[visionlist] CRT monitor solutions

Michael Bach Michael.Bach at uni-freiburg.de
Tue Nov 3 09:01:44 PST 2009

Dear Colleagues:

In this CRT-entombing discussion I thank you for your knowledgeable  
input on this itchy problem! One issue that has not been touched upon  
is absolute screen timing, which is somewhat separate from frame rate  
or pixel-change speed.

The question is: at what time, within one millisecond, does the image  
change (or begin to change at the top)?
This a totally important for some of my fields (Pattern ERG, VEP), but  
also pertinent for visual masking.

In the days of the CRT, the beam followed the timing of the video  
lines. You knew when it started at the top by monitoring the retrace  
signal, or vertical blanking signal, whatever you'd call it.
With any technology that has its own frame store, like nearly all LCDs  
(to my knowlege), it just begs for temporal aliasing: the screen is  
typically updated at 60 Hz from the frame store. Independent from  
this, the incoming video signal (be it analog, e.g. VGA, or digital,  
e.g. DVI) updates the frame store at a range of possible frame rates.
So when you monitor timing in the computer, you are, unfortunately,  
decoupled from the appearance on the screen by up to 17 ms.

So far I have only briefly looked into this myself with a photodiode  
picking up screen luminance and found such drift. What is the general  
wisdom on this?

Thanks for your time and your thoughts,
Prof. Michael Bach PhD, Ophthalmology, University of Freiburg,  
Killianstr. 5, 79106 Freiburg, Germany.
President of ISCEV <http://www.iscev.org/>
Visual illusions:  <http://www.michaelbach.de/ot/>

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