Implement command-line option for LNA gain.

NOTES.txt

@@ -160,18 +160,6 @@ radio4   2 MS/s  34 dB  default   ON     80 kHz     0.36 CLIP -38.0 dB/Hz
 Conclusion: AGC mode has little effect on quality.
 
 
-Local radio stations
---------------------
-
-radio2      92600000  (good)
-radio3      96800000  (good)
-radio4      94300000  (bad)
-qmusic     100700000  (medium)
-radio538   102100000  (medium)
-radio10    103800000  (bad)
-radio west  89300000  (medium)
-
-
 Stereo pilot frequency
 ----------------------
 
@@ -183,4 +171,47 @@ radio538 19 kHz pilot = 18999.78 Hz +- 0.04 Hz    (6 hours measurement)
 
 Conclusion: stereo pilot is not a reliable time source.
 
+
+Ferrites
+--------
+
+Claming a ferrite block on the USB cable, as close as possible to the DVB
+received, reduces disturbance peaks in the spectrum by ~ 6 dB.
+A second ferrite block at the PC side gives another small improvement.
+
+The ferrite causes a clearly audible improvement of weak stations.
+
+
+DIY antenna
+-----------
+
+Constructed  antenna from a vertical telescopic rod antenna (85 cm)
+and two steel wire ground radials ~ 85 cm.
+Antenna placed indoors close to window.
+Antenna connected to DVB receiver via 1.5 meter 75 Ohm coax cable.
+
+The ground radials are floppy and look ridiculous but they are essential.
+No ground radials, no reception.
+
+The DIY antenna has ~ 20 dB more signal than the basic DVB antenna.
+
+The DIY antenna causes notable improvement of weak stations.
+Radio4 reception improves from very bad to almost good.
+
+However, strong stations (radio3) sound slightly worse with the DIY antenna
+than with the basic DVB antenna.
+May be caused by clipping due to too strong signal from antenna.
+
+
+Local radio stations
+--------------------
+
+radio2      92600000  (good)
+radio3      96800000  (good)
+radio4      94300000  (bad)
+qmusic     100700000  (medium)
+radio538   102100000  (medium)
+radio10    103800000  (bad)
+radio west  89300000  (medium)
+
 --

RtlSdrSource.cc

@@ -1,4 +1,5 @@
 
+#include <climits>
 #include <cstring>
 #include <rtl-sdr.h>
 
@@ -59,7 +60,7 @@ bool RtlSdrSource::configure(uint32_t sample_rate,
         return false;
     }
 
-    if (tuner_gain < 0) {
+    if (tuner_gain == INT_MIN) {
         r = rtlsdr_set_tuner_gain_mode(m_dev, 0);
         if (r < 0) {
             m_error = "rtlsdr_set_tuner_gain_mode could not set automatic gain";
@@ -116,31 +117,25 @@ uint32_t RtlSdrSource::get_frequency()
 }
 
 
-// Return current tuner gain in dB.
+// Return current tuner gain in units of 0.1 dB.
-double RtlSdrSource::get_tuner_gain()
+int RtlSdrSource::get_tuner_gain()
 {
-    return 0.1 * rtlsdr_get_tuner_gain(m_dev);
+    return rtlsdr_get_tuner_gain(m_dev);
 }
 
 
-// Return a list of supported tuner gain settings in dB.
+// Return a list of supported tuner gain settings in units of 0.1 dB.
-vector<double> RtlSdrSource::get_tuner_gains()
+vector<int> RtlSdrSource::get_tuner_gains()
 {
-    vector<double> result;
-
     int num_gains = rtlsdr_get_tuner_gains(m_dev, NULL);
     if (num_gains <= 0)
-        return result;
+        return vector<int>();
 
-    int gains[num_gains];
+    vector<int> gains(num_gains);
-    if (rtlsdr_get_tuner_gains(m_dev, gains) != num_gains)
+    if (rtlsdr_get_tuner_gains(m_dev, gains.data()) != num_gains)
-        return result;
+        return vector<int>();
 
-    result.reserve(num_gains);
+    return gains;
-    for (int i = 0; i < num_gains; i++)
-        result.push_back(0.1 * gains[i]);
-
-    return result;
 }
 
 

RtlSdrSource.h

@@ -25,7 +25,7 @@ public:
      *
      * sample_rate  :: desired sample rate in Hz.
      * frequency    :: desired center frequency in Hz.
-     * tuner_gain   :: desired tuner gain in 0.1 dB, or -1 for auto-gain.
+     * tuner_gain   :: desired tuner gain in 0.1 dB, or INT_MIN for auto-gain.
      * block_length :: preferred number of samples per block.
      *
      * Return true for success, false if an error occurred.
@@ -42,11 +42,11 @@ public:
     /** Return current center frequency in Hz. */
     std::uint32_t get_frequency();
 
-    /** Return current tuner gain in dB. */
+    /** Return current tuner gain in units of 0.1 dB. */
-    double get_tuner_gain();
+    int get_tuner_gain();
 
-    /** Return a list of supported tuner gain settings in dB. */
+    /** Return a list of supported tuner gain settings in units of 0.1 dB. */
-    std::vector<double> get_tuner_gains();
+    std::vector<int> get_tuner_gains();
 
     /** Return name of opened RTL-SDR device. */
     std::string get_device_name() const

TODO.txt

@@ -1,6 +1,8 @@
+* (quality) solve issues with bad sound due to strong antenna signal
+            (experiment with adaptive LNA gain or adaptive IF gain)
 * (speedup) maybe replace high-order FIR downsampling filter with 2nd order butterworth followed by lower order FIR filter
-* figure out why we sometimes lose stereo lock
 * (feature) implement RDS decoding
 * (quality) consider FM demodulation with PLL instead of phase discriminator
+* (quality) consider pulse-counting discriminator
 * (speedup) consider downsampling of IF signal before FM detection
             (work in progress in branch "filterif")

main.cc

@@ -23,6 +23,7 @@
 #include <cmath>
 #include <csignal>
 #include <cstring>
+#include <algorithm>
 #include <atomic>
 #include <condition_variable>
 #include <memory>
@@ -216,6 +217,7 @@ void usage()
     "Usage: softfm -f freq [options]\n"
             "  -f freq       Frequency of radio station in Hz\n"
             "  -d devidx     RTL-SDR device index, 'list' to show device list (default 0)\n"
+            "  -g gain       Set LNA gain in dB, or 'auto' (default auto)\n"
             "  -s ifrate     IF sample rate in Hz (default 1000000, min 900001)\n"
             "  -r pcmrate    Audio sample rate in Hz (default 48000 Hz)\n"
             "  -a 0          Disable RTL AGC mode (default 1 = enabled)\n"
@@ -290,6 +292,7 @@ int main(int argc, char **argv)
 {
     double  freq    = -1;
     int     devidx  = 0;
+    int     lnagain = INT_MIN;
     double  ifrate  = 1.0e6;
     int     pcmrate = 48000;
     bool    stereo  = true;
@@ -308,6 +311,7 @@ int main(int argc, char **argv)
     const struct option longopts[] = {
         { "freq",       1, NULL, 'f' },
         { "dev",        1, NULL, 'd' },
+        { "gain",       1, NULL, 'g' },
         { "ifrate",     1, NULL, 's' },
         { "pcmrate",    1, NULL, 'r' },
         { "agc",        1, NULL, 'a' },
@@ -321,7 +325,7 @@ int main(int argc, char **argv)
 
     int c, longindex;
     while ((c = getopt_long(argc, argv,
-                            "f:d:s:r:MR:W:P::T:b:a:",
+                            "f:d:g:s:r:MR:W:P::T:b:a:",
                             longopts, &longindex)) >= 0) {
         switch (c) {
             case 'f':
@@ -333,6 +337,23 @@ int main(int argc, char **argv)
                 if (!parse_int(optarg, devidx))
                     devidx = -1;
                 break;
+            case 'g':
+                if (strcasecmp(optarg, "auto") == 0) {
+                    lnagain = INT_MIN;
+                } else if (strcasecmp(optarg, "list") == 0) {
+                    lnagain = INT_MIN + 1;
+                } else {
+                    double tmpgain;
+                    if (!parse_dbl(optarg, tmpgain)) {
+                        badarg("-g");
+                    }
+                    long int tmpgain2 = lrint(tmpgain * 10);
+                    if (tmpgain2 <= INT_MIN || tmpgain2 >= INT_MAX) {
+                        badarg("-g");
+                    }
+                    lnagain = tmpgain2;
+                }
+                break;
             case 's':
                 // NOTE: RTL does not suppor sample rate 900 kS/s or lower
                 if (!parse_dbl(optarg, ifrate) || ifrate <= 900000) {
@@ -388,6 +409,7 @@ int main(int argc, char **argv)
 
     vector<string> devnames = RtlSdrSource::get_device_names();
     if (devidx < 0 || (unsigned int)devidx >= devnames.size()) {
+        if (devidx != -1)
             fprintf(stderr, "ERROR: invalid device index %d\n", devidx);
         fprintf(stderr, "Found %u devices:\n", (unsigned int)devnames.size());
         for (unsigned int i = 0; i < devnames.size(); i++) {
@@ -427,8 +449,22 @@ int main(int argc, char **argv)
         exit(1);
     }
 
+    // Check LNA gain.
+    if (lnagain != INT_MIN) {
+        vector<int> gains = rtlsdr.get_tuner_gains();
+        if (find(gains.begin(), gains.end(), lnagain) == gains.end()) {
+            if (lnagain != INT_MIN + 1)
+                fprintf(stderr, "ERROR: LNA gain %.1f dB not supported by tuner\n", lnagain * 0.1);
+            fprintf(stderr, "Supported LNA gains: ");
+            for (int g: gains)
+                fprintf(stderr, " %.1f dB ", 0.1 * g);
+            fprintf(stderr, "\n");
+            exit(1);
+        }
+    }
+
     // Configure RTL-SDR device and start streaming.
-    rtlsdr.configure(ifrate, tuner_freq, -1,
+    rtlsdr.configure(ifrate, tuner_freq, lnagain,
                      RtlSdrSource::default_block_length, agcmode);
     if (!rtlsdr) {
         fprintf(stderr, "ERROR: RtlSdr: %s\n", rtlsdr.error().c_str());
@@ -436,12 +472,19 @@ int main(int argc, char **argv)
     }
 
     tuner_freq = rtlsdr.get_frequency();
-    fprintf(stderr, "device tuned for %.6f MHz\n", tuner_freq * 1.0e-6);
+    fprintf(stderr, "device tuned for:  %.6f MHz\n", tuner_freq * 1.0e-6);
+
+    if (lnagain == INT_MIN)
+        fprintf(stderr, "LNA gain:          auto\n");
+    else
+        fprintf(stderr, "LNA gain:          %.1f dB\n",
+                0.1 * rtlsdr.get_tuner_gain());
 
     ifrate = rtlsdr.get_sample_rate();
-    fprintf(stderr, "IF sample rate %.0f Hz\n", ifrate);
+    fprintf(stderr, "IF sample rate:    %.0f Hz\n", ifrate);
 
-    fprintf(stderr, "RTL AGC mode %s\n", agcmode ? "enabled" : "disabled");
+    fprintf(stderr, "RTL AGC mode:      %s\n",
+            agcmode ? "enabled" : "disabled");
 
     // Create source data queue.
     DataBuffer<IQSample> source_buffer;
@@ -458,8 +501,8 @@ int main(int argc, char **argv)
     // Prevent aliasing at very low output sample rates.
     double bandwidth_pcm = min(FmDecoder::default_bandwidth_pcm,
                                0.45 * pcmrate);
-    fprintf(stderr, "audio sample rate %u Hz\n", pcmrate);
+    fprintf(stderr, "audio sample rate: %u Hz\n", pcmrate);
-    fprintf(stderr, "audio bandwidth %.3f kHz\n", bandwidth_pcm * 1.0e-3);
+    fprintf(stderr, "audio bandwidth:   %.3f kHz\n", bandwidth_pcm * 1.0e-3);
 
     // Prepare decoder.
     FmDecoder fm(ifrate,                            // sample_rate_if
@@ -483,17 +526,17 @@ int main(int argc, char **argv)
         outputbuf_samples = (unsigned int)(bufsecs * pcmrate);
     }
     if (outputbuf_samples > 0) {
-        fprintf(stderr, "output buffer %.1f seconds\n",
+        fprintf(stderr, "output buffer:     %.1f seconds\n",
                 outputbuf_samples / double(pcmrate));
     }
 
     // Open PPS file.
     if (!ppsfilename.empty()) {
         if (ppsfilename == "-") {
-            fprintf(stderr, "Writing pulse-per-second markers to stdout\n");
+            fprintf(stderr, "writing pulse-per-second markers to stdout\n");
             ppsfile = stdout;
         } else {
-            fprintf(stderr, "Writing pulse-per-second markers to '%s'\n",
+            fprintf(stderr, "writing pulse-per-second markers to '%s'\n",
                     ppsfilename.c_str());
             ppsfile = fopen(ppsfilename.c_str(), "w");
             if (ppsfile == NULL) {
@@ -510,17 +553,17 @@ int main(int argc, char **argv)
     unique_ptr<AudioOutput> audio_output;
     switch (outmode) {
         case MODE_RAW:
-            fprintf(stderr, "Writing raw 16-bit audio samples to '%s'\n",
+            fprintf(stderr, "writing raw 16-bit audio samples to '%s'\n",
                     filename.c_str());
             audio_output.reset(new RawAudioOutput(filename));
             break;
         case MODE_WAV:
-            fprintf(stderr, "Writing audio samples to '%s'\n",
+            fprintf(stderr, "writing audio samples to '%s'\n",
                     filename.c_str());
             audio_output.reset(new WavAudioOutput(filename, pcmrate, stereo));
             break;
         case MODE_ALSA:
-            fprintf(stderr, "Playing audio to ALSA device '%s'\n",
+            fprintf(stderr, "playing audio to ALSA device '%s'\n",
                     alsadev.c_str());
             audio_output.reset(new AlsaAudioOutput(alsadev, pcmrate, stereo));
             break;