Timing Question

[Carlos Cortes]

Have you tried to sinc the stimulus to the vertical refresh?

>>> emzcoffey at gmail.com 04/26/09 6:17 AM >>>

I will try it, but the problem is not so much the timing, but the
consistency: if it were all 527ms it would be fine, but somehow I
always get 2 different timings which are quite different.

On Apr 25, 8:31 pm, ben robinson <baltimore.... at gmail.com> wrote:
> have you tried adding more than just 10 ms as your buffer?  that is, if you
> want something to display every 505 ms, and you set it to prepare the
> display at 495 ms, yet it ends up actually displaying at 527 ms, why not try
> preparing the display at 480 ms?  just a thought.
>
> On Thu, Apr 23, 2009 at 2:28 PM, Emz <emzcof... at gmail.com> wrote:
>
> > Hello,
>
> > I need some help with a timing problem in a reasonably complex set-up
> > (the complexity is in in-line scripts that output to another system,
> > which requires the use of the IsPending() function, the task itself is
> > just an N-back variant). I have tried and tested many minor changes in
> > the set up and have managed to eliminate all of the timing problems
> > except one: the actual duration of stimulus presentations are always
> > either one of two values, each different than expected (as based on
> > multiples of screen refresh rate).
>
> > The specifics are as follows:
>
> > The screen refresh rate is 75.031Hz, giving a duration of 13.3. I wish
> > to have presentation times of about 500 ms and 2000 ms, so as per
> > manual I have selected about 10 ms less than the nearest multiples
> > (505.4 and 2008.3), resulting in the set presentation duration times
> > set to 495ms and 2000ms.
>
> > Errors are consistently as follows:
>
> > 527.3438 - 505.4000 =  21.9438 (many more of this one)
>
> > 558.5938 - 505.4000 =  53.1938 (only about 15% of the ~500 ms display
> > items)
>
> > 2027.3483 - 2008.3000 = 19.0483 (equal proportions with below for the
> > ~2000ms items)
>
> >  1996.1 * 2008.3 =  -12.2
>
> > I am currently not using the pre-loading option as I read somewhere
> > this was not necessary for this sort of task. I also have some in-line
> > scripts between display items, and the before mentioned IsPending loop
> > in some places, for which I don’t know if the pre-loading would even
> > make sense. The aforementioned IsPending loop doesn’t seem to be the
> > problem as trials with and without it have exactly the same errors.
> > Deleting or moving the in-line scripts (which are used to output data)
> > appear to produce a bigger variety of errors.
>
> > Ideas ?
>
> > E.



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