[Mark Burton]

Got it. I gave this method a go and for me the resulting .mov file displays an audio track duration which is too long, its as ffmpeg would do on its own for me. If I take a 00:00:15.00 (15 seconds exactly) 24fps file and process with this method, the resulting file has a duration of 00:00:15.062. Using modmovie, the file is 00:00:15.00.
Are you getting a resulting file with an audio track which matches exactly the source file?

[Mark Burton]

Thanks mate. Not sure I understand the batch command though, why is the output from afconvert called ‘primed_acc.wav’? Are you not trying to make an AAC file to go back into the .mov at the end or was this just a typo?

I’m not sure there is a way for afconvert to handle a .mov file, it is called Audio File Convert after all!

[Mark Burton]

OK cool. How are you combining the two together?

[Mark Burton]

Hi mate, sorry I didn’t come to you on this. If I were you, I’d just do it all in ffmpeg in one hit and use another tool to rewrap the .mov. Its not so much the encode of the aac thats the issue with ffmpeg, but the way it creates the .mov wrapper where the problem occurs I believe.

[Mark Burton]

Use this to get the start TC value:

set start_tc to do shell script "/usr/local/bin/ffmpeg -i " & movie_name_quoted_posix & " 2>&1 | awk '$1 ~ /^timecode/ {print $NF;exit;}’ "

[Mark Burton]

Hi Frederico,

I understand what you want to do. For me I have been using Applescript to manipulate the timecode tracks for my FFmpeg encodes. Essentially its exactly as you said – you get the start timecode, convert it to frames, then convert it back to timecode using the framerate you need.

This will get you started:
https://macscripter.net/viewtopic.php?id=43530

Good luck.
Mark

[Mark Burton]

I’ve tested that before and its not 100% reliable unfortunately, you will end up losing a frame of picture in some instances. The issue is a structural issue with the container rather than there actually being an extra frame I believe.

[Mark Burton]

Would really love to see how to did this, we’re currently trying to find the best method to trim handles from shots. Looking into using the select filter, but not found the solution yet. Be great to see what you ended up using.

Thanks

[Mark Burton]

Unfortunately the mp4 route does not work for either. The platform we play on only accepts .mov files. I tried to see if making first as mp4 and then re-wrapping to .mov with ffmpeg worked, but no luck.

I may have found another workaround solution which is at least scriptable and can be included with the ffmpeg command. Using the QTCoffee toolkit (modmovie) to re-wrap the output into a new self contained .mov produces a file which is essentially the same as those that Quicktime creates. It does not have the ‘offset’ on the aac track and as a result plays without the white flash frame. Its doing the same as MPEG Streamclip, but since its command line based, its much more suited to our scripted workflows.

Still testing it to make sure there are no sync issues as a result, but I think this is probably the best workaround for now. Thanks for all the help everyone, much appreciated.

[Mark Burton]

Have now tested libfdk_aac and libfaac and found the same problem. It seems likely this is more a decoder issue with a specific playback engine we use.

Having looked into how compressed aac audio is actually stored in the container (in this case a .mov), it does look like there is an incompatibility with the way ffmpeg does it and our decoder. Thats likely the reason the ffmpeg ‘bug’ reports around this issue always end with the developer stating its as expected, which is right. The emphasis is on the decoder to understand whats called the aac priming I believe.

This is all well above my pay grade, could well be on completely the wrong track, but as far as I can see, there is nothing ‘wrong’ with the files in and of themselves, its just that the layout is not as widely understood as Quicktime .mov’s.

Thanks for all your ideas and help. The short of it is that there is nothing I can do in ffmpeg to change this behaviour. I need to either change the .mov container structure post encode (MPEG Streamclip ‘fixes’ the issue) or the decoder needs to be updated to handle ffmpeg files with aac audio better.