audio in video track to keep sync

[mic]

Hi,

I read two video tracks better keep sync if both have an audio track coded. Idid, but I notice after some minutes they are out of sync anyway.  Is it ok if i keep -INF audio level of one of the tracks cue? Should it guarantee sync, or is there a minimum audio level I should set both track to get desired sync?

regards

michele

[Chris Ashworth]

Hi Mic,

Synchronization is a tricky topic. Backing up a bit:

QLab lives in an environment with many independent clocks: the computer clock, each audio device, etc.

If a video file has no audio track, QLab delivers frames of the video based on the computer’s internal clock.  If two video files both have no audio, then they are both following the same clock.

If a video file has an audio track, QLab delivers frames of the video based on the audio device’s clock.  Thus, if you have one silent video and one video with audio, they will be following different clocks and could drift apart during playback.  This is why adding a silent audio track, and assigning the audio output device to be the same audio device for both cues, can affect synchronization of different video cues. 

The volume level of the audio does not matter, just the underlying clock (computer or audio device).  The synchronization will only be improved based on whether the cues were previously following different clocks and now follow the same clock. Adding the audio track therefore does not necessarily improve synchronization. (e.g. two videos with no audio track were already following the same clock, and thus were already synchronized.)

Thus, if two videos are out of sync, and depending on what version of “out of sync” you mean, things that might be happening:

1) they’re following different clocks 

2) they were started at different times and are not drifting but are also not in sync

3) the computer is under heavy load and is unable to keep up with both files fast enough to render them in sync

4) the files can’t be synchronized because of how they were generated (i.e. they are playing in sync but something about them doesn't line up because of how they were created)

5) other?

Best,

C