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The real part of the complex spectrum is shown in the second plot from the top in Figure 1 . It will become important later to note that the real part of the spectrum is symmetrical about the folding frequency near the center of theplot (at the eighth tick mark) .
Without attempting to explain why, I will simply tell you that the real part of the Fourier transform of a complex series whose imaginary part is all zeros (like a typical sampled time series for real-world data) is always symmetrical about the folding frequency.
The imaginary part of the complex spectrum is shown in the third plot from the top. Again, it will become important later to note that the imaginary partof the spectrum is asymmetrical about the folding frequency.
Once again, without attempting to explain why, the imaginary part of the Fourier transform of a complex series whose imaginary part is all zeros (like a typical sampled time series for real-world data) is always asymmetrical about the folding frequency.
It is also true that the values of the imaginary part of the Fourier transform of a complex spectrum whose real part is symmetrical about the foldingfrequency and whose imaginary part is asymmetrical about the folding frequency will all be zero. I will take advantage of these facts later to simplify thecomputing and plotting process.
The amplitude spectrum is shown in the fourth plot down from the top. Recall from previous modules that the amplitude values are always positive, consistingof the square root of the sum of the squares of the real and imaginary parts.
The output time series, produced by performing an inverse Fourier transform on the complex spectrum is shown in the bottom plot in Figure 1 . Compare the bottom plot to the top plot. As you can see, they are the same, demonstratingthe reversible nature of the Fourier transform.
I will discuss this program in fragments. A complete listing of the program is provided in Listing 14 near the end of the module.
The beginning of the class for Dsp035 , including the declaration of some variables and the creation of some array objects is shown in Listing 1 . This code is straightforward.
| Listing 1. The beginning of the class named Dsp035. |
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class Dsp035 implements GraphIntfc01{
final double pi = Math.PI;int len = 256;
double[]timeDataIn = new double[len];double[] realSpect = new double[len];
double[]imagSpect = new double[len];double[] angle = new double[len];//unused
double[]magnitude = new double[len];double[] timeDataOut = new double[len]; |
The constructor begins in Listing 2 . The code in Listing 2 creates the input time series data shown in the top plot of Figure 1 .
| Listing 2. Beginning of the constructor. |
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public Dsp035(){//constructor
//Create the raw data pulsestimeDataIn[0] = 0;timeDataIn[1] = 50;//...
//code deleted for brevity//...
timeDataIn[254]= -80;
timeDataIn[255]= -80;
//Create raw data sinusoidfor(int x = len/3;x<3*len/4;x++){
timeDataIn[x]= 80.0 * Math.sin(
2*pi*(x)*1.0/20.0);}//end for loop |
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