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Added a FFT based RRC filter

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This commit is contained in:
f4exb 2018-05-22 00:10:56 +02:00
parent 35c4d5a325
commit 74286a5767
2 changed files with 59 additions and 6 deletions
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35
sdrbase/dsp/fftfilt.cxx
View File

@ -28,6 +28,7 @@
// ----------------------------------------------------------------------------
#include <memory.h>
#include <algorithm>
#include <iostream>
#include <fstream>
#include <cstdlib>
@ -130,7 +131,7 @@ void fftfilt::create_filter(float f1, float f2)
for (int i = 0; i < flen2; i++)
filter[i] *= _blackman(i, flen2);
fft->ComplexFFT(filter);
fft->ComplexFFT(filter); // filter was expressed in the time domain (impulse response)
// normalize the output filter for unity gain
float scale = 0, mag;
@ -155,7 +156,7 @@ void fftfilt::create_dsb_filter(float f2)
filter[i] *= _blackman(i, flen2);
}
fft->ComplexFFT(filter);
fft->ComplexFFT(filter); // filter was expressed in the time domain (impulse response)
// normalize the output filter for unity gain
float scale = 0, mag;
@ -182,7 +183,7 @@ void fftfilt::create_asym_filter(float fopp, float fin)
filter[i] *= _blackman(i, flen2);
}
fft->ComplexFFT(filter);
fft->ComplexFFT(filter); // filter was expressed in the time domain (impulse response)
// normalize the output filter for unity gain
float scale = 0, mag;
@ -204,7 +205,7 @@ void fftfilt::create_asym_filter(float fopp, float fin)
filterOpp[i] *= _blackman(i, flen2);
}
fft->ComplexFFT(filterOpp);
fft->ComplexFFT(filterOpp); // filter was expressed in the time domain (impulse response)
// normalize the output filter for unity gain
scale = 0;
@ -218,6 +219,32 @@ void fftfilt::create_asym_filter(float fopp, float fin)
}
}
// This filter is constructed directly from frequency domain response. Run with runFilt.
void fftfilt::create_rrc_filter(float fb, float a)
{
std::fill(filter, filter+flen, 0);
for (int i = 0; i < flen; i++) {
filter[i] = frrc(fb, a, i, flen);
}
// normalize the output filter for unity gain
float scale = 0, mag;
for (int i = 0; i < flen; i++)
{
mag = abs(filter[i]);
if (mag > scale) {
scale = mag;
}
}
if (scale != 0)
{
for (int i = 0; i < flen; i++) {
filter[i] /= scale;
}
}
}
// test bypass
int fftfilt::noFilt(const cmplx & in, cmplx **out)
{

30
sdrbase/dsp/fftfilt.h
View File

@ -28,6 +28,7 @@ public:
void create_filter(float f1, float f2);
void create_dsb_filter(float f2);
void create_asym_filter(float fopp, float fin); //!< two different filters for in band and opposite band
void create_rrc_filter(float fb, float a); //!< root raised cosine. fb is half the band pass
int noFilt(const cmplx& in, cmplx **out);
int runFilt(const cmplx& in, cmplx **out);
@ -48,18 +49,43 @@ protected:
int pass;
int window;
inline float fsinc(float fc, int i, int len) {
inline float fsinc(float fc, int i, int len)
{
int len2 = len/2;
return (i == len2) ? 2.0 * fc:
sin(2 * M_PI * fc * (i - len2)) / (M_PI * (i - len2));
}
inline float _blackman(int i, int len) {
inline float _blackman(int i, int len)
{
return (0.42 -
0.50 * cos(2.0 * M_PI * i / len) +
0.08 * cos(4.0 * M_PI * i / len));
}
/** RRC function in the frequency domain. Zero frequency is on the sides with first half in positive frequencies
* and second half in negative frequencies */
inline cmplx frrc(float fb, float a, int i, int len)
{
float x = i/(float)len; // normalize to [0..1]
x = 0.5-fabs(x-0.5); // apply symmetry: now both halves overlap near 0
float tr = (fb*a)/2.0; // half the transition zone
if (x < fb-tr)
{
return 1.0; // in band
}
else if (x < fb+tr) // transition
{
float y = ((x-(fb-tr)) / (2.0*tr))*M_PI;
return (cos(y) + 1.0f)/2.0f;
}
else
{
return 0.0; // out of band
}
}
void init_filter();
void init_dsb_filter();
};