[Openchrome-users] VT1625 NTSC Modes

[Ken Huisman]

On Thu, 2006-02-11 at 09:47 -0600, Joel Means wrote:
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> I am looking for a little help.  I have tried to patch the via_vt162x.h
> file to add a mode for 640x480Over and to fix the 640x480 mode for the
> TV-out on my EN12000 board (CN700 chipset).  So I copied the register
> values from Via's code into the Openchrome code.  Then I copied the
> modeline from the vt1622 table.  The output on S-Video is close to
> right.  It is offset quite a bit to the left, but I know that is just a
> modeline problem that I should be able to tweak.  The other issue I have
> is that the image jumps up and down rapidly.  It almost seems as if the
> A and B fields are being drawn in different places.

I noticed this same problem when I was messing with the modelines for
720x480Over.  I haven't figured it out yet.  I tried messing with the
value in register 9 (the vertical start position) but it only made
matters worse.



>   Now, I don't know a
> whole lot about what all of the register values do and I have read the
> Xf86 guide to modelines, but that is all a bit fuzzy still.  So my
> question is, is the jumpy problem a modeline issue or a problem with the
> registers?

I'm not sure.  I've been reading about modelines a bit, and I've tried a
few ways to come up with ones that make sense.  Here are some things I
have learned, hopefully it will help:

1. NTSC TV is a 525 line signal.  You'd think that the last entry in the
modeline would be 525.  In fact, since NTSC is a standardized signal,
you'd think there would only be one valid modeline for it.  Is there a
definitive NTSC TV modeline?  I haven't found one yet.

2. there is a file in the via framebuffer driver called viafb.modes.
from the information in that file and from the information here:
http://tldp.org/HOWTO/Framebuffer-HOWTO-18.html
you should be able to reverse-engineer a modeline entry.  I tried this
for 720x480Over and I got a better result from the perspective that the
overscan resolution actually overscanned properly, but it is much more
flickery than it was before (the two fields appear to be slightly off).
I haven't figured out why changing the modeline info makes the fields go
off like that.  But maybe I made an error.

some helpful web links:
http://ryoandr.free.fr/english.html - homebrew project for getting tv
out from some ati video card.  has modelines for 640x480 and 720x480
that i haven't tried yet.
http://en.wikipedia.org/wiki/NTSC
http://www.tldp.org/HOWTO/XFree86-Video-Timings-HOWTO/fixes.html
http://www.knowplace.org/pages/howtos/xfree86_modelines_conversion_guide.php

cheers,

Ken