Film-Tech Forum ARCHIVE: Getting A/V sync right...

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Posted by Christian Hove (Member # 6743) on 09-04-2019, 06:46 AM:

Hi,
What procedure/tools/test DCPs are you guys using to get A/V sync right (or at least a happy medium)?
From time to time I am experiencing noticeable sync problems (at least to me) - Mostly with old restored films distributed on DCP.
I wonder if I can somehow make our Doremis adjust the A/V sync per film...but I have not been able to find a way to do it.

Thanks,
Christian

Posted by Carsten Kurz (Member # 5396) on 09-04-2019, 07:00 AM:

There's no way to set A/V sync per film/CPL on any server I know. Doremis allow to set a per-CPL audio mapping, but audio delay is not part of that.

Some audio processors allow to specify a delay per preset (e.g. AP20/25). As the Datasat offers loads of free presets, it shouldn't be a problem to make a few copies of existing presets and adjust global delay in them, to be addressed from within SPLs.

I think a CP750 can only adjust global delay per input. However, it may be possible to adjust global delay during runtime using automation commands (cp750.sys.dig_X_global_delay - 'X' for input number). So you could put a suitable delay value into an SPL associated with that CPL. Don't know if that value is applied right away, you would need to test it. The max value of 250ms should be easily discernible during a test.

While it's easy to find proper sync with a test-clip, I'd find it rather complicated to do that with varying live footage (unless the sync is severely off - which I would consider not a sync issue, but a broken DCP). If you take the time sitting in the auditorium and using some remote control option for your processor, it may work out well enough.

- Carsten

Posted by Christian Hove (Member # 6743) on 09-04-2019, 08:08 AM:

Hi Carsten,
Thanks for the reply.
Having AP20s, I will definitely give it a shot to solve it that way.
Which sync test clips are you using - Are there any of them that are downloadable somewhere?

/Christian

Posted by Marcel Birgelen (Member # 6801) on 09-04-2019, 08:25 AM:

The cp750.sys.dig_X_global_delay option Carsten mentioned takes immediate effect. I've used this in the past to fix an auto-of-sync DCP. Don't forget to reset it to the original value after you played the content.

The disadvantage of controlling audio delay via your audio processor is that you can only add "positive delay".

Posted by Christian Hove (Member # 6743) on 09-04-2019, 08:33 AM:

Is there a similar command for the AP20?

/Christian

Posted by Marcel Birgelen (Member # 6801) on 09-04-2019, 09:04 AM:

No, but you can create a new format by copying it from the existing format and set the global delay there.

You can then switch the format with the @FORMAT Format Name API command.

Posted by Steve Guttag (Member # 268) on 09-04-2019, 10:38 AM:

Marcel,

Since one normally has about 60-80msec of delay (about 2-frames)...you generally have at least that much to advance the audio so I don't see the audio processor method as much of a disadvantage and, like others I don't think most servers allow adjustment during playback (or not easily). And not able to do it via cue.

Posted by Marcel Birgelen (Member # 6801) on 09-04-2019, 11:01 AM:

I'd say what you say is true for the average professionally created DCP...

But in reality, it depends on what material you're working with. Professionally made DCPs seldomly have any A/V sync issues, but the DCP format is somewhat of an oddball format for indie producers. They often don't even have the proper tools to test their DCPs themselves, so they just export it or look up some on-line tutorial and are done with it. I actually prefer them to deliver some kind of MP4, MKV or other common format, instead of a DCP with issues.

But as a result, I've seen some crappy productions over the years and some have their audio so much out of sync, it becomes a challenge to get right and yes, I've had one DCP once that had the audio so much off-set to the "negative side" (as in too early), the only way to compensate it was on the server. Sure, that DCP was clearly defective, but sometimes you need to work with the stuff you're handed...

Like Carsten mentioned, getting A/V sync right on "live content" isn't always easy and can be quite a frustrating thing to do. I also tend to look for short sound effects with a high impulse like gunshots, doors closing, etc., as they're far easier to track and time than someones mouth movements.

Posted by Harold Hallikainen (Member # 5405) on 09-04-2019, 03:50 PM:

A sync pop DCP is available at http://ftp.uslinc.com/Products/LSS-200/Software/TestContent/181211%20-%20LSS200%20Test%20Content%20(cropped)/ . This is designed to work with the LSS-200, so there is a slow flash sequence at the start to start the measurement script in the LSS. You can see some test results at https://lss.hallikainen.org/report.php?db=dfs&tn=1&sn=3 (sync at bottom of page). You can also see a lamp becoming discolored and a few days they forgot to turn on the audio...

Harold

Posted by Christian Hove (Member # 6743) on 09-06-2019, 01:36 AM:

Thank you all for you answers.
As Marcel writes, sometimes we just have to show some DCP without the option to reject it or time to have it remade - In those cases we just want to give the audience the best possible experience.
I will try the DCP Harold links to - Quite a neat setup with the automated tests.