How to save the data of different rates in a single VI?

Hello

I tried to acquire 2-channel measurement of voltage at 10 kHz and analyze data, finally to save the data to both 1 kHz and 1 Hz. What I did is this loop of producer/consumer use to do the work of recording at different rates. I have queued all the data in the main loop, and then removed the data and write it to the file in other loops. I added "wait = 1 ms' a consumer loop for recording data at 1 kHz (loop of fast logging) and added" wait = 1000ms' in another loop to get the slow 1 Hz (logging loop) data record.

My problem is that the data record works fine in the case of 1 Hz, but it does not work for the case of 1 kHz. Although the time-out period defined as 1ms, the interval between two samples in the loop of fast data recording is about 100ms.  Subsequently, 1ms, 10ms, 100ms, 150ms tried in this loop to see what intervals are in these cases. I found that the intervals are same as assigned in the timeout greater than 100 ms, but if the time-out is set to 1ms or 10ms, the intervals are again around 100ms.

My question is - why I can't reach data record 1 kHz using loop of consumers/producers, while the same way works for the case of 1 Hz?

Thank you very much

Hao

You are sampling at 10 kHz and you want to back up your data "as if you sampled at 1 kHz and 1 Hz", if I understand correctly.  I also understand that you use the design of producer/consumer model to help with this (a good idea).

I'm assuming that the producer 'product' a segment of data at a time, to say the value of a second of data or a 2 x 10000 I16 (assuming a 16-bit integer A/D) table you put into a queue and wait for the consumer.

If you have data at 10 kHz, how do you get representations to 1 kHz or 1 Hz?  There are (at least) two possibilities - average data over the interval of proper time, or (re) sample data.  The good thing is that you can handle the situation 1 kHz and 1 Hz, essentially at the same time and in the same processing loop.  I'll discuss the case 1 kHz - with the assumptions I made, the case of 1 Hz becomes almost trivial.  I'll discuss treatment also just one channel, as it is similarly almost trivial to go from 1 to N-channel.

If you have 1 second of data sampled at 10 kHz, and will "reduce" 1 second of data sampled at 1 kHz.  Note that you have 10 samples for each data point that you want in the end.  The "Averaging" method said "Replace all 10 points in the average of these 10 points", while "resampling" method said "replace 10 points with, say, the first point, ignoring the others.  An easy way to do this is to use the Reshape feature to transform an array of 10 x 1000 elements of your array of 10000 items 1-D 2D.  Now, you can use a loop FOR an average of 10 items or simply extract the first.  [This is where almost everyone, including me, a mistake - when you put a 10 x 1000 2D array in a loop, you get 10 iterations of 1000 items and we want 1000 iterations of 10 items.]  Oops, what to do?  [Simple - converts 2D array before sending in the loop FOR.]

So when you have processed your 10,000 points and got both 1000 samples "1 kHz" and 1 "1 Hz" sample, you write each of them in their respective folder and wait for the next item in the queue of the consumer.  Consumer focus on the queue loop, it will automatically 'block', waiting for the producer of the 'feed' some data.  Note that the producer generates data once per second, producing 10000 points for the consumer in the process, but the time to deal with these points and perform the required disk i/o should take only a few milliseconds (certainly not more than 100, even if writing to a floppy!), so you don't have much time to do other interesting things (such as display data).

Bob Schor

Tags: NI Software

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