Added a FFT based RRC filter

sdrbase/dsp/fftfilt.cxx

@@ -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)
 {

sdrbase/dsp/fftfilt.h

@@ -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();
 };