Other: weird audio issue

L

LennyLevy

Veteran
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
 
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-...au0nk8oa2W8L4sqwA2zQ4R2eDUmxAhLhoCtU0QAvD_BwE

Chris Young


















Chris Young
 
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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 ?
 
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!

LennyLevy said:
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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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 .
 
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.
 
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.
 
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?
 
I don't have the footage and the producer is in New Jersey . I'm in San Francisco and the event was in San Jose, so not possible to replicate and I can't check the footage that carefully. The producer does work with that company frequently and so will investigate on his return, but it happened about 30 min into an event so my guess is it will be hard to replicate. I know he has asked if anything odd was going on at that moment. It didn't affect any of the wireless handheld mics that presenters were using.

As I recall it was not either hiss or hum - just white noise sudden and loud completely obscuring the actual audio. I assumed at first it was my camera only and only found a few minutes later that it had hit both cameras when I turned to tell the other Op. Oddly it seemed I could still see the level meter moving as though it was seeing the normal audio , so I thought perhaps it was only in headphone circuit , but indeed it was on the recording. I'll ask if I can get a sample to post here.
 
Did you check at the time to see whether audio channels 3 and/or 4 experienced the problem as well?
 
That's an interesting question . I'll try to pose a few of these questions to the producer. But are you asking because you have any thoughts about what the cause might have been or just fishing for more info? ( Nothing wrong with that though.)
 
Didn't realize that the sound was such a piercing loud white noise. If that's the case I would tend to agree paulears. One spurious unexplainable burst of RF interference of some kind or something else happening on a circuit close by somewhere that just happened to have had an impact on your audio. The only other thing that comes to mind and we think we worked out what caused it was on a stadium job during setup where long cables runs had lazily been dragged over carpet down long hall ways. They had had built up a load of static charge and suddenly something added to the system caused them to discharge causing a loud white noise spike but once that had happened everything was fine until another cable was plugged in, same again. One bright lighting crew member suggested it might be static and so the next cables runs were rolled out carefully and no further issues. Who knows if that was the cause. It's all a bit black magic to me, no pun intended!

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

Never done it Lenny. There is always a first time I guess. Don't think I would but then again the thought has never crossed my tiny mind. Sometimes audio is still a bit of a black art to me.

Chris Young
 
LennyLevy said:
That's an interesting question . I'll try to pose a few of these questions to the producer. But are you asking because you have any thoughts about what the cause might have been or just fishing for more info? ( Nothing wrong with that though.)
Click to expand...

Fishing for the big picture... I found it interesting that the audio meters seemed to be working correctly through this. To me that would indicate that the issue was introduced downstream of the audio signal being read in the meters -- which might also have meant that the introduction took place after the audio was split out to channels 3 & 4?

Also thinking that laverdir's question about the cellphone jammers might be a cause...

In the past I've run into issues where long xlr runs have acted as antennae to bring electrical rf interference into audio tracks, but never happened to me with a camera mic.

The fact that this is happening with two cameras, both affected only on camera mic channel, makes me favor some kind of electrical interference that suddenly came into play. But why it wouldn't show up in the meters is also strange???
 
Its weird all right. Kind of scares me about what I always consider a completely reliable backup audio . What if its some new kind of RF that will become more common.

I'm not entirely sure about the audio meters though.
 
LennyLevy said:
What if its some new kind of RF that will become more common.
Click to expand...

That's why on interviews I usually run one cable boom mic and one wireless. Touch wood I have never had drama with both at the same time. On really old frail people or in a case where there is no possibility to run a wireless, due to the type of clothing or some other reason I will run two mics on the boom. One with a wireless butt plug and the other on cable.

Chris Young
 
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