Steve Baker (steve++at++mred.bgm.link.com)
Wed, 29 Jan 97 12:25:01 -0500
> Anybody out there playing with the NTSC encoder on Inf.Reality?
>
> I'm trying to videotape some of my normal 1280x1024/60Hz simulations.
>
> When I set up ircombine on my iR box for 1280x1024 (60Hz) on one
> DG-2 output and NTSC on the other, I see an occasional horizontal
> discontinuity creeping up the NTSC display - no such artifact
> appears on the regular display.
Well, I think I now understand the problem fully (thanks to some help from an
old friend at Philips Research)...
NTSC *isn't* 60Hz - it's 59.96Hz, however, your normal SGI CRT
*is* exactly 60.00000000Hz. Hence, the buffer swap signal in the simulation
switches the double buffers at 60Hz - which is not in sync with the
NTSC vertical retrace - so you get a buffer swap partway down the screen.
Now all we need is an SGI Guru to tell us either:
1) How to get Performer to swap the double buffer in sync with the
NTSC frame rate.
or...
2) How to get the NTSC encoder to run at exactly 60Hz. (Admittedly if we
did this, it wouldn't be perfectly timed to the NTSC standard - but
a VCR wouldn't care about that).
Why isn't NTSC 60Hz? Well - that's an interesting subject in it's
own right. I think the story boils down to the fact that good old
monochrome television (back when "I Love Lucy" wasn't a rerun) *was*
60Hz, in order to avoid nasty interactions with 60Hz wall socket
power frequencies.
When colour came along, they needed some extra data within each
frame - and to avoid increasing the bandwidth needed in the
transmitters, they dropped the frame rate to 59.96Hz (ugh!) and
presumably just hoped that electrical isolation within the TV
was good enough to avoid interactions with the 60Hz power.
...and these are the same people who are defining the wonderous (NOT!)
new HDTV standards.
Steve Baker 817-619-1361 (Vox-Lab)
Hughes Training Inc. 817-619-8776 (Vox-Office/Vox-Mail)
2200 Arlington Downs Road 817-619-4028 (Fax)
Arlington, Texas. TX 76005-6171 Steve++at++MrEd.bgm.link.com (eMail)
http://www.hti.com (external) http://MrEd.bgm.link.com/staff/steve (intranet)
http://web2.airmail.net/sjbaker1 (external)
"You can't destroy the Earth - that's where I keep all my stuff!" - The Tick.
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