Noise in the audio - A tutorial for eliminating noise
I recently fielded a question about a noisy audio system in an installation. There were two ends of the system connected by balanced audio which shared space in the conduit with a video cable. Here were my thoughts.
Break the ground loop by using a battery powered mixer or just a microphone at the head end of the audio. The end you first described with the two XLRs, etc..
If you still get buzz and/or hum, then the problem lies elsewhere. If the buzz/hum goes away, then there's a difference in ground potential between where you have the mixer plugged in and where the gear on the other end of the conduit is plugged in.
Try operating battery powered devices on both ends of the conduit. If it's clean then you can begin to presume it's a power grounding problem. If it's not clean and you are off the grid, then something is radiating in or into the conduit system. Maybe the conduit is not grounded properly or runs past a big AC mains transformer or some other noise source.
You can suss that by running cables instead of using the conduit. PITA, but you may find it that way. I was recording audio for video at a site a few years ago and SOMETHING was under the floor in certain parts of the room. Whenever my balanced cables were there, I got buzz. In fact, even though my cables were balanced, someone accidentally moved a power cable so that it was parallel to my cables for about 10 feet. That's all it took to get the AC into my audio.
Or, there may be Silicon Control Rectifiers (SCR) dimmers for lighting control. If the lights are dimmable, listen for that noise while you turn on the lights and fade them up. Dimmer noise is usually worst when you're dimming a lot. It tends to go away when the lights are up full. SCRs may be in other rooms or even on other floors of a structure and they will still leak (radiate) into your audio.
Or, there could be something wrong with your house wiring. That screen may not be grounded after all. Regular electricians are not always aware of the "best practices" required for AV. Fun stuff. I love ghost hunting.
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We carry a cable ground lift adapter which is a barrel with XLRs on each end with the shield pin disconnected on one end. Many people don't realize that only one end of a shielded cable needs to be connected for it to work as a shield. However, it's important that the connected end of the shield is attached to a solid ground.
In our studio we use a star-point grounding scheme. All the chassis of all our gear are directly grounded to one star point. And all the gear use a/c ground lifts to avoid ground loops. Shields are connected at the gear-end of all cables and left open at the other end. In our case that's the console, which is also a/c ground lifted and chassis grounded to the star point. If you really want to go crazy also use a power conditioner like an Equi=Tech. Then you can crank up the knobs and hear virtually nothing.
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I am not a fan of lifting Pin 1, but will agree that sometimes it's the easier temporary solution. If you think about it, you're not treating the problem itself, though. You are only treating a symptom of it and the actual problem still remains. It's not always easy or possible to solve the actual problem, though, so sometimes the temporary solution becomes the permanent fix.
Whenever we go on film shoots, I carry 2 ProCo ISO boxes in our kit. I always take a good listen back at the camera to what the signal sounds like when I move the mic, cabling, etc. around. I use the Pin 1 lift on the ISO boxes when needed.
P.S. - I think it is a good idea to talk about "ground lifting", at least in regards to audio, as Pin 1 Lifting; whenever I think of a ground lift, I think of 2-prong AC adapters used incorrectly and 'death caps' (live sound guy, here).