Saturday, July 19, 2003

Audio/Visual Sync Manual Method

Captured an 18 minute clip (720x480 YUV2 PicVideo MJPEG Q19 48kHz stero audio no compression) and I'm getting some slip in the audio (it's just a touch ahead of the video by the end). Looking at my capture info file, I see that I did have some dropped frames, but also that the audio rate isn't quite exactly 48kHz:

Capture Stats
Frames Captured: 34039
Frames Dropped1: 0
Frames Dropped2: 10
Time : 00:18:56
Time Left: 01:02:58
Free Space: 11,895,775,232
Video Bytes: 3,359,814,888
Audio Bytes: 218,194,216
Total Bytes: 3,578,009,104
Video Bytes Sec: 2,956,279
Audio Bytes Sec: 191,987
Total Bytes Sec: 3,148,267
Video Compression: 7.003
Video Rate: 29.976161
Audio Rate: 48003.631317 (0.00756%)
AV Diff: 0.000130
AV Adjust: 0.000000
AV Actual: 0.000000

According to VDub, the video is 1136.146 sec in length, so an estimate that I need to make the audio track 0.00756% longer results in adding 85ms to the length of the audio, making the length 1136.232 sec. The question is, is that going to be close enough of a guess for bigger tasks.

After testing with adding 150ms to the clip, which resulted in the audio lagging the video by a tiny amount, I re-stretched using the 86ms number (1136.232) which should put the stetch spot-on. So using the Audio Rate number as a quick-n-dirty calculation seems to work well (going to try a longer clip and see).

posted by Wuphon's at 12:07 PM