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FCPX clip drifts out of sync

This has never happened before, so I'm a bit stumped.

I have a long clip, its about an hour. Its a ProRes 422 file with attached audio.

In QT player and everywhere else, it plays perfectly. When in FCPX, however, the audio falls out of sync by what seems like around 3 seconds by the end. What is most perplexing is that this is not in a timeline, and its not a transcoded file or a proxy or anything, its a file I'm importing but "leaving in place" so its the identical file that plays perfectly elsewhere. And this is in the browser that it falls out of sync, not in the timeline. What gives? So perplexed. Ideas? How is this even possible?


RELATED QUESTION: I tried to separate the audio and change the speed of it using the "duration" so as to retime it to be 3 seconds shorter, but that seems to not work. The input just keeps jumping back to the original duration. Is this too small a change for fcpx to handle? is there some other way to retime the audio by fractional percents?


Thanks for any advice.

Mac Pro, macOS 10.13

Posted on Feb 4, 2021 6:04 PM

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Posted on Feb 5, 2021 1:39 PM

Select the clip in the browser and use Clip>Open Clip. Rather than trying to do a speed change of the audio as a percentage, try changing the speed based on duration. You can drag the end of the retiming bar to change the length.

14 replies

Feb 5, 2021 2:04 PM in response to Tom Wolsky

THAT WORKS. So I kept trying to change the numerical duration and it kept not wanting to do it, but I had no idea you could drag the end of the retime bar AND THAT WORKED. with like 2 minutes of futzing back and forth the whole thing is now in near perfect sync at every point in the timeline. 24 hours of useless transcoding and importing later, this solution took 2 minutes. Thank you. Huzzah.

Feb 4, 2021 6:19 PM in response to Tom Wolsky

The file is an export from Wirecast.

Its 1080p ProRes422, 23.98fps, Linear PCM Audio at 48k. 1 channel of mono. It was exported as a single .mov file.

Its actually nearly two hours long. But it stays in perfect sync as a clip everywhere *except* in FCPX.

what other significant info would be helpful? As I mentioned, I'm playing the exact same clip in the Browser as I do in the QT player, so I just don't get what could be happening.


Feb 4, 2021 9:19 PM in response to Tom Wolsky

its perfectly in sync everywhere except FCPX, thats the problem. FCPX starts in sync and then slowly drifts out, even though in theory it is playing the exact same file.


There *is* a peculiar discrepancy, in that QT player sees this file as having a 23.94 framerate, and fcpx reads it as 23.98 (per the inspector) Such that the file in FCPX is 9 seconds shorter than the original (per both finder and QT player).


why that would be the case is beyond me, but that would be more than enough over two hours to make the difference.

Why is FCPX not seeing the proper framerate / duration of the clip? I mean, its an apple ProRes codec, not something esoteric.

Feb 5, 2021 1:39 AM in response to Javier Bonafont

The QT number usually means that the frame rate is variable. It’s not a standard fixed video frame rate needed for editing. You should use Compressor to convert the file to stabilize the frame rate.


I understand that it drifts out of sync. Did you try to play a portion from the end of the clip in the browser. Is that in sync? If it is it would mean it a really a playback problem, any area of the clip is in sync for a time, but then goes out of sync, not that the audio is displayed along the length of the clip.

Feb 5, 2021 7:33 AM in response to Tom Wolsky

I dont know what you mean by "stabilize the frame rate". It seems perfectly stable at 29.94. It plays perfectly and in perfect sync in every application except FCPX. 29.94 is not a common framerate, but its not totally bizarre, I seem to recall it being pretty common in the panasonic DVX workflow. I frankly dont know why it exported at this framerate, but what I don't understand is why FCPX doesnt SEE that framerate, when QT and Compressor both do, VLC does, even the finder Quicklook window stays in sync to the end. My actual pro editing app is the only one that can't.


In the FCPX Browser, the clip duration is reading as 9 seconds shorter than the clip actually is, and the framerate is reading as 29.98 instead of 29.94. Playback IN THE BROWSER, playing the same file, is several seconds out of sync by the end of the clip.


I did a re-import of the clip, this time "copying to library" to see if that would resolve anything. It did not. the clip in the library, when opened in QT, is in 23.94 and is 9 seconds longer than the Browser says it is. The clip in the library plays in perfect sync at every point in the clip in QT, but in Browser is way way off by the end. The clip in the library, in other words, is identical to the original, and misbehaves in fcpx exactly the same.


I also re-exported the clip from compressor, again in ProRes422. The framerate of that new clip reads 23.97 in QT and 23.98 in FCPX. Nonetheless, the difference in duration, and the drift in sync is exactly the same. no difference at all. As a matter of fact, FCPX is reading faulty information even *before* importing. In the Import Media window, the new clip reads the wrong duration, 9 seconds shorter than reality.


Nothing seems to help. I may just resort to frankensteining this sequence manually. But I just wish I understood the problem.

FCPX has usually been **** solid in handling media import.


I am doing another compressor export, this time reencoding the audio as AAC to see if that makes any difference. Frankly the clip duration is not important so long as its synced, but it does seem to be a symptom of something wrong.


I will update this post if I have any updates or results. I would appreciate any other advice on where this weird issue could reside.


Feb 5, 2021 9:07 AM in response to Javier Bonafont

23.94 is an incorrect frame rate. The correct rate is 23.976; FCP and most applications call it 23.98. Frame rates have to be exact and not variable. If you use MediaInfo it will show you what the variance is. 29.94 is the average. Devices (like the iPhone and others) that generate unstable frame rates cause serious problems in post production. Actual pro editing apps require actual pro media that conforms to internationally agreed standards.


Please, please will you try the playback test I have asked you twice to do. Play the clip near the end and see if it is sync, or if it is out of sync, it will tell you where the problem lies in the media.


Do not use AAC in Compressor, not with ProRes, or anything else for that matter, not for production, only for delivery. Use Linear PCM at 48K.

Feb 5, 2021 12:09 PM in response to Tom Wolsky

I do not understand why you do not understand what I keep saying. THE CLIP IS OUT OF SYNC AT THE END ONLY INSIDE FCPX. Every transcode, every import, every version, plays perfectly in sync both at the beginning and at the end everywhere except FCPX where by the end it is way out of sync by several seconds. This is NOT in the timeline. This is in the Browser. This is even inside the import window. It is OUT OF SYNC AT THE END. This was in my OP and in every subsequent one and you keep asking me the same question, so I dont understand what part is not clear. It does not have a point at which it suddenly falls out of sync, it DRIFTS out slowly. How does this help? This is the entire problem. It plays perfectly well inside my other "actual pro editing app" Premiere. It shows the correct duration in Premiere also. Also shows up correctly and plays correctly in Compressor. FCPX is the only place this is being an issue.

as i pointed out, Linear PCM was the original and non-syncing audio source, which is why I'm trying other transcodes.


Is there any suggestion you have for fixing the problem? What are the possible changes I can do to get this to work?

Feb 5, 2021 1:30 PM in response to Luis Sequeira1

As I have noted above, I HAVE TRANSCODED THIS MULTIPLE TIMES FROM COMPRESSOR.

I have used different codecs for both audio and video to no avail with the same result.

All the transcodes wind up working everywhere except FCPX. I now have a couple of other ones going with some other settings and also cutting it into two chunks. But I'm just doing random things to see if anything works. It just takes a long time with a 100+ minute clip.

Is there a specific thing I need to do in compressor to change the result? I keep doing this trial and error transcoding and reimporting and testing and I'm just asking is there something specific I should be looking at? Is there some best practice setting I should look at using? Is there anything in the FCPX import workflow that I can change?


My second question, in my OP was about retiming the audio track to conform, which I cannot seem to get to work either. I am guessing that maybe fcpx cannot retime something with such a small increment change? Or is there some better way? I have a sense that if I retimed the audio to be just 4 seconds shorter over the 100 minutes or so of the program, it would stay in sync, but I cant get that to happen in the retime window. Is there another way?

FCPX clip drifts out of sync

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