From 384c5d559d2398874e085db478d84e102b1201dc Mon Sep 17 00:00:00 2001 From: Cort Buffington Date: Wed, 12 Aug 2026 07:43:23 -0500 Subject: [PATCH] Reconstruction filter after ext-audio upsampling in FM mode CFMUpSampler zero-stuffs 8kHz to 24kHz with no lowpass after it, leaving imaging artifacts in the reconstructed ext audio. Filter it with a second, separately-stated pass of the same 300-2700Hz design already used for TX shaping, right where it comes out of the upsampler. --- FM.cpp | 8 ++++++++ FM.h | 3 +++ 2 files changed, 11 insertions(+) diff --git a/FM.cpp b/FM.cpp index 9928edd..d39bb46 100644 --- a/FM.cpp +++ b/FM.cpp @@ -71,6 +71,9 @@ 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_usFilterStage1( 724, 1448, 724, 32768, -37895, 21352),//same design, separate state +m_usFilterStage2(32768, 0,-32768, 32768, -50339, 19052), +m_usFilterStage3(32768, -65536, 32768, 32768, -64075, 31460), m_blanking(), m_accessMode(1U), m_linkMode(false), @@ -125,6 +128,7 @@ void CFM::repeaterSamples(bool cos, q15_t* samples, const uint16_t* rssi, uint8_ q15_t currentExtSample; bool inputExt = m_inputExtRB.getSample(currentExtSample);//always consume the external input data so it does not overflow + currentExtSample = m_usFilterStage3.filter(m_usFilterStage2.filter(m_usFilterStage1.filter(currentExtSample))); inputExt = inputExt && m_extEnabled; switch (m_accessMode) { @@ -270,6 +274,7 @@ void CFM::linkSamples(bool cos, q15_t* samples, uint8_t length) // few samples past that point q15_t currentExtSample = 0; bool inputExt = m_inputExtRB.getSample(currentExtSample);//always consume the external input data so it does not overflow + currentExtSample = m_usFilterStage3.filter(m_usFilterStage2.filter(m_usFilterStage1.filter(currentExtSample))); inputExt = inputExt && m_extEnabled; switch (m_accessMode) { @@ -423,6 +428,9 @@ void CFM::reset() m_dsFilterStage1.reset(); m_dsFilterStage2.reset(); m_dsFilterStage3.reset(); + m_usFilterStage1.reset(); + m_usFilterStage2.reset(); + m_usFilterStage3.reset(); m_squelch.reset(); m_needReverse = false; diff --git a/FM.h b/FM.h index 8d5f49b..29df859 100644 --- a/FM.h +++ b/FM.h @@ -83,6 +83,9 @@ private: CFMDirectFormI m_dsFilterStage1; // anti-alias filter for the downsampler, own state CFMDirectFormI m_dsFilterStage2; CFMDirectFormI m_dsFilterStage3; + CFMDirectFormI m_usFilterStage1; // reconstruction filter for the upsampler, own state + CFMDirectFormI m_usFilterStage2; + CFMDirectFormI m_usFilterStage3; CFMBlanking m_blanking; uint8_t m_accessMode; bool m_linkMode;