refresh cycles and LCD

[David Hairston]

Hey, I may know this one!
I believe that both of these problems stem from the same reason.  LCD
screens have a fixed, or "native", number of pixels, these days around
1024x768 or even 1280x1024 - they display an image by selectively
turning on (or not) each pixel. What this means is that if you have a
LCD that is 1024x768, and you select, say, 800x600 resolution, then this
can be resolved 1 of 2 ways. Generally the requested resolution is
interpolated across the native, so that in this case it is attempting to
make it look like there are only 4 pixels for every 5 or so.
Unfortunately this doesn't look so good.
The other solution is for the software to use only the central part of
the screen to illuminate 800x600 pixels, and say leave a black box
around it that is 124 on either side etc. Generally it seems that
E-prime will do the former, but on occasion we have had it do the latter
also, especially when using multiple displays or different display
drivers and so on.
CRTs don't do this b/c they don't have a "native" resolution, the pixel
size is determined by changing the number of stops that the electron gun
(or whatever you call that gizmo in there) makes as it steps across the
screen, painting line after line.

Just a hunch.

W. David Hairston
Neurobiology and Anatomy
 
Wake Forest University School of Medicine
Winston-Salem, NC 27157
(336) 716-4481 (lab)

-----Original Message-----
From: eprime at mail.talkbank.org [mailto:eprime at mail.talkbank.org] On
Behalf Of Henk Smit
Sent: Tuesday, November 23, 2004 9:39 AM
Cc: eprime at mail.talkbank.org
Subject: Re: refresh cycles and LCD

Another thing about LCD screens, in my experience, is that they either
1) produce a viewing area smaller than the actual screen, or
2) produce jaggered text/images due to low resolution.
Does anyone know why this does not happen on crt monitor? And maybe more

important: does anyone know how to avoid this, and have a normal 
full-screen presentation of the stimuli in proper resolution?

Sorry if some of my lingo is not correct, but I'm sure you get the 
picture :-)

Henk Smit
Bristol University

Andrews, A.S. wrote:
> Hi
> 
> No, that is not quite right. An LCD screen still receives the signal
in the same way, i.e. frame by frame at a particular frequency, usually
60 or 75HZ. If the computer changes the contents of the graphics memory
in mid-frame this will still lead to display errors. This is true for
analogue or digital connections.
> 
> Where LCDs differ is that they have a constant backlight and so are
not subject to flicker in the way that phosphor displays (CRTs) are. The
16ms you refer to is the response time of the LCD pixels. Various
manufacturers define this in different ways, but I think it is becoming
accepted for this to mean the time it takes to turn from fully-off to
fully-on and back again. 16ms is particularly fast, with many panels
that are currently in use (particularly cheaper ones) having response
times of e.g. 40ms. For this reason, CRTs are still my preferred display
device. Also, LCDs have a relatively short life, particularly the
backlight, so if you are planning a purchase, make sure you get a good
warranty plan.
> 
> Regards
> 
> Tony Andrews
> Senior Computer Officer
> School of Psychology
> University of Leicester.
> 
> -----Original Message-----
> From: eprime at mail.talkbank.org [mailto:eprime at mail.talkbank.org]On
> Behalf Of Yoav Bar Anan
> Sent: 23 November 2004 11:47
> To: e-prime unofficial forum
> Subject: refresh cycles and LCD
> 
> 
> Hi
> 
> I was told that unlike CRT screens, LCD screens do not use
refresh-cycles.
> Rather, any request to paint the screen is transmitted immediately to
the
> screen, and after a certain delay (16ms?) the screen applies the paint
> request. It means that on LCD, there is no obligation for the
durations to
> be a product of the refresh-duration. Is it true?
> 
> 
> 
> 


-- 
Dr. H.J. Smit
Department of Experimental Psychology
University of Bristol
8 Woodland Road
Bristol BS8 1TN
United Kingdom

tel +44 (0)117.954 6616
fax +44 (0)117.928 8588