[mythtv-users] How I got great quality TV-out on my nVidia MX4000

Will Dormann wd at pobox.com
Wed Mar 9 15:14:11 UTC 2005

Brian J. Murrell wrote:
> On the issue of hardware encoding and whether it preserves the
> interlacing, I do believe the PVR-250 I have in my machine does indeed
> preserve the interlacing in the MPEG2 stream it creates.

Yes it does.  The problem is, no video card appears to be able to do 
TV-Out while retaining that proper interlacing information.  The instant 
the card does any sort of scaling of the picture (to do overscan, for 
example), the interlacing will no longer match up with the TV scan lines.

Bob deinterlacing makes up for this by converting each field to a frame 
and then sending ~60frames per second, rather than 60fields per second. 
  This retains the smoothness of the original recording, but not 
requiring fancy output where everything lines up right with the interlacing.

The PVR-350 has dedicated hardware to output the video such that the 
interlacing matches up with the TV's scan lines, as that is what it was 
designed to do.  A video card with TV-out, however, seem to be designed 
to output your computer desktop (or games or whatever) at various 
resolutions and have them all look relatively OK.

I think I've probably used some wrong terminology here, but I think I've 
got the gist of things right.


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