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Strangest thing — when I delete either audio track, the remaining track sounds clearer
This isn’t really a problem, because this weird deleting thing actually solves the problem, but it’s the strangest thing I’ve ever encountered in FCP (and that’s saying a lot), so I was wondering if anyone had any idea why.
I have a bunch of interviews. Shot with lavaliere mics going through the camera. It seems it was shot on a normal SD camera. There’s a couple interviews where the sound is particularly bad. One, for instance, shot from a distance sounds like they’re using the mic mounted on the camera because the sound is so extremely quiet.
Then, when I deleted one of the two audio tracks (after putting it in a sequence), it came through loud and clear. So of course I assumed the two tracks were two different mics (one the clear lavaliere and one the far away camera mic) because that makes sense. But no. If I delete either of the tracks, it doesn’t matter which one, the sound goes from extremely quiet (around -42) with a lot of background noise to loud and clear (-12), with no background noise.
Weird right? Is there any possible explanation?
FCP 6.0.3/Mac G5 OS X 10.4.11
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10 Replies
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Phase cancellation. In essenece, when you listen to both channels, they sound in running in almost opposite phases and it essentially helps to cancel each other out. When you listen to one channel, there’s no more cancellation.
Jeremy
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Well that’s absolutely fascinating. Thank you!
Guess I should have paid more attention in sound class.
May I ask, what potentially causes this?
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You have encountered what is called PHASING, where one audio channel cancels out or nearly cancels out another.
Shane
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Read my blog, Little Frog in High Def -
And I have encountered what is called “forgetting to hit the POST button for 15 min.”
Shane
GETTING ORGANIZED WITH FINAL CUT PRO DVD…don’t miss it.
Read my blog, Little Frog in High Def -
You should also try panning the audio channels hard left and hard right and then decide which channel sounds best for that particular edit.
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Well that was a truly first class suggestion. Hard right and it’s seriously crystal clear. Thank you very much!
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[Eva Ikari] “Hard right and it’s seriously crystal clear. Thank you very much! “
This is essentailly as playing back one channel at a time. This way, that track is now playing out of the right speaker only and the phase cancellation is cancelled. You are cancelling the cancel.
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Be careful about panning full left and full right. It will appear to solve the problem if you are listening to playback in stereo (i.e. there is no canceling because there is no mixing of the out of phase signals). But if your viewer watches the program on a device with a single speaker, then the phase cancellation will be back!
Mike Raff
Richmond, VA -
[Eva Ikari] “May I ask, what potentially causes this? “
The cause can be the unequal length of the cables or a wrong connector.
For example when you branch together two XRL cables, the phase is shifted 180°.
If you use in a mic a 10 meters cable and in the other two 5 meters cables, the phase of both signal would have 180° difference. The cancellation would be total. No audio output at all.
rafael -
This kind of phase cancelation can happen a couple of ways. Many mixers have a phase reversal switch. If this switch is activated on ONE SIDE of a stereo mixer, then the two channels will cancel when mixed mono. It’s important to realize that even though the audio sounds good playing back as stereo, if it’s played in mono the cancelation will occur. This will usually result in the QC dept. rejecting the program.
Another cause of phase reversal is miswired connectors (mic or line level). The more-or-less standard wiring of an XLR (TRS) is pin 1 shield (sleeve), pin 2 hot (tip), and pin 3 cold (ring). If pins 2 & 3 (T & R) are reversed you get a phase reversal compared to a properly wired connector. You can use multiple cables together without phase reversal as long as they’re wired properly. While Rafael is correct that different length cables will cause phase differences, you’d have to have a tremendous difference in length to cause a reversal.
Other causes of phase differences (not necessarily cancelation, but resulting in a “hollow” sound) are using two mics at different distances from talent, or reflections from a hard surface like a table or desk. One test every soundperson should make is to listen to mono playback. If cancellation is occuring that’s the way to catch it.
John
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