Why is timer object firing at an incorrect frame rate?

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James Potter
James Potter 2013-1-16
I am using a timer object to try to enforce a frame rate of 25 Hz (0.04 seconds per period) for a few minutes. When I use tic/toc to see how long each 25-frame "second" takes, it is not one second. The time-per-second depends on what I have running in the TimerFcn. I have seen a string of 1.1-second "seconds", and other times there are strings of 0.92-second "seconds".
I am confused because when I use the "AveragePeriod" method, the timer always thinks it has perfect timing, and has an average period of 0.04 seconds. Even if the actual periods seem to be 10% off. Does the timer use a completely different system than the tic/toc functions?
I am running it in 'FixedRate' mode, so it should run the TimerFcn every 0.04 seconds. I have tried it with both 'drop' and 'queue' options for the BusyAction.
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James Potter
James Potter 2013-1-17
Here's how I'm defining the timer:
trialTimer = timer('ExecutionMode', 'FixedRate', 'Period', 0.04, 'TimerFcn', @RunOneFrame, 'TasksToExecute', 115*25, 'StartDelay', 0);
As an experiment, I made RunOneFrame contain almost nothing. In the Profile Viewer, timer.wait took up over 95% of the self time, so excessive computation time wasn't an issue. Using get(trialTimer, 'AveragePeriod) gave answers of 0.04 seconds, but the time measured with tic/toc (output every second) showed that 0.94 seconds was passing for each 25-frame set, instead of 1 second.
The computer is running MATLAB 2012a, with Windows XP operating system. It has 1 GB of RAM (it's a little old).
James Potter
James Potter 2013-1-18
Again, the main thing that confuses me is the difference between what the timer thinks its average period is, and what the actual average period is. I don't know what would cause these to be different.

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