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Thread: refresh rate, test online

  1. #1
    Join Date
    Oct 2007
    Posts
    13

    refresh rate, test online

    I'am a french PhD, in a french laboratory of Psychology.
    I'm designing an experimentation which evaluates the capacities of perception of speed, and perception of movement of vehicles.
    I have some videos (AVI) of one vehicle moving at a constant speed from left to right. Vehicle speed and vehicle distance varied across videos. These AVI were design at 60hz (1024x768).
    I fixed the refresh rate in Windows at 60hz (1024x768).
    I also set the 1024x768 mode in Direct RT.
    I test the refresh rate in Direct RT : 62.54 Hz.

    As, refresh rate is very important to the design of this experimentation (perception of speed),
    I would like to be sure that Direct RT runs my videos at this refresh rate, during all the session.
    These sets (in direct RT and in Windows) are an optimum at which Direct RT runs the session (sometimes 60hz, but sometimes less), or these sets really fix the refresh rate for the whole session (refresh rate constant, 60Hz, during the entire session) ?
    Is it possible to evaluate the refresh rate during the session, online, during Direct RT runs my video ?

    I hope that my questions are clear.

    Best,

    Aurelie Dommes.






  2. #2
    Join Date
    Nov 2005
    Posts
    3,309
    I would like to be sure I understand your question so let me paraphrase it:

    You would like to run your session at 60Hz--it's very important. So you would like some way to check that it's actually doing this while a session is running.

    Is that correct?

    If so, there are two ways off the top of my head. First, you can check the data in the log file. All of the display times for each stimulus are listed. If you are running the session at 60Hz, then every thing that is displayed must be on the screen for multiples of 16.67ms, about 17ms. So divide all the display times by 17 and see if you get an integer value (i.e., little or no remainder). That would be a good sign. Similarly, if you were running your session at 100Hz, then your display times should be in multiples of 10ms, and so forth.

    It is more complicated, but another test is to use an oscilloscope. That can show you, definitively, what refresh rate is being used at any given moment in your experiment. It's a great way to independently validate the exposure times you have requested.

    Hope that helps!

  3. #3
    Join Date
    Oct 2007
    Posts
    13
    Thank you for your answer.
    You well understood my question which concerns the refresh rate. I would like to be sure that my session actually runs at 60Hz when I fixed the refresh rate in Windows at 60hz (1024x768) and when I also set the 1024x768 mode in Direct RT.

    The first way to check it that you suggested me didn't work every time (divide all the display times by 16.67 and get an integer value); sometimes it worked, sometimes not.

    When I test the refresh rate in Direct RT, I get 62.54 Hz.
    It's perfect for me.
    But, I'm asking a question about Direct RT functionning.
    Can the refresh rate fluctuates accross the session, between trials ?
    Is there a reason to think that this rate is not the same (62.54Hz) during the whole session ?

    If I can't check it online, I would at least to be sure that the refresh rate doesn't fluctuate when I previously fixed it.

    Thanks

    aurelie





  4. #4
    Join Date
    Nov 2005
    Posts
    3,309
    To be super-certain, you could check it over the course of a session with an oscilloscope. I could also take a look at your log file if you post it as an attachment.

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