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Old November 17th, 2010, 11:06 AM   #1
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Buzzing noise while monitoring headphone return feed from camera

Hello all,

As the message title suggests, I'm having an annoying problem with a buzzing noise in the headphone return feed from the camera during video shoots. We use a Canon XL-2. When I first noticed the noise we were in our studio. I ran the mics through a SD-302 mixer to the camera and returned the headphone feed back into the return input on the mixer. I noticed a lot of noise: partly hiss and partly buzzing. I plugged the headphones directly into the camera and there was no noise. I was able to use a long headphone extension cable and work that way, but I was unable to get rid of the noise when monitoring through the SD-302.

Time went by, and I've been working around this problem. Then it came up again, with a different mixer, and got me wondering and more annoyed.

This time we were doing a live shoot. Similar setup: Canon XL2, except that I ran a variety of stage microphones through an Allen and Heath MixWizard, which fed the house sound system and the camera (using the matrix outputs). I thought I would be able to return the camera's headphone monitor feed back through the 2-track inputs on the mixer and monitor that way; however, same problem: hiss and buzzing. And again, if I bypassed the mixer and monitored the camera audio using an extension cable, no problems, no noise at all.

The noise sounds like it might be some type of grounding issue to me. But I really don't know for sure, and I'm a little surprised that I'm having the same problem with both the MixWizard and the SD-302, which leaves me wondering if there's something I'm missing here.

Any ideas what's causing the noise?

Thanks - Andrew
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Old November 17th, 2010, 12:37 PM   #2
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How long is the return cable? Is it running adjacent to power cables? (the most usual source of EMI buzzing)

-Try a known good 100% shielded audio cable with metal connector barrels speaced at least a foot away from from A/C cables and any dimmer packs. Consumer type lamp dimmers are also known to cause EMI.
Try a 1:1 iso transformer, like the popular Sescom IL-19, It's good to have a couple of those on hand when interfacing multiple systems.
Though It's likely not the problem, the Canon XLs have a notoriously low-output HP amp.
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Old November 17th, 2010, 01:08 PM   #3
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If the mixer is hardwired to the camera and the camera is connected to a monitor that is powered by AC and your A&H Mixer is also powered on AC then you possibly have a ground loop. Make sure that any gear both video and audio in the chain is all plugged into the same preferably clean AC circuit and not sharing a circuit with any production lights etc. If there is an AC powered video monitor in the chain then try disconnecting the video cable from the camera and see if the buzz goes away.

Andy
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Old November 17th, 2010, 05:07 PM   #4
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Thanks Rick and Andy for the suggestions. The cable is long, 20' or so, and isn't really shielded at all. I tried moving it away from any power or video cables and this had no effect on the noise. A ground-loop problem seems like a likely culprit, but the camera and mixer are plugged into the same power strip, which is plugged into the building's power, which is supposed to be "clean," but that's IT clean, ie no major voltage spikes, not studio clean.

We do use a video monitor, and I'll try disconnecting the video cable from the camera to see if that changes things. I might also try an iso transformer. The cable I'm using isn't particularly nice--I don't think it's shielded really.

Anyway, thanks for the useful suggestions. What really vexes me about this is that I don't get any of this noise in the other audio lines; it's not in the recording at all (thank god); just in the headphone line. And only when I go through the return feed of the mixer.

Have a great day - Andrew
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Old November 17th, 2010, 05:56 PM   #5
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Clue: "The cable is long, 20' or so, and isn't really shielded at all" 'Unshielded' cables are prone to pick-up EMI and RF type noise. That's what the shielding is there for.

FYI, 'Ground loops' are typically identifiable by a 50/60 Hz. hum. (+ harmonics) 1:1 Iso. transformers are usually the easiest and safest cure, or.. supply all A/C power from a single circuit.
(Not the lights though)
Cheers
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Old November 18th, 2010, 09:59 AM   #6
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"Unshielded' cables are prone to pick-up EMI and RF type noise. That's what the shielding is there for." - Yeah, but what puzzles me is that I'm only getting the noise when I go through the mixer. Using the same cable, routed in exactly the same place, if I listen directly, it's entirely quiet (well, quiet enough). When I try and monitor through the mixer, there's a pretty serious buzz. Undoubtedly it has something to do with the amp stages the signal is going through in the mixer, but I would have though EMI and RF noise such as that would have been apparent in both cases.
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Old November 19th, 2010, 06:10 PM   #7
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Quote:
Originally Posted by Andrew Hughes View Post
"Unshielded' cables are prone to pick-up EMI and RF type noise. That's what the shielding is there for." - Yeah, but what puzzles me is that I'm only getting the noise when I go through the mixer. Using the same cable, routed in exactly the same place, if I listen directly, it's entirely quiet (well, quiet enough). When I try and monitor through the mixer, there's a pretty serious buzz. Undoubtedly it has something to do with the amp stages the signal is going through in the mixer, but I would have though EMI and RF noise such as that would have been apparent in both cases.
Andrew, I just got my hands on my own SD 302 and I've observed nearly the same as you. In my case, I monitor through my Sony MDR-7506 headphones, plugging them directly to the SD 302, i.e., no extension. When the headphone selector is "ST", I hear almost no noise. Monitoring the return from my camera, however, I hear plenty of hiss. Freaked me out the first time I heard it. I mean, I dropped serious coin on SD, and I get hiss?!!

Just now I recorded a minute of me walking around my room, talking a little, etc. In my NLE, I don't hear ANY hiss (just my computer fans!). FWIW, I recorded audio through my new AT BP-4025.

I can't explain the hiss, but my tests suggest that my camera return will be for confidence only. The SD's headphone out must not be the quietest. <shrug>

Steve
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Old November 20th, 2010, 01:30 PM   #8
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I'm with Andy and Rick,

We had a similar issue, with our Canon.

When we where off mains, and on battery, we did not get the "noise".

Plugging into the same mains, eliminating the possible "ground Loop", worked for us.
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Old November 20th, 2010, 01:48 PM   #9
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Same experience as Sam, with a canon HV20 we set up to monitor interviews, with audio from an AC powered mixer.

I think this is caused by the difference in the ungrounded power brick that powers the camera and the grounded audio mixer, in our case...the signal on the power brick will always be at some voltage difference to its negative side as the ac powered device is to it's ground. When we ran on battery we had no problems.
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Old November 20th, 2010, 03:08 PM   #10
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Quote:
Originally Posted by Steven Reid View Post
Andrew, I just got my hands on my own SD 302 and I've observed nearly the same as you. In my case, I monitor through my Sony MDR-7506 headphones, plugging them directly to the SD 302, i.e., no extension. When the headphone selector is "ST", I hear almost no noise. Monitoring the return from my camera, however, I hear plenty of hiss. Freaked me out the first time I heard it. I mean, I dropped serious coin on SD, and I get hiss?!!

Just now I recorded a minute of me walking around my room, talking a little, etc. In my NLE, I don't hear ANY hiss (just my computer fans!). FWIW, I recorded audio through my new AT BP-4025.

I can't explain the hiss, but my tests suggest that my camera return will be for confidence only. The SD's headphone out must not be the quietest. <shrug>

Steve
The 302 headphone amp is plenty quiet and loud. You're hearing the hiss from the camera return amplified so it sounds noisier than monitoring the mixer itself (ie: ST). You can turn up the return level up or down beyond what's available with the main pot with the small input five/six pots on the side of the mixer. This way you can get the loudness from the return to be close to monitoring right off the mixer without adjusting the main headphone pot up and down all the time.
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Old November 20th, 2010, 03:16 PM   #11
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Originally Posted by Bernie Beaudry View Post
The 302 headphone amp is plenty quiet and loud. You're hearing the hiss from the camera return amplified so it sounds noisier than monitoring the mixer itself (ie: ST). You can turn up the return level up or down beyond what's available with the main pot with the small input five/six pots on the side of the mixer. This way you can get the loudness from the return to be close to monitoring right off the mixer without adjusting the main headphone pot up and down all the time.
Thanks! That makes sense. I'm still new to the unit, so I appreciate the suggestion.

Steve
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Old November 22nd, 2010, 11:23 AM   #12
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Steven - I'm guessing that this is some grounding problem/peculiarity with the Canon, not the SD, because I have exactly the same problem with our Allen and Heath mixer.

I've had my SD about a year and I really like using it. Just well made, all around. Lots of useful features, very little fluff (to my mind). Anyway, I'm glad somebody else has run into this--at least I know I'm not doing something stupid or going crazy.

Now I wonder if the isolation transformer would get rid of the noise? Hmm...


Bernie - Interesting point. I'll have to check that. But it seems like an awful lot of noise to get introduced in one gain stage.
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Old November 22nd, 2010, 01:04 PM   #13
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Steven - I'm guessing that this is some grounding problem/peculiarity with the Canon, not the SD, because I have exactly the same problem with our Allen and Heath mixer.

I've had my SD about a year and I really like using it. Just well made, all around. Lots of useful features, very little fluff (to my mind). Anyway, I'm glad somebody else has run into this--at least I know I'm not doing something stupid or going crazy.
I just recorded a concert this weekend -- my first real use of the SD 302 -- and managed to minimize the slight hiss in the camera return, which is most certainly coming from the Canon's headphone jack. Following advice above, I upward-adjusted the 302's 4/5 pots for the return headphone level; monkeying with the Canon's own headphone out volume helped, too. In the edit bay, I was pleased to hear nothing but pristine, hiss-free sound.

Steve
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Old November 22nd, 2010, 01:14 PM   #14
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Nice, I'll give that a try. It's definitely a little annoying to not be able to monitor through the mixer. Even if I know that the hiss will not be in the final recording, it makes me nervous not being able to hear the noise floor.
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