Weird audio phase cancellation when capturing with iSight

W

wickerman

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When I first got my iMac (OSX 10.5.2 with Leopard) I had no problem capturing audio/video in iMovie using my built-in iSight. Lately however I get a strange, undulating phase-cancellation sound appearing on the audio track about 10 seconds after I start recording. I can't hear this through my Mac when I'm recording, only during playback. It completely messes up my guitar movies.

I have since downloaded iMovie '06 but I get the same problem there too. Is there a fault with my internal mic? What else could be wrong?

Thanks in advance.
 
The audio comes and goes with a whooshing sound. Not exactly volume attenuation. If you know what a guitar sounds like through a phaser pedal, that's exactly like what I'm hearing.
 
when you record again check your cpu usage via activity monitor in utilities folder.

Also try booting in safe mode by holding down the shift key during boot up. this will rebuild the kext cache clearing out any stale bits.

btw phase cancellation attenuates the sound. two waves interacting out of phase, specifically 180 degrees will cause phase cancellation.
 
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Ok, This is what I get when I try to capture from iMovie after rebooting in Safe Mode:

"There's a problem accessing the iSight mic. Make sure no other applications are using it, or try disconnecting and reconnecting the iSight. If you record right now, your movies will be silent."

...!
 
I just shut down and rebooted normally and then tested iMovie. During recording CPU usage is between 32 and 35%. Attenuation isn't so much coming and going now as just getting quieter after 5 secs and remaining constant at that level.

I note that the attenuation occurs 5-10 secs after I begin generating sound for the mic to pick up, not from the beginning of recording.
 
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Look at a couple things:
1) The software you are using, is noise reduction on?
2) go to system preference/Sound/ Input tab and make sure the "use ambient noise reduction isn't checked." This can cause the problem you are having.
 
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