. By referencing the visual carrier frequency of each transmitter to a common frequency, the difference frequency can be maintained at one of those frequencies at which the interference is minimum. This paper explains how the LORAN-C signal can be used as the frequency reference for television transmitters and how the ideal difference frequency can be maintained to minimize the visibility of co-channel interference. Equipment required is commercially available, and no rule changes are required to employ LORAN-C referenced carrier operation.<
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