Video (NTSC) output from an IRIS

[Chris Ott]

     Ok, since no one seems to remember the article I posted last time
this question was asked, here is my input again. Let me explain now that
I am speaking of the 2000 and 3000 series IRISes. I know nothing about
the 4D machines, except that they have a resolution of 1280x1024, rather
than 1024x768, but I would still expect this stuff to apply.

     For those of you who think that you can't use the whole screen when
videotaping, let me confirm your suspicions. The IRIS screen's resolution
is 1024x768, while NTSC's resolution is something like 500x700, if you're
lucky. Also, the IRIS's video is 60Hz non-interlaced, while NTSC video is
interlaced. Obviously, this requires a completely different setup to
display hi-res than to display NTSC.

     Most IRIS systems (at least the 2000 and 3000 series) are shipped
with the ability to display two of four different customer-selected video
formats. The four formats are 60Hz non-interlaced, 30Hz interlaced, NTSC,
and PAL. Usually, the two selected (maybe the defaults) are 60Hz and NTSC.
This is the way all of our IRISes are set up. Since the video formats are
different, all you can see is garbage on the 60Hz screen when the IRIS is
in NTSC mode, and vice-versa. The simple solution is to get some equipment
so you can see something in NTSC mode.

     The equipment needed is a color encoder and an NTSC format monitor.
If you want _broadcast quality_ video (i.e. exact NTSC timings), you will
also need a sync generator. A color encoder is used to convert the RGB
signal from the IRIS to a composite video signal. Of course, for this to
work, NTSC format must be selected; the color encoder will not convert
60Hz to composite video. We got a pretty cheap color encoder for about
$1000. The picture is acceptable, but pretty fuzzy. The sync generator
doesn't help that much, either, except to improve some of the timings.
Of course, you get what you pay for. A friend of mine who works for a
local television station (an NBC affiliate, if anyone cares) says they
paid $6000 for their encoder. I've seen the picture, and it's flawless.
Our (cheap) sync generator is also about $1000. I can't speak for other
sync generators.

     Before I go any further, let me say that I do not recommend just
shooting an NTSC video camera directly at the screen of the IRIS. The
resolutions are different and the timings are different. The resulting
picture will look like trash. Ever wonder why TV screens in the movies
have that horizontal bar traveling from the top to the bottom of the
screen? Now you know.

     It seems strange to me that anyone would complain about the fact
that you can only display part of the screen. You can't display 1024x768
pixels on an NTSC screen, anyway. Anything over 700x500 is just a waste
of pixels. It certainly doesn't give you a problem with programming,
either. All you need to do is use the graphics library like so:

          viewport( 0,636, 0,485 );

I believe those are the correct coordinates. As far as I can remember,
nothing else needs to be changed. Everything that used to take up the
full 60Hz screen will now take up only the lower left-hand corner of the
60Hz screen, but it will take up the entire NTSC screen. Now, is that easy,
or what? I think this was the easiest and cheapest way for SGI to do it.
Conversion between two different video resolutions is _expensive_. Rather
than paying $1000 for a color encoder, you'd be paying between $10000 and
$20000 for a converter.

     So, that's how it's done. If you want any more information, like more
detail, an actual working program, or some company names, let me know.

Chris

-------------------------------------------------------------------------------
 Chris Ott
 Computational Fluid Mechanics Lab            Just say "Whoa!!" and
 University of Arizona                          vote for Randee!!

 Internet: chris at spock.ame.arizona.edu
 UUCP: {allegra,cmcl2,hao!noao}!arizona!amethyst!spock!chris
-------------------------------------------------------------------------------