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Re: FCP X destroys my audio

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Oliver Peters
Re: FCP X destroys my audio
on Feb 15, 2014 at 9:15:19 pm

I did a little more sleuthing on this issue. It appears to boil down to mixed sample rates. Although it seems to be a bug as to why it's happening. Here's the situation.

The points on the timeline that cause the distortion on export are places where I used a specific set of B-roll shots. These were 25fps H264 clips that I externally converted to ProRes 23.98 and imported into FCP X. (Converted via MPEG Streamclip and then conformed with Cinema Tools.) The transcoded video is ProRes at 23.98, but the resulting sample rate of audio in the new files is about 46KHz due to the frame rate change. The audio is never used and these clips appear in the timeline with volume reduced to nothing. Audio was not detach or removed - only silenced - but still present as part of the clip in the timeline.

When I detach and remove the audio from these clips, there is no distortion on export in these areas. If I copy-and-paste only the connected audio clips (my WAV dialogue and my AIF music cues) to a new timeline, also no distortion. So that leads me to pinpoint these specific B-roll clips as the source of the distortion - even though none of their audio was ever in the mix.

I have done this sort of thing for a long time with both FCP "legacy" and FCP X, so it would appear that with Mavericks and/or FCP X 10.1 something has changed regarding the tolerance for mixed sample rates. Maybe an AVF issue?

FWIW - this project has other clips that were transcoded/conformed externally from 59.94 and/or 29.97 to 23.98, as well. None of these seem to pose any issues. Only the clips that started as 25fps.

At least, that's the preliminary analysis as of today.


Oliver Peters Post Production Services, LLC
Orlando, FL

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