If you've ever experienced audio sync problems where the audio / video syncronisation is fine at the beginning, then becomes so out of sync by the end of the movie that it's unwatchable, I believe I've stumbled upon an extremely simple fix. I've spent countless hours over the past year searching for a simple, quick, and permanent resolution to this problem and was unsucessful. So don't flame me if my fix has been posted before... I'm just trying to help out the encoding community.
I've performed the following fix on DivX and XviD files (avi's), but it should work on any file that can be opened with VirtualDub.
STEPS (this is NOT difficult... only well explained ;-)
1. Determine "exactly" what the frame rate is on the problematic movie. There are many methods, however this is what I do:
a) open the movie in DivX Player
b) right click in the video window
c) click "File Information..."
d) the frame rate will be clearly displayed as something like "29,96999 fps"
*** Assuming the video is ahead of the audio at the end of the movie...
2. Open Virtual Dub (I'm running version 1.6.11 build 23774)
3. Click "File" >>> "Open video file..."
4. Navigate to the movie (AVI file) that has the audio that's out of sync and open
5. Click "Video" >>> "Direct stream copy"
6. Click "Video" >>> "Frame rate..."
7. Click the radio button beside "Change to ____ frames per second"
8. Enter a number in the box that is about 2 / 1000's lower than determined in step 1
(for our example 29.970 - 0.002 = 29.968)
9. Click "OK"
10. Click "File" >>> "Save as AVI..."
* In a 2 hour movie, decreasing the frame rate by 0.002 in step #8 will move the video 0.5 seconds behind the audio by the end of the movie.
As you can see the new AVI is saved in a matter of 2 minutes!!! Now open the new AVI and see if the audio video sync problem is corrected. If the audio is still lagging behind the video go back to step #8 and further decrease the frame rate.
I'm not sure exactly what method Virtual Dub is using to change the frame rate, however I can tell you that since the new movie is resaved in two minutes that it's certainly NOT "re"encoding the video... so don't concern yourself with quality loss. If it's dropping video frames to make up the frame rate difference, that works out to only 14 dropped frames in the span of 120 minutes. Even the most discerning eyes would not notice a dropped video frame every 8:20 minutes!
Please test for yourself, verify that it works and spread the word!
