[visionegg] Re: time and OS es

Christophe
just a short remark concerning latencies under linux, for visual stimulation. As
far as I remember when I used visionegg, I got latencies BUT only if the
hardware OpenGL acceleration was not enabled. After enabling this the timing was
really extremely precise (<2ms, I think).


I also wrote a visionegg program (must be somewhere in the archive), where I got
the MR scanner TTL signal and the mouse-button-press via the LPT (one pin for
the TTL signal, the other for the button-box). and it worked fine, since LPT
polling also worked with max-priority set (the USB-mouse doesn't work any more
with max-priority)
Cheers!

Christoph

Quoting Christophe Pallier <pallier@xxxxxxxxxxxxx>:

> Mark Halko a écrit :
> 
> > The other solution to this is to only use the first TTL pulse, as a 
> > "trigger" for your experiment.  You can then use the computer's own 
> > internal timing to display your stimulus.  If your experiment doesn't 
> > start on time, but starts 2 seconds late, you'll know you missed the 
> > pulse.
> 
> Yes. I use that approach now. That is how I discovered that one of our 
> scanners has an actual interscan delay which is different than the 
> requested one, leading to a drift which ruined an audio experiment where 
> stimuli were supposed to be presented in the one second gap between 
> consecutive scans... Now, we measure the interscan delay rather than 
> trust the vendor's program.
> 
> >> Which brings out one of the points I made but didn't emphasize.  Many 
> >> experimenters do not record the error of the measurement.  A long 
> >> tradition of canned packages that just give you back what you put in 
> >> has really promoted this.  I'm glad some of the newer ones like 
> >> Experiment Builder (SRI) and Presenter(?) record what happened.
> >
> Call me paranoid, but can you trust that the software knows everything? 
> The hardware could introduce some additional buffering, no?
> This can matter or not, depending on the experiment. Audio sound boards, 
> for example, have actual sampling rates that can differ markedly from 
> the requested one. If not taken into account, this can introduce 
> systematic differences in reaction times measured from targets appearing 
> at different moments in the audio stream.
> 
> In the end, for some experiments, it is necessary to check with external 
> means if the timing of stimuli matches what is intended.
> 
> A very useful contribution would be a toolbox to perform this sorts of 
> checks. It could be ran on second computer connected to the stimulation 
> PC (to its audio ouput, its parallel port, and to its screen via a 
> photodiode). Not everyone knows how to use an oscilloscope or wants to 
> use one.
> 
> One possibility may be to build some simple hardware to record the 
> various events in an experiment and mix them as an audio stream on the 
> second computer. This "poor man's" approach once allowed me to check the 
> delay between the display of an image and the start of a sound file. It 
> actually worked quite well (the stimulation program was a commercial 
> software which has not been cited here yet, and which I do not like at 
> all, but I must recognize it handled timing quite well in this case)
> 
> The idea of a stripped down distribution is a good one (I usually only 
> disable power management).
> Once latency problems are solved, a project that would get me excited 
> would aim at creating a Linux-Live CD for Psychophysics (a la Knoppix). 
> 
> Cheers (and apologies for this long series of off-topic posts)
> 
> Christophe Pallier
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
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-- 
========================================
Christoph Lehmann
University of Berne
christoph.lehmann@xxxxxxxxxxxx

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