Video Sync

[Richard Haley]

New User - I'm putting together a sizeable Musical show consisting of prerecorded music, pre-recorded conductor on video for the cast only to see, and some creative video for the audience to see. I have put a video test cue at the beginning, so we can time align our audio to the video system a venue so the conductor is spot on timing - with the thought to add the audio delay in the Mixing desk, not Qlab. My surprise was that playing the test video through Qlab, monitoring it on the built-in screen on my Mac screen introduces a 00:00:20 delay in the video.  This video delay is not there when I just play the video outside of Qlab on the same computer (13"Pro m1).  I can fix this by pre-delaying the click audio by the same amount, but as this is my reference, does that mean I should add 00:00:20 predelay to all my audio tracks before it leaves Qlab? Thanks

[Chris Ashworth]

Hi Richard,

Thanks for your email.  A couple of initial questions for you:

- How are you measuring the delay?

- Which version of QLab are you using? The latest v4 is 4.7. The latest v5 is 5.0.11

- If you are using version 5, is your video cue following the video clock or the audio clock? This setting is available in the I/O tab of the inspector: https://qlab.app/docs/v5/video/video-cues/#the-io-tab 

- If you are using version 5, does the delay change if you change which clock the cue is following?

Best,

Chris

[v27rjh]

Hi Chris, I have the video click with its own audio click as 1 video cue, I have also detached the audio from the video and run it as a separate audio cue.  If I run them together, you can hear the delay between the two click tracks.  I am running the latest v5 version.  I didn't know about the clock - I've tried both but it makes little difference, but I guess setting all the video to audio clock would be a wise move anyway. Thanks

[v27rjh]

Sorry Chris, running the video from the audio clock has solved it!  Thank you Awesome

[Chris Ashworth]

Thanks Richard,

I’m glad this helped, and I’m also glad to know about the measurement you made comparing the two — I’ve filed your observation in our tracker to investigate a little more. While the audio will necessarily have more variability when following the video clock, it sounds like there may be room for improvement in how we sync the two, so thanks for this.

Just wanted to confirm your measurement of the delay — you wrote about a 00:00:20 delay which to my eyes looks like 20 seconds, but I assume you mean a smaller delay?

Best,

Chris

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[Richard Haley]

00:00.20 was the pre-delay I added to the audio to realign - so I think this is 20ms?

[mackerr]

Don’t forget that for every 30’ of audience depth there is about 30ms of audio “time of flight” delay relative to video. Once you are in the theater you have to decide where you want the sync to be dead on, and where you need to fudge a little early and a little late.

Mac

[micpool]

There are 1000ms in a second  so a 20ms delay would be 0.020. 0.2s is 200ms, which unless you are in an arena, is a very large delay

Mic.

[Richard Haley]

The fix in Qlab was 00:00.20. Running the video from the audio clock fixed it perfectly :-)

To view this discussion on the web visit https://groups.google.com/d/msgid/qlab/a5107de7-6dfd-433a-9ce9-93c120068309n%40googlegroups.com.