From 4c7e8fbe46dfc4352c906cdae1decb5d93039285 Mon Sep 17 00:00:00 2001
From: Cort Buffington <n0mjs@me.com>
Date: Tue, 11 Aug 2026 09:57:47 -0500
Subject: [PATCH] Anti-alias filter before RF->network downsampling in FM mode

The 24kHz->8kHz decimation in CFMDownSampler had no lowpass ahead of
it, aliasing everything above 4kHz back into the audio band. Reuse
the existing 300-2700Hz filter design (separate instance/state) ahead
of all three addSample() call sites in FM.cpp.

Also: reset() wasn't clearing the ring buffer or in-progress sample
pack, so stale audio could bleed into the next transmission. And
addSample() packed samples into 12 bits with no saturation, so an
out-of-range value corrupted the neighbouring sample.
---
 FM.cpp            | 23 +++++++++++++++++------
 FM.h              |  3 +++
 FMDownSampler.cpp |  3 +++
 3 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/FM.cpp b/FM.cpp
index efe3e05..9928edd 100644
--- a/FM.cpp
+++ b/FM.cpp
@@ -68,6 +68,9 @@ m_needReverse(false),
 m_filterStage1(  724,   1448,   724, 32768, -37895, 21352),//3rd order Cheby Filter 300 to 2700Hz, 0.2dB passband ripple, sampling rate 24kHz
 m_filterStage2(32768,      0,-32768, 32768, -50339, 19052),
 m_filterStage3(32768, -65536, 32768, 32768, -64075, 31460),
+m_dsFilterStage1(  724,   1448,   724, 32768, -37895, 21352),//same design, separate state
+m_dsFilterStage2(32768,      0,-32768, 32768, -50339, 19052),
+m_dsFilterStage3(32768, -65536, 32768, 32768, -64075, 31460),
 m_blanking(),
 m_accessMode(1U),
 m_linkMode(false),
@@ -203,8 +206,10 @@ void CFM::repeaterSamples(bool cos, q15_t* samples, const uint16_t* rssi, uint8_
     if (m_duplex) {
       if (m_state == FS_RELAYING_RF || m_state == FS_KERCHUNK_RF || m_state == FS_RELAYING_EXT || m_state == FS_KERCHUNK_EXT) {
         currentSample = m_blanking.process(currentSample);
-        if (m_extEnabled && (m_state == FS_RELAYING_RF || m_state == FS_KERCHUNK_RF))
-          m_downSampler.addSample(currentSample);
+        if (m_extEnabled && (m_state == FS_RELAYING_RF || m_state == FS_KERCHUNK_RF)) {
+          q15_t dsSample = m_dsFilterStage3.filter(m_dsFilterStage2.filter(m_dsFilterStage1.filter(currentSample)));
+          m_downSampler.addSample(dsSample);
+        }
 
         currentSample *= currentBoost;
       } else {
@@ -214,9 +219,11 @@ void CFM::repeaterSamples(bool cos, q15_t* samples, const uint16_t* rssi, uint8_
         if (m_state == FS_RELAYING_EXT || m_state == FS_KERCHUNK_EXT) {
           currentSample *= currentBoost;
         } else {
-          if (m_extEnabled && (m_state == FS_RELAYING_RF || m_state == FS_KERCHUNK_RF))
-            m_downSampler.addSample(currentSample);
-          continue; 
+          if (m_extEnabled && (m_state == FS_RELAYING_RF || m_state == FS_KERCHUNK_RF)) {
+            q15_t dsSample = m_dsFilterStage3.filter(m_dsFilterStage2.filter(m_dsFilterStage1.filter(currentSample)));
+            m_downSampler.addSample(dsSample);
+          }
+          continue;
         }
     }
 
@@ -332,7 +339,8 @@ void CFM::linkSamples(bool cos, q15_t* samples, uint8_t length)
 
     if (m_rfSignal && m_extEnabled) {
       q15_t currentSample = m_blanking.process(currentRFSample);
-      m_downSampler.addSample(currentSample);
+      q15_t dsSample = m_dsFilterStage3.filter(m_dsFilterStage2.filter(m_dsFilterStage1.filter(currentSample)));
+      m_downSampler.addSample(dsSample);
     }
 
     if (!m_extSignal)
@@ -412,6 +420,9 @@ void CFM::reset()
   m_inputExtRB.reset();
 
   m_downSampler.reset();
+  m_dsFilterStage1.reset();
+  m_dsFilterStage2.reset();
+  m_dsFilterStage3.reset();
   m_squelch.reset();
   
   m_needReverse = false;
diff --git a/FM.h b/FM.h
index a1a198f..8d5f49b 100644
--- a/FM.h
+++ b/FM.h
@@ -80,6 +80,9 @@ private:
   CFMDirectFormI       m_filterStage1;
   CFMDirectFormI       m_filterStage2;
   CFMDirectFormI       m_filterStage3;
+  CFMDirectFormI       m_dsFilterStage1; // anti-alias filter for the downsampler, own state
+  CFMDirectFormI       m_dsFilterStage2;
+  CFMDirectFormI       m_dsFilterStage3;
   CFMBlanking          m_blanking;
   uint8_t              m_accessMode;
   bool                 m_linkMode;
diff --git a/FMDownSampler.cpp b/FMDownSampler.cpp
index 47fb120..04b2c96 100644
--- a/FMDownSampler.cpp
+++ b/FMDownSampler.cpp
@@ -34,6 +34,7 @@ m_sampleIndex(0U)
 
 void CFMDownSampler::addSample(q15_t sample)
 {
+  sample = __SSAT(sample, 12);//clamp before packing to 12 bits
   uint32_t usample = uint32_t(int32_t(sample) + 2048);
   //only take one of three samples
   switch(m_sampleIndex){
@@ -74,6 +75,8 @@ uint16_t CFMDownSampler::getData()
 void CFMDownSampler::reset()
 {
   m_sampleIndex = 0U;
+  m_samplePack  = 0U;
+  m_ringBuffer.reset();
 }
 
 #endif