Conditionally capturing certain data while continuously acquiring data

Question

In case my question has totally confused you, let me describe the situation, a general one which I'm sure some of you have already tackled.
Suppose you're acquiring data from several channels (let's say two waveforms). And you only want to capture their amplitudes (Y1, Y2,etc.) when either a button is pushed (i.e., things are steady state, now grab that data) or when something else has changed a certain amount (say their frequency has changed 2 Hz).
When I say "capture", I simply mean grabbing the data into a separate array of X,Y values for later processing or on-line plotting while still acquiring data.
Hopefully, this makes sense. Would greatly appreciate any thoughts on this, as it's a
situation that I seem to run into often.
All the best, Hunter

Answer 1

Your use of the terms "Waveforms" and "amplitude" implies a measurement of many samples into one number (for each channel).
For example, if your signal was pure 100 Hz, 1.000 Volts peak, and you captured a 0.1 second block of that signal, then you would have 10 cycles in the capture buffer, varying from +1V to -1V. You can use the RMS function to turn that block of samples into a number of 0.707, meaning the RMS value is 0.707V (= 1.0 V peak).
The basic philosophy would be:
1... Start a continuous acquisition. By specifying NUMBER oF SAMPLES = 0 in the AI-START vi, you will acquire data forever (until the buffer overflows, or until stopped).
2... Periodically (based on a timer), you ask the AI READ function to read zero samples. This does not remove data, but it does tell you how many samples are in the buffer.
3... You immediately read that many samples out of the buffer, and convert from volts to engineering units (if appropriate).
4... If you are measuring their frequency, you need to perform an FFT now. The highest peak in the magnitude of the FFT output is the dominant frequency in the signal. (Freq. of peak = Index of peak * Sample Rate / NPoints in block.
5... If you are doing FFTs, you get a big speed advantage if your block size is a power of two (128, 256, 512, 1024, etc.). If so, then replace step 3 with *-IF-* backlog >= N (power of 2) then read N points from buffer.
6... Apply your criteria - *-IF-* the button has been pressed, *-OR-* if the frequency of channel 2 is different, then compute the RMS or peak values and append to your final results array. Otherwise do nothing.
If you want to detect 2-Hz changes, you need to know the rules of how FFTs work - the frequency resolution is the BIN WIDTH, which equals the sample rate divided by the number of points in the block. If you have a 1000 Hz sample rate and a 1024 point block, your resolution is 1000 / 1024 or just less than 1 Hz. But if you have a 1000 Hz sample rate and a 32 point block, your resolution is 1000 / 32 or about 30 Hz.
Steve Bird
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