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5K Retina iMac & 4K (+1080) multicam editing performance issues - GPU?

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Max Tannone
5K Retina iMac & 4K (+1080) multicam editing performance issues - GPU?
on Dec 16, 2016 at 6:16:24 pm

I cannot smoothly playback a multicam project in Final Cut Pro X with the Angle Editor open.

I believe this is a GPU issue.

Some additional information:

iMac (Retina 5K, 27-inch, Late 2015)
3.2 GHz Intel Core i5
24 GB 1867 MHz DDR3
AMD Radeon R9 M390 2048 MB

Driving 3 displays (iMac 5K display, and two additional 1920x1080 displays)

This machine has a 1TB fusion drive installed (this fusion drive has 24GB of SSD, and 1TB of spinning storage on a Samsung ST1000DM003, which I believe is a 7200 RPM.

My FCP X library file is stored on an external USB 3 Samsung T3 1TB SSD drive. (I get sustained read/writes of 400 MB/sec on this drive)

The FCP X library file has all of the source media copied into it.

The media is left in its native format. It is a 3 camera shoot. 2 cameras of 1080p 30fps Canon AVCHD codec (max bitrate of 24Mbps), and 1 camera of 4K (UHD) in Sony XAVC-S at 100 Mbps.

So total bitrate of 148Mbps, or a data rate of 18.5 MB/sec.

I am unable to play back with the angle editor displayed. I know the bottleneck is not the external SSD, nor does it seem to be the CPU (as activity monitor doesn't show the CPU even coming close to being fully stressed.)

Therefore, is the bottleneck my GPU?

Also, what is the relationship between bitrate and codec? I assumed bitrate was the only thing that mattered, but clearly that isn't true, since I've read that if I converted all of my clips to ProRes, instead of AVCHD (or variants thereof), I would have no trouble playing back smoothly, DESPITE the overall bitrate being WAY higher than my native overall bitrate.

Am I correct in this assumption? Can someone explain what is going on here? Thank you.


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Noah Kadner
Re: 5K Retina iMac & 4K (+1080) multicam editing performance issues - GPU?
on Dec 16, 2016 at 7:01:40 pm

You would most likely need to switch to proxy mode to have a hope of decent multicam playback with those specs/codecs.

Noah

FCPWORKS - FCPX Workflow
FCP Exchange - FCPX Workshops
XinTwo - FCPX Training


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Jeff Kirkland
Re: 5K Retina iMac & 4K (+1080) multicam editing performance issues - GPU?
on Dec 16, 2016 at 7:01:40 pm

I'll let someone else chime in with a technical explanation but it's not so much about the bitrate as it is about the effort required to decode a specific codec.

AVCHD is an inter-frame compression codec so it is constructed from one full frame followed by several partial frames that are reconstructed on the fly as you play them back. That recreation of the intermediate frames can have a high system overhead.

ProRes on the other hand is an intra-frame codec. Every frame exists in its entirety and all the computer has to do is display one frame after another which requires way less effort.

----
Jeff Kirkland | Video Producer & Cinematographer
Hobart, Tasmania | Twitter: @jeffkirkland


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Max Tannone
Re: 5K Retina iMac & 4K (+1080) multicam editing performance issues - GPU?
on Dec 17, 2016 at 3:20:36 am

Thanks to both of you.


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Mark Smith
Re: 5K Retina iMac & 4K (+1080) multicam editing performance issues - GPU?
on Dec 17, 2016 at 12:02:18 pm

Recently I cut a project (FCPX 10.2.1) for a client that involved 3 -ish angles - two were Canon XAVC format and the third was pro res from a BMPCC . All angles were 1080 23.98. I did not convert to optimized media.

I was cutting on an older iMac - 2013 vintage with the camera files on an external drive that was USB 3 . I know doesn't sound so good. It wasn't, but then I switched over to proxy media and with this under - spec'ed set up it was remarkably easy to cut then switch to original media for the export.


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