refresh cycles and LCD

[Andrews, A.S.]

Hi Henk

1. The area of the picture will be fixed, and is usually to within about 1mm of the bezel. A point to note is that a 17" LCD has a diagonal size of 17". A 17" CRT will be ~16" because the bezel overlaps the front of the screen.

2. This is because LCDs have a fixed native resolution. A 17" panel is usually 1280*1024 pixels. If you supply it with fewer pixels than this, it has to internally 'scale up' the picture to fit, causing aliasing effects as you describe. If you ever see 'fuzzy' text on an LCD, it is probably being driven at too low a resolution. This can be a problem with people who have 'older' eyes, and don't like smaller text. Also, using the higher resolutions places greater demands on your graphics card and so can be a source of timing issues on older computers.

I hope that helps.

Regards

Tony.


-----Original Message-----
From: eprime at mail.talkbank.org [mailto:eprime at mail.talkbank.org]On
Behalf Of Henk Smit
Sent: 23 November 2004 14:39
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