[mythtv-users] Interlaced vs. noninterlaced: getting perfect output smoothness

[Cory Papenfuss]

On Thu, 14 Apr 2005, Sasha Z wrote:

> Yeah, I've tried using xvidtune, but since it couldn't control the
> clocks, it was rather useless. The positioning of the screen is
> superb. I'm using one of the NTSC modelines floating around on a
> javascript calculator page, and have been adjusting the dot clock to
> try and get the refresh just right. I'm constantly getting closer and
> closer, but never seem to be able to hit the magic number.
>
> I'm assuming that getting an HDTV with a digital progressive input
> would make this job oodles easier, no?

 	It sounds like you're doing what many people do with TVout 
cards... try to tweak the modeline to make a "perfect" NTSC modeline. 
Bottom line is if you're using a TVout card, it is *always* producing a 
"perfect" NTSC modeline... it's just that there isn't a 1-to-1 
relationship between the one it's using and the one you program up.  The 
TVOUT chip is doing funky spatial and temporal scaling to make the 
tv-happy modeline roughly similar to the computer-happy modeline.

 	If you're trying true, NTSC-frequencied modeline, 98% of the ones 
you find on the 'net are incorrect.  They might be happily 2x the rate (as 
is standard, 640x480 VGA resolution), but they are *NOT* NTSC rates.  A 
computer monitor will not sync to NTSC rates.  Basically you do not have 
the ability to truly configure what you want, so your "tweak until it's 
good enough" is probably the best approach.  I'd try one that is as close 
to exactly 480p as you can, but you still might get beat frequencies. 
Something like the NTSC modeline I use with my homebrew hardware:

ModeLine     "coryntsci" 14.318 720 760 824 910 480 484 492 525 interlace

 	... converted into a 480p version:

ModeLine     "coryntscp" 28.636 720 760 824 910 480 484 492 525

 	... and adjusted slightly since NVidia's drivers have certain 
"divisible by 8" requirements:

ModeLine     "coryntscp" 28.6989 720 760 824 912 480 484 492 525

 	... yields 28,698,900/912 = 31.468kHz = 2 * 15.734 kHz

 	That works if you have a 1-1 relationship between the actual 
modeline you program, and the one that's sent to the tv.  Since you don't 
all bets are off, and it very well may have little to no effect... except 
possibly blowing up your TV, monitor, computer, living room, house, 
family, pets, cars, etc.  YMMV... :)  Seriously, though... the 480p 
modeline should work find on a PC monitor... whether or not it maps over 
via the tvout part of the card is a whole 'nother kettle of fish.

-Cory

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