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Finding an elusive squeal

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Finding an elusive squeal

Postby kdoerbecker » Sat Dec 08, 2018 2:58 am

I'm live mixing a trio (guitar, violin/mando, drum kit 3 vocal mics) and am at my wits end to find the source of a very short duration (maybe 30 ms) squeal (1-2kHZ) in the FOH that happens very intermittently. Don't think its feedback because 1) It doesn't build up and decay - (its like a square wave envelop), and 2) I can raise every channel, foh, and stage monitor by 20db and can't make it happen. It's so infrequent, I can't catch it. I am beginning to think its from some of the electronics the musicians are using breaking into an oscillation. One of the guys uses 2 instruments all wirelesss and they play through a massive foot pedal effects board. BTW, this doesn't happen with any other group I work with. But, they keep looking at me like I have 2 heads when it happens. Everyone says the mix sounds great.
My config is: XR18 mixer, 2 Turbosound 1000 towers foh, 2 Behringer 125 watt floor monitors.
I plan to use the XR18 USB to record each channel, maybe I'll be able to find the source of the squeal after the show.
Any ideas???
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Re: Finding an elusive squeal

Postby ef37a » Sat Dec 08, 2018 7:52 am

Sounds like a burst of RFI to me, have you got a phone TX tower nearby?

My first thought would be to record the sound but since it is so random you will need some form of "logging" recorder. You could use a handheld recorder or a laptop and AI but also a video recorder of some sort. I have several DVD recorders and they give 4 hours per disc on LP.

Naturally all these things would have to be kludged into points in the system to "sniff" signals. Record the feed to the FOH speakers first then move back through the system.

Does the mixer have direct outs or inserts?

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Re: Finding an elusive squeal

Postby Sam Spoons » Sat Dec 08, 2018 10:04 am

The mixer can record 18 separate channels to a PC but the take off will be quite early in the channel signal path (IIRC post eq but pre fx but I think it's configurable) so it won't record anything picked up in the rest of the system. Of course, if the noise is loud enough the vocal mics will pick it up or you could stick an extra mic in front of a FOH cab (obviously with faders down on that channel) to record the FOH.
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Re: Finding an elusive squeal

Postby Wonks » Sat Dec 08, 2018 11:10 am

Any fast moving military jets passing overhead? You may be getting a burst of radar from one which is being picked up by someone's equipment when the plane is very close?

Any channel overload lights coming on when the squealing occurs?
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Re: Finding an elusive squeal

Postby Mike Stranks » Sat Dec 08, 2018 11:30 am

My hunch is that ther's something happening in the wifi/2.4GHz band.

I used to work a venue that was almost a Faraday Cage in its effectiveness in keeping out external RF. Consequently all devices inside the venue such as phones and tablets would push up their signal strength to try and get connection.

All sorts of warbles, squeals and squeaks could be detected in unbalanced or poorly screened kit.

So there's a tip... if you have any unbalanced jack-jack lines keep them as short as possible and use DIs to get the signal balanced as soon as possible on its journey from the instrument.
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Re: Finding an elusive squeal

Postby Wonks » Sat Dec 08, 2018 11:41 am

What is the wireless kit they are using? If it is wi-fi band stuff, then it could well be as Mike says, but if it's other RF kit, you need to check on the frequencies being used. Assuming you are in the UK, I'm sure there is still lots of UHF equipment still being used that is now illegal to use after the Government's frequency sell-offs, and so may be clashing with another service (say digital TV). A lot of the now useless kit is still being sold on eBay without any warnings.
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Re: Finding an elusive squeal

Postby Wonks » Sat Dec 08, 2018 11:46 am

Can you get them to just use standard instrument leads and ditch the wireless for a test period? If the noise goes away, then it's almost certainly down to the wireless kit.
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Re: Finding an elusive squeal

Postby Hugh Robjohns » Sat Dec 08, 2018 12:56 pm

With faults like this you need to be logical and systematic, rather than change everything you can think of at random.

So the first thing you need to do is identify if it's a noise coming into the console from the band on stage (which seems most likely from what you've said), or something in the console-amp-PA side of the rig. And having the ability to record each individual input channel on the XR18 will make that job very straightforward.

So, at your earliest opportunity, record the band sources individually, and when you hear the offending squeal, check the recording back to see which channel(s) is coming in on.

Like everyone else, my hunch would be that it's going to be the lines from the wireless instruments /digital effects pedal...

Of that's the case, and you have the recorded evidence to prove it, you can have a discussion with that musician to try and narrow the source down further.

Again, I'd start by replacing the wireless links with hard-wired connections. If that doesn't cure it, then I'd look at the digital effects pedal, and particularly its powering arrangements.

However, I suspect it will be the wireless links... I don't think you've said what the wireless systems are, but you can get all kinds of nasty noises if the two transmitters come too close to each other, or if there are other interference sources in the room, or if the receivers are placed too close to digital equipment (like the effects pedal) or too close to metalwork which causes brief dropouts.

But start with a source track recording to find out if the noises are from you or the stage!

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Re: Finding an elusive squeal

Postby Mike Stranks » Sat Dec 08, 2018 1:42 pm

:blush: I'd missed the 'wireless' element...

Even my wife who knows diddly-squat about anything to do with sound gets alarmed when she sees me packing my wireless kit prior to setting-out for a gig... "You know how unreliable that can be..." She's right. Even with decent spec kit I always feel apprehensive... give me hard-wired every time for peace of mind.

I refer the hon gentleman to: https://www.soundonsound.com/forum/viewtopic.php?f=26&t=63887
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Re: Finding an elusive squeal

Postby AlecSp » Sat Dec 08, 2018 2:52 pm

Hugh Robjohns wrote:But start with a source track recording to find out if the noises are from you or the stage!
Definitely the easiest start point to allow you to identify who is producing the noise - and it's so easy from an XR18. Also, if you're not using all 18 inputs, you could record the XR18's main outs too. That way *just in case* it was a problem within the mixer's signal chain, you'd have captured that - albeit that it's unlikely.

Assuming you do confirm that it's a single source that's at fault, identifying the root cause is going to be hard. You'll only know for sure if it's the wireless if you run both wired and wireless in parallel and then observe the problem on the wireless but not on the wired connection. Even then, you're only confirming that the wireless connection is the problem, and not what's actually causing the problem (external interference, faulty equipment, etc)
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