Opt-in channel-power squelch for AM audio RX (#3315)

* AM: add opt-in channel-power squelch for AM audio RX

Add an SDR++-style power squelch on the complex channel signal for the
Analog Audio RX app in AM modes (DSB/SSB/CW). The squelch measures the
mean channel power, converts it to dBFS, and mutes the demodulated audio
when the signal is below a user-set threshold. Being carrier-based it
keys on the AM carrier (e.g. airband/ATC) rather than on audio-band
noise, which the existing FM squelch could not do for AM.

A new 'SQ' field (0-99, 0 = off) is added to the AM options view and is
persisted per app via app_settings. Default is 0, so existing AM
behaviour is unchanged unless the user enables it. Threshold maps 1..99
to -80..-20 dBFS.

Touches: message (AMConfigureMessage.squelch_level), baseband_api
(AMConfig::apply), receiver_model (am_squelch_level get/set),
app_settings (persistence), proc_am_audio (DSP), analog_audio_app (UI).

* Refactor audio power calculation to use magnitude squared

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

firmware/application/app_settings.cpp

@@ -203,6 +203,7 @@ void copy_to_radio_model(const AppSettings& settings) {
             settings.wfm_config_index,
             settings.wfmam_config_index,
             settings.squelch);
+        receiver_model.set_am_squelch_level(settings.am_squelch);
     }
 
     receiver_model.set_frequency_step(settings.step);
@@ -227,6 +228,7 @@ void copy_from_radio_model(AppSettings& settings) {
         settings.vga = receiver_model.vga();
         settings.rx_amp = receiver_model.rf_amp();
         settings.squelch = receiver_model.squelch_level();
+        settings.am_squelch = receiver_model.am_squelch_level();
 
         settings.modulation = static_cast<uint8_t>(receiver_model.modulation());
         settings.am_config_index = receiver_model.am_configuration();
@@ -288,6 +290,7 @@ SettingsManager::SettingsManager(
         bindings_.emplace_back("wfm_config_index"sv, &settings_.wfm_config_index);
         bindings_.emplace_back("wfmam_config_index"sv, &settings_.wfmam_config_index);
         bindings_.emplace_back("squelch"sv, &settings_.squelch);
+        bindings_.emplace_back("am_squelch"sv, &settings_.am_squelch);
     }
 
     // Common model settings.

firmware/application/app_settings.hpp

@@ -40,7 +40,7 @@
 // Bring in the string_view literal.
 using std::literals::operator""sv;
 
-#define COMMON_APP_SETTINGS_COUNT 19
+#define COMMON_APP_SETTINGS_COUNT 20
 
 /* Represents a named setting bound to a variable instance. */
 /* Using void* instead of std::variant, because variant is a pain to dispatch over. */
@@ -150,6 +150,7 @@ struct AppSettings {
     uint8_t wfm_config_index = 0;
     uint8_t wfmam_config_index = 0;
     uint8_t squelch = 80;
+    uint8_t am_squelch = 0;  // AM channel-power squelch threshold (0 = off)
     uint8_t volume;
     // NOTE: update COMMON_APP_SETTINGS_COUNT when adding to this
 };

firmware/application/apps/analog_audio_app.cpp

@@ -72,8 +72,15 @@ AMOptionsView::AMOptionsView(
         &label_config,
         &options_config,
         &zoom_config,
+        &label_squelch,
+        &field_squelch,
     });
 
+    field_squelch.set_value(receiver_model.am_squelch_level());
+    field_squelch.on_change = [](int32_t v) {
+        receiver_model.set_am_squelch_level(v);
+    };
+
     zoom_config.on_change = [this, view](size_t, OptionsField::value_t n) {
         receiver_model.set_am_configuration(
             view->get_previous_AM_mode_option() + zoom_filter_offset(n),

firmware/application/apps/analog_audio_app.hpp

@@ -100,6 +100,17 @@ class AMOptionsView : public View {
          {"ZOOM x2", (int)AMSpectrumZoomOption::X2},
          {"ZOOM x3", (int)AMSpectrumZoomOption::X3},
          {"ZOOM x4", (int)AMSpectrumZoomOption::X4}}};
+
+    Text label_squelch{
+        {UI_POS_X(10), UI_POS_Y(0), UI_POS_WIDTH(2), UI_POS_HEIGHT(1)},
+        "SQ"};
+    NumberField field_squelch{
+        {UI_POS_X(13), UI_POS_Y(0)},
+        2,
+        {0, 99},
+        1,
+        ' ',
+    };
 };
 
 class AMFMAptOptionsView : public View {

firmware/application/baseband_api.cpp

@@ -70,11 +70,11 @@ static void send_message(const Message* const message) {
     }
 }
 
-void AMConfig::apply() const {
+void AMConfig::apply(const uint8_t squelch_level) const {
-    apply((AMConfigureMessage::Zoom_waterfall)spectrum_decimation_factor);
+    apply((AMConfigureMessage::Zoom_waterfall)spectrum_decimation_factor, squelch_level);
 }
 
-void AMConfig::apply(const AMConfigureMessage::Zoom_waterfall spectrum_zoom) const {
+void AMConfig::apply(const AMConfigureMessage::Zoom_waterfall spectrum_zoom, const uint8_t squelch_level) const {
     const AMConfigureMessage message{
         taps_6k0_decim_0,             // common FIR filter taps pre-decim_0 to all 6 x AM mod types.(AM-9K, AM-6K, USB, LSB, CW, AMFM-WFAX)
         decim_1,                      // var decim_1 FIR taps filter , variable values , to handle two spectrum decim factor 1 and 2 (zoom) and more APT LPF filtered .
@@ -82,7 +82,8 @@ void AMConfig::apply(const AMConfigureMessage::Zoom_waterfall spectrum_zoom) con
         channel,                      // var channel FIR taps filter , variable values, depending selected  AM mode, each one different  (DSB-9K, DSB-6K, USB-3K, LSB-3K,CW,AMFM-WFAX)
         modulation,                   // var parameter . enum class Modulation : int32_t {DSB = 0, SSB = 1, SSB_FM = 2}
         audio_12k_iir_filter_config,  // var parameter , 300 Hz hpf all except Wefax (1.500Hz lpf)
-        (size_t)spectrum_zoom};
+        (size_t)spectrum_zoom,
+        squelch_level};
     send_message(&message);
     audio::set_rate(audio::Rate::Hz_12000);
 }

firmware/application/baseband_api.hpp

@@ -43,8 +43,8 @@ struct AMConfig {
     const iir_biquad_config_t audio_12k_iir_filter_config;  // added to handle two var IIR filter types : 300 hpf(as before) , 1500Hz lpf for Wefax.
     const size_t spectrum_decimation_factor;
 
-    void apply() const;
+    void apply(uint8_t squelch_level = 0) const;
-    void apply(AMConfigureMessage::Zoom_waterfall spectrum_zoom) const;
+    void apply(AMConfigureMessage::Zoom_waterfall spectrum_zoom, uint8_t squelch_level = 0) const;
 };
 
 struct NBFMConfig {

firmware/application/receiver_model.cpp

@@ -251,6 +251,15 @@ void ReceiverModel::set_squelch_level(uint8_t v) {
     update_modulation();
 }
 
+uint8_t ReceiverModel::am_squelch_level() const {
+    return settings_.am_squelch_level;
+}
+
+void ReceiverModel::set_am_squelch_level(uint8_t v) {
+    settings_.am_squelch_level = v;
+    update_modulation();
+}
+
 void ReceiverModel::set_antenna_bias() {
     update_antenna_bias();
 }
@@ -486,7 +495,7 @@ void ReceiverModel::update_modulation() {
 }
 
 void ReceiverModel::update_am_configuration() {
-    am_configs[am_configuration()].apply(am_spectrum_zoom_);
+    am_configs[am_configuration()].apply(am_spectrum_zoom_, am_squelch_level());
 }
 
 void ReceiverModel::update_amfm_configuration() {

firmware/application/receiver_model.hpp

@@ -61,6 +61,7 @@ class ReceiverModel {
         uint8_t nbfm_config_index = 0;
         uint8_t wfm_config_index = 0;
         uint8_t squelch_level = 80;
+        uint8_t am_squelch_level = 0;  // AM channel-power squelch threshold (0 = off)
     };
 
     /* The frequency to receive (no offset). */
@@ -109,6 +110,9 @@ class ReceiverModel {
     uint8_t squelch_level() const;
     void set_squelch_level(uint8_t v);
 
+    uint8_t am_squelch_level() const;
+    void set_am_squelch_level(uint8_t v);
+
     void set_antenna_bias();
 
     volume_t headphone_volume() const;

firmware/baseband/proc_am_audio.cpp

@@ -27,6 +27,7 @@
 #include "event_m4.hpp"
 
 #include <array>
+#include <cmath>
 #include "dsp_hilbert.hpp"
 
 // Phase 2: Constructor to start threads AFTER object is fully initialized
@@ -72,6 +73,32 @@ void NarrowbandAMAudio::execute(const buffer_c8_t& buffer) {
     feed_channel_stats(channel_out);
 
     auto audio = demodulate(channel_out);  // now 3 AM demodulation types : demod_am, demod_ssb, demod_ssb_fm (for Wefax)
+
+    // SDR++-style power squelch on the complex channel signal (opt-in; 0 = off).
+    // Measures mean channel power, converts to dBFS, and mutes the audio when
+    // the signal is below the user threshold. Carrier-based, so it keys on the
+    // AM carrier (e.g. airband/ATC) rather than on audio-band noise.
+    if (squelch_level > 0 && channel_out.count > 0) {
+        uint64_t sum_mag_sq = 0;
+        for (size_t i = 0; i < channel_out.count; i++) {
+            const int32_t re = channel_out.p[i].real();
+            const int32_t im = channel_out.p[i].imag();
+            sum_mag_sq += static_cast<uint64_t>(re) * re + static_cast<uint64_t>(im) * im;
+        }
+
+        constexpr float full_scale_mag2 = 32768.0f * 32768.0f;
+        const float mean_mag2_norm =
+            static_cast<float>(sum_mag_sq) / (static_cast<float>(channel_out.count) * full_scale_mag2);
+        const float level_dbfs = (mean_mag2_norm > 0.0f) ? mag2_to_dbv_norm(mean_mag2_norm) : -200.0f;
+
+        // Map squelch_level 1..99 to a threshold of -80..-20 dBFS.
+        const float threshold_dbfs = -80.0f + (squelch_level - 1) * (60.0f / 98.0f);
+        if (level_dbfs < threshold_dbfs) {
+            for (size_t i = 0; i < audio.count; i++)
+                audio.p[i] = 0.0f;
+        }
+    }
+
     audio_compressor.execute_in_place(audio);
     audio_output.write(audio);
 }
@@ -151,6 +178,7 @@ void NarrowbandAMAudio::configure(const AMConfigureMessage& message) {
     spectrum_interval_samples =
         decim_0_output_fs / spectrum_rate_hz;
     audio_output.configure(message.audio_hpf_lpf_config);  // hpf in all AM demod modes (AM-6K/9K, USB/LSB,DSB), except Wefax (lpf there).
+    squelch_level = message.squelch_level;
 
     configured = true;
 }

firmware/baseband/proc_am_audio.hpp

@@ -67,6 +67,7 @@ class NarrowbandAMAudio : public BasebandProcessor {
     int32_t channel_filter_high_f = 0;
     int32_t channel_filter_transition = 0;
     bool configured{false};
+    uint8_t squelch_level{0};  // AM channel-power squelch threshold (0 = off), SDR++-style
     size_t spectrum_interval_samples{0};
     size_t spectrum_samples{0};
     bool spectrum_capture_active{false};

firmware/common/message.hpp

@@ -739,7 +739,8 @@ class AMConfigureMessage : public Message {
         const fir_taps_complex<64> channel_filter,
         const Modulation modulation,
         const iir_biquad_config_t audio_hpf_lpf_config,
-        const size_t channel_spectrum_decimation_factor)
+        const size_t channel_spectrum_decimation_factor,
+        const uint8_t squelch_level = 0)
 
         : Message{ID::AMConfigure},
           decim_0_filter(decim_0_filter),
@@ -748,7 +749,8 @@ class AMConfigureMessage : public Message {
           channel_filter(channel_filter),
           modulation{modulation},
           audio_hpf_lpf_config(audio_hpf_lpf_config),
-          channel_spectrum_decimation_factor(channel_spectrum_decimation_factor) {
+          channel_spectrum_decimation_factor(channel_spectrum_decimation_factor),
+          squelch_level(squelch_level) {
     }
 
     const fir_taps_real<24> decim_0_filter;
@@ -758,6 +760,7 @@ class AMConfigureMessage : public Message {
     const Modulation modulation;
     const iir_biquad_config_t audio_hpf_lpf_config;
     const size_t channel_spectrum_decimation_factor;
+    const uint8_t squelch_level;  // AM channel-power squelch threshold (0 = off)
 };
 
 // TODO: Put this somewhere else, or at least the implementation part.