Changing X-Scale of ComplexWaveform plotted with WaveformGraph control (Plotting FFT Data)

[Jabadoodle]

Development Environment:  

* Microsoft Visual Studio 2010 Pro 

* C# Language

* National Instruments Measurement Studio 2012

Summary of Question:

* I'm plotting data from a Fast Fouirer Tranform (FFT)

* The scale on the X-Axis is currently the number of BINS in the FFT.

* I want the scale on the X-Axis to be the Frequency (which should be from 0 Hz to 1000 Hz)

More Thoughts on the Question:

* Lets say the number of data-samples I'm collecting is 4096 at a frequency of 2000 samples/second.

* This results in an FFT that has 2048 BINS and should represent frequencies from 0 to 1000 Hz.

* If I just plot the data as-is...I get an X-Axis from 0 to 2048.

* There is a scaling factor that I could multiply X by to get the proper X values. (I have that)

* The problem is the X-Values for the plot are the ARRAY INDICES...and those obviously have to be whole numbers.

Code I'm Using...simplified with irrelevant thigns left out:

void PlotFFT(System.Numerics.Complex[] FFT_Result_Bins)

{

  // Create an object that describes the properties of how the plot should be created.

  ComplexDataPart complexDataPart = ComplexDataPart.Real;

  ComplexWaveformPlotOptions complexPlotOptions = new ComplexWaveformPlotOptions(

                ComplexWaveformPlotDisplayMode.Samples,

                ComplexWaveformPlotScaleMode.Scaled, complexDataPart);

  const int DataSamplingRate = 2000; 

  const int DiscernableFrequency = DataSamplingRate / 2;

  int NumberOfBins = FFT_Result_Bins.Length;   

  double HzPerBin = DiscernableFrequency / NumberOfBins;

 // Generate an array of doubles from the FFT_Result_Bins.

 var dataPoints = new ComplexDouble[NumberOfBins];

  for (int x = 0; x < NumberOfBins; x++)

  {

      dataPoints[x].Real = FFT_Result_Bins[x].Real;

      dataPoints[x].Imaginary = FFT_Result_Bins[x].Imaginary;

  }

  // Create a WaveForm data strcuture/object from the array of data (doubles).

  ComplexWaveform<ComplexDouble> waveform = ComplexWaveform<ComplexDouble>.FromArray1D(dataPoints);

  // Plot the graph using a separate thread.

  this.BeginInvoke(new MethodInvoker(delegate()

  {waveformGraphFreqDomain.PlotComplexWaveform<ComplexDouble>(waveform, complexPlotOptions);}));

}

Thanks for any help!

gw

[Jacob_R]

Hello Jabadoodle,

Thank you for the thorough post.

Setting up your graph, with ComplexWaveformPlotDisplayMode set for samples, and plotting a waveform with 2048 data points would work as you are seeing.

I haven't configured any complex data to test this with; however based on the PlotComplexWaveform help example, you should be able to set the DisplayMode to Time and then configure the X-Axis to not display the samples. This may be configurable to show what you want. Have you tried that? What have been the results?

ComplexWaveform<ComplexDouble> waveform;
ComplexWaveformPlotOptions options = new ComplexWaveformPlotOptions(ComplexWaveformPlotDisplayMode.Time,
                                                                    ComplexWaveformPlotScaleMode.Raw,
                                                                    ComplexDataPart.Real);

GetRandomComplexWaveformData(out waveform);

// Set the X-Axis to display time
wvfmGraph.XAxes[0].MajorDivisions.LabelFormat = timeFormatString;
// Set the Y-Axis to display samples
wvfmGraph.YAxes[0].MajorDivisions.LabelFormat = samplesFormatString;

if (wvfmGraph.Plots[0].Mode != WaveformPlotMode.PlotComplexWaveform)
    wvfmGraph.ClearData();
wvfmGraph.PlotComplexWaveform<ComplexDouble>(waveform, options);

[Jabadoodle]

Jacob, thanks for the suggestion.

I found I could get what I wanted by turning off the X-Axis Major & Minor division lines...then creating my own custom divisions on the X-Axis where I need them to be. 

This is working for now, but I'll try DisplayMode as Time becuase I'm curious if that will do it and be simpler.

Thanks again!