From 07be74701fe8a8ecea1500080f71b6982c104756 Mon Sep 17 00:00:00 2001
From: Kyle Reed <3761006+kallanreed@users.noreply.github.com>
Date: Wed, 30 Aug 2023 10:24:35 -0700
Subject: [PATCH] POCSAG Squelch and Filtering (#1424)

* Disable audio processing when not using squelch
* Add 2k4 lowpass fitler for 24Khz audio for POCSAG
---
 firmware/baseband/proc_pocsag.cpp  | 17 ++++++++++-------
 firmware/baseband/proc_pocsag.hpp  |  2 ++
 firmware/common/dsp_iir_config.hpp |  6 ++++++
 3 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/firmware/baseband/proc_pocsag.cpp b/firmware/baseband/proc_pocsag.cpp
index ebc3e0b92..48ec3959f 100644
--- a/firmware/baseband/proc_pocsag.cpp
+++ b/firmware/baseband/proc_pocsag.cpp
@@ -33,8 +33,6 @@
 #include <cstddef>
 
 void POCSAGProcessor::execute(const buffer_c8_t& buffer) {
-    // This is called at 1500Hz
-
     if (!configured) return;
 
     // Get 24kHz audio
@@ -43,12 +41,14 @@ void POCSAGProcessor::execute(const buffer_c8_t& buffer) {
     const auto channel_out = channel_filter.execute(decim_1_out, dst_buffer);
     auto audio = demod.execute(channel_out, audio_buffer);
 
-    // Output audio pre-smoothing so squelch actually works.
-    // NB: It's useful to output *after* when debugging the smoothing filter.
+    // When use_squelch_ is true, AudioOutput applies filters
+    // which should eliminate the need to do this smoothing.
+    if (use_squelch_ == false)
+        smooth.Process(audio.p, audio.count);
+
+    // NB: This applies audio filters in-place when use_squelch_ is true.
     audio_output.write(audio);
 
-    // Smooth the data to make decoding more accurate.
-    smooth.Process(audio.p, audio.count);
     processDemodulatedSamples(audio.p, 16);
     extractFrames();
 }
@@ -83,10 +83,13 @@ void POCSAGProcessor::on_message(const Message* const message) {
 
         case Message::ID::NBFMConfigure: {
             auto config = reinterpret_cast<const NBFMConfigureMessage*>(message);
+
+            use_squelch_ = config->squelch_level > 0;
             audio_output.configure(
                 audio_24k_hpf_300hz_config,
-                audio_8k_deemph_300_6_config,
+                audio_24k_lpf_2400hz_config,
                 config->squelch_level / 100.0);
+            audio_output.configure(use_squelch_);
             break;
         }
 
diff --git a/firmware/baseband/proc_pocsag.hpp b/firmware/baseband/proc_pocsag.hpp
index 57253f2d9..947ec7146 100644
--- a/firmware/baseband/proc_pocsag.hpp
+++ b/firmware/baseband/proc_pocsag.hpp
@@ -215,6 +215,8 @@ class POCSAGProcessor : public BasebandProcessor {
     int m_numCode{0};
     bool m_inverted{false};
 
+    bool use_squelch_ = false;
+
     /* NB: Threads should be the last members in the class definition. */
     BasebandThread baseband_thread{baseband_fs, this, baseband::Direction::Receive};
     RSSIThread rssi_thread{};
diff --git a/firmware/common/dsp_iir_config.hpp b/firmware/common/dsp_iir_config.hpp
index 177475df8..36133517f 100644
--- a/firmware/common/dsp_iir_config.hpp
+++ b/firmware/common/dsp_iir_config.hpp
@@ -77,6 +77,12 @@ constexpr iir_biquad_config_t audio_24k_deemph_300_6_config{
     {0.03780475f, 0.03780475f, 0.00000000f},
     {1.00000000f, -0.92439049f, 0.00000000f}};
 
+// scipy.signal.butter(1, 2400 / 12000.0, 'lowpass', analog=False)
+// NOTE: Technically, order-1 filter, b[2] = a[2] = 0.
+constexpr iir_biquad_config_t audio_24k_lpf_2400hz_config{
+    {0.03780475f, 0.03780475f, 0.00000000f},
+    {1.00000000f, -0.92439049f, 0.00000000f}};
+
 // scipy.signal.butter(1, 300 / 8000.0, 'lowpass', analog=False)
 // NOTE: Technically, order-1 filter, b[2] = a[2] = 0.
 constexpr iir_biquad_config_t audio_16k_deemph_300_6_config{