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Old 22 Apr 2002, 11:29 AM   #1
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Default Virtual Dub and DivX 5.0.1

Ok. Here's the deal. I was running an older version of Virtual Dub. Something like 1.4.8. I took several movies and used Virtual Dub to save segments of these movies. I encoded using DivX 5.0.1.

I went to the Virtual Dub site recently and they had version 1.4.10 and it spoke of correcting a problem with audio sync and DivX 5.0.1. Now I'd checked my output but not all the way to the end with scrutiny so I did this time and sure enough they slowly go out of sync.

I'm currently running 1.4.10 and I'm trying a video to see if it no longer has this problem, but my problem is that I deleted the originals of the segments.

Is there any way to fix the audio sync problems that were caused by a problem Virtual Dub had with DivX 5.0.1's B frames or something like that? Or am I just SOL and chalk it up to experience and move on?
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Old 22 Apr 2002, 12:13 PM   #2
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To the best of my knowlede using B-frames in divx5 can't cause the audio to slowly go out of sync, there is a constant one frame delay but AFAIK the DirectShow filter will delay the audio by a similar amount of time.

So far I have done about 10-15 movies with divx5 and divx5.01 I have yet to see any sync problems.
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Old 22 Apr 2002, 05:36 PM   #3
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OK, try a different encode like vidomi.

I am not ditching Virtualdub, just to see if it is a bug in there or something.

No harm trying other alternatives, right?

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Old 22 Apr 2002, 07:35 PM   #4
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I'm not video saavy. I'm still a newbie so I'm still learning. I'm pasting what was on the Virtual Dub site whic explains my situation. I just verified that the problem no longer exists. I encoded a 42 minute segment and checked and the audio stayed in sync. So now I have to figure out whether to just leave the other encoded stuff as is or if there is a way to fix it. I don't know how though. If it's lagging behind at a constant interval how would I be able to tell?

Anyway here's what the Virtual Dub site had to say.

DivX 5.0 introduced B-frames into the system without any knowledge or intervention from client applications. The problem is that it used frame delay in the decoder. This severely broke VirtualDub's video decoding (decoding forward and backward across the same keyframe would get you different images from the codec), and also caused the video to lag behind the audio. In 5.0.1, the behavior was changed so that the decoder does not delay frames at all, but the encoder still "eats" several frames before beginning to compress. This lag gives the codec the future frames it needs to encode B-frames. What VirtualDub 1.4.10's B-frame support does is skew the audio forward to match the delayed video, and then kills the dummy frames generated at the beginning of the video stream by the codec ramp-up. This is the same scheme used by client applications that supported Indeo 4 bidirectional prediction, incidentally.
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Old 23 Apr 2002, 01:21 AM   #5
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This seem to be pretty much inline with what I said, there will be a 1 frame delay when encoding with vdub lower than version 1.4.10. Any other sync problems are most likely caused by something else.

You can delay the audio in an avi file, with virtualdub, select direct streamcopy for both video and audio, and select audio->interleaveing and set the desired audio delay.
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Old 23 Apr 2002, 04:16 AM   #6
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Go here and check out fixing audio sync problems. There is a tool AVIinfo that can change the frame rate in very small increments to keep your file in sync. Has worked great for me in the past.

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http://nickyguides.digital-digest.com/audio-synch.htm
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Old 23 Apr 2002, 08:21 AM   #7
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Thanks for the help guys.

khp
I'm trying your method. But how do I tell what interleave settings I should use. Is there an accurate way or do I just have to play it and sortof guess and try and see?
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Old 23 Apr 2002, 08:47 AM   #8
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Quote:
Originally posted by Stormblade
khp
I'm trying your method. But how do I tell what interleave settings I should use. Is there an accurate way or do I just have to play it and sortof guess and try and see?
Sync issues are never easy, my primary way of dealing with it, is to avoid it . I use GordianKnot as descriped at www.doom9.org, and have never had any problems.

You need to figure out excactly how your audio is out of sync, if it's a constant delay or if it slowly goes of sync. But everything is covered in great detail this guide http://nickyguides.digital-digest.com/audio-synch.htm
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Old 23 Apr 2002, 08:56 AM   #9
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*nod* I'm leaning towards leaving the ones I did alone. Reason being the sync isn't off by too much and I have a hard time telling with some of them....not alot of talking It's why I missed the audio sync problem before. So it's just a minor annoyance.

At least now I know that I won't be causing any audio sync problems now because Virtual Dub 1.4.10 and Divx 5.0.1 seem to work fine. I tested it and the audio sync was just the same as the original so I'm good.

Heh I figure if its so difficult for me to see how the sync is off it's not that big of a deal. From what I saw though it appears just what we said. The video lags behind the audio but in the beginning I have to look really hard to see it and towards the end I can see it clearly.
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Old 23 Apr 2002, 08:56 AM   #10
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One other thing, make sure you have set you postprocessing to a the lowest possible setting.
In WMP6.4 select file->properties->Advanced->'Divx Decode Filter'->Properties and move the quality slider to the left.

At max quality with B-Frames and GMC and Q-pel turned on you would need a 2GHz processor for smooth playback.
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Old 23 Apr 2002, 09:29 AM   #11
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Well it wasn't on max. It was on default setting. I have a 1.1 ghz machine and it plays back smooth enough. Does this setting affect just the video's playback or all videos?
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Old 23 Apr 2002, 09:36 AM   #12
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Quote:
Originally posted by Stormblade
Well it wasn't on max. It was on default setting. I have a 1.1 ghz machine and it plays back smooth enough. Does this setting affect just the video's playback or all videos?
The default setting is whatever it was set for when the video was encoded, I usually set mine at zero.
This settings is relevant for all divx movies. Try checking your cpu load durring playback, if it ever reaches 90% you should think about using a lower postprocessing quality level.
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