Linking video playback between macs - staying in sync.

[Pruitt]

Hey gang,

I did a setup in a festival this past weekend in which a video artist was showing 1080 video on three screens. In order to do the playback I ran Qlab 3 with two screens on 2012 iMac i7 (16GB RAM) and a single screen on a 2012 Macbook Pro i7 (8GB RAM), and linked them together via 3' ethernet cable and ipmidi, then triggered via MSC. I'd have preferred to run all three off of one computer, but we didn't have access to any Mac Pros or older systems that could recognize three screens at the same time. Anyway, the system worked exceptionally well and everything about it was excellent, except that I was getting a bit of drift between the two computers - nothing consistent, and I'd estimate that it was never more than a quarter second off (maybe 4-5 frames, really) and more often just a couple frames, but it was somewhat consistent in that it was always the laptop that was behind. In practice, none of the audience noticed and even the festival directors never really were able to see it, but it had the video artist in tears at one point, and she was considering cancelling the remaining showings, and I did feel bad because it wasn't perfect.

In various testing, I switched what the various computers were doing, the order of cue firing, substituted in a different laptop (same basic model but an i5 processor, so in theory it was less capable, but in practice, we didn't see any real difference), etc and nothing seemed to really solve it. There was a vague sense that it was cumulative - lags were noticeable later in the video, which was about an hour and 10 minutes long, while it always seemed that they started perfectly in sync, but to be honest, what we were looking for was so small, and the opportunities to see it so infrequent (when all three screens shifted at the same time), that it seemed to me that they'd come back to sync sometimes after they appeared to be out ie. at minute 52 the center screen would appear to be slightly behind, but then at minute 55 all three screens appeared to jump to white at the same time, etc.

Anyway, the festival is over, but I'm certain I'll end up doing this again, and I'm curious about others who have used multiple computers this way, and your own experiences in terms of keeping sync across machines.

Cheers.

Stephen

[Patrick Andrews]

I really have no idea if I am on the right track but just guessing here. I would think that there would need to be a master clock that the other machine(s) would slave to??? Or is that even possible to do with this setup?
I guess the better option is to get the gear necessary to have all outputs coming from one machine. Although I can think of a few different ways syncing a couple computers together would be useful and fun to do.
Watching this thread with interest.

[Dan]

If you had to have multiple machines then a third machine (master) sending SMPTE or MIDI timecode to both slave machines would have improved the sync between them significantly.

You could have also used something like a Matrox Triple Head or Datapath X4 to get your 3 outputs from one machine. These would have had a much better chance of staying in sync without any external input.

[Rich Walsh]

How would TC help? QLab doesn’t chase TC, and I think it only guarantees internal video timing for media that contains an audio track for clock reference – although I haven’t checked if that’s still true.

A common external _wordclock_ (not TC) would at least guarantee that the two computer timing clocks were running at the same speed – so the offset delay from simultaneous triggering would at least be maintained throughout the clips. TC would not do this.

Rich

[Sam Kusnetz]

Hello all

QLab does not sync to timecode, so in fact a third Mac or a master timecode clock would probably not have actually helped much.

The trick lies in using audio interfaces with word clock connections on both Macs. In QLab, if a video cue contains an audio track, then the clock for that cue follows the clock of the audio device the cue is assigned to. Therefore, if there are Video cues on both Macs which have audio tracks, and those audio tracks are assigned to interfaces which are synced with word clock, then the videos will not drift.

Best
Sam

--
Sam Kusnetz | Figure 53
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> On Apr 22, 2017, at 2:41 PM, Dan <daniel.t...@gmail.com> wrote:
>
> If you had to have multiple machines then a third machine (master) sending SMPTE or MIDI timecode to both slave machines would have improved the sync between them significantly.
>
> You could have also used something like a Matrox Triple Head or Datapath X4 to get your 3 outputs from one machine. These would have had a much better chance of staying in sync without any external input.
>

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[Dan]

Well then it would be entirely useless!

Dan

[Pruitt]

Thanks for the updates...

I do own a Matrox triple-head, but I have had problems with it in the past keeping up when trying to send 3 displays at full 1920x1080 output, which was crucial on this gig, so I didn't use it. In retrospect, I probably would have been better off using it with the iMac.

If I do this sort of thing again, then I'll look into syncing via audio devices, but I generally try to keep my set-ups as simple as possible, so adding more gear doesn't usually jump out at me as the first solution when I feel like I'm already taxing the machines. Also, most of the troubleshooting was done on Easter Sunday, so it wasn't like I could have gone out and rented something...

thanks!

-s