CRT vs. LCD

[Peter Quain]

Date: Thu, 01 Nov 2007 09:08:43 -0700
To: DMDX at psy1.psych.arizona.edu
From: "j.c.f." <jforster at psy1.psych.arizona.edu>
X-ASG-Orig-Subj: [DMDX] LCD flat panel displays and experiments
Subject: [DMDX] LCD flat panel displays and experiments
Reply-To: DMDX at psy1.psych.arizona.edu
Sender: DMDX-owner at psy1.psych.arizona.edu

---------------------------------------------------------------

   I've noticed several machines in the last couple of weeks running 
DMDX with flat panel displays in the department, no great surprise as 
it's getting pretty challenging to buy a machine with a CRT.  Just 
buying stand alone CRTs is challenging.  But people need to be aware 
that using an LCD flat panel is not the same thing as using a CRT is 
as far as experimental displays are concerned, regardless of whether 
you're using DMDX or some other program.

   If you're not really interested in accurate display timing you can 
hit control D now and I apologize for the interruption to your email 
reading experience.

   However those of you that are interested in tachistoscopic 
displays, masked priming and anything else measuring RTs down to tens 
of milliseconds and less need to be aware that an LCD flat panel's 
display is going to be lagged by some arbitrary and variable amount 
of time unless the video mode that is being used is exactly the 
native resolution of the display.  Worse, it can skip whole retraces 
of displayed data.  This applies to laptops as well as desktops, 
however it's faintly possible given a laptop's integration the 
problem may not be so acute.  Wouldn't surprise me if it was just as 
acute, put it that way.  So a 17" flat panel really needs to use 
1280x1024, a 20" one 1600x1200 and so on.

   It gets worse, recently I've seen two sets of displays that both 
accept 60, 70 and 75 Hz displays.  One set of them didn't even 
display things properly at 70 Hz (which is fine, you'd hardly miss 
it) but one set only worked properly without lag and dropped frames 
at 60 Hz and the other set only at 75 Hz.

   I'm updating the TimeDX documentation to make the steps clearer 
but it's basically a matter of getting the Refresh Rate's display to 
be smooth.  A sure but counter intuitive sign that you haven't got 
the right settings is TimeDX being able to correctly determine the 
retrace interval (as that test was designed to do) but having the 
display flicker.  Here the hardware that TimeDX is looking at is fine 
and it's able to be timed correctly but the monitor on the end of all 
that that TimeDX doesn't have access to is doing it's own rendering 
of what it's being sent and taking it's own sweet time to do it and 
dropping frames as it needs to when it's too slow.

   So they can be made to work as scientific instruments, they just 
take more work and only run at one display mode and refresh rate.  To 
date anyway, who knows what tomorrow may bring ;)
                                                /"\
  -jonathan (j.c.f.)                            \ /
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     ASCII RIBBON CAMPAIGN - AGAINST HTML MAIL  / \

   Standard parts are not.
                                       - Murphy's Technology Laws
                                             www.murphys-laws.com


At 01:40 AM 5/03/2010, you wrote:
>I had a discussion yesterday with a collegue from another lab about
>display types. I am always using CRT displays for my experiments
>because they have higher refresh rates and no decay in which color
>values might differ from what you programmed.
>However, this guy was bringing forward an argument I have not thought
>about so far: For LCDs you have a stable overall picture at any given
>moment. A CRT display virtually draws one point after another, i.e.
>you never see an entire, let's say circle but your brain makes you see
>it as the point and lines are drawn so rapidly. I was shown photos
>made by a digital camera with really short shutter times (less than 5
>ms) and for CRT displays you can never see entire stimuli whereas for
>LCDs you do.
>
>So I was wondering what kind of display you are using in your labs and
>what your experiences are. Maybe there will develop a fruitful
>discussion.
>
>Cheers,
>Tobias
>
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