OT: Audio Question -- Phasing...?

Jay Gladwell wrote on 10/27/2006, 10:33 AM

Recently, I did a series of talking heads for a client. The miniDV was transferred to Beta SP.

The client just sent me an e-mail stating: "Many times throughout the interviews, the audio would go silent in the middle of comments. That usually is a phasing problem but I don’t really know."

Nor do I. I was under the impression that phasing was a problem between two competing sounds (tracks?).

The client was monitoring the sound through a set of headphones. He never mentioned any audio dropouts during the course of the video taping.

Any ideas as to what may have happened? Is there anything I can do to test it?

Thanks!


Comments

richard-courtney wrote on 10/27/2006, 10:52 AM
We subcontract our conversions. Obviously check the DV file using your
own equipment. SP like many format record audio using separate heads,
dirty audio heads on the SP deck?

Check the DV file by zooming in on the timeline section in Vegas. Out of phase
you will see a "valley" in one track and the same height "hill" on the other. When
these are summed together onto a mono track it will cancel each other out.

Why would the phase change during a recording? Check wiring of your mic cables.
1 to 1 2 to 2 3 to 3 if XLR cables were used. Make sure you din't invert a track in vegas.
kdm wrote on 10/27/2006, 10:56 AM
Phase describes the amount a waveform cycle is offset from it's 0 crossing start poing. If two identical audio files are out of phase by 180degrees, they will cancel completely (since one is an inverted version of the other). Phase problems with mics are due to reversed or loose connections between hot and cold pins, which can occur at the mic, preamp, mixer, camera, or transfer.

As far as the situation you are describing, it would be hard to know for sure what was happening without knowing where they were monitoring, and which point in the path the audio was dropping out. (Was this happening while monitoring audio from a mixer, camera (pre or post record?), etc?)

Obviously if the audio doesn't drop out, or sound "odd" (phased) at certain points in the footage you are working with, it may have just been a problem with their monitoring. If at some points something in the audio sounds like it flips, or changes position in the sound stage (even if mono - usually depth will appear to move), then it could be a phase problem in one of the audio cables during transfer, if done analog. If done digitally, this wouldn't happen - you would either have audio or not.
Jay Gladwell wrote on 10/27/2006, 4:00 PM

Just reviewed the masters, there were no audio dropouts.

Either it was a bad tranfers, or the client's equpiment is having problems.

Thanks, guys, for your help!


farss wrote on 10/27/2006, 4:07 PM
I wouldn't be in such a hurry to say all is well.
Have you checked your masters with a mono mixdown?
This is a HUGE trap!

Senny have a socket for some of their kit that wires +ve to one channel and -ve of the balanced pair to the other on a minipin cable. Plug that into a DV camera and an ocean of grief awaits you.
Bill Ravens wrote on 10/28/2006, 7:03 AM
Jay...

Bob is right. Several ways you can check the phasing besides playing it back in the mono mode...which you should ALWAYS do, by the way. Phase errors many times make the audio pump in volume.

So, one way to check phase is to use the Volume Maximizer in Ozone. It has a phase meter that works really well. If you don't have Ozone, or if you want a visual indicator, there's a nice piece of freeware available. Do a google search for VUmeter. It shows a "scope" presentation of phase vs. frequency. Interesting that so many audio comps have a phase reversal under ~100Hz. Many professional audio people don't even see this, except for doing the mono playback check that Bob mentions. I think this might be a throwback to the days of LP records when phase reversal under 100Hz was de riguer, to keep the playback needle from jumping out of the record grove.
rs170a wrote on 10/28/2006, 9:35 AM
Do a google search for VUmeter.

Bill, Google returns a bunch of hits. Do you mean this one?

Mike
Bill Ravens wrote on 10/29/2006, 5:55 AM
yeppers, that's the one