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Date: Fri, 18 Oct 91 12:05:21 -0500
From: Greg Newby <gbnewby@alexia.lis.uiuc.edu>
Message-Id: <9110181705.AA15909@alexia.lis.uiuc.edu>
To: jdb9608@cs.rit.edu
Subject: Re: sega glasses available?
Cc: glove-list@karazm.math.uh.edu
For the circuit, has anyone tested it out?  It appears that
you hook in the Sega glasses, and then send a signal to switch
the LCDs (from left-on, right-off to left-off, right-on).  

The signal is sent via pin 4, which is not the regular pin
for reading or writing data.

So, the question is, what's involved in switching the signal?
If it's a call to termio() or somesuch, what sort of speed
limitations are we talking about?  I didn't check the
RS232 standard just now, but I wonder about the +/- 10V switching
signal -- isn't that rather 0-12V?  (effectively, probably
+/-3 to 8-12V).

Finally, does anyone have the tech specs on the glasses (or 
an estimate, as I suspect Sega doesn't include such things
with the glasses)?  Switching speed, %dark, etc...

With the brain trust we have on this list, I'm not too worried
that someone will figure out how to hook in the glasses.  I'm
just wondering in advance if performance (especially switching
speed) will be up to snuff.  
	(For example, to give some numbers, we need a minimum
	 of about 10 frames per second to perceive continuity
	 between frames of a moving scene.  That means the glasses
	 need to switch 20 times per second, right?  A left-right
	 sequence for each frame.
		If there's more than a few u-second lag for the
	 glasses to switch, we'll have to compensate in the
	 display program.  Being that we're usually using all
	 the computing power we have just to keep the graphic
	 images coming fast enough, incorporating some sort
	 of delay for the glasses would be undesirable.)

Oh, BTW: I've done a bit of experimenting (back at Syracuse) with
some Tektronics shutter glasses -- a very similar beast, tho
presumably higher performance.  I found that the flicker was
perceivable at 30Hz (60 switches / second), and intolerable
(read, "doesn't work") at about 7-15 Hz.  I didn't experiment
with the full range....this was because the software-driven
switching circuit never operated more quickly than about 15Hz.

Ok, I'm rambling now:  the alternative, to get 30Hz, is to 
NOT switch the glasses via software.  Just let them operate
at the frequency of the power supply (60Hz in the US, which, in
the circuit we used @ Syracuse, was run through a rectifier).
Then, no coordinating between your graphics program and the
glasses are necessary, PROVIDED that your program is able to 
operate at "top-speed," switching the image on the screen
as often as possible (as in, the speed of the power supply).
 	I don't know if this technique works for all platforms,
but it worked on the Amiga and Iris we used at Syracuse.  I even
synched the glasses off of a Sony VCR/editor to look at 
the display on the Iris.

'Nuff said.
-- Greg
gbnewby@alexia.lis.uiuc.edu