Wireless interference

[macsound]

This past weekend at the church I mix at, we had some nasty RF interference on two Sennheiser wireless.

2 part Question.
Isn't the pilot tone on the receiver supposed to prevent interference from a strong signal on the same or close frequency? If so, is the interference so great that it overcomes what the pilot tone is supposed to do and I just get a blast of noise on a receiver that isn't on anyway?

On one mic, I get a high pitch whine, around 12-14k that I thought was feedback or a hearing aid, but when listening in cans, it gets louder as the speaker talks louder and isn't feedback.

Any thoughts?

 

[FMEng]

How many mic and IEM systems were in use at the time?

 

[macsound]

Just the one.
I did a sweep that morning and actually changed the freq to something around 480mhz. I didn't take a picture of that.

 

[FMEng]

I don't know for sure, but I suspect tone squelch can be fooled by white noise from something like a digitally modulated carrier. If there's some kind of carrier or intermod product on the channel. Tone squelch won't save you.

The modulation of the interference by audio is a result of the companding used by analog, wireless mics for noise reduction. The transmitter compresses the audio and the receiver applies complimentary expansion to restore the dynamic range. The pumping suggests that the interference is entering the system at the receiver, not at the transmitter, because you are hearing the expansion. So, it is not a cell phone in a pocket, getting into the transmitter.

You have three, pretty hot TV transmitters on your spectrum sweep. Perhaps you experienced an intermod product of some sort. This is why it is good to let software do the math to pick the channels. Just because a channel appears empty does not mean it is a good pick.

 

[macsound]

I'm going to do a quick youtube refresher on assigning channels automatically using wireless workbench or whatever the senn version of that is.
I did it in a hurry last week and got a popup about no available channels.