[mythtv-users] Re: Anyone tried MPEG2 to MPEG2 transcoding?

Cory Papenfuss papenfuss at juneau.me.vt.edu
Thu Jul 21 13:41:59 EDT 2005

> On Thursday 21 July 2005 17:09, Cory Papenfuss wrote:
>> The problem (as I've painfully discovered and discussed before) is
>> that the ivtv cards change the audio and video relative timestamps (PTS,
>> DTS, etc).  That information is at the PS level, and avidemux blindly
>> strips PS into A and V.  If the sync offset isn't constant, it gets off as
>> the clip progresses
> have you asked the ivtv bods if they could "fix" this?

 	I haven't but it's not the fault of the driver.  I believe that 
the chip itself does it when it gets a bogus packet.  Basically, since 
everything is a convoluted mess of interleaved audio and video frames, 
when a video frame comes up short (like an analog glitch in a VHS tape 
capture for instance), it's much easier to just throw that frame away and 
change a time offset.  All of the other frames are still in buffers and 
don't need to be messed with.

 	... or at least that's how I rationalize it.  If the ivtv driver 
were to look for this, it would basically have to do everything we're 
talking about, but in the driver kernel space (not good).  Basically, the 
streams produced aren't incorrect, they're just a little more complicated 
than the simplistic streams that 95% of the linux software mpeg utilities 
out there expect



* Cory Papenfuss                                                        *
* Electrical Engineering candidate Ph.D. graduate student               *
* Virginia Polytechnic Institute and State University                   *

More information about the mythtv-users mailing list