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Mysterious Clicks on Compressor Output from FCPx

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Julie Turner
Mysterious Clicks on Compressor Output from FCPx
on Apr 29, 2019 at 6:57:10 am

I have a nearly done 45m piece and am now testing renders out to Compressor. I'm finding that this compressor output has occasional clicks/pops in the audio that do not occur in the FCPx project itself. I've tracked one down for experiment's sake, and interestingly, if I grab an offending clip and put it in a new project and render it out, I get a click here once again, though not in precisely the same location (within a second or so)!

I'm compressing using the Video Sharing-->HD1080p profile, but with 320kb selected for audio.

My only idea is that it has to do w/ the fact that the source clip is h264 720p 29.97fps aac 44.1khz, while my project is 1080p 30fps aac 48khz. I didn't do anything but drag the file in and work on it (as I did with everything else I'm working with--all of which are all over the place). Could this be a source of problem for me?

Any suggestions for how to troubleshoot are appreciated!


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Julie Turner
Re: Mysterious Clicks on Compressor Output from FCPx
on Apr 29, 2019 at 8:13:47 am

Well, I exported AIFF audio to Audition so I could take a look and it's a horror show. There are drop-outs everywhere. These aren't audible or evident in FCPx, but are present in the output. Like every few seconds. It's like the signal just disappears, and it happens in the worst ways (e.g. going from signal to 0 in 1 sample).

Any ideas are really appreciated ... not sure how these months of work isn't trash right now....


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Julie Turner
Re: Mysterious Clicks on Compressor Output from FCPx
on Apr 29, 2019 at 8:30:43 am

Just to give a sense of what I'm seeing/hearing, here's a screenshot from Audition:



Neither the original media—nor the playback/timeline in final cut—has this dropout. And these are everywhere...


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Julie Turner
Re: Mysterious Clicks on Compressor Output from FCPx
on May 3, 2019 at 6:07:53 am

Hi All,

I wanted to follow up on my own post in case anyone finds this via search.

In my case, I narrowed down the problem clips and tried to find what was in common amongst them. In the end, the only thing in common was that they all used the aac high efficiency codec (AAC-HE) or version 2 of the HE codec. I don't know why this would cause a problem—or even for sure whether it was the thing causing the problem—but that was most in common amongst them.

I also found that a bunch of the problem clips had 44.1khz sample rates while my project was 48khz. But this wasn't true of all clips and I had the same glitchy problems from clips that were sampled at 48khz.

I *did* realize once I narrowed down the problem that there were some clues in the audio waveform representation on the timeline ... a sort of drop-out was visible, even though it usually caused no audible issue. This wasn't true universally, but it was much of the time.

So anyway, in the end my strategy for repair became an iterative process something along these lines:
- re-encode the original clip's audio to aac (not high efficiency) using ffmpeg
- if that didn't work, upsample any 44.1khz audio to 48khz.
- relink

One of those worked for about half my problem clips. From there, I had to:
- sometimes open the clip in QT and export back out (fixed a missing audio issue in FCPx)
- sometimes open the clip in VLC and do the same (if QT trick didn't work)
- re-encode both audio and video

Occasionally no matter what I did the new version was a tiny bit shorter than the old and FCPx wouldn't relink it. Also, sometimes, especially when sample rate changed from 44.1 to 48 I had to mod the segment because the audio shifted position by around 1 frame.

Finally, when all else failed (and it did a few times), I ended up re-encoded the source video to webm/opus and then back to h264/aac. Somehow this seemed to "cleanse" the file, removing whatever had been buried in it that gave FCPx fits. I'm sure I lost quality here, but this was better than the glitchy clicks I got on export.

HTH someone else some day.


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Joe Marler
Re: Mysterious Clicks on Compressor Output from FCPx
on May 3, 2019 at 10:43:56 am

Julie, sorry about all the problems. Thanks for taking the time to post this in case it helps someone in the future.

Could you also post the model year and type of your Mac and version of macOS?

There have been several reports of *external* audio issues (say to USB output) for certain Macs. These are thought related to newer Macs having the T2 chip. But I don't recollect any issues when processing or transcoding internal audio data.

It's theoretically possible a 3rd-party kernel extension could induce latency when processing audio data. Here's an article which describes how to list those: http://osxdaily.com/2010/08/03/list-all-third-party-kernel-extensions/


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