What Is Television Cross Modulation?

Symptoms

• Television cross modulation is often caused by a strong local television signal being received at the same time as a weaker, more distant signal. Television cross modulation manifests itself in various ways. It may cause vertical or horizontal framing bars from the interfering channel to be superimposed on the desired channel̵2;a phenomenon known as the "windshield wiper effect"̵2;or cause a negative image, or ghost, of the interfering channel to appear in the background of the picture. Other symptoms of television cross modulation include moving wavy or herringbone patterns of lines and the picture's freezing or breaking up.

Amplifiers

• Television amplifiers, especially broadband distribution amplifiers, which supply multiple outlets, are a common cause of television cross modulation. Amplifiers amplify both desirable and undesirable signals, so if one or more signals exceed the rated output capacity of the amplifier, cross modulation will occur. Furthermore, if the amplifier gain̵2;the ratio of the amplitude of the output signal to the input signal̵2;is excessive, it will overload the equipment. Note that digital terrestrial television signals are more susceptible to cross modulation than analog signals, simply because they are broadcast at power levels some 20 decibels lower.

Test

• Distribution amplifiers for very high frequency (VHF) signals, between 30 and 300 megahertz, and ultrahigh frequency signals, between 300 and 3000 MHz, have separate inputs for VHF and UHF, but combine the signals internally. One simple test for cross modulation is therefore to disconnect the VHF aerial from the amplifier; if television interference ceases, cross modulation from the VHF aerial is the cause.

Elimination

• Cross modulation can typically be eliminated with a highly directional antenna̵2;an antenna that focuses radio frequency in one or two directions̵2;and filters and taps to attenuate (decrease the amplitude of) the interfering signal. If you̵7;re using a television amplifier, make sure that any unused outlets are properly terminated.