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Author Topic: Audio/video sync question  (Read 1550 times)

Trumpetguy

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Audio/video sync question
« on: January 22, 2012, 01:09:43 pm »

I wonder if you guys could explain how a/v sync is handled. 99.9% of my movies are MPEG2 or VC1/h264 in mkvs, and previously I used ConvolverVST, which has a proceesing time and causes audio delay. Sync has thus always been a manual adjustment issue, where video needed to be delayed. I have never really thought how sync was managed without DRC filtering in the signal chain. It seems to me that a/v sync is managed pretty good with no settings from me, even if I am not using VideoClock. When I say "pretty good", I mean that I would probably have adjusted a few millisecs if I bothered, but it does not seem to be identical for all movies, and the out-of-sync is so far within my tolerance limits.

Is the sync good by coincidence, or is this as it should be? If so, how? If not, how should I manage it without using VideoClock (I need fixed and known audio sample rate)?

With the new JRiver convolver, the audio DRC filter is calculated in several x real time, and if I understand correctly, the filtering does not add any time delay to the audio. Right?
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Matt

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Re: Audio/video sync question
« Reply #1 on: January 22, 2012, 10:22:54 pm »

Lip-sync is always handled automatically.

VideoClock decides if the audio or video gets adjusted to maintain sync (so you get perfect sync whether you use it or not).

If your display or audio post-processor has a delay, you can correct for this in Options > Video > Advanced.
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audunth

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Re: Audio/video sync question
« Reply #2 on: January 22, 2012, 10:55:56 pm »

I guess every audio and video stream has a timecode and any media player will align those timecodes together, right?

And what you mean, Matt, is that the default method is to adjust the video to match the audio, and enabling Videoclock adjusts the audio to match the video instead to acheive smooth video?
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Trumpetguy

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Re: Audio/video sync question
« Reply #3 on: January 23, 2012, 12:57:17 pm »

Lip-sync is always handled automatically.
Strangly enough it hasn't occurred to me before that that's the case. Must be because the filter has always created some delay. Thanks.

VideoClock decides if the audio or video gets adjusted to maintain sync (so you get perfect sync whether you use it or not).
Please explain this once more. I get perfect sync whether I use VideoClock or not? Why then use VideoClock? I don't get that. BTW I used VideoClock once. The DRC filter then was fed by an audio stream with varying sample rate. The result was - to put it mildly - strange. I have never before had nausea because of audio, even if it can happen because of certain visual phenomena.

If your display or audio post-processor has a delay, you can correct for this in Options > Video > Advanced.

Sure, that is what I have been doing. The new convolver is brilliant, though, as it does not create any audio delay.
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Matt

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Re: Audio/video sync question
« Reply #4 on: January 23, 2012, 01:17:30 pm »

Please explain this once more. I get perfect sync whether I use VideoClock or not? Why then use VideoClock?

Hopefully this helps:
http://wiki.jriver.com/index.php/VideoClock
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Trumpetguy

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Re: Audio/video sync question
« Reply #5 on: January 23, 2012, 04:28:54 pm »

It did, thanks, I should have checked the wiki first.
Only thing is that the wiki states that the tiny change in audio rate is not audible. That is probably true as long as you do not apply any filtering that requires e.g. FFT/iFFT. Now, anyone using drc/convolver would probably be aware of that, but maybe there should be a note in the wiki telling users about this limitation?
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Trumpetguy

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Re: Audio/video sync question
« Reply #6 on: January 23, 2012, 04:29:45 pm »

I guess every audio and video stream has a timecode and any media player will align those timecodes together, right?

And what you mean, Matt, is that the default method is to adjust the video to match the audio, and enabling Videoclock adjusts the audio to match the video instead to acheive smooth video?
Exactly what the wiki states  :)
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