Add software adaptive gain control, ported from dump1090-fa

.gitignore

@@ -4,3 +4,4 @@ dump978-fa
 skyaware978
 faup978
 fec_tests
+adaptive_tests

Makefile

@@ -14,7 +14,7 @@ LIBS_SDR=-lSoapySDR
 
 all: dump978-fa skyaware978
 
-dump978-fa: dump978_main.o socket_output.o message_dispatch.o fec.o libs/fec/init_rs_char.o libs/fec/decode_rs_char.o sample_source.o soapy_source.o convert.o demodulator.o uat_message.o stratux_serial.o
+dump978-fa: dump978_main.o socket_output.o message_dispatch.o fec.o libs/fec/init_rs_char.o libs/fec/decode_rs_char.o sample_source.o soapy_source.o convert.o demodulator.o uat_message.o stratux_serial.o adaptive.o
 	$(CXX) $(CXXFLAGS) $(LDFLAGS) $^ -o $@ $(LIBS) $(LIBS_SDR)
 
 faup978: faup978_main.o socket_input.o uat_message.o track.o faup978_reporter.o
@@ -23,14 +23,18 @@ faup978: faup978_main.o socket_input.o uat_message.o track.o faup978_reporter.o
 skyaware978: skyaware978_main.o socket_input.o uat_message.o track.o skyaware_writer.o
 	$(CXX) $(CXXFLAGS) $(LDFLAGS) $^ -o $@ $(LIBS)
 
-test: fec_tests
+test: fec_tests adaptive_tests
 	./fec_tests
+	./adaptive_tests
 
 fec_tests: fec_tests.o libs/fec/init_rs_char.o libs/fec/decode_rs_char.o libs/fec/encode_rs_char.o
 	$(CXX) $(CXXFLAGS) $(LDFLAGS) $^ -o $@ -lstdc++
 
+adaptive_tests: adaptive_tests.o adaptive.o
+	$(CXX) $(CXXFLAGS) $(LDFLAGS) $^ -o $@ -lstdc++ -lm
+
 format:
 	clang-format -style=file -i *.cc *.h
 
 clean:
-	rm -f *.o libs/fec/*.o dump978-fa faup978 skyaware978 fec_tests
+	rm -f *.o libs/fec/*.o dump978-fa faup978 skyaware978 fec_tests adaptive_tests

README.md

@@ -65,6 +65,39 @@ The main options are:
 
 Pass `--help` for a full list of options.
 
+## Adaptive gain control
+
+When receiving samples from an SDR (`--sdr` only), dump978-fa can adjust the
+SDR gain automatically. This is a port of
+[dump1090-fa's adaptive gain feature](https://github.com/flightaware/dump1090/blob/master/README.adaptive-gain.md)
+and works the same way. Two independent modes are available; both are off by
+default and can be enabled together:
+
+ * `--adaptive-range` enables **dynamic range mode**: dump978 measures the
+   noise floor and searches for the highest gain that still provides a target
+   dynamic range (by default, 60% of the SDR's gain range in dB -
+   approximately 30dB for an RTLSDR). This suits most installs.
+ * `--adaptive-burst` enables **burst mode**: dump978 watches for loud,
+   message-length bursts of signal that failed to decode - a sign that a
+   nearby transmitter is overloading the receiver - and reduces gain when
+   they arrive too often, raising it again once things quieten down.
+
+Useful tuning options:
+
+ * `--adaptive-min-gain <dB>` / `--adaptive-max-gain <dB>` restrict the gain
+   range that adaptive gain control will use. If you know an approximately
+   correct gain for your installation, setting limits around it speeds up
+   convergence.
+ * `--adaptive-duty-cycle <percent>` controls the fraction of received
+   samples inspected (default: 50). Lower values reduce CPU use on slow
+   machines at a small cost in measurement accuracy.
+ * `--adaptive-range-target <dB>` overrides the target dynamic range.
+
+Further tunables mirroring dump1090-fa's options are available; pass `--help`
+for the full list. Adaptive gain control requires manual gain control of the
+SDR and cannot be combined with `--sdr-auto-gain`. Gain changes and a
+once-a-minute status line are logged to stderr.
+
 ## Third-party code
 
 Third-party source code included in libs/: