Video cutting out when loud noises happen

[The JC]

Hiya team,

This problem is well and truly stumping me. My venue has 2x Panasonic AW-HE120 cameras which are used as the show relay for backstage monitors etc. 

A fault has developed whereby whenever a loud noise happens in the space (e.g. audience applause, pyrotechnics, stamping on the ground, door slams etc) one of the video feeds momentarily cuts out. Power to the camera remains consistent, it's just the image that goes for about 3 seconds. The routing is camera SDI out to facilities panel, into a patch panel and then distributed by a BlackMagic 12x12 SmartHub. Our audio is fed by a separate microphone, going directly into a Symetrix Radius 12x8, out into amplifiers and then into backstage speakers. At no point are the two signals combined; they share the same comms rack but are otherwise separate. The microphone is also over 10m away from the cameras. We have tried altering microphone levels, changing the framerate and resolution of the video feed and power supply to the camera in question but to no avail.   

As a wild guess, I'm thinking that somewhere/somehow the mic signal is causing some sort of issue within the comms rack itself??? But I don't know how I can trace/prove that. 

Any suggestions for where I can go next? Questions welcome so we can work through this together...

[DrV]

My first test would be to switch off the whole of the audio link, then go and slam some doors to determine whether it is an electrical problem or a mechanical one. Depending on how accessible the dodgy camera is I'd also go and jiggle the camera and its cables as a start.

[sunray]

We once had an issue with VGA. A pair of plasma screens immediately adjacent to loudspeakers, one had a VDA behind it and cabling from rack to VDA and then from there to the other side was PSF1/3 on BNC's.

Loud bass sounds modulated one colour on both screens. Supplier of plasma screens reckoned it was a magnetic issue affecting screens.

 

However it turned out one of the BNC's pins had not been crimped and vibrated with the noise.

Edited November 27, 2021 by sunray

[timsabre]

Do you have an induction loop in the space with ambient mic. If so try turning it off 

[sunray]

1 hour ago, timsabre said:

Do you have an induction loop in the space with ambient mic. If so try turning it off 

I've been out tonight with other AV guys and having discussed this they reminded me of a venue. Iwas going to suggest the same thing.

[The JC]

Thank you for all the feedback (No pun intended...). I'll give all of your suggestions a go and see if we get some good results. 

[The JC]

On 11/27/2021 at 9:38 PM, timsabre said:

Do you have an induction loop in the space with ambient mic. If so try turning it off 

Thanks for this. Any advice on what I can do if it turns out to be the loop causing the issue? Obviously still want to keep performances accessible to people with hearing aids. If it is just a straight forward either/or, with no workarounds, then I think the loop would have to take priority. 

[Stuart91]

I had a near-identical problem at an event in a cathedral. There was a long SDI run adjacent to where the loop cable was laid. 

I think all you can do is re-route the SDI cable so it is further away from the loop.

Might also be worth checking the loop level itself, what sometimes happens is the gain gets tweaked up a little every time somebody complains. (And most of the time the fault is with the user, not the loop) Before long, the amp is pegged and the loop level is outrageously hot. It may be you can bring the level down quite a bit before hearing aid users are affected, in fact it may improve matters if you're not clipping like crazy. 

[david.elsbury]

1 hour ago, The JC said:

Thanks for this. Any advice on what I can do if it turns out to be the loop causing the issue? Obviously still want to keep performances accessible to people with hearing aids. If it is just a straight forward either/or, with no workarounds, then I think the loop would have to take priority. 

You might be able to use SDI-fibre optic converters for the run, to eliminate induced noise (if indeed that’s the issue)

[timsabre]

In my painful experiences with induction loops and video it's normally some sort of earthing fault on the loop cable which causes video breakup. As it's unlikely you'll be able to fix that, you can get sdi isolating transformers or as David suggests, though more expensive, convert to fibre and back. 

[richardash1981]

Tim, to unpack your comment, you think there is a connection between the induction loop cable and earth somewhere? I can certainly see that causing problems (although I would expect it to also destroy the amplifier in not too short order, as most of the designs I have seen do the current sensing on the earth side of the loop).

I have also seen VGA with artifacts on it with a satisfactory loop install (or at least a not-earthed one). The magnetic field up close to the loop cable (e.g. in the same trunking) is large given the loop current drive needed, and so no unbalanced signal is going to like it. Even 100mm of spacing brings the magnetic field down a lot and may make a difference, so I'd be inspecting your problematic cable run carefully.

That reminds me. I thought I read someone pointing out it was easy to damage SDI kit when earth testing, because the BNCs on SDI kit are transformer isolated. A BNC feed which is only grounded at one end will pick up a lot less interference than one which is connected to mains earth at both ends. So maybe check the video wiring for ground loops as well?

[timsabre]

Usually it's a section of loop wiring has got damp. It doesn't seem to affect the loop amp at all but it certainly affects audio and video equipment in the room. 

You're right about problems if you run the cables close to the loop cable, I've had that too on vga.

[Bazz339]

On 11/27/2021 at 9:38 PM, timsabre said:

Do you have an induction loop in the space with ambient mic. If so try turning it off 

My first thought as well.