VF captions lagging

[ednamillion]

Hi, just tried for the first time to make an open-caption version file of a DCP O Matic DCP to use in screenings for the hearing impaired. I followed the instructions in the manual and it all seemed pretty easy, and then I took everything to a local cinema to check it out and make sure everything was playing back properly. A few strange things happened. First, the projectionist (who works at a commercial cinema and seemed inexperienced with VF files) told me that he was able to ingest only the OV (he called it the "feature") and that the VF would not ingest. But then when he played it back for me, strangely, it played back with the captions running. So that doesn't make sense to me... the OV did not have any subtitles or captions. Could there be some way that his playback system grabbed the VF even though he didn't understand that it happened? And then, the other more worrying thing is that the captions were great at the beginning of the 30 minute film, but by the end they were really lagging, appearing about 2-3 seconds later than they should have. I thought maybe something was off with the actual SRT file I had used, but I've just opened up the 5.1 Quicktime ProRes file with the open caption SRT in VLC, and in VLC everything looks perfectly in sync. So I think the caption file is fine—something about either the playback situation or the way I made the DCP has caused this problem. If it's something about the cinema and their system, that's not a problem, since that's not the venue that will be showing the film. But how do I know if it's my problem or their problem and what is causing the issue?

[ednamillion]

I will add, that if I skip through the VF with OV loaded in DCP O Matic player, I also see the captions lagging in the later part of the film. So it must be something wrong with the DCP itself, since it all runs in sync in VLC... but I don't know what.

[ednamillion]

and, one more thought: the original video file that the captions were timed to was 23.98 fps. So maybe the drift / lag is getting introduced because the DCP is 24fps but it is still pulling the original time code from the caption file. If that's what's causing the problem, how do I conform the SRT file correctly?

[carl]

I think you have worked out the problem correctly... the SRT file does not contain any information about the frame rate it was timed for, so DCP-o-matic will assume it's the same as the DCP.

If you select your .srt file in DCP-o-matic, then go to the timing tab and enter 23.976 it should time the subtitles correctly.

It's hard to know what went on with the OV/VF confusion; perhaps his system ingested everything off the drive you gave, and the projectionist played the VF without realising?

[Carsten]

DCI systems deal differently with OVs and VFs during ingest. On some systems, both need to be ingested separately. On some, the server collects the OV/VF references automatically. If your OV plays in DCP-o-matic player without subtitles, and the VF with subtitles, this 'projectionist' probably somehow got it right by chance.

[ednamillion]

Thank you both, that all makes sense. In the DCP-o-matic player, the OV and VF play back correctly (OV has no captions, VF has captions). So, what you are saying about the playback system automatically capturing both makes sense—it seems like the guy who ran the test for me did not understand that this is what was happening. I asked him at the end if he usually work with VF files and he said not really—it's a commercial cinema—so maybe he just isn't used to it and misunderstood what was happening in his system. In a film festival, where the DCP will ultimately be shown, the projectionists should be much more familiar, I think.

And, yes, I finally figured out that I could use https://www.nikse.dk/SubtitleEdit to convert all the timecodes in the srt file to correctly run with a 24 fps file. And, that did fix the problem!

[ednamillion]

Carl, I actually did not retime the subtitles inside DCP O Matic, like you've suggested—maybe I will try that too as a test to see if it works equally well? I did try to use that timing adjustment feature, but I was putting in 24 instead of 23.98...I thought if I put in 24 that would make the captions run at 24, but that actually did not work. But you are saying I should actually put in 23.98 instead?

I may do a quick experiment, because I am curious if that is another way to solve this issue in the future. Thank you!

[carl]

But you are saying I should actually put in 23.98 instead?

Yes, that should be the frame rate that the content is prepared for. Then it will be made to fit whatever the chosen DCP rate is.

[IoannisSyrogiannis]

My bet would be 23,976 fps instead of 23,98.
The difference might be small, about 1/10 of a second for every ten minutes, if my calculations are correct, but if the feature is, say, 90 minutes, it will be close to a second before the end.