[I][edited from support email][/I]
For some of my images, the presentation time differs from what I requested. E.g., I request 3000ms but the images are displayed for 2992ms.
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[I][edited from support email][/I]
For some of my images, the presentation time differs from what I requested. E.g., I request 3000ms but the images are displayed for 2992ms.
This is most likely due to your system's [I]refresh rate[/I] which is one of the most fundamental concepts in high precision computerized experimentation. The screen refresh rate refers to how long it takes your computer to redraw what you see on the monitor. It is often expressed in Hz (e.g., 60Hz, 100Hz) which refers to the number of times your screen is refreshed (or redrawn) in a single second. This can easily be converted to milliseconds by dividing it into 1000, e.g., 60Hz = 1000/60 = 16.67ms per refresh; 100Hz = 1000/100 = 10ms per refresh.
Why is this so critical?
Because everything DirectRT displays will be in multiples of the refresh rate and in perfect synch with the refresh rate. For example, if your refresh rate is 10ms, then DirectRT can display stimuli for 10ms, 20ms, 30ms, 40ms, 2000ms, or 2010ms, etc. So if you have a 15ms refresh rate and you want to show a stimulus for 12ms? Not going to happen. Instead you can have display times of 0, 15, 30, 45, 60… etc. Fortunately you don't have to worry much about this. You can ask DirectRT to display stimuli for any amount of time you like. Based on your refresh rate, DirectRT will display your stimulus for as many screen refreshes as it takes to come closest to your requested display time and will create a log data file for you so that you can compare requested against actual presentation times.
Other factors can also influence display times causing an image to stay onscreen for an extra refresh, or in rare cases, longer than that. One important thing to remember is to have a sufficiently long trial interval as set in the options of your style file. By default it's 1000ms which is plenty of time for DirectRT to prepare the next trial. If your trial interval drops below 100ms then you risk not providing DirectRT with enough time to prepare the trial and that can cause delays [I]during[/I] the trial as some on-the-fly work will need to be done.
Also remember that a system's refresh rate may be different when using different screen resolutions. e.g., it may be able to handle 100Hz in 800x600, but only 80Hz in 1024x768. It's best to manually set your display to the desired resolution and then manually set the refresh rate as desired.
See also: [URL="http://www.empirisoft.com/support/showthread.php?t=55"]why can I see my subliminal stimuli?[/URL]