Scrolling timeline jerky while from browser ok, do you recognize this?

[Joe Marler]

[Vincent Cabview Holland] “large file 4K clip I can scroll through it amazingly fluently from within the browser by hovering the mouse over it. But when I place it into the timeline I cannot scroll as fluently anymore all of a sudden. This has always been the case for me on several iMacs and different versions of FCP, and I am wondering why this is, as it should be playing from the same file within the library. Currently on the iMac Pro, SSD or HDD makes no difference. Project settings at automatic, but have tried all different settings. Different cameras too. When I use optimized media, it is OK, but this is severely time consuming for hours of film each day. “

This is an excellent question. Here are three factors which affect this:

(1) The execution code path used for Browser skimming/playback is apparently different from timeline skimming/playback. Even if Browser waveforms are enabled and timeline waveforms disabled, and the Viewer is in Better Performance, the difference exists.

I don’t know why FCPX cannot use the same low latency code path for timeline skimming. Maybe it must monitor for various user editing events unique to the timeline.

(2) Certain codecs magnify this effect. Your material is maybe Sony XAVC-S 4k which is H264. By contrast similar 4k H264 material from a Panasonic GH5 at the same bit rate, bit depth and chroma sampling behaves much smoother in FCPX. The internal composition of each H264 codec can vary. E.g, GOP size (fixed or variable), number of frames between I or P frames, etc.

(3) You are currently using the iMac Pro, which does not have Quick Sync but uses hardware acceleration in the T2 chip for H264 decoding. It will be a little more sluggish than the Kaby Lake or later version of Quick Sync in the 2017 or later iMac. The same Browser vs timeline difference exists with Quick Sync, but it is lessened.

If you are not on the latest version of macOS, FCPX and Apple Pro Video Formats, it is good to upgrade. Late last year there were some significant improvements in H264 handling unique to the iMac Pro. However it appears this was mostly encode-related, not decode.

It appears the Panasonic material uses a fixed GOP size whereas the Sony XAVC-S material uses a variable GOP size. My guess is the variable GOP encoding is more difficult to smoothly decode. This is not unique to FCPX; Premiere is also much slower on 4k XAVC-S.

I tested a 4k XAVC-S clip, then exported it from FCPX using Master File>Computer>H264>Best Quality, then re-imported it. The timeline performance was smoother. Inspection of the header showed it was fixed GOP, size=30, vs the Panasonic material of size=8.

I then encoded the same file using Compressor and selected GOP size 8. When imported to FCPX the file was yet faster and smoother.

I also tested re-wrapping the XAVC-S with EditReady before import and importing the clips using “copy to library” vs the bare files extracted from the camera’s folder tree. I didn’t observe a major difference.

There is nothing you can do about the difference in skimmer latency between the Browser and timeline. That exists with various codecs but is more noticeable on “difficult” variants of H264 such as Sony XAVC-S. Your only other options are transcode the XAVC-S to proxy or change to cameras which emit better-performing H264.

[Vincent Cabview holland]

Thanks for the explanation Joe! Although I only understand half of it, it is a consolation that there is a reason. Indeed it is Sony’s xavc. I thought the Osmo Action would be THE solution because it can save files as .mov, but it is even worse..

I am desperately waiting for the next version of the Sony FDR-X3000, hopefully with HEVC support, should cure the problem I think.