Radio Intereference

[liamk40]

Hi Guys,

 

A venue I work in has quite a problem. We have recently switched to digital radios which initially was great but we have discovered a major problem. Using shotgun microphones (usually as chorus overheads) which are Beyerdynamic MCE 86II.2, when broadcasting on radio we get a low rumble in the mics which of course goes into the PA. It only occurs when using these mics so I'm assuming its something to do with the way its earthed or how the casing is made. Either way its pretty much an unnacceptable problem as we need to use radio for FOH stewards and in emergency cases onstage.

 

Anyone got any idea how to resolve this/what the best move would be?

 

Many thanks

[dbuckley]

First question: Do they do it when the Beyers are plugged in to the mixer with a two foot cable?

[paulears]

Welcome to the nasty world of digital comms. Two main types are in use - belonging to two rather VHS v Betamax camps. The type based on dPMR uses a continuous carrier with the data on it, and is in everyday terms, fairly similar to analogue radio in it's interference characteristics. Close in, I can induce a feint 'thump' on susceptible audio kit. The other type, DMR, uses a shared slot system so transmissions from mobile equipment is pulsed and these make a real mess of my troublesome audio kit when in close - a nasty motorboat/deisel engine puttering as the damn things turn on and off. The design also needs very sharp rise time on the RF, so there's no ramping up of power from off to full, they bang up to full very quickly, then cut, then up again, very fast. There's no extra harmonic output, that's well controlled, but any audio kit that uses commanders seems to suffer a bit more. The cabled Beyers I'm a bit surprised by. Beyer normal have pretty good shielding, so I'm guessing the trouble is the RF being 'collected' by the cable shield and the interference gets in at the other end where the equipment is? Does swapping them out temporarily cure the problem?

 

I've now got a repeater working at my home, where I work from, and using it's aerials to test some digital kit for a client, DMR messes up my TV reception badly, dPMR doesn't. Most manufacturers have both systems in their product range at the moment, so it's going to get worse, I suspect. I'm amazed it's so popular, because the audio quality of digital comms radios is pretty horrid, but people like the idea of privacy (or almost privacy).

[liamk40]

Thanks for the great reply!

 

In answer to both questions, yes we have tried the mics with a short cable into the mixer and have pretty much the same amount of interference, I had thought it may have been cable length at first, but alas no. It is specifically with these mics, We have had many other shows in with other mics, boundaries, large diaphram condenser, etc, but yeah it is specifically these shotguns which are suseptable.

 

Obviously a temporary fix is to say not use those mics, but I don't think its accpetable that any pro audio environment should now have to limit the use of microphones because of comms, especially when analogue doesn't interfere. I read up about these mics using a shilding cable that earths the PCB to the casing, which is low pass filtered for analogue RF, however with it being digital the frequency obviously is low enough to affect it. Just wondering if anyone had more knowledge on this?

 

Thanks

[paulears]

Not sure if the filtering is any different for digital? The fundamental frequency is the same, and the change in mode doesn't alter the bandwidth (well, it sort of does with dPMR as it actually has two 6.25KHz channels side by side in a standard 12.5KHz one, but the more annoying of the two uses the usual 12.5KHz channel, just using half the time. You can slot a digital channel in to replace an analogue one with no side-effects to other radio equipment. Somewhere in your system, it's open to RF getting in - might be at the mic end, but equally possible at the other end?

[dbuckley]

specifically these shotguns which are suseptable.

Before setting fire to the mics; try an alternative mixer. There is an outside chance that the mixer is the one not doing rejection properly, because of some peculiarity of the mics.

 

I don't think its accpetable that any pro audio environment should now have to limit the use of microphones because of comms, especially when analogue doesn't interfere

Agreed. If the mics are CE marked, which one would assume they are, then the rejection should be certified to be adequate. There is some process to go through when CE marked kit is inadequate in EMC terms, but I cant remember what it is! Been out of the UK too long...