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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.<br><br>
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.<br><br>
Additionally, any concurrent audio would be out of sync because the audio
is not routed through the display, and so would experience no
delay.<br><br>
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).<br><br>
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.?<br><br>
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.<br><br>
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Good old Wikipedia defining input lag:<br><br>
<a href="http://en.wikipedia.org/wiki/Input_lag" eudora="autourl">
http://en.wikipedia.org/wiki/Input_lag<br><br>
</a>A basic primer on how LCDs / CRTs function, and differences:<br><br>
<a href="http://www.bit-tech.net/hardware/2006/03/20/how_crt_and_lcd_monitors_work/" eudora="autourl">
http://www.bit-tech.net/hardware/2006/03/20/how_crt_and_lcd_monitors_work/<br>
<br>
</a>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:<br><br>
<a href="http://www.tweakguides.com/Graphics_7.html" eudora="autourl">
http://www.tweakguides.com/Graphics_7.html<br><br>
</a><u>How to test your monitors for input lag?<br><br>
</u>Here is a brief description, and a little counter program you can
download:<br><br>
<a href="http://www.flatpanelshd.com/focus.php?subaction=showfull&id=1229335064" eudora="autourl">
http://www.flatpanelshd.com/focus.php?subaction=showfull&id=1229335064<br>
<br>
</a>Here is another description with some useful info about type of
camera that is suitable:<br><br>
<a href="http://www.avsforum.com/avs-vb/showthread.php?t=1131464" eudora="autourl">
http://www.avsforum.com/avs-vb/showthread.php?t=1131464<br><br>
</a>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:<br><br>
<a href="http://www.digitalversus.com/duels.php?ty=6&ma1=35&mo1=121&p1=1303&ma2=284&mo2=326&p2=3097&ph=12" eudora="autourl">
http://www.digitalversus.com/duels.php?ty=6&ma1=35&mo1=121&p1=1303&ma2=284&mo2=326&p2=3097&ph=12<br>
<br>
</a>
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Peter<br><br>
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