Set RTL AGC mode by default (no improvement), with command-line option to disable.

NOTES.txt

@@ -29,9 +29,7 @@ settings during a run. The LNA gain seems to switch between ~ 24 dB
 and ~ 34 dB without intermediate steps.
 
 With RTL in AGC mode, the level of the digital sample stream is normalized
-to -6 dB FS.
-Unknown whether this is an analog or digital gain stage.
-Does this improve SNR or not?
+to -6 dB FS. Unknown whether this is an analog or digital gain stage.
 
 At first I suspected that AGC mode may be a cooperation between the RTL and
 the Elonics tuner. I thought that the RTL would monitor the level and send

RtlSdrSource.cc

@@ -39,7 +39,8 @@ RtlSdrSource::~RtlSdrSource()
 bool RtlSdrSource::configure(uint32_t sample_rate,
                              uint32_t frequency,
                              int tuner_gain,
-                             int block_length)
+                             int block_length,
+                             bool agcmode)
 {
     int r;
 
@@ -78,6 +79,13 @@ bool RtlSdrSource::configure(uint32_t sample_rate,
         }
     }
 
+    // set RTL AGC mode
+    r = rtlsdr_set_agc_mode(m_dev, int(agcmode));
+    if (r < 0) {
+        m_error = "rtlsdr_set_agc_mode failed";
+        return false;
+    }
+
     // set block length
     m_block_length = (block_length < 4096) ? 4096 :
                      (block_length > 1024 * 1024) ? 1024 * 1024 :

RtlSdrSource.h

@@ -25,13 +25,14 @@ public:
      *
      * sample_rate  :: desired sample rate in Hz.
      * frequency    :: desired center frequency in Hz.
-     * gain         :: desired tuner gain index, or -1 for auto-gain.
+     * tuner_gain   :: desired tuner gain in 0.1 dB, or -1 for auto-gain.
      * block_length :: preferred number of samples per block.
      *
      * Return true for success, false if an error occurred.
      */
     bool configure(uint32_t sample_rate, uint32_t frequency, int tuner_gain,
-                   int block_length=default_block_length);
+                   int block_length=default_block_length,
+                   bool agcmode=false);
 
     /** Return current sample frequency in Hz. */
     uint32_t get_sample_rate();

TODO.txt

@@ -1,12 +1,9 @@
 * (experiment) consider reducing IF filter bandwidth to ~ 80 kHz
 * (experiment) consider downsampling IF signal before FM detection
-* (experiment) try if RTL AGC mode improves FM decoding
 
 * (feature) support 'M' 'k' suffixes for sample rates and tuning frequency
 * (feature) implement stereo pilot pulse-per-second
 * (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
-* it looks like IF level sometimes varies so much that it saturates the receiver; perhaps this can be solved by dynamically managing the hardware gain in response to level measurements
-* (quality) figure out if hardware gain settings can improve weak stations
 * (feature) implement RDS decoding
 * (quality) consider FM demodulation with PLL instead of phase discriminator

main.cc

@@ -217,6 +217,7 @@ void usage()
             "  -d devidx     RTL-SDR device index, 'list' to show device list (default 0)\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"
             "  -M            Disable stereo decoding\n"
             "  -R filename   Write audio data as raw S16_LE samples\n"
             "                use filename '-' to write to stdout\n"
@@ -266,6 +267,7 @@ int main(int argc, char **argv)
     string  filename;
     string  alsadev("default");
     double  bufsecs = -1;
+    int     agcmode = 1;
 
     fprintf(stderr,
             "SoftFM - Software decoder for FM broadcast radio with RTL-SDR\n");
@@ -275,6 +277,7 @@ int main(int argc, char **argv)
         { "dev",        1, NULL, 'd' },
         { "ifrate",     1, NULL, 's' },
         { "pcmrate",    1, NULL, 'r' },
+        { "agc",        1, NULL, 'a' },
         { "mono",       0, NULL, 'M' },
         { "raw",        1, NULL, 'R' },
         { "wav",        1, NULL, 'W' },
@@ -284,7 +287,7 @@ int main(int argc, char **argv)
 
     int c, longindex;
     while ((c = getopt_long(argc, argv,
-                            "f:d:s:r:MR:W:P::b:",
+                            "f:d:s:r:MR:W:P::b:a:",
                             longopts, &longindex)) >= 0) {
         switch (c) {
             case 'f':
@@ -328,13 +331,24 @@ int main(int argc, char **argv)
                     badarg("-b");
                 }
                 break;
+            case 'a':
+                if (!parse_opt(optarg, agcmode)) {
+                    badarg("-a");
+                }
+                break;
             default:
                 usage();
-                fprintf(stderr, "ERROR: Unknown option\n");
+                fprintf(stderr, "ERROR: Invalid command line options\n");
                 exit(1);
         }
     }
 
+    if (optind < argc) {
+        usage();
+        fprintf(stderr, "ERROR: Invalid command  line options\n");
+        exit(1);
+    }
+
     vector<string> devnames = RtlSdrSource::get_device_names();
     if (devidx < 0 || (unsigned int)devidx >= devnames.size()) {
         fprintf(stderr, "ERROR: invalid device index %d\n", devidx);
@@ -377,7 +391,8 @@ int main(int argc, char **argv)
     }
 
     // Configure RTL-SDR device and start streaming.
-    rtlsdr.configure(ifrate, tuner_freq, -1);
+    rtlsdr.configure(ifrate, tuner_freq, -1,
+                     RtlSdrSource::default_block_length, agcmode);
     if (!rtlsdr) {
         fprintf(stderr, "ERROR: RtlSdr: %s\n", rtlsdr.error().c_str());
         exit(1);
@@ -389,6 +404,8 @@ int main(int argc, char **argv)
     ifrate = rtlsdr.get_sample_rate();
     fprintf(stderr, "IF sample rate %.0f Hz\n", ifrate);
 
+    fprintf(stderr, "RTL AGC mode %s\n", agcmode ? "enabled" : "disabled");
+
     // Create source data queue.
     DataBuffer<IQSample> source_buffer;