Need help understanding A/V Relationship

[mb571]

In order to get better at video editing, conversion, and correction, I need help in understanding the relationship between video and audio. This comes up when dabbling in audio sync correction methods. I have a Xvid that has audio sync problems. The video track is 4 seconds longer than the audio track.
Q: How does a media player software handle this? Does it start both at the same time and the video at the end has no audio? Or are the two synchronized and there is video without audio at each end?

Q: When the audio is extracted and re-joined in TMPGEnc, the green audio line (in source range setting) is present throughout the movie but disappears while the last 4 seconds of video plays. Could the audio in the original start after some soundless video and therefore TMPGEnc is not lining up the audio correctly?

It would be nice to know how things work so that I don't have to just follow instructions. Any insite would be appreciated.

Mark

[ZippyP.]

In order to get better at video editing, conversion, and correction, I need help in understanding the relationship between video and audio. This comes up when dabbling in audio sync correction methods. I have a Xvid that has audio sync problems. The video track is 4 seconds longer than the audio track.
Q: How does a media player software handle this? Does it start both at the same time and the video at the end has no audio? Or are the two synchronized and there is video without audio at each end?

Q: When the audio is extracted and re-joined in TMPGEnc, the green audio line (in source range setting) is present throughout the movie but disappears while the last 4 seconds of video plays. Could the audio in the original start after some soundless video and therefore TMPGEnc is not lining up the audio correctly?

It would be nice to know how things work so that I don't have to just follow instructions. Any insite would be appreciated.

Mark

Q.#1. I assume we're talking about playing the Avi. It really depends how the source file was created. The audio could start 4 seconds in or it could be short 4 seconds at the end. Ideally they should match, but in this case it's messed up and there's no way to tell how except by viewing. The audio and video are interleaved (hence AVI) and the syncing is done when the file is created.

Q.#2. When you recombine the audio and video in mpeg form with TMPGEnc, you will be short 4 seconds of audio at the end.


There are ways to correct the problem that you have, but it needs to be diagnosed correctly.

If the audio is out by a constant amount throughout the movie then that is corrected by adjusting the audio start time. Under the Source Range function there is a box called "Audio Gap Correct." The increments are milliseconds (1,000=1 sec.), positive numbers start the audio sooner, negatives start it later.

If the audio starts out in sync at the beginnning of the movie and gets progressively worse towards the end, then the audio duration must be altered. Goldwave has a function called "Time Warp" in which the audio can be shrunk or stretched to fit the video.

A few test encodes (1 or 2 minutes long) will be required to get the adjustments fine tuned to fit. I always do a test encode about 3/4 of the way through the movie before full encoding. It tells me if I have a problem right from the start.

Hope that's clear enough! 8)