Merge pull request #252 from cjcliffe/ssb_fix

SSB mode FIR filter side-band supression

src/modules/modem/analog/ModemLSB.cpp

@@ -2,8 +2,20 @@
 
 ModemLSB::ModemLSB() : ModemAnalog() {
     // half band filter used for side-band elimination
-    ssbFilt = resamp2_crcf_create(12,-0.25f,60.0f);
     demodAM_LSB = ampmodem_create(0.5, 0.0, LIQUID_AMPMODEM_LSB, 1);
+    // options
+    float fc = 0.25f;         // filter cutoff frequency
+    float ft = 0.05f;         // filter transition
+    float As = 90.0f;         // stop-band attenuation [dB]
+    float mu = 0.0f;          // fractional timing offset
+    
+    // estimate required filter length and generate filter
+    unsigned int h_len = estimate_req_filter_len(ft,As);
+    float h[h_len];
+    liquid_firdes_kaiser(h_len,fc,As,mu,h);
+    ssbFilt = firfilt_crcf_create(h,h_len);
+    ssbShift = nco_crcf_create(LIQUID_NCO);
+    nco_crcf_set_frequency(ssbShift,  (2.0 * M_PI) * 0.25);
 }
 
 Modem *ModemLSB::factory() {
@@ -15,7 +27,8 @@ std::string ModemLSB::getName() {
 }
 
 ModemLSB::~ModemLSB() {
-    resamp2_crcf_destroy(ssbFilt);
+    firfilt_crcf_destroy(ssbFilt);
+    nco_crcf_destroy(ssbShift);
     ampmodem_destroy(demodAM_LSB);
 }
 
@@ -45,8 +58,12 @@ void ModemLSB::demodulate(ModemKit *kit, ModemIQData *input, AudioThreadInput *a
     
     liquid_float_complex x, y;
     for (int i = 0; i < bufSize; i++) { // Reject upper band
-        resamp2_crcf_filter_execute(ssbFilt,input->data[i],&x,&y);
+        nco_crcf_step(ssbShift);
-        ampmodem_demodulate(demodAM_LSB, x, &demodOutputData[i]);
+        nco_crcf_mix_up(ssbShift, input->data[i], &x);
+        firfilt_crcf_push(ssbFilt, x);
+        firfilt_crcf_execute(ssbFilt, &x);
+        nco_crcf_mix_down(ssbShift, x, &y);
+        ampmodem_demodulate(demodAM_LSB, y, &demodOutputData[i]);
     }
     
     buildAudioOutput(akit, audioOut, true);

src/modules/modem/analog/ModemLSB.h

@@ -16,6 +16,7 @@ public:
     void demodulate(ModemKit *kit, ModemIQData *input, AudioThreadInput *audioOut);
     
 private:
-    resamp2_crcf ssbFilt;
+    firfilt_crcf ssbFilt;
+    nco_crcf ssbShift;
     ampmodem demodAM_LSB;
 };

src/modules/modem/analog/ModemUSB.cpp

@@ -2,8 +2,20 @@
 
 ModemUSB::ModemUSB() : ModemAnalog() {
     // half band filter used for side-band elimination
-    ssbFilt = resamp2_crcf_create(12,-0.25f,60.0f);
     demodAM_USB = ampmodem_create(0.5, 0.0, LIQUID_AMPMODEM_USB, 1);
+    // options
+    float fc = 0.25f;         // filter cutoff frequency
+    float ft = 0.05f;         // filter transition
+    float As = 90.0f;         // stop-band attenuation [dB]
+    float mu = 0.0f;          // fractional timing offset
+    
+    // estimate required filter length and generate filter
+    unsigned int h_len = estimate_req_filter_len(ft,As);
+    float h[h_len];
+    liquid_firdes_kaiser(h_len,fc,As,mu,h);
+    ssbFilt = firfilt_crcf_create(h,h_len);
+    ssbShift = nco_crcf_create(LIQUID_NCO);
+    nco_crcf_set_frequency(ssbShift,  (2.0 * M_PI) * 0.25);
 }
 
 Modem *ModemUSB::factory() {
@@ -15,7 +27,8 @@ std::string ModemUSB::getName() {
 }
 
 ModemUSB::~ModemUSB() {
-    resamp2_crcf_destroy(ssbFilt);
+    firfilt_crcf_destroy(ssbFilt);
+    nco_crcf_destroy(ssbShift);
     ampmodem_destroy(demodAM_USB);
 }
 
@@ -45,7 +58,11 @@ void ModemUSB::demodulate(ModemKit *kit, ModemIQData *input, AudioThreadInput *a
     
     liquid_float_complex x, y;
     for (int i = 0; i < bufSize; i++) { // Reject lower band
-        resamp2_crcf_filter_execute(ssbFilt,input->data[i],&x,&y);
+        nco_crcf_step(ssbShift);
+        nco_crcf_mix_down(ssbShift, input->data[i], &x);
+        firfilt_crcf_push(ssbFilt, x);
+        firfilt_crcf_execute(ssbFilt, &x);
+        nco_crcf_mix_up(ssbShift, x, &y);
         ampmodem_demodulate(demodAM_USB, y, &demodOutputData[i]);
     }
     

src/modules/modem/analog/ModemUSB.h

@@ -16,6 +16,7 @@ public:
     void demodulate(ModemKit *kit, ModemIQData *input, AudioThreadInput *audioOut);
     
 private:
-    resamp2_crcf ssbFilt;
+    firfilt_crcf ssbFilt;
+    nco_crcf ssbShift;
     ampmodem demodAM_USB;
 };