From 77aead675ac45112c66de49d77f0110589bf362c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Niccol=C3=B2=20Izzo?= <izzo.niccolo@gmail.com>
Date: Tue, 15 Feb 2022 18:59:25 +0100
Subject: [PATCH] Improve runtime performance of M17 Demodulator

Tweaked the exponential moving average alpha factors to improve the
reactivity of the M17 demodulator.

TG-81
---
 openrtx/include/protocols/M17/M17Demodulator.h |  2 +-
 openrtx/src/protocols/M17/M17Demodulator.cpp   | 15 ++++++++-------
 tests/unit/M17_demodulator.cpp                 |  4 ++--
 3 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/openrtx/include/protocols/M17/M17Demodulator.h b/openrtx/include/protocols/M17/M17Demodulator.h
index 9ea5cce4..df232074 100644
--- a/openrtx/include/protocols/M17/M17Demodulator.h
+++ b/openrtx/include/protocols/M17/M17Demodulator.h
@@ -113,7 +113,7 @@ private:
     static constexpr size_t M17_INPUT_BUF_SIZE     = 2 * M17_FRAME_SAMPLES_24K;
     static constexpr size_t M17_FRAME_BYTES        = M17_FRAME_SYMBOLS / 4;
     static constexpr float conv_stats_alpha        = 0.001f;
-    static constexpr float qnt_stats_alpha         = 0.99995f;
+    static constexpr float qnt_stats_alpha         = 0.9f;
     static constexpr float conv_threshold_factor   = 3.40;
     static constexpr size_t M17_BRIDGE_SIZE        = M17_SAMPLES_PER_SYMBOL *
                                                      M17_SYNCWORD_SYMBOLS;
diff --git a/openrtx/src/protocols/M17/M17Demodulator.cpp b/openrtx/src/protocols/M17/M17Demodulator.cpp
index 4cf4ebe1..58fa3a7f 100644
--- a/openrtx/src/protocols/M17/M17Demodulator.cpp
+++ b/openrtx/src/protocols/M17/M17Demodulator.cpp
@@ -315,6 +315,7 @@ void M17Demodulator::update()
                     locked = true;
                     isLSF = syncword.lsf;
                     offset = syncword.index + 1;
+                    frameIndex = 0;
                 }
             }
             // While we are locked, demodulate available samples
@@ -330,7 +331,7 @@ void M17Demodulator::update()
                         getQuantizationMax() / 2,
                         getQuantizationMin() / 2,
                         symbol * 666,
-                        symbol_index);
+                        frameIndex == 0 ? 2300 : symbol_index);
 #endif
                 setSymbol<M17_FRAME_BYTES>(*activeFrame, frameIndex, symbol);
                 decoded_syms++;
@@ -341,12 +342,12 @@ void M17Demodulator::update()
                     std::swap(activeFrame, idleFrame);
                     frameIndex = 0;
                     // DEBUG: print idleFrame bytes
-                    //for(size_t i = 0; i < idleFrame->size(); i+=2)
-                    //{
-                    //    if (i % 16 == 14)
-                    //        printf("\r\n");
-                    //    printf(" %02X%02X", (*idleFrame)[i], (*idleFrame)[i+1]);
-                    //}
+                    for(size_t i = 0; i < idleFrame->size(); i+=2)
+                    {
+                        if (i % 16 == 14)
+                            printf("\r\n");
+                        printf(" %02X%02X", (*idleFrame)[i], (*idleFrame)[i+1]);
+                    }
                 }
                 // If syncword is not valid, lock is lost, accept 2 bit errors
                 if (frameIndex == M17_SYNCWORD_SYMBOLS &&
diff --git a/tests/unit/M17_demodulator.cpp b/tests/unit/M17_demodulator.cpp
index 8b45ab89..391e3853 100644
--- a/tests/unit/M17_demodulator.cpp
+++ b/tests/unit/M17_demodulator.cpp
@@ -149,13 +149,13 @@ int main()
     syncword = m17Demodulator.nextFrameSync(offset);
     for(unsigned i = 0; i < baseband_samples - m17Demodulator.M17_SYNCWORD_SYMBOLS * m17Demodulator.M17_SAMPLES_PER_SYMBOL; i++)
     {
-        if ((int) i == syncword.index + 2) {
+        if ((int) i == (syncword.index + 1)) {
             if (syncword.lsf)
                 detect = -4000;
             else
                 detect = 4000;
             syncword = m17Demodulator.nextFrameSync(syncword.index + m17Demodulator.M17_SYNCWORD_SYMBOLS * m17Demodulator.M17_SAMPLES_PER_SYMBOL);
-        } else if (((int) i % 10) == ((syncword.index + 2) % 10)) {
+        } else if (((int) i % 10) == ((syncword.index + 1) % 10)) {
             m17Demodulator.updateQuantizationStats(i);
             symbol = m17Demodulator.quantize(i) * 1000;
             detect = 3000;