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Audio phase cancellation on output to betacam/ any easy fix?
I’m dubbing off a spot from FCP thru AJA IO to a beta SP recorder and when I test play the dub back, I’ve definitely got a phase-cancellation problem on the dub. In the room where I do the test playback the levels are too low until I hit aphase button on either channel and bing, levels are good. I was trying to make a 2-track mono mix output, but obviously have something wrong. My master is two tracks, track one is just vocals, track two is music. Really I think the problem is in the audio mixing board that is between the FCP system and the Beta deck, (though I haven’t tracked it down yet) but is there some handy way to fix this easily on my timeline?
Embarassedly yours:-)
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11 Replies
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Any chance you are using mic outs instead of line outs? Or maybe a setting on the deck?
Sounds like the difference between line and mic levels.
CaptM
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No, not an impedance problem, I know what that sounds like, and I don’t have distortion or excess hiss that might suggest that. When you have phase cancellation, the waveforms of the left and right chanels (1 and 2) beat against each other in a manner that cancels out most of their energy. What you hear is a very attenuated version of the sound, even when both channels are cranked to eleven. However, if you throw one channel out of that phase relationship, using a handy little button on the mixer bus, or by cutting one of the two channels, BAM, the waves are no longer cancelling each other, and your audio is at full loudness level, and clean.
What I’m trying to figure out is where the phase cancellation is happening, the mixer that’s feeding the dub deck has phase invert buttons for inputs but not any outputs, so I was looking to see if there was something I could do aboutt his in my timeline and output settings rather than tear up the edit bay and run xlr’s direct from the IO to the deck to bypass the mixer. Which I think might fix it, but is extreme. Or I could just ship a dub with one channel empty, it’sa mono audio mix anyhow. Somehow while that works I don;t think it’s good practice since in some plants nobody knows or cares to flip a “mix” switch and so the spot may air with zero sound fromt he empty channel instead of good sound off the good channel. No, the real fix is to kill the pahse cancellation. Any ideas? Anyone?
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Hi Mark,
You haven’t plug two XLR to make a “longer cable” ?
This would produce a 180 degree phase shift.
Rafael -
Tried it, but perhaps the added cable was not long enough. Anyway, I’m looking for a more fundamental fix. But thatnks for the suggestion.
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Hi Mark,
Was not a suggestion but a question. Is something to be avoided because it reverse the phase.
May be a bad XLR. I think with a balanced signal a wrong contact can do something like that.
But just to say something because the audio is not my strong at all.
Good luck,
rafael -
Went into sequence settings menus and changed my audio output settings to mixed mono instead of stereo, we’ll see what difference it will make. My shop is kind of behind the times and doesn’t do/isn’t really wired for stereo. For our kind of work it has never been an issue till now.
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[Mark Suszko] “changed my audio output settings to mixed mono instead of stereo, we’ll see what difference it will make.”
Hi Mark,
is possible that you found the problem. You were feeding the AJA with a stereo imput.
rafael -
There’s a connector or patch point where the low and high signals are reversed giving you a 180 degree phase shift. It’s a pain running that down. A quick fix would be to take one of your outputs and use a mult in your patchbay to send it to both inputs of your deck. If that preoduces an out of phase result, then one of the cables to the deck has reversed XLR pins 2 & 3.
John
MacPro 8-core 2.8GHz 8 GB RAM OS 10.5.5 QT7.5.5 Kona 3 Dual Cinema 23 ATI Radeon HD 3870
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Hi John,
I wish I would have the things as clear as you about audio.
For me is a real “black hole”.
Best,
rafael -
Rafael, thanks for your kind words. I’ve read many of your posts over the past couple of years and have benefited from your knowledge and experience. BTW your idea about the XLR connector was a good one. An improperly wired XLR on one side (channel 1 or 2) will put the signal 180 degrees out of phase. It’d be the first thing I’d check since it’s easy to open the connector and check which wires are going to which pins. If there’s a patchbay involved, that’s a bit more involved.
John
MacPro 8-core 2.8GHz 8 GB RAM OS 10.5.5 QT7.5.5 Kona 3 Dual Cinema 23 ATI Radeon HD 3870
ATTO ExpressSAS R380 RAID Adapter, PDE Enclosure with 8-drive 6TB RAID 5
24″ TV-Logic Monitor
Final Cut Studio 2 (up to date)Pro Tools HD w SYNC IO, Yamaha DM1000, Millennia Media HV-3C, Neumann U87, Schoeps Mk41 mics, Genelec Monitors, PrimaLT ISDN
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