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The amplitude of the input video may be too high. This can cause the sound carrier to cut in and out at the vertical rate (making a 60ish Hz buzz) and the horizontal rate (usually not noticeable). White areas in the video makes it louder.
An easy to reduce the video amplitude is to add an additional 75 ohm termination to the video line. Some consumer equipment has the amplitude set too high, by about 20%. Makes a consumer think that the equipment makes a brighter and higher contrast image on the screen.
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