I am trying to use a USB-6009 to determine the response time of a lcd monitor. To do this I have a matlab script that toggles the screen between black and white, I note the time the flip command on the gpu is completed. At the same time I have a photodiode attached to the 6009 analog input, pressed against the screen. I do a continous background sampling of 1khz during the trial, 20 seconds in total, so far all good. But when I try to synchronize the two dataseries, flip-time and sampletime, I get into some trouble. I get an absolute system time from the matlab data acquisition library for the first sample (TriggerTime), but this time seems off. What I have noticed is that it seems to get better if I lower the threshold for calling the callback function that receives my data and stores it.
So my first question is how accurate this TriggerTime actually is? Is it set in the computer, and not in the daq itself!? And if this is the case, how is one supposed to sync two timeseries like I am trying to do?