Electronics > Beginners
Interference problem on a wireless mic system
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ogden:

--- Quote from: Towger on May 05, 2018, 08:40:51 pm ---It was from the computer's bus.  The lights are hard wired with multiple cat5 up to the stage, where they change over to traditional DMX/110ohm cable.

--- End quote ---

Whatever. Your case was static interference, OP describes spatial effects which is more like acoustic feedback or RF multipath.
chris_leyson:

--- Quote ---First thing, as has been mentioned, is that the ringing and whistling you are getting is more than likely audio feedback
--- End quote ---
From the description it's probably aucoustic feedback and has nothing to with it being a wireless mic. There will be some extra delay between the mic and driving amp because of the delay through the wireless harware so that will change the audio feedback characteristics. Turn down the mic gain to fix the problem or maybe use frequency shifting hardware between the mic and drive amplifier. Frequency shifting was and maybe still is a technique for reducing or eliminating howl around.

RF multipath will just give you peaks and drop outs but in practice multipath is not an issue. The mic is most probably FM so you wouldn't really hear any variation unless the signal strength drops to below the FM demodulator capture range and then it will drop out. The RF part of the audio feedback loop is insignificant in terms of phase delay, basic physics and common sense will tell you that.

Try it with a wired mic and see how that works, you will probably get the same result depending on how you set the overall gain in the audio path. Stabillized feedback amplifiers was published by Harold Black from Bell Labs over 80 years ago and still these days most so called electronics engineers haven't got a clue about feedback.

WaveyDipole:

--- Quote from: chris_leyson on May 05, 2018, 09:03:31 pm ---From the description it's aucoustic feedback and has nothing to with it being a wireless mic. There will be some extra delay between the mic and driving amp because of the delay through the wireless harware so that will change the audio feddback. Turn down the mic gain to fix the problem or maybe use frequency shifting hardware between the mic and drive amplifier. Frequency shifting was and maybe still is a technique for reducing or eliminating howl around.

--- End quote ---

Thanks. Looking at adjusting the mic gain is one possible option to consider. The manufacturers user manual has some guidance on this, in summary reducing the gain to the point that the peak light never comes on. As far as we are aware, we have never seen the peak light come on although we will ask the sound technician to monitor this. Different positions have been tried for the receivers around the sound console, although the suggestion now is to move the receivers into the ceiling space. This would be a fairly costly process as it will require mains supply to be provided and due to the increased length (est 20-25m), balanced line wiring of the receiver to amplifier connection, which, in turn will, I think, need some kind of unbalanced to balanced converter at the receiver end.

I do know that the amplifier has some form of accoustic feedback prevention built in, although I'm not sure what type.

My question still remains though. If this is an accoustic feedback issue (and I accept that this is likely to be the case) then why does the radio signal also drop significantly? Surely the radio signal should not be affected by the accoustic feedback loop? I'm asking because this has been asked of me and I do not yet have a convincing answer to give them.

I perhaps should also point out - in case it is relevant - that the rostrum microphone is wired as are a number of other microphones in use, including two "roving" microphones on long leads that were used prior to the wireless ones, and these never give a problem.
Brumby:
I'm only guessing - but there might be an AGC control somewhere in the mix and when you get a high volume signal (such as when you go into feedback), the level is automatically dropped.  My only reservation with this is that I've never encountered such a control on the RF transmit power.  On the audio signal, yes, but not the transmitter.
Electro Detective:
Have you tried the mics separately, with either one totally disconnected, i.e. no hookup to anything including power  ? same deal?  :-//

Perhaps try power the receivers via a battery source or a sine wave UPS disconnected from mains if possible


This one is a whopper, and I've had some that made no sense to know   |O 

In some cases the clients were just over it so wired mics came back into play, and a few wireless models tested till a good one worked as a rover

In most cases there was too much electrical and electronics stuff going on and off at different times, plus phones and wireless stuff going on to lock down the problem.
Not that there would be any assurance of a solution anyway...   :horse:

 
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