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The sound issue that disappeared when you switched modulators was probably video over modulation....audio is FM and video is AM...AM modulation at 0% is full carrier strength and at 100% modulation is no carrier ( proper AM normally stops just shy of 100% modulation) when AM modulation exceeds 100% the carrier turns back on but is phase inverted 180degrees (or reverse polarity)....that effectively creates phase modulation of the carrier that depends on video and video has a 60Hz rate. Phase modulation and frequency modulation are closely related...any FM modulator creates phase modulation and any phase modulator creates frequency modulation you can switch a transmitter between sending one or the other by applying an integral or derrivative mathematical function to the signal and the Same applies to a reciever just run the output through a circuit to do the math to make is sound correct... that PM noise will make it through the FM detector as easy as a proper FM signal.
Many cheap modulators over modulate the video and use the same carrier for both audio and video resulting in the PM noise being on the sound carrier and having an EASY path through the set.
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