Audio Hum while recording scene

J

jmcgilvary

New member
Using a DVX-100, with several lights in the area.
During our filming, while monitoring sound, we are hearing a fairly loud hum. Now sometimes moving the mic cable can mitigate a little bit of the noise, but not completely.
We are using home made rheostats to control the lights, and I have a strong feeling these may be the culprit.
Any advice on a) building quieter "dimmers" or a decent power filter to help quiet this situation?
Or, is there something else we can look at as the culprit?
I have also considered using a good quality UPS with a high quality power filter on it.
Ideas? Help?
Thanks
 
jmcgilvary said:
Using a DVX-100, with several lights in the area.
During our filming, while monitoring sound, we are hearing a fairly loud hum. Now sometimes moving the mic cable can mitigate a little bit of the noise, but not completely.
We are using home made rheostats to control the lights, and I have a strong feeling these may be the culprit.
Any advice on a) building quieter "dimmers" or a decent power filter to help quiet this situation?
Or, is there something else we can look at as the culprit?
I have also considered using a good quality UPS with a high quality power filter on it.
Ideas? Help?
Thanks
Click to expand...

Can't you just test by shutting off the lights with the home made dimmers and see if the fairly loud hum still exists? If you find out the dimmers are the problem, then you can do a search here and find all the discussions about dimmers.
 
What is your audio setup? Are you running a mic straight into the camera? Through a mixer? Are you using an external audio recorder?

The 2 most common sources of hum, come from a ground interference, and cable interference. Check that your microphone wires aren't too close to any other cabling or lights. Then if you're using external equipment, like a mixer, or an external recorder, make sure they're all on the same circuit, or lift the ground (using those 99 cent ground lift adapters you can get at a hardware store.)

-Steve
 
fleaguit said:
external recorder, make sure they're all on the same circuit, or lift the ground (using those 99 cent ground lift adapters you can get at a hardware store.)
Click to expand...
I was having the same problem. Like everyone mentioned, AC cables and other electrical interference were the biggest culprits.

Excuse my ignorance, but what do you mean by "lift the ground."
:dankk2:
 
justin_newlands said:
Lifting the ground, literally means removing the ground connection.
The reason this can stop the hum is that if you have a ground loop, caused by more than one connection to ground. The only problem is that your setup is dangerous, as you have no safety ground
The correct way to stop ground loops is to use an audio transformer, so that your audio signal is now "Floating" ie, no connection to ground, but the rest of the set up is grounded. By using a transformer, you ensure the ground connection is not contiguous.
I can explain more if you email me.
Click to expand...
Sure. I tried emailing from DVXforums but it gave me the incorrect page...

Can you explain the solution. What would you do to have a safe ground but stop ground loops. what do you need from the 99 cent store? Can you send some links or pics so I can get an idea. Is it something you would lay on the ground...
 
Could you copy me on that email?

I am experiencing similar problems. I have done many tests. I have the camera running on battery power, and I have turned off everything in the house. I am using an external countryman B3 hardwired lavalier mic that is attached to the camera with a star quad cable. When I have absolutely no lights turned on, there is no humming. As soon as I turn on an Arri 150 Watt light, I begin hearing some humming. It does not appear to matter if the Arri is plugged into a grounded socket or I use an adapter that converts the three prong light into a two prong. The humming is almost inaudible, but when I edit it, the humming from different camera angles is slightly different and it becomes very noticable.

Currently, the only mic I have is the countryman lav, and the scene I am shooting calls for two people to be seated at a table. I am trying to fasten the cable to the ceiling with gaffers tape and drop the mic just out of the frame between the two actors to act as a pseudo-boom. One odd thing I did notice, however, is that if I hold the lavalier mic in my hand, the humming appears to go completely away.

Any suggestions would be greatly appreciated. Thanks guys.
 
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Whoa... this thread went in a lot of seperate directions fast.

A couple people mentioned that their issue seems to be tied to lights/ dimmers/ other high current devices. If that's the case, forget about ground lift. This is caused by good ol' EMFs. (The reason the hum went away when you held the lav in your hand was that your hand was acting as an RF shield.) Instead, in what ever order seems easiest to you:

• Make sure all your audio cables (and devices they're connected to) are balanced with good shielding. (Matt's are excellent for this.)
• Make sure that you don't have ANY audio cables running close (1.5') to other electrical or lighting cables. This includes power in walls, and also includes your own electrical or powered (loudspeaker) cables, if you have any.
• If you must cross a powered line at any point, do it at a 90° angle.
• Make sure your audio equipment is on it's own circuit if not self-powered.
• Replace cheap dimmers with higher quality ones. (This is one of the reasons pro-lighting dimmers are more expensive; they're designed not to spit out gobs of interference.)

If you're hearing the infamous 60Hz hum/buzz in your audio equipment regardless of location or what non-audio equipment is turned on, then it's possible you do have a ground loop. Ground is just what it sounds like: the dirt underneath you. It's what that third round (in the US) prong of most electrical outlets is connected to. It's purpose (rough analogy) is like a lightning rod: if there's a static charge in your equipment, it discharges into the ground instead of frying your stuff. A ground loop means something has two connections to the ground, thus forming a "loop": electricity can (and does) flow from the ground through your equipment back into the ground.

There are several ways to cut or "lift" (same thing) the loop. The cheap and dirty way is to get some of those 3- to 2-prong electrical adapters discussed above. I would NOT recommend this at all. If you do, and there is a discharge in your equipment, instead of flowing directly out as designed, it will flow through your audio cables and circuitry, to a secondary piece of equipment and then out, possibly damaging the other piece. Unless of course you lifted your entire setup, in which case the charge will have no where to go and zap!

What I would recommend is figuring out where the ground is actually looping. If your equipment is metal, it could simply be conducting through the chassis; try putting thin plastic or rubber spacers between each piece. It could also be through balanced cables; if so just cut (literally) the ground wire at one end of the cable and mark it so you know that cable is lifted.
 
shawnmaddock said:
Ground is just what it sounds like: the dirt underneath you. It's what that third round (in the US) prong of most electrical outlets is connected to. It's purpose (rough analogy) is like a lightning rod: if there's a static charge in your equipment, it discharges into the ground instead of frying your stuff. A ground loop means something has two connections to the ground, thus forming a "loop": electricity can (and does) flow from the ground through your equipment back into the ground.
Click to expand...

So that means you want to maintain that third prong? What do you do if the structure itself has electrical outlets without the third prong. Does that mean it has no protection? And that we shouldn't use the adapter? How can we get rid of the hum while maintaining protection?

shawnmaddock said:
There are several ways to cut or "lift" (same thing) the loop. The cheap and dirty way is to get some of those 3- to 2-prong electrical adapters discussed above. I would NOT recommend this at all. If you do, and there is a discharge in your equipment, instead of flowing directly out as designed, it will flow through your audio cables and circuitry, to a secondary piece of equipment and then out, possibly damaging the other piece. Unless of course you lifted your entire setup, in which case the charge will have no where to go and zap!

What I would recommend is figuring out where the ground is actually looping. If your equipment is metal, it could simply be conducting through the chassis; try putting thin plastic or rubber spacers between each piece. It could also be through balanced cables; if so just cut (literally) the ground wire at one end of the cable and mark it so you know that cable is lifted.
Click to expand...
So then we shouldn't use the 3 to 2 prong adapters because it's like taking away the extra protection in case of static charge?

I didn't exactly understand the solution you mentioned with rubber pieces and cutting the wire. Put what kind of rubber? Between which pieces? Cut a ground wire? COuld you explain it to someone who knows nothing about electricity.

Thanks! :dankk2:
 
Shawn, thanks for the response. Unfortunately, I am at work today. Work does seem to be getting in the way of my budding film career. In any case, I will do some more experiments tonight. One question, what did you mean by Matt's? Thanks for your time.
 
Very nice and complete response Shawn. I usually just post a link whenever this issue comes up.

For safe removal of ground loop, this works well: http://www.zzounds.com/item--EBTHUMX

But jmc's original posting sounds like rf noise and not a ground loop problem. As Shawn said, dimmers (even the expensive ones can be noisy) will "f" with your audio almost everytime. Variac dimmers will be pretty quiet but they are VERY expensive and you really don't need them unless you are doing an in-scene lighting effect. Anything else that you can do with a dimmer can be done by moving the light back.

Cheers
 
Another unexpected place to find power running that would cause a hum is in the floor. I was working on a project as an actor (and a consulting sound mixer - they didn't really have a mixer, just a PA holding the boom - poorly, I might add...) and at one point they were shooting a scene and had a bad hum in the audio. Lifting the cable off the floor got rid of the hum. Turns out this was a second floor conference room with outlets in the floor under the table. Also, they were using cheap Guitar Center cables.

So, even if you can't see the power cables, that doesn't mean that power isn't the interference.

Phil
 
ptalsky said:
Turns out this was a second floor conference room with outlets in the floor under the table. Also, they were using cheap Guitar Center cables.

So, even if you can't see the power cables, that doesn't mean that power isn't the interference.

Phil
Click to expand...

So even outlets cause static ... Also, where do you recommend we get our cables from?
 
One of the more experienced users, MattinSTL, makes very high quality XLR cables for more than reasonable prices. They have quite a few advantages over cheap cables, including good RF rejection. He's posted his rates in at least a couple threads you can search for. PM him with what you need.
 
Um, I didn't mean to "upstage" you Justin. I assumed you had emailed omar as you offerred to do, but I thought that because there were at least three folks who had seemingly different issues, it would also be good to clarify the different causes on the forum for anyone else who may be reading. In fact I purposely didn't go into the correlation between ground and hum to avoid repeating anything you may have or would say. I didn't mean to step on any toes or steal your limelight. And I definitely don't think I "know it all," which is why I and most people on this board depend on you and others like you who have the experience to educate those of us who don't. I was merely trying to pass on the knowledge others had passed on to me. Sorry for any hurt feelings.
 
Just now arriving on this thread.

One question is not being asked: what microphones are being used?

If the mics are lavalier, they are usually unbalanced and maybe getting hum through. In fact any microphone that is not completely balanced will get hum, particularly if the mic cables cross through any AC cable in the room.

Even if cables are balanced, they might be crossing AC cables which might induce hum. Being XLR does not warrant you are automatically balanced.

About dimmers, only specific types will be silent, in hum RFI. Stay away from home types, or be prepared to deal with them in post.
 
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