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usrp: try to bring back sanity to pre- and post-processing of FFT data
The pre- and post-processing of FFT data was tweaked based on results obtained from a mis-configured measurement setup, in an only partially successful attempt to right the wrong data. - usrp/fft.c (fft_complex): square the magnitude on output but don't square the scaling - usrp/fft.c (fft_real): square the magnitude on input (does this make sense ?) - usrp/fft.c (fft_real): scale the output with the square root of the FFT size (dubious)
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usrp/fft.c
14
usrp/fft.c
@ -31,8 +31,8 @@ static void fft_complex(int n, const float *re, const float *im, double *res)
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fftw_execute(plan);
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for (i = 0; i != n; i++) {
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a = hypot(out[i][0], out[i][1])/n;
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// a = a*a;
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a = hypot(out[i][0], out[i][1]); // /n;
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a = a*a/n;
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res[i] = a;
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}
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}
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@ -46,16 +46,18 @@ static void fft_real(int n, const float *re, const float *im, double *res)
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double a ;
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in = fftw_malloc(sizeof(double)*n);
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(void) a;
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for (i = 0; i != n; i++)
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in[i] = hypot(re[i], im[i]);
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for (i = 0; i != n; i++) {
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a = hypot(re[i], im[i]);
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in[i] = a*a;
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}
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plan = fftw_plan_r2r_1d(n, in, res, FFTW_REDFT10, FFTW_ESTIMATE);
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fftw_execute(plan);
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/* @@@ not sure at all about the scaling */
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for (i = 0; i != n; i++)
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res[i] /= n;
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res[i] = res[i]/sqrt(n);
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}
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