LVM file time stamps - time stamps are not uniform

LVM file time stamps - time stamps are not uniform

Post by hprd » Fri, 02 Feb 2007 08:10:09


My LVM files do not have evenly separated time stamps.  Is this by design?  I want to record data from multiple channels at say 100S/s and see this in my LVM file.  Ideally each of the 100 data points would be separated by 0.01s.  This is not what I get.
 
I have tried changing the samples per second and the samples per channel inputs on my timing and read VIs but I don't get even spacing.  I have figured out that the DAQmx Read VI always cuts the number of samples in the file in half.  So sampling at 100S/s gives me 50 data points per second in my LVM file.  Given that it cuts the points in half, I would like the spacing to be .00, .02, .04 etc., but I get this:  .00, .01, .02, .03, .08, .09, .10, .11, .16, .17, .18, .19... 
 
Is this normal or am I missing something?
 
HP
 
 
 

LVM file time stamps - time stamps are not uniform

Post by hprd » Fri, 02 Feb 2007 08:40:13

I am using hardware timing and Windows XP.  The only knowledge I have of the timing is from the file.  I realize that there is only one adc so nothing is truly being measured at the same time, but I thought it could do better.  I currently have my adc running at 20kS/s (ai.convert.rate) which suggests each channel should only be 50us apart.  That's 200 channel scans before the next 100S/s interval.  In my mind, there should be plenty of time for it all to fit.
Are you familiar with the DAQmx Read chopping the samples per second in half, or could this be happening just in the file write?
I know some people on the forums suggest using write to spreadsheet instead of lvm, but now I am wary of the timing.  If I assume a time stamp/delta t for a spreadsheet file, I could be drastically wrong.
Your thoughts?

 
 
 

LVM file time stamps - time stamps are not uniform

Post by hprd » Sun, 04 Feb 2007 06:40:10

I've solved the problem.  After posting my example code, I realized my example code doesn't fully reflect what I am doing.  I actually have 2 DAQmx Read VIs on the same task.  One reads a waveform, the other a 2D DBL.  I believe having 2 Read VIs on the same task screws up the timing for both.  By disabling the second Read VI I was able to record data as expected.
Thanks for your help as I learn what, and what not to do in LabView.
HP