[mythtv-users] Hardware for Best TV-Out?

Cory Papenfuss papenfuss at juneau.me.vt.edu
Sun Oct 31 13:12:21 UTC 2004

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

On Sat, 30 Oct 2004, Shawn wrote:

> I would be honored to have your help if you'd give it. I do indeed have
> my GeForce FX 5200 hooked into the A960 doing 640x480 from Xorg via
> nvidia-kernel-1.0.6111-r2 on gentoo.
 	So it's being run via component.  You really should check the 
specifications on the TV to see what frequencies it likes on the component 
input.  You can damage some devices if you feed them the wrong frequency. 
*SO*... any modelines that I mention here should be used on the 
understanding that if you blow up your TV from feeding it the wrong 
signal, you were warned and it wasn't my fault... :)

> I can't do higher resolutions,
You're not telling it to use "TVOUT" or anything, right?  That will limit 
your resolutions that the VGA card will do.

>and I seem to get a little pincushion
That's a monitor/TV problem, not a modeline problem.

> (right side of screen curving inward) and the lower edge of my screen is
> off screen.

That's a monitor/TV problem (overscanning).  It can be band-aided by using 
an underscanning modeline, but it's a TV geometry adjustment.

> I have the following modeline, which is the only one I could get to work
> in powerstrip:
> "640x480" 25.18 640 704 800 864 480 483 489 524 +hsync +vsync
 	That's only 29.1kHz... a little too slow to be 480p resolution. 
You should start with a standard mode for VGA as given by

e.g 640x480 at 60Hz
Modeline "640x480"   25.17   640 656 752 800   480 491 493 524  -hsync -vsync
25,170,000 / 800 = 31.4kHz

Then, if you need more underscanning, change the dotclock and the 
horizonal line width by the same percentage and you'll get another 
modeline with the same frequencies, but different widths of black at the 
edges.  Strictly speaking, you should adjust the sync widths as well, but 
practically most monitors don't care so much about it, and you end up 
wanting fracional pixels or something you can't do anyway..  For example, 
adjust for 5% underscanning:

25.17*1.05	 = 26.428
800*1.05	= 840
Modeline "640x480"   26.428   640 656 752 840   480 491 493 524
26,428,000/840 = 31.46kHz

That'll put lots of black on the left side of the screen, but keep the 
frequency the same.  To move the picture to the left side, add some (the 
same amount) to the sync-describing number, keeping their difference the 
same.  This adjustment will split the underscanning (which added 40 pixels 
to the line count) equally on the L+R side:
656 + 20 = 676
752 + 20 = 772

Modeline "640x480"   26.428   640 676 772 840   480 491 493 524

Remember, xvidtune does *NOT* change the dotclock.  It will only move the 
visible around on the screen.  Also, some video card drivers (e.g. NVIDIA 
for one) enforce some constraints on the width of the pulses to be 
multiples of 8, etc.  All you can do then is try to get one that's close 
that the driver will use.

To change the visible horizonal resolution, you can do the same thing.  If 
it's more than a few percent like the previous example, however, you 
really need to scale the sync-related ones as well.  I'm not sure why 
you're using 640x480, but if you're looking for something higher for 480p, 
you can try a 720x480 with 480p timings e.g.

ModeLine     "480p" 28.6 720 760 824 910 480 484 492 525
28,600,000/910 = 31.42kHz (see a pattern here?)

If you want higher resolutions than 480p, you'll need to find out what's 
acceptable for your TV.  I don't have one that'll do any higher than 480p 
so I haven't looked into it.  540p, 720p, and 1080i should all be doable 
if the TV does it, but I won't presume to know anything about the 
modelines to use.


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