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Old June 10th, 2008, 09:44 AM   #1
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Why does MP4 throw my video out of sync with my audio?

Kinda strange issue here.

I've been cutting together a multi-camera edit of a concert in FCP 6 and by the looks of the preview everything looks perfectly in-sync with everything else.

i'm rendering it out as an mp4 because i'd like to keep my footage at an HD resolution (in this case 1280X720) and keep it a small file size to put on my vimeo account.

for some reason though, once i rendered it out the final product goes in and out of sync with the audio.
when the footage cross fades to another camera angle the audio jumps by about a second, and when it cuts to a new camera angle the audio syncs back in.
again, the preview matches perfectly with the audio. is mp4 the problem? any other recommendations for compression? this is going on my vimeo account so i don't think divix will work.

here are the settings i used (not sure if it matters)
MP4 > h.264 > 3000 kbits/sec > 1280X720 > frame rate: 30 > key frame automatic
multi-pass was selected in video options
in audio i selected 192KBS and the encoding quality was set to best
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Old June 11th, 2008, 07:30 AM   #2
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Sorry. I'm having a similar problem of a different nature. I've been focusing on a potential problem between the frame rate of the mp4 (30fps) and the audio rate- maybe 44.1 instead of 48. I have more clues than answers.
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Old June 11th, 2008, 08:55 AM   #3
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ok,
so i've done some further work on the footage.

mp4 isn't the problem, because i rendered it out as an .mov and i had the same issue.

so like i said, it's a multi camera edit. specifically it's 3 cameras.

i've narrowed the problem down to one of the cameras. for some reason, when i cut to that specific camera, it jumps out of sync, and as soon as i cut to another... it's back in sync.

again, the preview is perfectly synced with the rest

what's really strange is at some points i'll crossfade between 2 cameras and kind of hold it so that 2 camera angles are on the screen at once... not sure what the technique is called... but when i do that, the 'bad' camera syncs up again with everything, and when i fade the 'good' camera out, the 'bad' one jumps out of sync...

what in the hell happened to this camera? can i do anything about it?
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Old June 12th, 2008, 02:46 PM   #4
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anyone? i hope i didn't just lose one angle.

i've never seen or heard of this happening before.
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