Hum/Buzz in audio on shoot yesterday

B

Brian_Broz

Active member
Hi,
I normally record audio and video right into the camera. Yesterday, I shot a dance recital where I had a PD170 connected to a DSR-25 VTR via firewire. I was getting a hum in the audio. A low level hum...discernable with my 7506 headphones turned way up.
I had 1 channel coming from the house board (XLR line) and 1 channel on the camera mic (for applause/FX). The camera and VTR were both plugged into the same AC power supply. When I unplugged the VTR (and went straight into the camera only) there was no hum...so it wasn't in the house feed.
The hum was there whether I fed firewire out of the camera to the DSR-25 or if I went S-Video out of the camera with RCA audio. It didn't matter.
I thought I tried everything. I called the client who I was shooting for (and his gear) and he had never heard of this....of course! :-)
Does anyone have any ideas how to avoid this in the future? I've been shooting for years but never had this problem. I've heard of humbuckers and ground lift adaptors for XLR but would that have really helped.
I want the sound to be as perfect as possible, as I used to record sound on documentaries!
Sorry for the long setup/explanation. Thanks for any suggestions,

Brian Broz
 
You got yourself a ground loop. It is actually quite common. Your equipment is getting two different grounding levels. You need to get them all on the same page or disconnect the second grounding path.

Sometimes just plugging the equipment into a different circuit will work, othertimes it is more difficult to isolate.

Good luck
 
Thanks gentlemen...
Both the camera and VTR were plugged in to AC (the same outlet).
When I switched the camera to battery power the hum was still there, but te deck had to be plugged into AC. When I unplugged any connection between the camera and VTR no hum. And the audio was perfect on the camera only.
I plugged both into a different outlet/circuit at halftime but no difference.
Anything I missed, or is there a product to purchase that will help this (in the future)?
Thanks,

Brian
 
There are isolating transformers, poss the "humbuckers" you mentioned earlier that will break the ground loop. It's also poss. to use a ground lifter on one of the equipt. that plugs in. There are two likely reasons for the hum. THe most likely was already mentioned, a ground loop. Ground loops are difficult to anticipate but the solution is usually to find the second ground connection and break the loop. The loop is not always the AC, you can get a ground loop from your audio connections also. THe other poss cause is a dirty ground line (electronic dirt, ie RF etc) and a box that cross-connects the signal ground and the power ground. The old SPX90 for instance was notorious for this problem. If it's in a rack with metal rails it needs to be isolated from the rack or you will get all kinds of hum. So it might be a design problem with a box. Again isolators are a good way to go.

SK
 
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