As someone else pointed out, there has been a plausible explanation (by me and a few others) awhile back on why this is so. Basically, when the audio and video streams are multiplexed together into program streams, there's some additional sync information with finer granularity than a single number. The PTS (presentation time stamp) and DTS (display time stamp) can change throughout the stream. Players are supposed to honor this, but avidemux does not. It can only deal with static A/V offsets.
On Thu, 14 Apr 2005, Niels Dybdahl wrote:
Sounds about right. That's what I had while copying from 8mm family video tapes.Hi,
I have started using nuvexport (using mpeg2cut v1.6) to cut my MPEG2 (PVR-250) files for archival. However around half of them (5 out of 12) have the audio significantly out of synch with the video.
All 5 with synch problems are copies from VHS tapes. The uncut PVR-250 files do not have any synch problems.What did you use to play them? Players like Mplayer honor the PTS/DPTS, so the original seems to play fine.
All 4 films that were taken from the air do not have any synch problems after nuvexport.As far as I can tell, when the ivtv chip gets a glitch in the video, it tends to fix broken frames by just adjusting the PTS/DTS offset between the video and audio. Broadcast video is required to be of very good quality, so frame drops are very infrequency. VCR's OTOH, tend to suck royally when it comes to the quality of the video they output. It's typically not perceivable by people, but the line/frame timings are horrid from a computer sampling standpoint.
The remaining 3 VHS films are almost in synch.
Can the cause be the lower quality of the VHS recordings ? Should I record from VHS with different settings ? (e.g 288 lines instead of 576 lines ? Or with higher bitrate ?). Are there any parameters in nuvexport that can be adjusted to solve the problem ?Not likely. When you capture with fewer lines than the source has, you introduce all sorts of ugliness as the card interpolates between scanlines. It's much better to reduce the *horizonal* bandwidth requirement if you want to retain video quality.... aspect ratio for video doesn't have to have square pixels. For an NTSC example, if you can't capture at 640x480, you'll get much better quality capturing at 480x480 or 360x480 than at 480x360.
The bottom line is that nuvexport uses avidemux to process MPEG2 captures. Avidemux is broken when it comes to dealing with MPEG streams with changing sync. What's needed is a cutter that doesn't demux the A/V streams... something like GOPchop or gopdit.
-Cory
************************************************************************* * Cory Papenfuss * * Electrical Engineering candidate Ph.D. graduate student * * Virginia Polytechnic Institute and State University * *************************************************************************
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