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In Cont Acq&Graph(buffered), if the scan rate is changed slightly, the display update rate slows dramatically. Why?

I am using Labview6 on a Windows2000 PC with DAQcard-AI-16E-4 and a recent version of NI-DAQ. When I run Cont Acq&Graph(buffered)with sample rate of 1000Hz and scans to read 40 (buffer size 4000), the graph updates smoothly. When I set the scan rate to 1001 Hz, the graph updates slowly. It's even a little slower at other scan rates, like 4000Hz. Synchronous display is "on". What's going on?
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What kind of signal are you viewing with your DAQCard? Could it be that when sampling at the lower rate it is aliasing the signal making it appear to be moving faster/slower?

If the frequency that you are sampling at is less than twice the frequancy of your signal, you will run into this issue.

Or is it that how it updates on the screen is not smooth? i.e. updates the left of the graph then the right.

Brian
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Thanks for replying so quickly to my question. It turns out that it was a problem with my DAQcard. My signal was a 1.3Hz sinewave from a pulse generator, and the effect stayed when I changed the frequency. The graph updated smoothly, it would just do it jerkily -- the entire screen would refresh, it would pause, then refresh again. It was very dramatic and happened for a variety of frequencies, not just 1001Hz.
I used this as an axample of a larger problem I'm having, in which I use the signal from a continuous analog input acquisition to update the location of a spot in a picture control at 60Hz. I put AI read in a loop and read a certain number of scans at a time to ensure that I get a 60Hz update rate. (i.e. if my scan rate is 6000Hz and update rate
of the picture control is 60hz, I read 100 scans at a time to ensure hardware timing of the loop.) The weird thing is that the appearance of the picture control is dependent on scan rate, regardless of update rate/number of scans to read at a time. If my scan rate is 1000Hz, the spot moves smoothly (at admittedly high speed). If I set it to 4000Hz, the spot jerks around the screen and it appears to update only every tenth of a second or so. I am confused as to how the scan rate of my daq card could affect the software in this way.
By the way, I appreciate all of the help you labview gurus give all of the rest of us who try to use it for really weird stuff. 🙂
--Melinda
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