HI All, I need a little help in understanding how to dynamically change the x scale of a waveform chart. Scenario: User would collect 1-4 channels of a biosignal from a USB-6001 DAQ board and sampling rate would be 1000 - 2048 Hz. The duration of the collection could range from 5s to 30s. If I wanted to display data on a waveform chart in real-time, what would my x scale properties be? For example, if I was collecting at 2048 Hz, set my update mode to strip chart, want to display 5s window worth of data, and have X axis represent # of samples collected, what would my properties be? - no values for XScale.Minimum and XScale.Maximum? - how do I determine the range? - Xscale.Multiplier = 1/2048 ? What if I wanted my x axis to reflect time (s or ms) instead of # of samples collected? What if I have an additional wait(ms) function in my display loop? My program has an event handling loop, an acquisition loop, and display loop. Looking through a number of threads, I have determined that XScale.Min, Max, Multiplier would be important in dynamically configuring my Waveform chart but I need clarification in how they are related to each other. Thanks in advance, Anton

Hi, Anton. Sometimes the best way to learn how things work is to build a Test Case and do some Experiments. As it turns out, there is a funny relationship between the Maximum and Multiplier properties, which you can experiment with using the following code I just cobbled together: The Signal Chart is a Waveform Chart operating as a Scope Chart (mainly so you can see it "sweep"). The top three controls, Sample Freq, # Samples, and Signal Freq control a Triangle Wave Generator -- the default settings of 10, 100, and 100 means that it generates a 10 Hz Triangle Wave sampled at 100 Hz (so 10 points per cycle) and generates 100 points at a time. The Wave part is fed into a timing loop to generate points every 1000/(Sample Freq) milliseconds (i.e. at a frequency of Sample Freq) and feed it to the Signal Chart. The other two controls influence the X axis. When the Chart is initialized (by having its History set to an empty Array), X Minimum is 0 (the default value) and we set X Maximum to the value at the right edge of the plot (default 100). X Multiplier (default 1) determines how many "plot points" each X input takes on the Chart. Ignore for now that I set both of these twice (we'll come back to this!). If you run this with these default settings, it will "paint" 10 Triangle Waves across the screen, starting from 0 to 100, then 100 to 200, then 200 to 300, etc. Here, the X Axis is clearly "generated points". Try setting X Max to 50, then try setting it to 200. What do you predict would happen? Did it? But suppose you want the X axis to be Time, not Points. The default settings generate 10 waves every second, and plot 10 waves, so the plot updates every second. Thus to show Time, X Max should be 1 (so Time goes from 0 to 1, 1 to 2, etc.). But now X Mult, the amount of "plot" each X point takes, is only 1/100 of the plot, so X Mult should be 0.01. So set X Max to 1, X Mult to 0.01, and before you run the test, make a prediction of (a) how the Waveform plotted differs from the X Max = 100, X Mult = 1 case, and (b) how the X "labels" will differ. Were you correct? So why did I set the Maximum and Multiplier twice? Good question, best answered by doing more experiments. Change the Block Diagram by deleting those two duplicate settings and see how the Plot changes. You'll have to make a change, then "unmake" the change to "get a surprise". Why does this happen? It's a bit of a Chicken-and-Egg problem -- the two parameters are not really independent, but interact in a funny way. It is also the case that when setting Properties, the order of the Property Nodes can matter (and does, in this case). Anyway, explore, and teach yourself how to do this to solve your own problem. I'll also attach this VI so you don't have to recreate it from the Snippet (in case Snippets aren't working ...). Bob "Learn by Experimenting" Schor

LabVIEW

Need explanation of waveform chart x scale properties

Solved!

Author

Hi Bob,

I have been playing around with your demo and the program I most recently uploaded and am still having difficulties getting the x scale right. I think what is complicating it is the Channel Writer is sending data packets of 128 samples (assuming sampling freq is 2048). This was recommended in another thread so that I wouldn't run into buffering issues. I am stuck and not sure what to do next.

If I am collecting a trial at 2048 Hz and for 5s and want my chart to have an Xaxis reflecting the entire duration of a trial, this is what I would expect for waveform chart properties:

- XScale.Maximum = 5

- XScale.Multiplier = 0.000488281; how does this get changed if the Channel Reader is spitting out 128 samples every iteration? I know the display loop runs 80X.

If I wanted my X axis to reflect the total number of samples, I would assume:

- XScale.Maximum = 10,240

- XScale.Multiplier = 1; again, how does this get influenced by the Channel Reader spitting out 128 samples every iteration?

Alternatively, I wouldn't need to display every sample of data but am not sure what would be the first step in decimating the data to display.

Thanks in advance,

Anton

Message 21 of 21

(364 Views)

What do you need our team of experts to assist you with?

Request a quote Find the right product Place an order Get support on a product

How can we help?

Please enter your information below and we'll be intouch soon.

This field is required

Preferred communication method

Email Phone call

What do you need our team of experts to assist you with?

Request a quote Find the right product Place an order Get support on a product

LabVIEW

Need explanation of waveform chart x scale properties

Re: Need explanation of waveform chart x scale properties