Daniel,
Thank you! You are running LabVIEW 2021 (which, fortunately, I have installed on one of my PCs, so I can open your VI, which consists of two Express VIs (not useful for what you are trying to do), and explain many of the problems you are having. And it also tells me that you have not learned the "Three Rules of Data Flow" (the programming paradigm that LabVIEW uses).
Here's the basis of Data Flow. First, there are "things", and "wires". "Things" include Functions (like "Add" or "Write Text File"), Structures (like a Case statement, a While or For Loop), and VIs (or sub-VIs) that you might write. "Things" usually have "Inputs" and "Outputs" -- data comes into the "Thing" through its "Inputs" and goes/ out through its "Outputs" (you can have zero or more Inputs or Outputs). "Wires" carry data, can go into and out of "Structures", and can go from the Output of a "Thing" to the Input of another "Thing". Got that?
So what are these "Three Rules" (that's what I call what comes next)?
- A Structure, Function, or VI (i.e. a "Thing") doesn't start to do anything until all of its Inputs have data present. I forgot to mention that VIs get their data in and out through their Front Panels, where the inputs are called "Controls" and the Outputs are called "Indicators".
- "Things" don't put data on any of their Outputs until the Thing is done with whatever it does (e.g. an Add doesn't give you a sum output until it finishes adding the data presented to its inputs).
- If you have multiple "Things" that aren't connected by "Wires" such that data flows from one to the other, you cannot say which one starts (or finishes) first. You need to treat them as if they run simultaneously, as in "parallel programming".
So I looked at your first DAQ Assistant, which is reading Voltages. You have set it up to take Continuous Samples (good, you don't want the data to be "interrupted" between samplings), acquiring 50,000 of them at a frequency of 44.1 kHz. It is connected by a strange wire, a Dynamic Data wire (which I never use, though, when they were first introduced, I tried them for about two weeks, then swore "never again!") to another DAQ Assistant (I call these the "Dreaded DAQ Assistant", or "DDA" for short) that accepts 10,000 data points and writes them out as a Voltage signal. I don't know what device you are using, but it appears that the data rate for output might be 1 kHz.
And finally, there's a Stop Button, wired to the Stop input of the first DDA, with its output going to the Stop input of the second DDA, and the final output going to the Stop Control of the While Loop.
So let's talk Data Flow. Assume you start the program with the Stop button not pushed. The While Loop starts, the first DDA starts, it collects 50 k samples at 44.1 kHz (which takes 50/44.1 = 1.13 (approx.) seconds, then puts those samples into the Dynamic Data Wire (DDW?). At this point, DDA #1 can start generating the next batch of 50 k samples. Meanwhile, DDA #2 accepts 10,000 (or 1/5 of the DDW's content) and starts playing it out at some rate (which might be 1 kHz). This takes 10 seconds, right? And that's only 1/5 of the data, so it will take 50 seconds to output the first 1.13 seconds of sampled data.
But we're not done -- there's the Stop button to consider. We assumed it was not pushed when we started, but let's say you pushed it 1 second after you started this routine running. Will this value be transferred to the While Loop's Stop Control? Remember the Third Rule -- everything in the While Loop that is not connected output-to-input runs "simultaneously". But the only thing that is not so connected is the Stop button, so it runs first, and when it does, it is not pushed. So it puts "False" on the input to DDA #1, which 1.13" later puts "False" on the output of DDA #1, which puts False on the input of DDA #2, which 50 seconds later puts False on the Stop Control of the While loop, so the Loop tries to run again. The second time, the Button has been pressed, so after another 1.13 + 50 seconds, the While loop will finally stop.
Some of my timing might be off because I don't know what device you are using, but you see the problem, I hope. But not to worry -- if you learn a little bit more about LabVIEW, you can sample DVD Audio (44.1 kHz) and play it out through speakers (with a faster D/A converter, like in a Sound Card) without a 50 second "lag".
To get a sense of how to wean yourself from the DDA, do a Web Search for "Learn 10 Functions in NI DAQmx and Handle 80 Percent of Your Data Acquisition Applications". Ignore Section 1, which talks about the DDA. I haven't looked at this White Paper recently, but as I recall, it tells you "How To" do things in LabVIEW and in other languages like C and C++ -- pay attention only to the LabVIEW sections.
Here's a little exercise to get you thinking Data Flow and learning the Joy of LabVIEW.
- Let's stick with the DDA and the DDW for the time being (even though you want to avoid them as soon as you possibly can, which is why I pointed you to the DAQmx White Paper). Design a While Loop that acquire data at 44.1 kHz, but delivers a data sample every 10 milliseconds. That's all it does -- it samples continuously, outputting data, which you should bring to the right-hand edge of the While Loop, where it will present itself as an "Output Tunnel" (don't connect this output tunnel to anything). Put the Stop Button inside this While Loop and wire the Stop output to the Stop Control.
- When you run it, verify that you can now stop the loop "almost instantly". Experiment a bit with the Loop speed -- what do you have to do to make the Loop run at 1 Hz (instead, as above, when it runs at 100 Hz)?
- Imagine, now, you had a second While Loop (placed to the right of the first, "Producing" loop) that could accept the (sound) samples and "play" them at a rate of 44.1 kHz, effectively "Consuming" the data produced by the first loop. Now look up "Consumer/Producer Design Pattern".
That's all for now.
Bob Schor
