[QLab] Audio/Video sync workaround

[Michel Antoine Castonguay]

Hi all,

back in June 2010, it was clearly explained by Sean in this post, that it is technically impossible to play in loop a, say 10 minutes long, audio/video clip all day long (museum installation) without loosing sync between audio and video.

Considering this, we stopped asking us "why are we loosing sync ?" and started asking "how to deal with ?"

We ended up with a "Goto" solution :
- Cue 1 : Audio/Video Clip, auto-stops at end, auto-follow triggering Cue 2
- Cue 2 : Pre Wait 0.2 sec. then Goto Cue 1, auto-continue triggering Cue 1

To fill the 0.2sec. gap between end of the clip and its start, we set a still frame on a lower layer.

Using this, Audio/Video sync is "refreshed" every time the loop starts.
As this might be an interesting workaround for video folks out there, one question remains :
is there any other way to achieve this ?

If not, could we expect any improvement about this on version 3 ?
I must say I'm NOT programmer and don't have any idea of what coding really is.
Even though, in can imagine a check box "Lock audio with video".
Am I dreaming ?

mac

[Keith Smith]

On 20 Jun 2011, at 04:09, Michel Antoine Castonguay wrote:

> We ended up with a "Goto" solution :
> - Cue 1 : Audio/Video Clip, auto-stops at end, auto-follow triggering Cue 2
> - Cue 2 : Pre Wait 0.2 sec. then Goto Cue 1, auto-continue triggering Cue 1

> [snip]

> is there any other way to achieve this ?

Well, if all you have is an a/v file that is just going around and around for ever, another option would be to not use QLab at all and play the file in Quicktime Player with continuous loop turned on.

Just out of interest, does the problem described in the thread apply to you? i.e. an external audio device who's clock may be drifting, or (and more probably from my re-read of the thread) a video stream that was created at 25fps (and muxed with 44kHz audio) and then played back at 29.97fps - which will definitely drift.

Regards,
Keith.

[Michel Antoine Castonguay]

2011/6/20 Keith Smith <keith...@keiths-place.com>

Well, if all you have is an a/v file that is just going around and around for ever, another option would be to not use QLab at all and play the file in Quicktime Player with continuous loop turned on.

If your outputting straight to a monitor on a wall, QuickTime is fine (in fact VLC would be even cheaper).
But for its geometric correction capabilities we choosed QLab.
 

Just out of interest, does the problem described in the thread apply to you? i.e. an external audio device who's clock may be drifting, or (and more probably from my re-read of the thread) a video stream that was created at 25fps (and muxed with 44kHz audio) and then played back at 29.97fps - which will definitely drift.

We use two different audio device models on the expo.
Interrestingly,  the setups unsing MOTU Microbook don't drift while those using MOTU Ultralite do.
The problem is that the Microbbok is not reliable and audio eventually stops playing.
For the rest, we shooted in AVCHD 29,97, logged and transfered in FCP using ProRes 422 HQ 29,97 and exported into H.264 29,97.

Actually, I'm wondering if QuickTime would drift or not.
If not, then there must be a way to prevent QLab doing so.

mac

[Michel Antoine Castonguay]

Forgot to mention : audio was recorded and is playing back at 48kHz 24 bits.

mac

[Christopher Ashworth]

On Jun 19, 2011, at 11:09 PM, Michel Antoine Castonguay <mantoi...@gmail.com> wrote:

could we expect any improvement about this on version 3 ?

Yes, in the future we should have video syncing to the selected audio clock rather than just he internal clock.

-C