From 4fb607a826bd3702f0dbad317054638c8dc22904 Mon Sep 17 00:00:00 2001
From: Oliver Jowett <oliver@mutability.co.uk>
Date: Thu, 22 Jan 2015 23:38:21 +0000
Subject: [PATCH] Noise floor measurement is too expensive to be worth it,
 disable it.

---
 demod_2400.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/demod_2400.c b/demod_2400.c
index ab78c92..b25f32b 100644
--- a/demod_2400.c
+++ b/demod_2400.c
@@ -19,6 +19,12 @@
 
 #include "dump1090.h"
 
+//
+// Measuring the noise power is actually surprisingly expensive on an ARM -
+// it increases the CPU use of the demodulator by 1/3. So it's off by default.
+// You can turn it back on here:
+#undef MEASURE_NOISE
+
 // 2.4MHz sampling rate version
 //
 // When sampling at 2.4MHz we have exactly 6 samples per 5 symbols.
@@ -153,14 +159,18 @@ void demodulate2400(uint16_t *m, uint32_t mlen) {
     struct modesMessage mm;
     unsigned char msg1[MODES_LONG_MSG_BYTES], msg2[MODES_LONG_MSG_BYTES], *msg;
     uint32_t j;
+#ifdef MEASURE_NOISE
     uint32_t last_message_end = 0;
+#endif
 
     unsigned char *bestmsg;
     int bestscore, bestphase;
 
+#ifdef MEASURE_NOISE
     // noise floor:
     uint32_t noise_power_count = 0;
     uint64_t noise_power_sum = 0;
+#endif
 
     memset(&mm, 0, sizeof(mm));
     msg = msg1;
@@ -172,6 +182,7 @@ void demodulate2400(uint16_t *m, uint32_t mlen) {
         int initial_phase, first_phase, last_phase, try_phase;
         int msglen;
 
+#ifdef MEASURE_NOISE
         // update noise for all samples that aren't part of a message
         // (we don't know if m[j] is or not, yet, so work one sample
         // in arrears)
@@ -182,6 +193,7 @@ void demodulate2400(uint16_t *m, uint32_t mlen) {
             noise_power_sum += s * s;
             noise_power_count++;
         }
+#endif
 
         // Look for a message starting at around sample 0 with phase offset 3..7
 
@@ -462,14 +474,18 @@ void demodulate2400(uint16_t *m, uint32_t mlen) {
         //  where the preamble of the second message clobbered the last
         //  few bits of the first message, but the message bits didn't
         //  overlap)
+#ifdef MEASURE_NOISE
         last_message_end = j + (8 + msglen)*12/5;
+#endif
         j += (8 + msglen - 8)*12/5 - 1;
             
         // Pass data to the next layer
         useModesMessage(&mm);
     }
 
+#ifdef MEASURE_NOISE
     Modes.stats_current.noise_power_sum += (noise_power_sum / MAX_POWER / noise_power_count);
     Modes.stats_current.noise_power_count ++;
+#endif
 }