leonid_makarovsky
14th January 2005, 00:48
I don't see many posts about A/V sync anymore. So I guess somehow folx don't have problems with it. The question is how?
My solution is pretty simple. I capture a 2 hour long video tape without worrying about gradual A/V dissynchronization. Then when the capture is done, I capture the very end of the footage for like 30 seconds (with the sound of course). I then load both AVI files into VirtualDubs and find the same frame from both footages. From this frame forward (like 15 seconds forward) I create 2 AVI files from these footages and load them into sound editor SoundForge. I notice that my gradual loss of sync is 200ms (audio is behind). Then I just copy the WAV file from original footage and squeeze it by 200ms. I captured about 10 times and the pattern was precisely the same. I then observed very interesting things:
The setup I originally used:
Soundcard: M-Audio 2496
VCR: JVC
Then I tried another VCR - Samsung. My A/V sync was perfect. Then I changed my soundcard to Terratec Aureon Universe and tried the first VCR JVC. My A/V was in perfect sync (and I did about 4 captures with such setup). And then I tried Samsung with Terratec soundcard. Now I got audio ahead of video for like 200ms. So I came to conclusion that A/V sync really does depend on Soundcard quartz clock and VCR speed. The VCR may play at slightly different speeds and the video doesn't have to perfectly 25fps or 29.976fps. However, on computer it is still stored at correct 25fps or 29.976 interval. But the audio is sampled differently depending on speed - thus there are out of sync issues. Correct me if I'm wrong.
Another thing I noticed when I was capturing from LaserDiscs with S/PDIF in (either coaxial or optical) for the sound. The sound sample rate was 44.1kHz to match the digital sound on LaserDisc. Both sound cards had the same pattern. The sound was in almost perfect sync. Well the difference is 13ms after 1 hour of capturing. I guess on LaserDisc the number of video frames and the number of audio samples match. So there's sort of direct copy of video frames and aud audio samples to the hard drive (soundcard doesn't sample it - it's just a pass through). Thus you get the same sync as the source.
Just my observation.
--Leonid
My solution is pretty simple. I capture a 2 hour long video tape without worrying about gradual A/V dissynchronization. Then when the capture is done, I capture the very end of the footage for like 30 seconds (with the sound of course). I then load both AVI files into VirtualDubs and find the same frame from both footages. From this frame forward (like 15 seconds forward) I create 2 AVI files from these footages and load them into sound editor SoundForge. I notice that my gradual loss of sync is 200ms (audio is behind). Then I just copy the WAV file from original footage and squeeze it by 200ms. I captured about 10 times and the pattern was precisely the same. I then observed very interesting things:
The setup I originally used:
Soundcard: M-Audio 2496
VCR: JVC
Then I tried another VCR - Samsung. My A/V sync was perfect. Then I changed my soundcard to Terratec Aureon Universe and tried the first VCR JVC. My A/V was in perfect sync (and I did about 4 captures with such setup). And then I tried Samsung with Terratec soundcard. Now I got audio ahead of video for like 200ms. So I came to conclusion that A/V sync really does depend on Soundcard quartz clock and VCR speed. The VCR may play at slightly different speeds and the video doesn't have to perfectly 25fps or 29.976fps. However, on computer it is still stored at correct 25fps or 29.976 interval. But the audio is sampled differently depending on speed - thus there are out of sync issues. Correct me if I'm wrong.
Another thing I noticed when I was capturing from LaserDiscs with S/PDIF in (either coaxial or optical) for the sound. The sound sample rate was 44.1kHz to match the digital sound on LaserDisc. Both sound cards had the same pattern. The sound was in almost perfect sync. Well the difference is 13ms after 1 hour of capturing. I guess on LaserDisc the number of video frames and the number of audio samples match. So there's sort of direct copy of video frames and aud audio samples to the hard drive (soundcard doesn't sample it - it's just a pass through). Thus you get the same sync as the source.
Just my observation.
--Leonid