Factor out the sample -> magnitude conversion code and make everything a little less sample-rate-dependent.

Add optional noise measurement (cheaper than the old version)
Add optional DC filter (expensive, not really needed with rtlsdr input)

convert.h

@@ -0,0 +1,40 @@
+// Part of dump1090, a Mode S message decoder for RTLSDR devices.
+//
+// convert.h: support for various IQ -> magnitude conversions
+//
+// Copyright (c) 2015 Oliver Jowett <oliver@mutability.co.uk>
+//
+// This file is free software: you may copy, redistribute and/or modify it  
+// under the terms of the GNU General Public License as published by the
+// Free Software Foundation, either version 2 of the License, or (at your  
+// option) any later version.  
+//
+// This file is distributed in the hope that it will be useful, but  
+// WITHOUT ANY WARRANTY; without even the implied warranty of  
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU  
+// General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License  
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+#ifndef DUMP1090_CONVERT_H
+#define DUMP1090_CONVERT_H
+
+struct converter_state;
+typedef enum { INPUT_UC8=0, INPUT_SC16, INPUT_SC16Q11 } input_format_t;
+
+typedef void (*iq_convert_fn)(void *iq_data,
+                              uint16_t *mag_data,
+                              unsigned nsamples,
+                              struct converter_state *state,
+                              double *out_power);
+
+iq_convert_fn init_converter(input_format_t format,
+                             double sample_rate,
+                             int filter_dc,
+                             int compute_power,
+                             struct converter_state **out_state);
+
+void cleanup_converter(struct converter_state *state);
+
+#endif