real-time and 3D workstations

[Aries Arditi]

I've been following the discussion following your request to info-iris for
a machine on which to conduct your experiments.  I'm afraid that most of
the discussants don't quite understand what the demands of an RT
experiment are.  The IRIS machines, and most other 3D workstations are
UNIX based, which almost by definition means they are NOT real-time in the
sense you need for accurate measurement of time.  Depending on what is
happening at any given time, any number of processes are competing for the
CPU's time. This means that you will always have some uncertainty as to
the onset of your stimulus. Additionally, you will have some uncertainty
as to the button time, unless you have an external real-time clock hung on
the back (Personal IRISes have only one VME bus slot, so only 1 such
device can be on your system at once), and even then, since you don't know
exactly when the stimulus onset was, you still have error.

I think the IRIS sounds like the machine in your price range to generate
your stimuli, but may or may not be depending on how accurately you need
to measure RT. Here are 2 possible solutions:

1. There are versions of "real-time" UNIX available (I know not where, but
someone at AT&T Murray Hill developed one of them for DEC PDP 11's.
Possibly an SGI sales rep will do some legwork to find one for you if he
thinks that's necessary to make the sale.  In the PDP 11 market, these
real-time versions of UNIX varied quite extensively both in quality, I
understand, and in the extent to which they are "real-time." So be careful
here.  You will need to know that you can still run the software for your
experiment, under this operating system.

2. The solution I have used with some success, on another multi-tasking
(i. e. not real time) machine, is to count video frames when measuring
time, rather than waiting for an interrupt from a clock or other device.
It's very easy, but your accuracy is limited to +/- 16.66667 msec.  If
that's OKAY for your application. Sometimes you can degrade your stimulus
or task in some irrelevant way in order toraise all your RT's so 17 msec
isn't such a big deal.

Anyway, computer graphics people usually think that real-time just means
animated, so watch out for that.

Good luck.
-Aries Arditi
 Vision Research Laboratory
 The Lighthouse
 111 E 59th Street
 New York, NY 10022