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    weird audio issue
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    I was shooting a corporate meeting recently with 2 Sony PMW-300's side by side ( those are successors and upgraded versions of the trusty old EX-3's) We took a wired feed from the house board and split into the A1 of both cameras and that worked fine throughout the shoot. Both cameras put internal camera mic on A2 as backup. The speakers were using wireless mics. And I recall both cameras were on AC. About halfway through a 1 hour event both of the cameras suddenly got loud steady noise on the internal mic channel. It continued throughout the event and after it was over at least for a few minutes. The noise was on the recorded track as well as the headphones during the event.

    Neither me nor the producer (who are both 30+ year veterans) had ever encountered anything like that on the internal camera mics. Have any of you? Any idea what could cause that? The internal Mic is what I often think of as the protection if anything goes wrong with the feed.
    Didn't make any difference if it was on auto or manual level control.

    Unfortunately we had to leave the room immediately and couldn't do any further testing.

    Lenny Levy


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    What kind of noise, Lenny?
    Ben Giles GBFTE BAFTA

    www.matobo.co.uk

    @MatoboLtd


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    #3
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    Just white noise as far as I could tell . Pretty nondescript but loud.


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    Over the years I've come across this situation occasionally, not often, and find it normally happens when two or more cameras are connected to AC mains that don't have a common ground point. When two or more devices are connected to a common ground through different paths, ground path noise, or a ground loop can occur. A system grounded at two different points, with a potential difference between the two grounds can cause unwanted noise voltage in the circuit paths. Currents flow through these multiple paths and develop voltages which can cause noise or 50Hz/60Hz buzz or hum in audio or video equipment. The ground loop can be eliminated in one of two ways. In a two cam situation remove one of the ground paths, thus converting the system to a single point ground. Or isolate one of the ground paths with an isolation transformer, common mode choke, optical coupler. Basically what you need to achieve is one common ground between all the bits of equipment that are connected together in any way by any audio or video cables. That means all cameras, monitors, recorders and audio desks . A quick way to check if it is a ground loop issue is to put the cameras on batteries and generally the problem will disappear. If it is still there disconnect the shared audio feed from the desk and in 99% of cases the sound magically seems to disappear. Indicating the earth loop was coming via the audio desk. A very bad case I had was at the opening of a new casino in Sydney where the problem was so bad and we didn't have time to chase down the problem. I ended up putting in wireless hops between the casino's sound desk and our audio mixer that was feeding the cameras and the problem disappeared. The casino techs were dismayed their circuits had caused problems. It was later discovered that one of their "technical clean earth circuits" which were specifically designed for all the AV equipment and the one that was powering the main audio desk in the place had somehow been grounded along with one of the normal house 'dirty' circuits and it turned out to be a hot water unit for making tea/coffee on that circuit that had a bad earth and it was that unit causing the ground loop feeding back through everything in the place. Nasty. When ever I can in multi-cam situations I try to make sure everything is on the one ground plane and that it is all on one phase. Having equipment connected together in a big building with some of it one one phase of a three phase system and other equipment on another phase can also cause this potential difference between the two grounds and that again can cause unwanted noise.

    Since my last experience of these sorts of issues I now have a couple of these things, different brand though, in the kit. They have saved me a few times. I now run them as a matter of course... like a security blanket

    https://www.storedj.com.au/art-pro-a...hoCtU0QAvD_BwE

    Chris Young


















    Chris Young
    Last edited by cyvideo; 08-16-2019 at 08:27 AM.


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    Chris, that's an interesting possibility , but the 2 cameras also shared a clean audio feed that was a wired feed from the sound mixer about 100 feet away then run through a little DA that split the single to both cameras. I think the DA was powered off the same circuit though I'm not sure, I'll ask the client.

    Wouldn't it be likely that the wired feed would have that noise as well if it was a ground loop?

    In this case the 2 cameras were side by side powered from what I'm guess were adjacent AC plugs if not the same plug.

    I carry an isolation transformer as well and it has occasionally saved my butt usually when working with an event mixers feed.

    BTW - Would you ever use that isolation transformer for a mic that mounted on the camera ?


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    Ground loop crossed my mind. But with modern building controls? And both cams on the same power spur? I doubt it. Or was it an outdoor event with potentially dodgy electrics/generator etc? Would also expect ground loop noise to be at same frequency as AC (ie 60hz in USA). RF interference the only thing I could imagine would cause simultaneous white noise on both int mic channels.

    Weird!

    Quote Originally Posted by LennyLevy View Post
    Chris, that's an interesting possibility , but the 2 cameras also shared a clean audio feed that was a wired feed from the sound mixer about 100 feet away then run through a little DA that split the single to both cameras. I think the DA was powered off the same circuit though I'm not sure, I'll ask the client.

    Wouldn't it be likely that the wired feed would have that noise as well if it was a ground loop?

    In this case the 2 cameras were side by side powered from what I'm guess were adjacent AC plugs if not the same plug.

    I carry an isolation transformer as well and it has occasionally saved my butt usually when working with an event mixers feed.

    BTW - Would you ever use that isolation transformer for a mic that mounted on the camera ?
    Ben Giles GBFTE BAFTA

    www.matobo.co.uk

    @MatoboLtd


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    Indoors , no generators just a fairly small meeting in a corporate building . Maybe 200 people or so .wouldn't a ground loop be hum or some sort ? This was sudden loud white noise obliterating the actual room sound . Nothing audible in the room . We also figured some kind of weird RF but it was a first for both of us .


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    I once had an issue similar to yours. It turned out to be interference from the gps unit in the camera, which is now permanently turned off.


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    You try them again, side by side and do the test. My guess is simply a fault that will either return or never show up again. Hiss I have never found from a grounding issue, only hum. Unexpected hiss can be caused by accidental phantom power applied the wrong way, as in back to a camera through a mic input. I've had that a couple of times, but mostly hiss as in hiss that suddenly starts points to two things. Somebody did something perhaps quite remotely that upset things, or a real honest temporary fault. They happen.

    Can you post a clip - my hiss might be different from your hiss in our heads.


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    Two other questions --
    Did the noise start lower and gradually ramp up to full intensity - or did it suddenly appear at full (like turning on a switch?)
    Did it actually start at the same frame on both cameras?


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