问题
I want to plot the frequency spectrum (like they do for example in Audacity). Hence I want the frequency in Hertz on the x-axis and the amplitude on the y-axis. My input is a periodically sine wave with 0,7 as amplitude and 500HZ as frequency.I use FFTW to compute the magnitude and QWT to plot. My problem , what parameters should I put in setSamples to get a pic on 500 HZ ? Any help would be appreciated here is my code
回答1:
From documentation you must set two pointers to data that build the curve and the size (number of samples in your case).
void QwtPlotCurve::setSamples(const double *xData, const double *yData, int size)
In your case yData is the data from FFT, the xData is an array that sweep from min to max/2 frequency values of the FFT discarding the other half when the signal is a real signal:
curve->setSamples(signalx, magnitude, N/2);
this should work if I'm correct.
EDIT
Change also:
signalx[i]=i;
magnitude[i]=sqrt(reout[i]*reout[i] + imgout[i]*imgout[i]); //calculate magnitude
to:
signalx[i]=(double)(Fs * i) / (double)N;
magnitude[i]=sqrt(reout[i]*reout[i] + imgout[i]*imgout[i]) / ((double)N / 2.0);
to represent the frequency for magnitude[i] element in plot.
You should check also for the nyquist frequency. Sampling 500Hz sine wave at 1000Hz produce aliasing and if you plot your signal[] data (at discrete steps) you can see it.
来源：https://stackoverflow.com/questions/24353795/how-to-plot-spectrum-using-fftw3-qwt