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Wifi interfering with Radio mics Strange goings on

#1 User is offline   DonkiDonki 

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Posted 28 September 2008 - 10:43 AM

I am currently on a conference roadshow and have encountered an unusual problem. (Sorry if there is already a thread for this, I couldn't find one)
We have a fairly typical conference setup for about 100-200 people with basic stage, set, projection and PA.
We have Sure dual channel UHF radio mics with DA and active ariels using 8 channels of group 69. (used frequencies vary depending upon local conflicts with each venue)

There is also an audience responce system which uses laptops with a wifi network (not sure which wifi type), there are usually about 20 laptops running.

The strange problem we are having is that at some venues we seem to have a conflict between the radio mics and wifi, whilst at other venues there is none.
When there is conflict, the signal stength on the Sure recievers is all over the place (normally rock solid in this size venue) and the signal drop out is very high, this occurs over all frequencies in the group.
With all radio mic transmitters off, there is no sign of a signal being picked up from the wifi on the Sure recievers.

Looking at the frequencies used by the mics and the wifi there should not be a conflict, but there most definately is at times.
Proving it is simple, when we have the problem; it goes away if all the laptops are turned off. (Typicaly, the wifi doesn't seem to suffer, or perhaps it does but not enough for anyone to notice.)

I am guessing that the size, shape and/or construction of the room must have an influence on whatever is taking place and possibly the physical positioning of the laptops too. Perhaps certain conditions cause intermodulation between the two systems? (we have tried moving the Sure ariels drastically when the interference is there, but with no success)

The audience response is an integral part of the event and it is not an option to turn it off for the shows. Likewise, the position of the laptops is dictated by the table layout in the room, often with not enough space to change the layout.

Does anyone have an explanation for this or encountered a similar issue?
In particular, any useful suggestions to fix the problem would be greatly appreciated.
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#2 User is offline   Andrew C 

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Posted 28 September 2008 - 11:58 AM

View PostDonkiDonki, on 28 Sep 2008, 11:43 AM, said:

Looking at the frequencies used by the mics and the wifi there should not be a conflict, but there most definitely is at times.
Proving it is simple, when we have the problem; it goes away if all the laptops are turned off.
Have you tried the laptops on, but with their WiFi off? I ask as I (long a ago) had a similar situation where the colour monitor oscillator in a BBC Micro was nailing a radio mic. Only one location on the stage was effected, but that was downstage centre. With one leg of the crystal lifted, all was well.

If it isn't the WiFi, you may be able to knobble various bits of laptop hardware to see if you can cure the interference.
Andrew

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#3 User is offline   Bobbsy 

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Posted 28 September 2008 - 12:22 PM

Well, as you know WiFi at 2.4Ghz is well separated from the UHF Channel 69 frequencies. Some quick mental math suggests to me that intermod products also shouldn't be an issue and, even if they were, they should affect individual channels, not your whole system.

How close does any of the computer gear get to your radio mic receivers or the antenna/antenna lead/RF DA? It occurs that computer gear can be electrically very noise and spew out all sorts of crap--hold a transistor radio up near your computer if you want to see what I mean. WiFi doesn't even have to be turned on for this to be any issue--it's the computer itself rather than the RF part of the system. This being the case, getting the offending part of the kit away from your radio mic system is the cure.

Troubleshooting this could be fun if your set up time is as short as any corporate gig I've done but, if at all possible, next time you get the interference I'd try turning off the wifi itself (while leaving the computers on) to see if the problem goes away or remains. This should help identify if the issues are with the 802.11 system or the computers themselves.

Bob
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#4 User is offline   DonkiDonki 

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Posted 28 September 2008 - 12:43 PM

Thanks for those comments, I will try to get the AR guys to disable the wifi for a while next time the problem occurs.
The AR control is usually setup at the front of the room near the stage while the sound desk and recievers are at the back of the room.

It may be a while before the problem rears its head again, it's something like 1 in 6 where it happens. (or at least where its bad enough to notice)
Also, as Bobbsy picked up on; we don't have much troubleshooting time on site. The roadshow schedule has us do a morning show, derig, travel and re-rig for the next morning.
It doesn't help that the AR is from a seperate company who aren't too bothered about the mics being affected. (And are sceptical of it being anything to do with their kit)

Out of interest, does anyone know if it makes any diference, in terms of frequencies; which channel the wifi router is on? If some venues already have wifi on the AR's normal channel, would it automatically switch to a different one? (I'm still looking for an explanation why the same setup has issues at some venues but not most)
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#5 User is offline   Reid 

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Posted 01 October 2008 - 12:26 AM

Are you certain the interference is from wifi and not from Blackberries? We've had to stop meetings and have all of the Blackberry users turn their units off because email alerts were constantly causing "raspberries" on our conference sound systems. -Reid

#6 User is offline   DonkiDonki 

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Posted 01 October 2008 - 01:59 PM

View PostReid, on 1 Oct 2008, 12:26 AM, said:

Are you certain the interference is from wifi and not from Blackberries? We've had to stop meetings and have all of the Blackberry users turn their units off because email alerts were constantly causing "raspberries" on our conference sound systems. -Reid



Ah, the old "didit, didit, didit fart" noise eh? We always ask for all mobile devices to be switched off, rather than silent. However it's obvious that most delegates and sometimes presenters ignore this; either when you get that lovely sound, or they answer a call/text mid show! (I'm told the noise only breaks through from GPRS, but that was over a pint or two so not sure how acurate that is)

To be honest at this stage I can't say for definate what is the root of the problem other than it is something to do with a large number of wifi networked laptops interacting with a bank of Sure UHF radio mics with DA and active ariels. Unfortunately, thanks to the tight schedule and the fact this problem only happens on a seemingly random basis, it is proving difficult to troubleshoot.

I find it intersting that not many other people seem to have encountered this problem, I'm sure it must meen something but I'm buggered if I can think what.
The fact that some people seem to have had somewhat quirky sources of radio mic problems makes me think it may be something a little more complicated than simply the wifi interfering.
Can anyone recommend a reasonably cheap radio scanner, preferably laptop based? It would be interesting to monitor what is going on, especially if anything obviously different shows up when we encounter the problems.

I guess that then, if there is no obvious diference in radio activity then we might be looking at an even more obscure source to the problem. (Anyone got much experience at mains harmonics?.....)
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#7 User is offline   Bobbsy 

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Posted 01 October 2008 - 02:15 PM

THIS TOPIC should help with your request for a radio scanner/spectrum analyser. Alternatively, there are specialist rental houses out there that would hire you a stand alone spectrum analyser. Certainly if you can check for spurious RF of one sort or another this would go a long way towards your diagnosis--you could see if there are specific frequencies interfering or if all the computer equipment is just raising your noise floor unacceptably (my theory).

Bob
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#8 User is offline   Chappie 

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Posted 01 October 2008 - 05:32 PM

Could the problem be the room full of laptops itself? That many cheap Chinese PSUs can cause havoc with a buildings power supply and any one of them could potentially be faulty, feeding noise back into the mains. Radio mics can get very upset with flaky power.

I had a faulty PSU generating so much harmonic crap on the mains it took the Broadband of an entire street back to dial-up speeds for six months until discovered. It still functioned normally and there was no apparent fault, until you got near it with an RF detector, which went mental.

Does the problem persist if the laptops are running on batteries?

This post has been edited by Chappie: 01 October 2008 - 05:35 PM

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