Stimuli/ Triggers/ Timing difficulties

[Peter Quain]

If LCD monitor is being used can you please post back to the list 
results of the photodiode tests. It will be interesting to see (a) 
whether there is any input lag; and (b) the relationship of this (if 
present) to trigger accuracy. Any input lag of the monitor will cause 
trigger to occur prior to display onset (on screen, not according to 
e-prime which will log when frame sent from display adaptor). If 
input lag is there it will most probably differ across trials which 
will give a degree of trigger slop - meaning you can't adjust the 
trigger position posthoc by a constant.

Peter


At 01:55 AM 19/05/2012, you wrote:
>Any local electronics expert can handle this for you, you would do 
>best to find one at your own institution.  Also, PST Web Support 
>might have some advice.  One hurdle I will mention -- peak light 
>output of modern LCD/TFT displays is much less than the old CRT 
>displays, so if you use an LCD display then you will need a good 
>sensitive photodetector.
>
>-- David McFarlane
>
>
>At 5/18/2012 11:46 AM Friday, Douglas wrote:
>>Hello again Mr. McFarlane,
>>
>>I want to use Photodiodes.
>>Would you please tell me what kind of photodiodesI have to buy and how
>>I should install it?!
>>
>>Thanks
>>
>>On Apr 20, 12:12 pm, David McFarlane <mcfar... at msu.edu> wrote:
>> > 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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