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overdriving the components in the signal chain

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Bill
overdriving the components in the signal chain
by
on Apr 2, 2005 at 4:56:39 pm

Hi,
I am fairly new to the video/audio field, and I recently shot a video using a Panasonic AGDVX100A camera with Sennheiser G2 wireless mics feeding directly to camera. I did not hear any audio problems with the headphones, but when I got got back to the studio the audio was horrible.
From what I understand, the signal level was too high and was overdriving one of the components in the signal chain. I guess at this point I am trying to find out why this happened, and what can I do to ensure that it never happens again. Any suggestions would certainly be appreciated.




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David Jones
Re: overdriving the components in the signal chain
on Apr 2, 2005 at 5:06:48 pm

What was the level on the audio meters when recording?


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Bill
Re: overdriving the components in the signal chain
by
on Apr 2, 2005 at 5:52:09 pm

Audio levels during recording were about -18 db.


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Seth Bloombaum
Re: overdriving the components in the signal chain
on Apr 2, 2005 at 6:12:37 pm

By a process of elimination that would seem to indicate that the audio was overmodulated at the transmitter (which is easy to do). (less likely - broken camcorder, head clog, error in transfer from camera tape to computer, impedance mismatch... peaks at -18db should be pretty safe)

I believe the G2 have an LCD display; when you set up a transmitter for a particular person you want them to speak as loudly as they might during a take, and adjust attenuation until their peaks just go to the (bad) meter reading. I'm not sure what that is on a G2, but if there is a mark about 2/3's of the way up the scale you only want the occassional loudest peaks to briefly (instantaneously) reach above that.

Obviously it's a safer setting to peak below the mark, but you need to do testing while listening to and recording the signal, monitor it very carefully to establish when it overmodulates, because too low a setting may tend to raise the overall noise. This testing should be done when you have access to good monitoring and some time on your hands (not on location!), it's part of getting to know what your equipment will do.

Not sure about the G2, but on the previous generation of inexpensive Senn wireless the squelch control was pretty fussy too. Experiment with that and read the manual until you know what it does and when it does it.

Also, there is NO SUBSTITUTE for monitoring ALL the audio during a shoot. The overmodulation should have been caught at that time.


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Bill
Re: overdriving the components in the signal chain
by
on Apr 2, 2005 at 8:14:11 pm

Thanks for the help!


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Ty Ford
Re: overdriving the components in the signal chain...and then
on Apr 9, 2005 at 4:10:51 pm

Regardless of where the distortion was taking place in the chain, it would have been heard at the camera input (camera headphone jack) if it happened before the camera. A distorted transmitter input, for example, would also be heard at the camera headphone jack.

However, if the distortion was taking place inside the camera -- after the camera headphone jack -- the distortion would not be detectable until playback.

I have been on shoots when the director, in haste, chose not to allow me to plug in a set of headphones or my mixer's camera return into the camera. This is a major rookie mistake. He set what he thought was the proper level on the camera input meters. BECAUSE HE DIDN'T ALLOW ME TO LISTEN, HE RECORDED THE ENTIRE FIRST SETUP WITH ME FEEDING HIM LINE LEVEL TO HIS MIC LEVEL INPUT.

Fortunately, I was relentless in asking to hear playback. When it sounded like crap, he discovered what I already feared. Audio is not trivial. You can easily set -18dB work of line level into the mic input and on the meters, it looks fine. In the headphones, however, it sounds like crap.

When do you listen to the camera headphone output? Always, every stinkin' second. You don't shoot the scene without looking through the view finder or eye piece.

I'm sure all of us have had our "oops!" moments with this issue. How about it folks? :)


Regards,

Ty Ford



Ty Ford's "Audio Bootcamp Field Guide" was written for video people who want better audio. Find out more at http://www.tyford.com


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JRF
Re: overdriving the components in the signal chain
by
on Apr 14, 2005 at 9:34:56 pm

This happened all the time with Sony BVW35 Beta decks. The meters monitored AFTER the input pre and it was easy to over modulate and distort, particularly with voice (lots of peaks). The tip-off was seeing the input pots down around 3 instead of 5 or higher.

Like Ty said, ALWAYS listen to playback.

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