LCD vs CRT for dot probe task

Fri Jun 29 07:58:51 UTC 2012

ssshhhhhh ... or you will frighten the (machine 
variance) elephant, in our room  :)))

OP, search the group archive for "LCD Input lag 
", there was something on this a while ago. 
Follow some of the links and decide for 
yourselves. We have scrounged and now have a nice 
cupboard full of CRT monitors to replace any failures, into the future.

In fact, here is the post (from 2011):

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LCD monitors and Input lag

The issue of LCD monitors has been raised 
periodically on this list. Because CRTs have gone 
way of the dinosaur many labs use LCDs now for 
experiment presentation, and my guess is that 
some aren't aware of some possible pitfalls of 
doing so. My understanding is, basically, LCDs 
work differently to CRT monitors, and the key 
issue for display timing accuracy is that LCD 
monitors have onboard image processing engines 
that do their own thing with the frames sent to 
them by graphics card, prior to displaying them, 
and this processing can take (varied) time, with 
range that can be perhaps 0 to 70 ms across 
frames, and average lag which can be in the 30-40 
ms range. The variation compounds the problem LCD 
screens may pose for some types of experimental psychology.

This means 1) that you don't have a clue what is 
going on with display timing unless you test your 
LCD monitors for input lag (and then it likely 
will fluctuate across trials anyway) ; and 2) 
Just because it shows a picture, you can't just 
treat an LCD monitor as though it is a CRT 
monitor for purposes of time critical paradigms.

Additionally, any concurrent audio would be out 
of sync because the audio is not routed through 
the display, and so would experience no delay.

Also, without testing the monitor, it may not be 
possible to trust the "refresh rate" setting at 
anything other than the native refresh (mostly 
60Hz) even though Windows may provide an option 
for the monitor to run at a higher "refresh" (say 
75Hz), which some researchers may choose in their 
experiment. I have seen results showing that when 
refresh is set to 75Hz on a 60Hz native refresh 
LCD, frames are redrawn every ~13ms (instead of 
~17ms), however 1 in every 6 frames was skipped 
(no display). So, in 167ms only 10 frames were 
displayed, not 12. Looked like the engine was 
correcting back to native refresh. You test this 
with high speed camera (same as to test input lag).

As far as e-prime goes, this means that you could 
write a tidy paradigm where timing was tested as 
perfect on a good PC, and e-prime would log all 
durations as being so. However, at the display 
level the timing could be all over the place. 
Nobody would know, and effectively all the time 
taken to use e-prime for millisecond precision 
would be wasted. On monitors that have a big 
range of input lag, some paradigms would really 
be impossible to implement accurately.?

I'm no expert in this, have just been fishing 
round on the net. For anyone who might be 
interested, below are some useful links providing 
a little digestible background on how LCDs work, 
how they differ from CRTs, and how to go about 
testing for input lag. Note that to do this 
properly you need a CRT monitor as baseline. 
Don't be tempted to use another LCD, which would 
include using a laptop screen, and my advice 
would be to definitely use a PC (with dual head 
graphics card in clone mode) not a laptop.

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Good old Wikipedia defining input lag:

<http://en.wikipedia.org/wiki/Input_lag>http://en.wikipedia.org/wiki/Input_lag

A basic primer on how LCDs / CRTs function, and differences:

<http://www.bit-tech.net/hardware/2006/03/20/how_crt_and_lcd_monitors_work/>http://www.bit-tech.net/hardware/2006/03/20/how_crt_and_lcd_monitors_work/

Here is an interesting site describing CRTs and 
LCDs (from gamer perspective). Navigate through 
the next few pages forward / backward using buttons down the bottom:

<http://www.tweakguides.com/Graphics_7.html>http://www.tweakguides.com/Graphics_7.html

How to test your monitors for input lag?

Here is a brief description, and a little counter program you can download:

<http://www.flatpanelshd.com/focus.php?subaction=showfull&id=1229335064>http://www.flatpanelshd.com/focus.php?subaction=showfull&id=1229335064

Here is another description with some useful info 
about type of camera that is suitable:

<http://www.avsforum.com/avs-vb/showthread.php?t=1131464>http://www.avsforum.com/avs-vb/showthread.php?t=1131464

A neat site who say they have done a lot of 
testing re: input lag, and provide comparison 
output for many LCD monitors. Note, lags might be 
different on your monitor even though it is same 
model tested. You need to test each individual monitor:

<http://www.digitalversus.com/duels.php?ty=6&ma1=35&mo1=121&p1=1303&ma2=284&mo2=326&p2=3097&ph=12>http://www.digitalversus.com/duels.php?ty=6&ma1=35&mo1=121&p1=1303&ma2=284&mo2=326&p2=3097&ph=12

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At 06:53 AM 29/06/2012, you wrote:
>Also see the post at https://support.pstnet.com/forum/Topic6087-2-1.aspx .
>
>-- David McFarlane
>
>
>At 6/26/2012 12:20 PM Tuesday, Peter Quain wrote:
>
>>If you have access to a CRT monitor you may as 
>>well use it. You can be pretty sure then that 
>>you will have no hard to find timing issues from the monitor.
>>
>>At 03:07 PM 26/06/2012, you wrote:
>>>Hi, I've been researching the whole LCD vs CRT debate and I still am
>>>confused.  I am putting together a simple dot probe task that will be
>>>looking at very small response time differences so I am concerned with
>>>getting it right.  I have two monitors to choose from:  A brand
>>>spankin' new Dell P2210 LCD or an old (2002) Gateway EV700 CRT.  I'd
>>>like any advice on which you'd recommend.  I will be using the most
>>>recent version of Eprime running on a new Dell computer with windows
>>>7. Here are the relevant specs:
>>>
>>>**************Dell P2210 Specs*********************
>>>Display Type:  LCD monitor / TFT active matrix
>>>Diagonal Size: 22"
>>>Viewable Size: 22"
>>>Panel Type: TN
>>>Aspect Ratio: Widescreen - 16:10
>>>Native Resolution: 1680 x 1050 at 60 Hz
>>>Pixel Pitch: 0.282 mm
>>>Brightness: 250 cd/m2
>>>Contrast Ratio: 1000:1
>>>Response Time: 5 ms
>>>Horizontal Viewing Angle: 170
>>>Vertical Viewing Angle: 160
>>>Features: 83% color gamut, HDCP
>>>
>>>*************Gateway EV700 (EV700AA on back) Specs***************
>>>Gateway EV700 17-Inch SVGA Color Monitor
>>>  17-inch diagonal with 15.9 inches viewable area
>>>22.5 mm neck CRT
>>>0.28 mm dot pitch
>>>90° deflection
>>>Resolution: 1,280 dots maximum horizontal
>>>1,024 lines maximum vertical
>>>Scanning Frequency:
>>>Horizontal: 31 to 69 kHz
>>>Vertical: 50 to 160 Hz
>>>NOTE: I also found on gateways site the same model listed with
>>>slightly different frequencies:
>>>Scanning frequency:
>>>Horizontal, 30 - 70 KHz (automatic)
>>>Vertical, 50 - 120 Hz (automatic)
>>>Also a link to some sort of timing table:
>>>https://support.gateway.com/s/MONITOR/7003421/700342103.shtml
>>>
>>>I'd really appreciate any advice on which I should choose (or if it
>>>even matters?), thanks.
>
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