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Bizarre Audio Pickup on HVX200

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D. Scott Dobbie
Bizarre Audio Pickup on HVX200
on Feb 3, 2008 at 10:28:51 pm

On a shoot yesterday, when checking the audio prior to rolling for the first time, I heard on the headphones what sounded like a very faint Hispanic radio or TV station. What was more unusual was that I wasn't using a remote lav - I was using my standard, wired Sennheiser 416.

I've never even heard of anything similar happening. Never even experienced it with a remote lav, though I could imagine it. But to encounter it with a wired mic made no sense. I also changed headphones and we rearranged all the cables - coiling them, laying them out, whatever.

This was a regular studio shoot on a 40 x 40 sound stage - so it's not like there were any unusual conditions.

I switched audio inputs and I also went on battery, thinking it might be a circuit issue (even though the camera and monitor were initially on an isolated circuit), but no luck.

In discussions with the editor, who was present, we decided to go ahead since the ghost audio was faint enough that it was overridden by keeping the levels up for the talent. Fortunately the performance was loud and dynamic. I'd hate to think what the result would be if talent had to whisper.

FWIW, this was the first time we used this particular stage. We're scheduled for a series of shoots there, unless we can't resolve the problem. I don't think it has anything to do with the stage, but the producers don't want to risk it if we can't pin it down.

Any ideas?

Thanks,

-Scott



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Tom Brooks
Re: Bizarre Audio Pickup on HVX200
on Feb 4, 2008 at 3:35:42 am

I've heard the same problem since getting my HVX200. Using a wired lavaliere mic, I was getting a faint pickup on a radio station. By eliminating the XLR mic cable and plugging the mic directly into the HVX input it was fixed. I tried various cables until I found one that was completely free of the radio pickup. It seems to depend on the quality of the shielding on the mic cable.


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D. Scott Dobbie
Re: Bizarre Audio Pickup on HVX200
on Feb 4, 2008 at 5:38:48 am

Thanks, Tom. I'll try a different mic cable, though I use this one all the time without (until this instance) any difficulty.

-S.



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Nate Stephens
Re: Bizarre Audio Pickup on HVX200
on Feb 4, 2008 at 3:39:58 am

Scott, It is audio, expect anything. I was on the 13 floor of an office building for a 2 day shoot last week. We kept getting radio thru our Sennheiser shotgun mic, but only in the morning,, not in the afternoon.. And the placement of the mic in the room also made a difference in the reception of our friendly Country Western radio station.

Now how do you explain audio mysteries to a clueless client? They will never want to hear about it.


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Chris Clephane
Re: Bizarre Audio Pickup on HVX200
on Feb 4, 2008 at 5:50:16 am

Eeesh!

This is the first I have hear of this with a video camera.

The quality of the cables would definitely make a difference.

Just FYI- as a musician for over 20 years...I have found that this IS a common problem with unbalanced cabling. (The type usually used to connect guitars to amplifiers and such.) Balanced cabling is supposed to be less succeptible.

Anyways...enough with the ramble...as I get to my point....
Two years ago I played a show in a band at a local college campus. The campus had a relatively high wattage AM station on campus...and the second we plugged in the guitar amplifiers onstage in the student union building...you could CLEARLY hear the broadcast through the amps. Our final solution was to WRAP the amp heads in aluminum foil (shielding) and plug-in THROUGH the aluminim foil. Creative contact between the foil/cable and grounded parts of the amp cabinet created an effective soak that shielded all the signal. IT was fun watching my guitar players perform without being able to fiddle with their amps between every song.

So anyways...better cables usually means better shielding. Worst case...be prepared to get creative with aluminum foil. (Effectively creating a small faraday cage around the offending unit/device.)
-C


I edit video. I post sometimes.
I fix things. I eat marshmallows.
I play drums. I drink scotch.
I like TV.

Done typing now.


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D. Scott Dobbie
Re: Bizarre Audio Pickup on HVX200
on Feb 4, 2008 at 6:03:40 am

Great idea, Chris. So, if I head to a Sam Ash or Guitar Center and ask for a balanced cable (I'll bring in what I currently use to confirm that it's NOT balanced), that should do the trick in this location? And if that doesn't work, what would I wrap with the foil - the connector at the XLR input of the camera?



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Rennie Klymyk
Re: Bizarre Audio Pickup on HVX200
on Feb 4, 2008 at 9:48:25 am

If your cable has XLR's it is balanced. Maybe it's been stepped on too often. Maybe it has a poorer design or shielding quality or a faulty end. You can spend a lot of dough on oxygen free cables and higher end shielding. Was the mic mounted on the camera or a suspension system?

I've seen this happen in a particular environment, weeks apart it still occurred.

It doesn't hurt to have a variety of XLR balanced cables in different lengths so I would get a new one with high end shielding and borrow or rent a second mic just to try (the process of elimination).

You could also post this on the audio pro forum and see what the audio gurus advise.

"everything is broken" ......1st. coined by Esther Philips I believe.


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Chris Clephane
Re: Bizarre Audio Pickup on HVX200
on Feb 4, 2008 at 7:45:37 pm

Thanks Rennie for clarifying....

That was really my point.

Unbalanced cables (being 2 conductor) are more suceptable to interference...and for production, you should use quality shielded/balanced (XLR) cables (that are in good shape) whenever possible.

Back to my "story"....
FYI-guitar cables are 2 conductor UNBALANCED...and are therefore PRONE to RF interference by nature of the design. The cable manufacturers try to shield them really well...but do to the nature of the regular use (abuse) they endure...the shielding breaks down quickly.

FINAL POINT - So sometimes on location....we have to resort to extreme mesaures (foil) to counter RF problems.

-C


I edit video. I post sometimes.
I fix things. I eat marshmallows.
I play drums. I drink scotch.
I like TV.

Done typing now.


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D. Scott Dobbie
Re: Bizarre Audio Pickup on HVX200
on Feb 4, 2008 at 8:01:51 pm

Based on what you said, it's balanced, Rennie. The cable is 25' and plugs right into the XLR connector on the camera with the other end directly into the mic. NEVER been stepped on. ALWAYS perfectly coiled afterwards. And this has never happened in the year and a half that I've been using it. Perhaps I was too close to some antenna and I just need a cable with a thicker sheath in the odd chance it happens again.

I'm going to go back to the studio in a couple days and see if I get the same problem.

To answer your other question, though, the mic is suspended in a shock mount on a K-Tek 9' boom pole.



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Chris Clephane
Re: Bizarre Audio Pickup on HVX200
on Feb 5, 2008 at 7:59:08 am

Does the K-Tek boom-pole have INTERNAL routing?

The internally routed poles utilize a tightly coiled internal coil (bad for RF interference and stray magnetic fields) that tends break down and wear out a LOT quicker than a standard cable. (We have worn out 3 internally routed booms in the past 4 years.)

So....if it is internally routed..has been used a LOT...AND the boom is carbon fiber... (an aluminum pole would offer an appreciable amount of shielding)...that could be your problem.

Again...this is really pushing the boundries of potential causes...but it is possible.

Good luck.
-C



I edit video. I post sometimes.
I fix things. I eat marshmallows.
I play drums. I drink scotch.
I like TV.

Done typing now.


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D. Scott Dobbie
Re: Bizarre Audio Pickup on HVX200
on Feb 5, 2008 at 6:52:12 pm

Another good thought. The cable is routed internally, however it's an aluminum, not carbon fiber, pole. When I do my next test, assuming I can still hear the station, I'll bypass the pole and plug the cable directly into the mic to see if that makes a difference.



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