[mythtv-users] nuvexport cut only with synch problem

[Cory Papenfuss]

 	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:

> 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.
>
 	Sounds about right.  That's what I had while copying from 8mm 
family video tapes.

> 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.
>
> The remaining 3 VHS films are almost in synch.
>
 	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.

> 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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