[mythtv-users] Custom Modeline

[Cory Papenfuss]

On Tue, 22 Jun 2004, Iain Waugh wrote:

>> Total horiz line is 910/14.318e6 = 63.556us => 15.73 kHz (sound familiar?)
>
> Not immediately obvious, but see below.

 	Quite true, but anyone doing this would most likely know that there's a 
difference and what it would be.

>
>> Total vert frame is 525/15.73e3 = 33.36ms => 29.97 Hz (sound familiar?)
>
> NTSC = 60Hz (interlaced) = 30Hz ~= 29.97Hz

 	The 30->29.97 is a (very clever) artifact of going to color.  To keep 
the two color channels in quadrature from clobbering each other in the 
frequency domain, they had to offset the scan rate ever so slightly.  When 
modulated on the video carrier, the inherently sampled spectrum of the horiz 
interlaced sweeps fit nicely in between each other.  Very friggin' clever... I 
knew exactly how it worked once.

  >
> For the rest of the world, the frequencies differ.
>
> Refer to this page for NTSC/PAL/SECAM frequencies.
> http://www.alkenmrs.com/video/standards.html
>
> Previous cut/paste modelines didn't work.  I'm calculating a PAL modeline of 
> my own now...

 	Although you can generate anything you want (provided your vid card 
supports it), I've noticed that 4x the video carrier (3.579545 MHz NTSC) is a 
*very* popular frequency to have as a base frequency on video cards.  That's 
why I'm using 14.318 MHz dotclock.  14.318/910 is exactly 15.73kHz.  For PAL, 
(some PAL formats at least... excuse my ignorance on what the "standard 
standard PAL is), the video carrier is 4.433618 MHz.  That doesn't go nicely 
into the 15.625 kHz horiz frequency (roughly 916 dots at 14.318 MHz).  I'd take 
a look through the X logs and see what you video card can/does do:
(II) NV(1): Clock range:  12.00 to 350.00 MHz
You might even look on the card to see if you can program one directly. 
4*Vc=17.734 MHz... there might be a crystal for that on there.

 	I've also noticed that programming the dotclock doesn't necessarily 
result in exactly what you ask for.  I've tried to get 14.318, and gotten 14.3 
and 14.4 before on different vid cards.  That's not a problem if you're cooking 
up a modeline that's going through a tvout anyway, it's still being "scanline 
converted."  For my application, I needed it *exact*, so the 14.318 has enough 
significant digits to accomplish that.  I also brewed up an ultra-high 
resolution by doubling everything horizonal (28.636MHz, 1440x480)... talk about 
crisp!  Running through s-vid I put up a test pattern and saw 600 lines of 
resolution on the projection TV.  If you're doing this via the SCART direct, 
you'll be able to take advantage of it too.  Running through a TVOUT on a vid 
card means you're stuck with whatever tradeoffs they decided to put into the 
card.



>
> Cheers,
>
> Iain
>

No, Cheers,
-Cory

:)