Incidental Carrier Phase Modulation (ICPM)
Incidental Carrier Phase Modulation (ICPM) – This is a distortion of the picture carrier phase caused by changes in either the chrominance or luminance video signal levels. This distortion is described in degrees using the following definition: ICPM = arctan (quadrature amplitude/video amplitude) The picture effects of ICPM will depend on the type of demodulation being used to recover the baseband signal from the transmitted signal. ICPM shows up in synchronously demodulated signals as differential phase and many other types of distortions, but the baseband signal is generally not as seriously affected when envelope detection is used. The effects of ICPM are therefore rarely seen in the picture in home receivers, which typically use envelope detection. However ICPM may manifest itself as an audio buzz at the home receiver. In the intercarrier sound system, the picture carrier is mixed with the FM sound carrier to form the 4.5 MHz sound IF. Audio rate phase modulation in the picture carrier can therefore be transferred into the audio system and heard as a buzzing noise. An unmodulated 5 to 10 stair step signal or unmodulated ramp can be used to test for this distortion.
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