Re: NTSC output from iR.

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Rémi Arnaud (remi++at++remi.asd.sgi.com)
Wed, 29 Jan 1997 11:38:01 -0800 (PST)

Steve Baker wrote:
>
> 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).

 Ed hutchins pointed to me that since patch1355 the NTSC frequency is
 quietly coerced to 60Hz.

 But you should tell ircombine what is the source channel for the NTSC output
 (knowned as vampire mode) by selecting the source channel in the encode
 attributes of ircombine.

 This also has the advantage to let you map the NTSC video to the full
 frame buffer, instead of being limited to 640x486.

>
> 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.
>

 Is it the difference between the two NTSCs ? Japan vs Us ?

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