Potential export bug

Hi all.

I think may have found a bug in 10.0.9 and earlier, and before I let apple know I was hoping someone could confirm the bug, or suggest a solution.

I'm conforming a half hour show from 24fps to 25fps by speeding it up of 105% on the timeline. This works well, and plays back as it should. On the exported file the audio falls out of sync with the video. The problem gets worse over the duration of playback, so I can only assume the audio is still playing at 100%.

All sample rates etc are correct and as they should be.

Has anyone experienced similar issues with retimed audio?

Is you project set to 24fps?
What happens if you copy your timeline into a 25fps project and export?

No it's set to 25. That's my target framerate for broadcast.

If your Timeline is set for 25fps, They why are you speeding things up? It's already 25fps in the Timeline.

For the usual reason. I dont want telecine judder. Optical flows a bit slow for conforming. Do you have a different technique? Is there a voodoo button on the timeline that I'm missing?

If you just drop the 24fps clip into the 25fps timeline, you should be all set? It will adjust accordingly. Only if you conform the speed of the original clip it will not be retimed.

Yes. It plays back at 25 fps, but it retains the clips original duration. It does so by repeating a frame each second, so you get juddering in smooth motion i.e. telecine judder. That's what I see when I drop a 24fps clip into a 25fps timeline in any NLE, FCPX included.

That's why the conform speed option exists surely? It forces the clip to play back at the timeline framerate, and as an inevitable consequence, changes the duration of the clip.

Have I got the wrong end of the stick here? I don't mean to be cheeky, but I thought this was pretty standard stuff? It's detailed in the standards conversion section of the BBC delivery specs as the preferred method of delivering 25fps from a 24fps source. I imagine the guides are similar in most 25fps countries.

www.bbc.co.uk/guidelines/dq/pdf/tv/Techn...ryStandardsBBCv3.pdf

If anyone has a better solution however, I'm all ears.

Not sure if there's a misunderstanding going on here. I'll try again, sorry if I sound weird, I'm German...

What if you do the following?: Drop the 24fps clip in the 25fps timeline. Select the clip IN the timeline. Choose "conform framerate" from the timing section. Now the clip will play all its 24 frames, no frames are added. This results in smooth playback and no stuttering while the clip is sped up to 105%. Isn't that what you want to achieve?

Yes, this is exactly what I'm doing already, which works fine in the timeline and everything plays back as it should, however on the exported file the audio slips out of sync over the course of the clip. This is why originally posted, to ask if any one else had experienced a similar problem with retimed footage and audio. I'm trying to establish if it's a bug in FCP or if I'm doing something silly.

Oh right, silly me, I lost it over the ongoing conversation. I never had that problem. Then again I mostly used this technique for single short clips in an edit, not for long stuff. It really sounds like a bug. Maybe it helps if you first edit the entire project as a 24fps timeline and export it and then later transcode to 25fps using compressor?

Conforming long-form 24p to 25 for television screening is something we do on a regular basis. What's the format (video+audio) of your 24p media? What is the format of your exported 25p file?

In: 1080p24 h264 VBR, AAC 48khz 16bit 360kps

Out: ProResLT

Any clues gratefully received!

Sorry, I don't get this. You say you want to conform 24p source video to 25p but your source video is 1080p25?

Typo: 1080p24

Thought so, but I wanted to be sure

It's not a bug and you are not doing something silly, but just like any other long-gop format H.264 is notorious for audio drifting issues when conforming the media to different frame rates.

I won't go into overly technical details as to why this is the case, but the bottomline is: if you want to conform H.264 video with AAC audio to a different framerate by retiming the video (which is indeed the advised workflow when going from 24fps to 25fps) you first should convert the 24p H.264 file to an all I-frame 24p format before dropping it onto a 25p timeline.

You could Optimize your H.264 file to ProRes 422 (R-click the file in the Event Browser and choose Transcode Media > Create Optimized Media). Then you create a 25p Project, you drop the optimized file onto the 25p timeline, you press the Retime button and select Conform Speed. Now when you export to ProRes (or ProRes LT) your audio should stay in sync.

But for some codecs, and depending on your system, FCPX might not allow you to Optimize the imported clip (option greyed out). In this case you can convert the original H.264 file to ProRes 422 using a third-party app (like the free MPEG Streamclip encoder) and import the 24p ProRes file in FCPX. Make sure to check the video/audio sync of the ProRes file in QT player before going any further. If the converted file is also out of sync you may have other issues with your source file.

Now the obvious question is: then why does the H.264 file seem to play fine on the 25p timeline, but it is out of sync in the exported ProRes file?

Actually on your 25p timeline each frame of the 24p H.264 video is always very, very slightly out of sync with the audio track. The drift is so little that you cannot hear it when you play the timeline frame by frame. And as the video is decoded and conformed on the fly one frame after the other when you play the timeline you will never notice the drift, wether you are at the start or at the end of your timeline.

But when you export the timeline each frame is decoded, conformed and added to the exported video file. As every video frame that is added to the file is very slightly out of synch with the audio track of the file, the more video frames are added the more the video track will get out of sync with the audio track and you will start hearing the issue.

Thanks so much for that. It had been driving me insane, but it all makes sense! Thanks for taking the time to write it up. It's much appreciated.