Re: [QLab] Video and Audio falling out of sync
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Keith Smith
Dec 1, 2012, 3:01:58 PM12/1/12
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There are a few things that can cause this but the two most likely are
1) A bug in the codec
2) Failure or inability to resync after frame drop(s)
and of those two the most common would be the second, usually triggered by a performance bottleneck somewhere. Interestingly, modern containers (eg: mp4 and mkv) are slightly more prone to this than older ones (eg: avi and mpg) due to the way the streams are handled internally.
You say that you have a Mac Pro but you also mention Thunderbolt. Given that there are no Mac Pro's with Thunderbolt I am going to assume that you have a recent Mac Pro with the Mini DisplayPort - which will use the same adapter you mention. This would also mean that, all things being equal, you have plenty of grunt at your disposal, which in turn suggests the data pipeline is close to a component limit somewhere and that you have another process running on the machine which steals time from this component occasionally, which gives you your random problem.
You can probably get around the problem by lowering the streams data rate and/or making sure that nothing else is running on the Mac. But just in case, some questions:
* What are the audio and video codecs?
* What is the resolution of the video encode?
* What is the sample rate of the audio encode?
* What is the projection resolution?
* What is the audio and video bit rates - or if you don't know what is the file's size?
* Is the video file on an internal or external disk?
* If internal: is it the same one that the operating system is on?
* If external: how is it connected?
Regards,
Keith.
On 1 Dec 2012, at 16:18, AMendelson wrote:
> Dear all,
> Having an inconsistent problem with video and its embedded audio falling out of sync (ie the lips move at a different time than the word is heard). What is mostly odd is that it seems to come and go as a problem. Same system running Thursday's performance no problems - Friday had sync issues. Any thoughts?
>
> MacPro
> Echo Audio fire Pre8 running firewire 400 from firewire 800 port
> Word clock to Audiofire coming from Yamaha DM1000 at 48k
> Video over thunderbolt to HDMI adapter
1) A bug in the codec
2) Failure or inability to resync after frame drop(s)
and of those two the most common would be the second, usually triggered by a performance bottleneck somewhere. Interestingly, modern containers (eg: mp4 and mkv) are slightly more prone to this than older ones (eg: avi and mpg) due to the way the streams are handled internally.
You say that you have a Mac Pro but you also mention Thunderbolt. Given that there are no Mac Pro's with Thunderbolt I am going to assume that you have a recent Mac Pro with the Mini DisplayPort - which will use the same adapter you mention. This would also mean that, all things being equal, you have plenty of grunt at your disposal, which in turn suggests the data pipeline is close to a component limit somewhere and that you have another process running on the machine which steals time from this component occasionally, which gives you your random problem.
You can probably get around the problem by lowering the streams data rate and/or making sure that nothing else is running on the Mac. But just in case, some questions:
* What are the audio and video codecs?
* What is the resolution of the video encode?
* What is the sample rate of the audio encode?
* What is the projection resolution?
* What is the audio and video bit rates - or if you don't know what is the file's size?
* Is the video file on an internal or external disk?
* If internal: is it the same one that the operating system is on?
* If external: how is it connected?
Regards,
Keith.
On 1 Dec 2012, at 16:18, AMendelson wrote:
> Dear all,
> Having an inconsistent problem with video and its embedded audio falling out of sync (ie the lips move at a different time than the word is heard). What is mostly odd is that it seems to come and go as a problem. Same system running Thursday's performance no problems - Friday had sync issues. Any thoughts?
>
> MacPro
> Echo Audio fire Pre8 running firewire 400 from firewire 800 port
> Word clock to Audiofire coming from Yamaha DM1000 at 48k
> Video over thunderbolt to HDMI adapter
AMendelson
Dec 1, 2012, 8:46:19 PM12/1/12
to ql...@googlegroups.com, keith...@keiths-place.com
Keith,
Thanks for your reply. I am away from the computer until Monday and away from all of that information, but I will get back to you on Monday with all of it.
When I used Qlab for video previously, I have gone through a lot of the considerations that you mention and, when I have had a problem, it always seemed to be a consistent one. The confusing part for me right now is that the "bottleneck" comes and goes. Some performances are flawless, some are completely out of sync and some are just barely off. This is why I cannot get my head around what is happening.
In either case, I will get you more information about your considerations on Monday.
Thanks for the reply,
Adam
Thanks for your reply. I am away from the computer until Monday and away from all of that information, but I will get back to you on Monday with all of it.
When I used Qlab for video previously, I have gone through a lot of the considerations that you mention and, when I have had a problem, it always seemed to be a consistent one. The confusing part for me right now is that the "bottleneck" comes and goes. Some performances are flawless, some are completely out of sync and some are just barely off. This is why I cannot get my head around what is happening.
In either case, I will get you more information about your considerations on Monday.
Thanks for the reply,
Adam
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