Odd export behavior

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Level 4

3,340 points

Odd export behavior

We have a pretty standard for us, 12 minute HD project with 99.9% Sony FX-6 4k 23.976 source footage, graphics, titles, music and VO. With the library opened on an M4 Pro MacMini, an export to a small, review mp4 preset created in Compressor that has worked for years, appears to get all the way to the end but then both the working mp4 and the final mp4 files disappear. It's as though the export fails when finalizing the working file or moving it to the final mp4 file.

I was able to export the first minute and the last few minutes without issue.

Opening that library on a Mac Studio M1 Ultra and exporting with the same Compressor preset, I noticed 5 working export mov files. The Background process viewer said something like, "Exporting (transcoding segments)" which I don't recall ever seeing before. Once the export was done the final mp4 appeared and the 5 working files disappeared.

I've never seen 5 working mov files before when exporting, just the 1 working file that's typically called theOutputFilename.mp4randomcharacters and 1 mp4 file that's the name chosen when exporting.

Note: there is one, 3 second, HEVC GoPro shot a few minutes into the 12 minute project. First time we've used HEVC in a long time. Does this now cause "(transcoding segments)" during export in FCP 11.1.x? I optimized this GoPro clip but the rooms are busy so I can't try another export test.

Of course I deleted the .fcpcache file and reset FCP preferences on the MacMini, but no change.

Very odd to me. Anyone come across this before? joema? Luis? BenB? Both machines are running 15.5, FCP 11.1.1.

[Re-Titled by Moderator]

Original Title: Odd export behavior

Posted on Oct 29, 2025 7:08 PM

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Answer 1

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joema

Level 4

1,095 points

Oct 30, 2025 5:34 AM in response to terryb

Terry there are two possible areas to examine:

(1) As Luis mentioned, check if additional Compressor instances are enabled. However, I think that only works for ProRes exports if the input is a single file or timeline. This is an older feature and only works for Max and Ultra machines (M4 Pro unknown, see below). It handles two sub-cases:

(a) If transcoding a batch of files, it will run them in parallel using multiple encoders. Each file is handled separately.

(b) If exporting a single timeline to ProRes, it will segment that and run those segments in parallel, then combine the output to a single file. There is overhead in that process, so the speedup (while useful) is much less than linear. Traditionally, it was difficult to segment Long GOP files so this was only attempted for ProRes.

(2) The HEVC/H.264 segmented export feature, added in 10.7, but only for Max and Ultra machines, which have multiple encoders (M4 Pro unknown). This feature implies the input and output Long GOP files are constructed with "independent" or "closed" GOPs, which do not reference each other. For dependent or open GOP files, I don't think those can be parallelized. There is no visibility or control over open vs closed GOPs. There are also automatic constraints on how long the timeline must be before it attempts this, which cannot be configured. On FCP's export dialog, there's a checkbox for "Allow export segmentation." I don't know about Compressor.

It's interesting you report this on M4 Pro. I thought all previous Mx Pro versions only had a single non-ProRes encoder, but Apple's tech specs says the M4 Pro has a single Video decode engine, two video encode engines and two ProRes encode and decode engines. That implies an FCP on M4 Pro might attempt segmented HEVC/H.264 encoding, whereas regular M4 or prior M1/M2/M3 Pro machines might not.

The M1 Ultra has two decoders and four encoders, plus four ProRes encode/decode engines.

This implies the M4 Pro is a bit unique. Also starting with M4, the non-ProRes encode/decode engines were optimized (details unknown) so that's another variable. While the same FCP software should work similarly on the full range of Apple Silicon Macs, we've had a few bugs that were unique to the Max or Ultra (all now fixed to my knowledge). This implies a possibility of an FCP problem unique to the M4 Pro that only happens in certain export segmentation cases. Such things are rare but not unheard of.

Here is an immediate step to gather more info the next time you can try it on the M4 Pro:

Get FCP ready to reproduce the problem.

Get this terminal command ready to run: sudo log show --last 30s > ~/Documents/LogShow30s.txt

Reproduce the problem in FCP

Once it happens, immediately run the above 'log show' command. The problem need not happen while running the command, since it looks back in time for 30 seconds, but it must have happened within the previous 30 seconds.

Zip the log file and send it to me.

Beyond that, anything you can do to narrow down the replication scenario would help, with a goal of sending that to me. That said, this class of problem is typically timing-dependent, so it can be difficult to narrow this down.

I have an M1 Ultra and M4 Pro so I can run tests on those, but it's rarely beneficial to take blind "pot shots" at reproducing it for this class of problem.

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Answer 2

terryb Author

Level 4

3,340 points

Nov 6, 2025 10:17 AM in response to terryb

Follow up:

I discovered what appears to be the issue - the project has a very tall PNG of a web page (~37,000 pixels high) that causes the export to fail on an M4 Pro MacMini, but not an M1 Ultra Mac Studio. If I disable the PNG, the project exports fine. The editor created the PNG using a Chrome extension on his laptop called FireShot that saves an entire web page as a PNG. (I don't allow Chrome on our edit machines.)

Joema was a big help trying to get to the bottom of this, but ultimately it was exporting small sections of the project to narrow down where the problem was. We will export from the M1 Ultra until I can figure next steps.

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Answer 3

Oct 30, 2025 12:22 AM in response to terryb

I can only speculate on the "transcoding segments" part.

The M1 Ultra has a lot of cores (20, I believe), and it is possible to set Compressor to run multiple instances

Compressor->Settings->Advanced: Enable additional Compressor instances

where the number of instances is bound to be larger for the Ultra (it maxes out at 3 on an M1 Max). It may be that this is set to 5 on that M1 Ultra, and the job is being split amongst them.

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Answer 4

terryb Author

Level 4

3,340 points

Oct 30, 2025 11:15 AM in response to joema

Thank you Luis and joema. When I am able to get onto the machines I will do some more investigating and report back. In might not be until later next week. To be clear, the FCP export is not using the Send to Compressor method, it's using an imported Compressor setting that exports to a small, H.264 mp4 for client review.

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