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Old February 21st, 2008, 05:35 PM   #1
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Distortion from a sound board!

I was doing video for a friend in a recording studio the other day and asked them if I could take one line out of the board to my wireless (uwp-c1). They hooked it up but said they often get distortion. I checked the levels and they appeared to be peaking at about -20 db on my camera (pd170). When they came out, bingo: distortion. I've taken sound out of a board at a club with no problems. What could have caused this?

I'll be offline for awhile and will be checking this on my cell phone, so it may be awhile before I post back. Thanks in advance.
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Old February 21st, 2008, 06:33 PM   #2
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Simply put, the board is too hot for your inputs. There are a number of things that could goof it up (is it Mic or Line level?) but the board op's just need to turn down their sends.
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Old February 21st, 2008, 06:44 PM   #3
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Sounds indeed like a line level to mic lever (or vice versa) issue. Unbalanced signals need to be attenuated
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Old February 21st, 2008, 08:49 PM   #4
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next time flip the switch on the 170 to LINE and it should take care of the problem.

Don
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Old February 22nd, 2008, 10:18 AM   #5
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Quote:
Originally Posted by Victor Kellar View Post
Unbalanced signals need to be attenuated
WHAT? Why would balanced vs. unbalanced have anything to do with the signal's level? A signal needs to be attenuated if it's too hot - either balanced or unbalanced signals can be too hot. If you have an unbalanced signal and you need to hook it up to balanced equipment, it may be a good idea to use a transformer to balance it, but that is for other reasons than to attenuate the signal. I have never heard of a generic recommendation to attenuate a signal just because it's unbalanced.

The OP wrote that he checked the levels in his camera, and they were peaking at -20dB (I assume that's -20 dBFS, Kell?). If the clipping occured in his camera (due to lack of necessary attenuation, mic/line level confusion), wouldn't he have observed higher levels? [I should add that I do not know the PD170, I can only relate to my experience with the Z1U, where an accidental switch to mic level would immediately show as peaking/clipping audio levels.]

My money is on this theory: the signal from the board was clipped in the process of transmitting it wirelessly to your camera. Once it's clipped, there's no way to correct it. The camera levels looked OK, but only because the peaks of the original waveform had already been cut off. If this was indeed the case, the solution would be to lower the board's output level (as Patrick suggested), to attenuate the signal before it enters your wireless transmitter, or to lower the sensitivity of the wireless transmitter (or a combination of the above). You can't fix it just in the camera - whereas with a wired connection (mic cable), that would have been possible (audio cables don't clip easily :-) ).

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Old February 22nd, 2008, 11:04 AM   #6
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"to attenuate the signal before it enters your wireless transmitter, "

That's what I was trying to say, just throwing out an option such as that
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Old February 24th, 2008, 12:29 PM   #7
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Thanks you guys,
If it matters, he said the board is a Sony DMXR100, 600 ohms, +4DB.
I'll see if I can bring this to them and try reducing the signal and see if that helps.

Just curious. If their signal doesn't clip anywhere else in what they are doing, and doesn't show up clipped anywhere, what would cause it to clip coming to my camera? Also, how can I tell if it's clipping before it gets to the camera, since the cameral levels can't show that? It's possible he just didn't notice that, but not likely it seems since it's a recording studio and sound is what they do.

Also, I need to learn about balanced/unbalanced signals, transformers etc since I don't know anything about that. Can you suggest a good starting place? And under what other job conditions might I encounter that issue or the former, just so I am aware and ready?

Thanks =)
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Old February 24th, 2008, 12:39 PM   #8
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Try this-

Hardwire your camera to the board and see what happens. If it sounds clean, then the levels in you wireless system need to be adjusted (normally the gain on the transmitter). A favorite tool of mine (and indispensable to say the least) is an in line attenuator - link below.

If the signal is still distorted, then check the levels on your camera. If its peaking and clipping, bring them down to see if that solves the issue.

If its distorted in camera but the levels are not peaking, then its the sound board. Have them turn their sends down.


http://www.bhphotovideo.com/c/produc...ttenuator.html
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Old February 24th, 2008, 01:53 PM   #9
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Originally Posted by Kell Smith View Post
Also, how can I tell if it's clipping before it gets to the camera, since the cameral levels can't show that?
By monitoring the signal that you record in the camera. Does your camera have a headphone jack? It's one of the golden rules of audio work - ALWAYS monitor what you record.

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Old February 24th, 2008, 02:04 PM   #10
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According to the Sony specs, the bodypack transmitter can accept an audio input between -60dBv and -39dBv, ie, mic level. If you're sending it a +4dBu line level from the sound board, you're severely overloading the transmitter input to the point of clipping. The recording levels on the camera are of no consequence - the audio was ruined long before it got there. All the camera meter is telling you is that your levels coming off the receiver and into the camera are fine for recording the distorted audio the transmitter is sending it without messing it up even more. The solution is a pad or DI Box between the sound board and the transmitter to drop the board output back down to mic level.

It's easy to fall for the trap of thinking the wireless radio link is sort of an invisible wire and you're dealing with one audio chain. It's not. You're dealing with two separate audio chains. One terminates at the transmitter. The other originates at the receiver and terminates at the recorder. You need to set the first chain's gain staging so the transmitted signal is clean and properly modulated. Then starting fresh with the receiver you need to set the second chain's gain staging so it records the receiver's ouput properly. Think of trying to record a radio station off the air. If the DJ at the station screws up the levels playing the record so it's being transmitted with distortion there's nothing you can do at your end to repair his screwup. But if it's being sent properly, you can still misadjust your receiver's output and recorder gain so it gets screwed up on your end and nothing the radio station does will fix that. You can't fix a problem introduced in one segment of the chain with any of the adjustments possible in the other.
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