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I've been having problems with my clip mics for a little while. I use the Audio Technica ATW- T1801 mics and seem to have a lot of problems with noise. The noise floor is higher than I would like anyway but intermittently I get a kind of "pumping" noise when any speech is delivered. This doesn't happen all the time which makes it even more of a pain.
Does anyone know if this is interference? maybe something wrong with the receiver? or maybe just old/bad mics? I'm not always here when the recording is happening so there could be an amount of user error in there as well!
squelch can sound differently especially on sennheiser systems.
there might have been interference due to very close freq usage of a different system.
if it was an audio ltd. system, then it was the low battery....
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The pumping that you hear is probably the "Squelch" function engaging in the wireless system. This is basically a noise gate for RF transmission that cuts out the signal every time the signal gets too weak. The system could also have a built-in audio noise gate that is causing the noise to pump. Like Larry said, a better RF signal will help.
If you have the ability, you might try to connect the mic directly to the camera and bypass the wireless system to test if the mic is adding significantly to the noise.