View Full Version : Vidcap A/V Synchronization Boggle

03-08-2009, 07:19 PM
I have a wierd problem that evades solution. Regardless the window size, frametrate, quality, codec, etc. I always experience A/V de-synchronization when using video capture software.

The source is Flash in a browser. It is seemingly without fail that a 21 minute video will have an audio stream 8 seconds shorter than the video. I can't reproduce this with RVC as any corresponding issue in a one minute eval video is sort of imperceptible.

Continuous audio recording with Audacity in the background during a capture also results in the same issue. A crop before the first sound and after the last sound for a 21 minute video episode and the resulting audio is about 8 seconds shorter than the video captured during the same period.

What could this be and how to fix it?

The dirty fix now is "change video framerate so audio and video durations match." This does result in A/V synchronization throughout the length of the video.

Cheryl Wester
03-09-2009, 07:02 PM
This is a common issue that people often have as you have determined. A suggestion is to try using windows media instead of MPEG 2 or vice versa and see if that makes a difference. You make this change in the settings of the program.

03-09-2009, 11:02 PM
Wow, I thought I had some unique problem. Sometimes these are very hard problems just trying to come up with the terms to use in a proper Internet search.

try using windows media instead of MPEG 2 or vice versa

Yes that did help. Using VC-1 WMV3 is less CPU intensive and produces better results. Probably an XP MCE is suited for WMV and I never use it. I recall the MPEG-2 encoder for RVC was very swift and a better choice when available. But I have a Realtek chipset / MCE that doesn't play as well as it could with RVC. So I am on my own at the moment. I'm not using that other company that shall not be named's very expensive capture software.

Also I went to capturing a frame every 40 milliseconds, interleaving PCM audio every 40 milliseconds, with a keyframe every 125 frames. I was trying more standard 29.97 and 30 fps before.

What I have should work out to 25 fps and a pretty standard audio interleave at every frame. I may have missed the math, but this seems compatible as I'm shooting at AVI hardware players. I will need one transcode regardless. So the PCM at this step also is easier on my bottlekneck, the CPU. Big expendable, capable HDD space using SATA and RAID 0 makes this workable too.

Transcodes are a breeze all done overnight and results look presentable.

So for the moment my problem is gone. Thx!