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For an experiment (training study with human subjects), I'm trying to record pen position on at tablet PC (Thinkpad X61t, Windows 7) at a constant and high (e.g., 100 Hz) sampling rate. Unfortunately we are currently getting only variable sampling intervals (15-16ms, with some considerably longer outliers) - probably related to the screen's refresh rate plus some measurement noise. Our programmer had managed to get constant sampling rates (100Hz) on a previous setup (X60t, Windows 7), but can't seem to get that to work on the current system (the identical EXE-File produces constant vs variable output on the two systems). We are not sure the same drivers are installed on both systems (he didn't keep track of that when working on the first), but we tried different drivers and systems (e.g., Windows XP) in the past weeks.

I'd greatly appreciate any help with this - suggestions for drivers to try, programming tricks (the software is written in C), explanations for the problem...

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I'm no tablet expert, but I know that Windows is generally not a "real time" operating system and timing variations / jitter on the order of tens-of-milliseconds (or more) are not uncommon. The operating system takes every running process and gives it a slice of time and decides when to switch to another task and which task should be next by means of some proprietary algorithm that you have little-to-no control over.

If you could somehow sense the pen position independent of the software running on the tablet, you could use a microcontroller to do the data-logging job for you and get really predictable and reliable results easily within your requirement of 100Hz. How to do that without compromising the behavior of the tablet (or affecting your human test subjects perceptions) is a completely different question, likely to have many viable solutions.

One approach that comes to mind as potentially viable would be to overlay another touch screen on top of the tablet. If it is sufficiently thin it might allow the underlying touch screen to continue to operate as usual. The foreground touchscreen would only be interfacing with a microcontroller / monitoring / data logging system.

vicatcu
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