Subject: Re: CVNet - An LCD monitor for research in vision
From: "Ian M Andolina" <[email-masked]>
To: "CVNet" <[email-masked]>
Date: Mon, 26 Sep 2011 11:55:27 +0100

<quote who="vincent ferrera">
> I found that the onset of the stimulus was delayed no less than 11 ms,
> and no more than 14 ms relative to the vertical sync signal.  In my
> experience, this is more reliable than most CRTs which have up to 15
> msec delay between vsync and stim onset, depending on where the stimulus
> is on the screen.

Surely the problem here is the variance (which is compounded when changing
amongst different grey levels). CRTs obviously have scanning artifacts,
but there is no variance (it is a fixed lag difference vertically). My
measurements using a photodiode and a TTL sent just after VBL (and
estimating the time to stimulus on using scanline position) using Psych
toolbox is that there is submilliscond latency and no variability on a CRT
(apart from the vertical position lag). Looking at the 2D grey-grey
transition time variances for most LCDs suggest they are will have
problems over and above the latency lag. The 120Hz Samsung is a somewhat
different beast, and Danko used a blanking trick (simulating a CRT!) to
optimise the temporal characteristics.

The other major issue is that 120Hz displays are twisted nematic displays,
which if I understand correctly are always 6-bit per pixel resolution, and
therefore unsuitable for the more demanding contrast level type stimuli.

There are 120Hz 10-bit LCD panels, I assume Viewpixx (which has a built-in
bit-depth extender) use one:

http://www.vpixx.com/products/visual-stimulus-displays/viewpixx.html

And Barco reference monitors are another (which also has a scanning
backlight to mimic CRTs):

http://www.barco.com/en/product/2146

Ian Andolina, UCL Dept. of Visual Neuroscience, London

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