16 Mar
2015
16 Mar
'15
11:21 p.m.
John Foust <jfoust at threedee.com> wrote:
I'm trying to understand at a low level how some
early computers
and game consoles generated a non-standard form of NTSC.
The Wikipedia http://en.wikipedia.org/wiki/Low-definition_television
says: (...)
Adam Sampson <ats at offog.org> wrote:
There's a pretty good description here, with
diagrams of the video
waveforms involved for both PAL and NTSC (both use the same idea):
http://martin.hinner.info/vga/pal.html
(...) in the kind of non-interlaced
signal you're talking about, every frame starts with the odd field
vertical sync, so the monitor always pulls the electron beam back to
the same place.
A-ha, interesting to see that this sort of shortcut was actually taken
in commercial products. I pretty much accidentally ended up with that
sort of signal when I, back in 2010ish, tried to coax a Sun cg3 style
framebuffer (onboard FB of a SPARCclassic) into outputting a (50Hz) TV
displayable RGBs Signal by feeding it a hand-crafted "mode line" after
working out what the registers on the video timing ASIC do. I'm
pretty sure there will be some accounts of that adventure in the list
archive as a list member helped me through it.
I got the picture to display on my Commodore 1081 Monitor via a 13W3
to SCART cable I had fashioned therefore, but of course wasn't very
impressed with the vertical resolution, and couldn't find out how to
enable interlaced mode on the ASIC - if it's capable of that at all.
So Long,
Arno