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That is an interesting response. As I read it, Ryan Whitfield has replied
in terms of <i>synchronisation</i> of frames displayed on two monitors
with the claim that clone mode cannot provide this. However, he states
that the <i>primary display's</i> refresh request (vertical blanking
signal; VBS) is given precedence (by e-prime). In plain language I read
this to mean that e-prime architecture (where identification of VBS is
fundamental to visual display timing accuracy) is built to call on the
primary display VBS. So the t<i>iming accuracy</i> of primary displays
should not be influenced by a second cloned (secondary) display, where
exact timing accuracy could not be assumed because of out of synch
vertical refresh. I have tested accuracy of primary display with
thousands of trials where we collected timing information on vertical
refresh intervals during time critical display events (presented using
Canvas, dual-head video card, VGA output, cloned, 'identical' CRT
monitors). Only a couple of VBS were missed, and the refresh intervals (3
of them) each averaged to the appropriate refresh interval. These timing
'marks' were taken immediately following Display.WaitForVerticalBlank
commands. We were careful to ensure that the monitor the subjects saw was
connected to the primary display port of the dual-head graphics card,
which we identified from documentation, and checked empirically - the
card would not output any signal to a monitor connected to the secondary
port unless there was also a monitor connected to the primary
port.<br><br>
David Nicholson's original query was regarding timing <i>accuracy</i>
while allowing experimenters to view on a second monitor what the subject
was seeing. So, exact display synchronisation between cloned displays not
possible because of nonsynchronised refresh cycles, but timing accuracy
of primary display is possible (given proper identification of the
primary display port, and an appropriate video card / drivers) because of
e-prime architecture (Or have I got this wrong?). This should also be the
case, in principle, using LCD monitors because they also generate a VBS
(for compatibility with graphics systems developed with CRTs) and so
e-prime should listen for the VBS from primary monitor. But then, you
have onboard image processing engines and possible display lag -
something a display splitter wouldn't fix.<br><br>
<br>
At 07:06 AM 14/12/2012, you wrote:<br>
<blockquote type=cite class=cite cite="">Scott,<br><br>
I went ahead and submitted the question myself to PST Web Support on 10
Dec, and got a very full reply on 12 Dec. I post PST's full
response here for everyone's benefit, along with my question because I
posed it in a slightly different way.<br><br>
First, my form of the question:<br><br>
/-----------<br>
What if we want to share the same video output to two screens, e.g., one
LCD screen for a subject in the fMRI scanner, and another in the control
room to mirror (echo?) the subject display so that researchers can
monitor what appears to the subject? In principle, we have no guarantee
that both screens even use the same refresh rate, and if they do, we have
no guarantee that both screens refresh at the same time. (Or do we?) In
that case,<br><br>
1) Suppose we have a video card with both VGA and DisplayPort outputs,
and we can use "clone" mode to send the same video to both
outputs. How could E-Prime know which of the two screens to use for
refresh synchronization? (I think this is exactly the problem addressed
at
<a href="http://www.pstnet.com/support/kb.asp?TopicID=5014" eudora="autourl">
http://www.pstnet.com/support/kb.asp?TopicID=5014</a>, and the answer is,
EP cannot know, hence the problem.)<br><br>
2) Suppose instead we connect the video card's VGA output to a video
splitter, which then goes to the two screens. Again, how could EP know
which of the two screens to use for refresh synchronization?<br>
<br>
3) Same as #2, except using the video card's DisplayPort output. How
could EP know which of the two screens to use for refresh
synchronization?<br>
\-----------<br><br>
And then the full response from Ryan Whitfield at PST Web
Support:<br><br>
/-----------<br>
The answer to any questions regarding the use of mirror/clone mode with
E-Prime experiments and multiple monitors is that this can cause timing
issues since there is no way to accurately synchronize between multiple
displays. The reason for this is because - when using a machine that has
multiple display outputs - the current technology is such that each of
the outputs has its own pixel clock. These pixel clocks are configured in
such a way that (99% of the time) they do not communicate and the primary
display's "refresh request" is given precedence in the cycle.
Because of this, multiple-output video cards should not be used in clone
mode if display synchronization is a concern.<br><br>
That being said, it IS possible to share the same video output to two
screens with display synchronization with the use of a DVI splitter. The
reason this is possible - while the multiple outputs are not - is again
related to the pixel clock. With a single display output, only one pixel
clock is being used and the display is being pushed out to the splitter -
and in turn the monitors - at the same time. However, working with a DVI
splitter requires that a few things be kept in mind so that
synchronization is possible. First, both monitors must be able to display
at refresh rate intended by the video card. This means that a video card
pushing out a 120Hz video signal cannot be split to a monitor capable of
60Hz and a monitor capable of 120Hz. However, a video card will be able
to push a 60hz signal to both of those monitors without issue (the 120Hz
monitor will likely just double-sample here). Second, the video card must
be capable of pushing out the correct refresh rate. This simply means (as
expected) that a video card only capable of 60Hz will not be able to push
out 120Hz video to two 120Hz-capable monitors. Lastly, (and likely the
most overlooked one if you are using higher refresh rates and/or larger
resolution monitors) the splitter must be of high enough quality to push
out enough bandwidth from the video card to the two monitors. Lower
quality splitters will be fine for 1024x768@60Hz on two monitors.
However, if you are looking at displaying 1920x1080@120Hz, you will
likely need a higher-quality DVI splitter - as lower quality ones will
not have the necessary bandwidth to push through that amount of
information.<br><br>
Most of my answer has revolved around the use of DVI for your display
source, as this is what we have tested in-house because of the work
necessary for the implementation in the Hyperion system. This likely
applies to VGA as well, since similar information transfer is at work
there. With the DisplayPort, it may be easier to synchronize than DVI
because of the internal functioning of the technology - but we haven't
tested this functionality in-house, so I would recommend just using a
DisplayPort to DVI converter and then a DVI splitter once the signal is
in DVI.<br>
\-----------<br><br>
I hope that helps.<br><br>
-----<br>
David McFarlane<br>
E-Prime training online:
<a href="http://psychology.msu.edu/Workshops_Courses/eprime.aspx" eudora="autourl">
http://psychology.msu.edu/Workshops_Courses/eprime.aspx</a><br>
Twitter: @EPrimeMaster
(<a href="https://twitter.com/EPrimeMaster" eudora="autourl">
https://twitter.com/EPrimeMaster</a>)<br><br>
<br>
At 12/12/2012 01:01 AM Wednesday, Scott wrote:<br>
<blockquote type=cite class=cite cite="">Hi David,<br><br>
Actually, I submitted this basic question to E-Prime support a couple of
hours after I posted here. Unfortunately, the body of my support request
did not get submitted; don't know why but guess I goofed somehow. Even
without the detailed explanation I posted here, I did get a decent
response to only the title of my support request (nothing else got
submitted): "What's better: mirror mode or splitter?"<br>
Reply:<br>
Cloning or mirroring can cause timing issues since vertical blank
synchronization is not accurate between multiple displays. Determining
which display is considered primary can cause confusion and may not be
consistent between computer configurations. Instead of cloning the
display through software, it is better to split it physically using a VGA
splitter cable. If it is necessary for you to view the experiment on
multiple screens, you should use a splitter to direct the
output.<br><br>
I hope this helps. Please let me know if you have any other
questions.<br><br>
Sincerely,<br>
David Nicholson, Technical Consultant<br><br>
I've asked for clarification and amended my request as follows:<br>
Thanks. You more or less answered my question from the support title,
even though (for reasons I don't understand) the actual text of the
question was not submitted.<br><br>
I'd like a little clarification and an answer to another part of my
question (which did not get posted). You are saying if I use a splitter,
then E-Prime will see only one monitor (connected to the display card)
and thus only one vertical blank signal; is that correct? Part of my
question that wasn't posted was that I will be using two LCD monitors.
Considering this, will it make any difference whether I divide the analog
signal from my display card, using a VGA splitter or the digital signal,
using a DVI or Display Port splitter? A digital connection can transmit
additional information about the display; I wonder if this might cause
any synchronization problems. On the other hand, the VGA signal will have
to be converted to digital data by the LCD display, possibly increasing
the lag. -- Thanks for clarification & recommendations regarding the
preferred type of splitter.<br><br>
I'll post this support response whenever I get it.<br><br>
Thanks.<br><br>
Scott<br><br>
On Monday, December 10, 2012 12:49:02 PM UTC-6, McFarlane, David
wrote:<br>
Scott,<br><br>
Good questions, wish I had an answer, hope someone else does.
You<br>
should submit this to PST Web Support at<br>
<<a href="http://support.pstnet.com/e-prime/support/login.asp%3Ehttp://support.pstnet.com/e-prime/support/login.asp" eudora="autourl">
http://support.pstnet.com/e%2Dprime/support/login.asp>http://support.pstnet.com/e%2Dprime/support/login.asp</a>
, after all it<br>
is their business to know these things. Then please post back
here<br>
with the answer.<br><br>
Thanks,<br>
-- David McFarlane<br><br>
<br>
At 12/9/2012 05:08 PM Sunday, Scott wrote:<br>
>I don't expect perfection, but would like feedback to help
choose<br>
>how to setup a new lab using E-Prime using physiological
equipment<br>
>in two rooms. One room has two PCs, one recording physiological<br>
>measurements, connected (via the parallel port) to a second PC<br>
>running E-Prime. Each will be monitored by experimenters in that<br>
>room. The other room has the subject's video display and button
box.<br>
>My question has to do ONLY with the E-Prime PC. We want<br>
>experimenters to be able to see what the subject sees, while
they<br>
>monitor physiological recording, so we want the E-Prime video<br>
>mirrored on LCD displays in the two rooms. Of course this is a
very<br>
>common situation for such labs (using EEG, fMRI, EDR, etc). Our<br>
>E-Prime computer has a video card (AMD RADEON HD 7470) with one<br>
>Display Port and one VGA port. Using this computer and display
card,<br>
>there are 3 simple ways (afaik) to accomplish what I want and<br>
>display the same thing on two monitors:<br>
> * Connect one LCD to the Display Port and the
other to the VGA<br>
> port, and set the display card to clone mode;<br>
> * Connect a VGA Y-splitter to the computer's VGA
port, and<br>
> connect one LCD monitor to each side of the splitter; or<br>
> * Connect a Display Port Y-splitter to the
computer's VGA port,<br>
> and connect one LCD monitor (via Display Port-to-HDMI or
-to-VGA<br>
> converter) to each side of the splitter.<br>
><br>
>Using method #1, the E-Prime 2.0.10.184 or later Experiment
Advisor<br>
>will warn that "Mirroring can cause timing issues since
vertical<br>
>blank synchronization is not accurate between multiple
displays..."<br>
>(see
<<http://www.pstnet.com/support/kb.asp?TopicID=5014>http://www.pstnet.com/support/kb.asp?TopicID=5014>KB
5014 -<br>
>FEATURE: Experiment Advisor Module - Display adapter set to
clone<br>
>mode). However, I do not know whether using a splitter will
actually<br>
>improve timing consistency or solve any issues related to
vertical<br>
>blank synchronization. If a splitter would be better, Also, I
don't<br>
>know whether the type of splitter would make any difference in
this<br>
>regard. (I do know that video resolution and quality might
differ).<br>
>Note that I am aware of various issues (eg., input lag, rise
time)<br>
>that can compromise timing accuracy of LCD displays. As a said
at<br>
>the outset, I'm not expecting perfection, or even anything close
to<br>
>the accuracy of a single CRT monitor. My goal is simply to
minimize<br>
>the variability of the time between the onset sync, detected by<br>
>E-Prime, and the onset of the visual display seen by the subject.
A<br>
>constant lag does not concern me. Also, the two monitors to do
NOT<br>
>need to be synchronized with each other, except to the extent<br>
>necessary for even one to be (relatively) synchronized to the<br>
>vertical blank. Also note that I am NOT concerned or asking
about<br>
>synchronization between physiological recordings and the
vertical<br>
>blank time detected by E-Prime, signaled via the parallel port;<br>
>that's a different and less troublesome issue, imho. Also, I<br>
>understand that timing accuracy is more or less critical,
depending<br>
>on experimental paradigms and physiological measures. All I'm
asking<br>
>about is which of the 3 methods listed above can be expected to<br>
>yield the most consistent and reliable (or least bad) timings, as
a<br>
>general rule, when connecting two LCD displays, both showing the<br>
>same content, to one PC running E-Prime. If anyone knows any
better<br>
>methods, other than these 3, I'd like to hear about them as
well.<br>
><br>
>Thanks, in advance, for sharing relevant experience, knowledge,
and<br>
>advice. --Scott</blockquote><br>
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