Stimuli/ Triggers/ Timing difficulties

[David McFarlane]

First, I must ask how exactly you measure "Stimulus OnsetTime" and 
"Trigger OnsetTime", please provide details.  AFAIK EP does not log 
those separately, so you seem to know something that I do not.

But ultimately, if you really want to know the relationship between 
the time of a visual stimulus and the time of a trigger output, then 
you must get out a dual-channel oscilloscope, aim a high-speed 
photodetector (e.g., photodiode or phototransistor) at your display 
screen, tap a line into your trigger output, and then look at those 
signals on the oscilloscope.  You might need to find someone local 
with some electrical engineering experience to help with this.

And then, to get the best timing performance, you *must*, MUST 
thoroughly study Chapter 3 of the User's Guide that came with 
E-Prime!!!  I cannot stress that enough!  Until you do that, you 
might as well be using a handheld stopwatch.

-----
David McFarlane
E-Prime training 
online:  http://psychology.msu.edu/Workshops_Courses/eprime.aspx
Twitter:  @EPrimeMaster (twitter.com/EPrimeMaster)


At 4/20/2012 11:52 AM Friday, you wrote:
>Hello David,
>
>Thanks a lot for your help.
>We made the PreRelease 400 and it seems like a miracles happened. Many
>thanks to you.
>
>Now, As our first problem was the delay of appearing the stimuli and
>triggers. We realized we still have that Delay and that the delay is
>not the same during the whole experiment among all Stimuli. As we read
>your posts regarding the timings and delay. We have realized we might
>have been measuring the delay in a wrong way. I am quoting what I read
>with your signature and then I will ask you my new question!
>
>"- OnsetTime:  Time when E-Prime actually submitted the stimulus data
>for
>    presentation (e.g., proceeded to copy data to display memory or
>load
>    sound buffer).  This may not coincide with when the stimulus
>actually
>    got presented, e.g., if data are submitted in the middle of a
>display
>    refresh cycle then they may not get presented until the next
>refresh."
>
>So, We have been trying to make the delay between the time stimulus is
>Presented by E-Prime on the screen and the time Trigger appears,, as
>least as possible and we wanted that delay to be fixes or within the
>least range possible. But As we read your post, we realized that what
>we were measuring which was the difference between Stimulus OnsetTime
>and Trigger OnsetTime, is the wrong way to measure that delay.
>
>Do you have any ideas  that what can we add in out inline to measure
>this delay or where we should look if we want to see the time of
>presentation of the stimulus and trigger?
>
>Looking forward to hearing from you.

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