Static in the Audio when using the VidiU device

Conversations around originating a webcast for conference, including cameras and mixers.
justincy
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Static in the Audio when using the VidiU device

Postby justincy » Fri Mar 20, 2015 11:26 am

We ran a full test with the VidiU device on Wednesday. One problem we ran into is that there was static in the audio whenever we plugged the VidiU device into the audio mixer to capture the audio. Without the VidiU plugged in, the audio mixer was producing great sound. But once we plugged the VidiU in it added static to the audio. The static was heard on all outputs from the mixer, not just on the VidiU feed.

What would cause this?

I'm no audio expert but the most probable cause is from incorrectly mixing mic level and line level input. Would that do it?

russellhltn
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Re: Static in the Audio when using the VidiU device

Postby russellhltn » Fri Mar 20, 2015 11:33 am

justincy wrote:But once we plugged the VidiU in it added static to the audio. The static was heard on all outputs from the mixer, not just on the VidiU feed.


Most likely a ground-loop problem. It may not be the VidiU itself, but something plugged into it (video feed?).

Perhaps you can fix it by making sure everything is plugged into the same outlet and not power from different outlets. But you might have to use a ground loop isolator. Like this.
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justincy
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Re: Static in the Audio when using the VidiU device

Postby justincy » Fri Mar 20, 2015 11:57 am

I think you're right. The static sounded very similar to the 60Hz mains hum described here: http://en.wikipedia.org/wiki/Mains_hum

justincy
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Re: Static in the Audio when using the VidiU device

Postby justincy » Fri Mar 20, 2015 11:59 am

BTW, the audio mixer, VidiU, and camera were all plugged into the same outlet.

russellhltn
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Re: Static in the Audio when using the VidiU device

Postby russellhltn » Fri Mar 20, 2015 1:15 pm

Odd. Usually ground loops come from connecting devices to different building grounds. Perhaps the VidiU is just the return path and it's the ground from the sound system or other lines creating the loop. What happens if you unplug the network from the VidiU?
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justincy
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Re: Static in the Audio when using the VidiU device

Postby justincy » Fri Mar 20, 2015 2:47 pm

russellhltn wrote:Odd. Usually ground loops come from connecting devices to different building grounds. Perhaps the VidiU is just the return path and it's the ground from the sound system or other lines creating the loop. What happens if you unplug the network from the VidiU?


We'll have to try that.

I'm pretty sure that the VidiU doesn't have a ground on it's power cord. It comes with an international adapter and the US end only has two prongs.

Also, the VidiU appeared to be running off the battery while we were broadcasting, which was weird. It lasted just about two hours. We have to find a way to fix that or we'll be in trouble.

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Re: Static in the Audio when using the VidiU device

Postby russellhltn » Fri Mar 20, 2015 3:11 pm

justincy wrote:I'm pretty sure that the VidiU doesn't have a ground on it's power cord. It comes with an international adapter and the US end only has two prongs.


Looking at the pictures, I agree. However, you might want to see if one prong is wider than the other. It might be a polarized plug. In which case, a mis-wired outlet could make things interesting.

The wide prong should be neutral, and some two-prong devices will put a high value resistor to to that to keep the output from straying off into la-la land. But if the outlet is miswired, then you have a resistor to "hot" and it will introduce some AC into your ground system. You can test it with a volt meter or a device designed to check 3-prong plugs. Some power strips will have a light to indicate a problem as well.

But bottom line, the VidiU is just part of the loop. You need to figure out what else is part of that loop to decide where to break it.

Uhhhhhh, but another question. What kind of output and cable are you using? It see the VidiU is using a 3.5mm phone plug (which frankly I detest), but no specs on if it's balanced or unbalanced. You might be grounding one side of a balanced or floating output of the board. That can cause a number of problems.
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justincy
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Re: Static in the Audio when using the VidiU device

Postby justincy » Sun Mar 22, 2015 6:12 am

justincy wrote:Uhhhhhh, but another question. What kind of output and cable are you using? It see the VidiU is using a 3.5mm phone plug (which frankly I detest), but no specs on if it's balanced or unbalanced. You might be grounding one side of a balanced or floating output of the board. That can cause a number of problems.


Yeah, you could be right about that being a problem too. I never realized audio was so complicated. I'm used to the simple world of consumer electronics with universal 3.5mm plugs.

We tried using the mixer last night to hook a laptop into the chapel audio system (our crab box doesn't work). That didn't go too well either. We didn't have too much time so we settled putting a mic up to an external speaker the presenter had brought with them.

justincy
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Re: Static in the Audio when using the VidiU device

Postby justincy » Sun Mar 22, 2015 2:54 pm

I'm at the chapel now and just figured a few things out.

1) The output we're using is under the clerks desk; the old recording outputs. We were using a mic level output so we were boosting the signal a ton. That likely amplified any interference or signal problems.

2) I was able to reproduce the static problem only when the VidiU device was connected via an ethernet cable. The power cable had a minor effect on the static but whether the ethernet cable was plugged in was the primary factor.

3) Under the clerk's desk there is also a port labeled "Aux Level." It takes an RCA style plug and appears to be line level output. The VidiU does not induce static when using this port, likely because we don't have to boost the signal so much.

russellhltn
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Re: Static in the Audio when using the VidiU device

Postby russellhltn » Sun Mar 22, 2015 3:59 pm

Given a choice, I'll take line level any day. Sounds like you've found your fix.

One caution. The line level out may be a "pro level" line out and can cause distortion in consumer level line inputs especially at high volume. Be sure to test for that.
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