Add sliding-frequency audio receiver tuning

firmware/application/apps/analog_audio_app.cpp

@@ -280,6 +280,12 @@ AnalogAudioView::AnalogAudioView(
     field_frequency.on_show_options = [this]() {
         this->on_show_options_frequency();
     };
+    field_frequency.changing = [this](rf::Frequency frequency) {
+        return this->on_frequency_changed(frequency);
+    };
+    field_frequency.entered = [this](rf::Frequency frequency) {
+        this->set_frequency_absolute(frequency);
+    };
 
     field_lna.on_show_options = [this]() {
         this->on_show_options_rf_gain();
@@ -308,8 +314,10 @@ AnalogAudioView::AnalogAudioView(
     };
 
     waterfall.on_select = [this](int32_t offset) {
-        field_frequency.set_value(receiver_model.target_frequency() + offset);
+        field_frequency.set_value(
+            field_frequency.value() + offset * receiver_model.frequency_step());
     };
+    waterfall.set_live_tuning(true);
 
 #ifdef PRALINE
     button_pro.on_select = [this](Button&) { this->on_show_options_praline(); };
@@ -319,6 +327,7 @@ AnalogAudioView::AnalogAudioView(
 
     // This call starts the correct baseband image to run
     // and sets the radio up as necessary for the given modulation.
+    sliding_center_frequency = receiver_model.target_frequency();
     on_modulation_changed(modulation);
 }
 
@@ -328,7 +337,7 @@ AnalogAudioView::AnalogAudioView(
     : AnalogAudioView(nav) {
     // Settings to override when launched from another app (versus from AppSettings .ini file)
     // TODO: Which other settings make sense to override?
-    field_frequency.set_value(override.frequency_app_override);
+    set_frequency_absolute(override.frequency_app_override);
     on_frequency_step_changed(override.frequency_step);
     options_modulation.set_by_value(toUType(override.mode));
 }
@@ -573,10 +582,14 @@ void AnalogAudioView::update_modulation(ReceiverModel::Mode modulation) {
     receiver_model.set_sampling_rate(is_wideband_spectrum_mode ? spec_bw : 3072000);
     receiver_model.set_baseband_bandwidth(is_wideband_spectrum_mode ? spec_bw / 2 : 1750000);
 
-    receiver_model.set_hidden_offset(modulation == ReceiverModel::Mode::AMAudioFMApt ? -2200 : 0);  // wefax needs to be shifted, see wefax rx app.
+    reset_sliding_frequency(modulation);
 
     receiver_model.enable();
 
+    if (sliding_enabled) {
+        baseband::set_audio_ddc_frequency(0);
+    }
+
     // TODO: This doesn't belong here! There's a better way.
     size_t sampling_rate = 0;
     switch (modulation) {
@@ -610,7 +623,64 @@ void AnalogAudioView::handle_coded_squelch(uint32_t value) {
 }
 
 void AnalogAudioView::on_freqchg(int64_t freq) {
-    field_frequency.set_value(freq);
+    set_frequency_absolute(freq);
+}
+
+void AnalogAudioView::set_frequency_absolute(rf::Frequency frequency) {
+    if (!sliding_enabled) {
+        field_frequency.set_value(frequency);
+        return;
+    }
+
+    sliding_center_frequency = frequency;
+
+    /* set_value() does not call on_change when the displayed frequency is
+     * already equal, so reset the hardware and DDC explicitly in that case. */
+    if (field_frequency.value() == frequency) {
+        receiver_model.set_target_frequency_with_hidden_offset(frequency, 0);
+        baseband::set_audio_ddc_frequency(0);
+    } else {
+        field_frequency.set_value(frequency);
+    }
+}
+
+int32_t AnalogAudioView::sliding_limit() const {
+    const bool zoom_x2 =
+        receiver_model.modulation() == ReceiverModel::Mode::AMAudio &&
+        previous_zoom != 0;
+    return zoom_x2 ? sliding_limit_zoom_x2 : sliding_limit_zoom_x1;
+}
+
+void AnalogAudioView::reset_sliding_frequency(ReceiverModel::Mode modulation) {
+    sliding_enabled =
+        modulation == ReceiverModel::Mode::AMAudio ||
+        modulation == ReceiverModel::Mode::NarrowbandFMAudio;
+    sliding_center_frequency = receiver_model.target_frequency();
+
+    /* AMFM keeps its existing Wefax offset; sliding applies to AM and NFM. */
+    receiver_model.set_hidden_offset(
+        modulation == ReceiverModel::Mode::AMAudioFMApt ? -2200 : 0);
+}
+
+bool AnalogAudioView::on_frequency_changed(rf::Frequency frequency) {
+    if (!sliding_enabled)
+        return false;
+
+    const auto limit = sliding_limit();
+    int64_t offset = frequency - sliding_center_frequency;
+    if (offset > limit) {
+        sliding_center_frequency = frequency - limit;
+        offset = limit;
+    } else if (offset < -limit) {
+        sliding_center_frequency = frequency + limit;
+        offset = -limit;
+    }
+
+    /* Store the displayed frequency and retune the hardware centre atomically. */
+    receiver_model.set_target_frequency_with_hidden_offset(
+        frequency, sliding_center_frequency - frequency);
+    baseband::set_audio_ddc_frequency(static_cast<int32_t>(offset));
+    return true;
 }
 
 #ifdef PRALINE

firmware/application/apps/analog_audio_app.hpp

@@ -269,6 +269,10 @@ class AnalogAudioView : public View {
     uint8_t zoom_factor_amfm{0};             // initial zoom factor in AMFM mode
     uint8_t previous_AM_mode_option{0};      // GUI 5 AM modes :  (0..4 ) (DSB9K, DSB6K, USB,LSB, CW). Used to select proper FIR filter (0..11) AM mode  + offset 0 (zoom+1) or +6 (if zoom+2)
     uint8_t previous_zoom{0};                // GUI ZOOM+1, ZOOM+2 , equivalent to two values offset 0 (zoom+1) or +6 (if zoom+2)
+    static constexpr int32_t sliding_limit_zoom_x1 = 50000;
+    static constexpr int32_t sliding_limit_zoom_x2 = 30000;
+    rf::Frequency sliding_center_frequency{0};
+    bool sliding_enabled{false};
 
     app_settings::SettingsManager settings_{
         "rx_audio",
@@ -354,6 +358,10 @@ class AnalogAudioView : public View {
     void handle_coded_squelch(uint32_t value);
 
     void on_freqchg(int64_t freq);
+    bool on_frequency_changed(rf::Frequency frequency);
+    void set_frequency_absolute(rf::Frequency frequency);
+    int32_t sliding_limit() const;
+    void reset_sliding_frequency(ReceiverModel::Mode modulation);
 
     MessageHandlerRegistration message_handler_coded_squelch{
         Message::ID::CodedSquelch,

firmware/application/baseband_api.cpp

@@ -343,6 +343,11 @@ void set_spectrum(
     send_message(&message);
 }
 
+void set_audio_ddc_frequency(int32_t frequency) {
+    const AudioDDCConfigMessage message{frequency};
+    send_message(&message);
+}
+
 void set_time_sink(
     const size_t sampling_rate,
     const size_t trigger) {

firmware/application/baseband_api.hpp

@@ -100,6 +100,7 @@ void set_rds_data(const uint16_t message_length);
 void set_spectrum(
     const size_t sampling_rate,
     const size_t trigger);
+void set_audio_ddc_frequency(int32_t frequency);
 void set_time_sink(
     const size_t sampling_rate,
     const size_t trigger);

firmware/application/receiver_model.cpp

@@ -88,6 +88,15 @@ void ReceiverModel::set_target_frequency(rf::Frequency f) {
     update_tuning_frequency();
 }
 
+void ReceiverModel::set_target_frequency_with_hidden_offset(
+    rf::Frequency f,
+    rf::Frequency offset) {
+    persistent_memory::set_target_frequency(f);
+    settings_.frequency_app_override = f;
+    hidden_offset = offset;
+    update_tuning_frequency();
+}
+
 uint32_t ReceiverModel::baseband_bandwidth() const {
     return settings_.baseband_bandwidth;
 }

firmware/application/receiver_model.hpp

@@ -66,6 +66,7 @@ class ReceiverModel {
     /* The frequency to receive (no offset). */
     rf::Frequency target_frequency() const;
     void set_target_frequency(rf::Frequency f);
+    void set_target_frequency_with_hidden_offset(rf::Frequency f, rf::Frequency offset);
 
     uint32_t baseband_bandwidth() const;
     void set_baseband_bandwidth(uint32_t v);

firmware/application/ui/ui_freq_field.hpp

@@ -39,6 +39,8 @@ class BoundFrequencyField : public FrequencyField {
 
    public:
     decltype(FrequencyField::on_change) updated{};
+    std::function<bool(rf::Frequency)> changing{};
+    std::function<void(rf::Frequency)> entered{};
 
     BoundFrequencyField(Point parent_pos, NavigationView& nav)
         : FrequencyField(parent_pos) {
@@ -47,7 +49,8 @@ class BoundFrequencyField : public FrequencyField {
         set_value(model->target_frequency());
 
         on_change = [this](rf::Frequency f) {
-            model->set_target_frequency(f);
+            if (!changing || !changing(f))
+                model->set_target_frequency(f);
             if (updated)
                 updated(f);
         };
@@ -57,7 +60,10 @@ class BoundFrequencyField : public FrequencyField {
                 on_edit_shown();
             auto freq_view = nav.push<FrequencyKeypadView>(model->target_frequency());
             freq_view->on_changed = [this](rf::Frequency f) {
-                set_value(f);
+                if (entered)
+                    entered(f);
+                else
+                    set_value(f);
             };
             nav.set_on_pop([this]() {
                 if (on_edit_hidden)
@@ -76,4 +82,4 @@ using TxFrequencyField = BoundFrequencyField<TransmitterModel, &portapack::trans
 
 }  // namespace ui
 
-#endif  // __UI_FREQ_FIELD_H__
+#endif  // __UI_FREQ_FIELD_H__

firmware/application/ui/ui_spectrum.cpp

@@ -88,16 +88,29 @@ void FrequencyScale::set_spectrum_sampling_rate(const int new_sampling_rate) {
 }
 
 void FrequencyScale::set_channel_filter(
+    const int offset,
     const int low_frequency,
     const int high_frequency,
     const int transition) {
-    if ((channel_filter_low_frequency != low_frequency) ||
+    const bool shape_changed =
+        (channel_filter_low_frequency != low_frequency) ||
         (channel_filter_high_frequency != high_frequency) ||
-        (channel_filter_transition != transition)) {
+        (channel_filter_transition != transition);
+    const bool offset_changed = channel_filter_offset != offset;
+
+    if (shape_changed) {
+        channel_filter_offset = offset;
         channel_filter_low_frequency = low_frequency;
         channel_filter_high_frequency = high_frequency;
         channel_filter_transition = transition;
         set_dirty();
+    } else if (offset_changed) {
+        const auto old_offset = channel_filter_offset;
+        channel_filter_offset = offset;
+        if (live_tuning && spectrum_sampling_rate && drawn())
+            redraw_filter_cursor(old_offset);
+        else
+            set_dirty();
     }
 }
 
@@ -123,12 +136,14 @@ void FrequencyScale::paint(Painter& painter) {
     draw_filter_ranges(painter, r);
     draw_frequency_ticks(painter, r);
 
-    const Rect r_cursor{
-        (screen_width / 2 - 2) + cursor_position, r.bottom() - filter_band_height,
-        5, filter_band_height};
-    painter.fill_rectangle(
-        r_cursor,
-        Color::red());
+    if (!live_tuning) {
+        const Rect r_cursor{
+            (screen_width / 2 - 2) + cursor_position, r.bottom() - filter_band_height,
+            5, filter_band_height};
+        painter.fill_rectangle(
+            r_cursor,
+            Color::red());
+    }
 }
 
 void FrequencyScale::clear() {
@@ -184,9 +199,79 @@ void FrequencyScale::draw_frequency_ticks(Painter& painter, const Rect r) {
     }
 }
 
+void FrequencyScale::redraw_filter_cursor(const int old_offset) {
+    const auto r = screen_rect();
+    const auto x_center = r.width() / 2;
+    const auto trans =
+        channel_filter_transition * spectrum_bins / spectrum_sampling_rate;
+
+    const auto cursor_left = [&](const int offset) {
+        return r.left() + x_center +
+               (offset + channel_filter_low_frequency) *
+                   spectrum_bins / spectrum_sampling_rate -
+               trans;
+    };
+    const auto cursor_right = [&](const int offset) {
+        return r.left() + x_center +
+               (offset + channel_filter_high_frequency) *
+                   spectrum_bins / spectrum_sampling_rate +
+               trans;
+    };
+
+    const auto dirty_left =
+        std::min(cursor_left(old_offset), cursor_left(channel_filter_offset)) - 1;
+    const auto dirty_right =
+        std::max(cursor_right(old_offset), cursor_right(channel_filter_offset)) + 1;
+    const Rect dirty{
+        dirty_left,
+        r.bottom() - filter_band_height,
+        dirty_right - dirty_left,
+        filter_band_height};
+
+    Painter painter;
+    painter.fill_rectangle(dirty, Theme::getInstance()->bg_darkest->background);
+    draw_filter_ranges(painter, r);
+    restore_tick_lines(painter, r, dirty);
+}
+
+void FrequencyScale::restore_tick_lines(
+    Painter& painter,
+    const Rect r,
+    const Rect dirty) {
+    const auto draw_if_dirty = [&](const Coord x) {
+        if (x >= dirty.left() && x < dirty.right()) {
+            painter.fill_rectangle(
+                {x, dirty.top(), 1, dirty.height()},
+                Theme::getInstance()->bg_darkest->foreground);
+        }
+    };
+
+    const auto x_center = r.left() + r.width() / 2;
+    draw_if_dirty(x_center);
+
+    constexpr int tick_count_max = 4;
+    float rough_tick_interval = float(spectrum_sampling_rate) / tick_count_max;
+    int magnitude = 1;
+    while (rough_tick_interval >= 10.0f) {
+        rough_tick_interval /= 10;
+        magnitude *= 10;
+    }
+
+    const int tick_interval = std::ceil(rough_tick_interval);
+    auto tick_offset = tick_interval;
+    while ((tick_offset * magnitude) < spectrum_sampling_rate / 2) {
+        const Dim pixel_offset =
+            tick_offset * magnitude * spectrum_bins / spectrum_sampling_rate;
+        draw_if_dirty(x_center - pixel_offset);
+        draw_if_dirty(x_center + pixel_offset);
+        tick_offset += tick_interval;
+    }
+}
+
 void FrequencyScale::draw_filter_ranges(Painter& painter, const Rect r) {
     if (channel_filter_low_frequency != channel_filter_high_frequency) {
-        const auto x_center = r.width() / 2;
+        const auto x_center = r.width() / 2 +
+                              channel_filter_offset * spectrum_bins / spectrum_sampling_rate;
 
         const auto x_low = x_center + channel_filter_low_frequency * spectrum_bins / spectrum_sampling_rate;
         const auto x_high = x_center + channel_filter_high_frequency * spectrum_bins / spectrum_sampling_rate;
@@ -220,6 +305,11 @@ void FrequencyScale::on_blur() {
 }
 
 bool FrequencyScale::on_encoder(const EncoderEvent delta) {
+    if (live_tuning) {
+        if (on_select) on_select(delta);
+        return true;
+    }
+
     cursor_position += delta;
 
     cursor_position = std::min<int32_t>(cursor_position, screen_width / 2 - 1);
@@ -326,9 +416,11 @@ WaterfallView::WaterfallView(const bool cursor) {
         frequency_scale.focus();  // focus on frequency scale to show cursor
 
         if (sampling_rate) {
-            // screen x to frequency scale x, NB we need two widgets align
-            int32_t cursor_position = x - (screen_width / 2);
-            frequency_scale.set_cursor_position(cursor_position);
+            const int32_t cursor_position = x - (screen_width / 2);
+            if (!frequency_scale.is_live_tuning()) {
+                // screen x to frequency scale x, NB we need two widgets align
+                frequency_scale.set_cursor_position(cursor_position);
+            }
         }
     };
 
@@ -405,6 +497,7 @@ void WaterfallView::on_channel_spectrum(const ChannelSpectrum& spectrum) {
     sampling_rate = spectrum.sampling_rate;
     frequency_scale.set_spectrum_sampling_rate(sampling_rate);
     frequency_scale.set_channel_filter(
+        spectrum.channel_filter_offset,
         spectrum.channel_filter_low_frequency,
         spectrum.channel_filter_high_frequency,
         spectrum.channel_filter_transition);

firmware/application/ui/ui_spectrum.hpp

@@ -83,8 +83,10 @@ class FrequencyScale : public Widget {
     bool on_touch(const TouchEvent touch) override;
 
     void set_spectrum_sampling_rate(const int new_sampling_rate);
-    void set_channel_filter(const int low_frequency, const int high_frequency, const int transition);
+    void set_channel_filter(const int offset, const int low_frequency, const int high_frequency, const int transition);
     void set_cursor_position(const int32_t position);
+    void set_live_tuning(const bool enabled) { live_tuning = enabled; }
+    bool is_live_tuning() const { return live_tuning; }
 
     void paint(Painter& painter) override;
 
@@ -94,15 +96,19 @@ class FrequencyScale : public Widget {
     int32_t cursor_position{0};
     int spectrum_sampling_rate{0};
     const int spectrum_bins = std::tuple_size<decltype(ChannelSpectrum::db)>::value;
+    int channel_filter_offset{0};
     int channel_filter_low_frequency{0};
     int channel_filter_high_frequency{0};
     int channel_filter_transition{0};
+    bool live_tuning{false};
 
     void clear();
     void clear_background(Painter& painter, const Rect r);
 
     void draw_frequency_ticks(Painter& painter, const Rect r);
     void draw_filter_ranges(Painter& painter, const Rect r);
+    void redraw_filter_cursor(const int old_offset);
+    void restore_tick_lines(Painter& painter, const Rect r, const Rect dirty);
 };
 
 /* NB: These visualizations rely on having a baseband image running.
@@ -147,6 +153,7 @@ class WaterfallView : public View {
     void set_parent_rect(const Rect new_parent_rect) override;
     void show_audio_spectrum_view(const bool show);
     void load_gradient();
+    void set_live_tuning(const bool enabled) { frequency_scale.set_live_tuning(enabled); }
 
    private:
     void update_widgets_rect();

firmware/baseband/dsp_decimate.cpp

@@ -337,6 +337,66 @@ buffer_c16_t FIRC16xR16x16Decim2::execute(
         src.sampling_rate / decimation_factor};
 }
 
+// FIRC16xR16x63HalfbandDecim2 ////////////////////////////////////////////
+
+void FIRC16xR16x63HalfbandDecim2::configure(
+    const std::array<int16_t, taps_count>& taps) {
+    std::copy(taps.cbegin(), taps.cend(), taps_.begin());
+    reset();
+}
+
+void FIRC16xR16x63HalfbandDecim2::reset() {
+    samples_.fill({});
+    samples_head_ = 0;
+}
+
+buffer_c16_t FIRC16xR16x63HalfbandDecim2::execute(
+    const buffer_c16_t& src,
+    const buffer_c16_t& dst) {
+    auto* dst_p = reinterpret_cast<uint32_t*>(dst.p);
+
+    for (size_t output = 0; output < src.count / decimation_factor; ++output) {
+        for (size_t i = 0; i < decimation_factor; ++i) {
+            const auto sample = src.p[output * decimation_factor + i];
+            samples_[samples_head_] = sample;
+            samples_[samples_head_ + taps_count] = sample;
+            if (++samples_head_ == taps_count)
+                samples_head_ = 0;
+        }
+
+        int64_t real = 0;
+        int64_t imag = 0;
+        for (size_t tap = 0; tap < taps_count; tap += 4) {
+            const auto sample_0 =
+                *reinterpret_cast<const uint32_t*>(&samples_[samples_head_ + tap]);
+            const auto sample_1 =
+                *reinterpret_cast<const uint32_t*>(&samples_[samples_head_ + tap + 2]);
+            const auto real_pair = __PKHBT(sample_0, sample_1, 16);
+            const auto imag_pair = __PKHTB(sample_1, sample_0, 16);
+            const auto taps_pair = uint32_t(uint16_t(taps_[tap])) |
+                                   (uint32_t(uint16_t(taps_[tap + 2])) << 16);
+            real = __SMLALD(real_pair, taps_pair, real);
+            imag = __SMLALD(imag_pair, taps_pair, imag);
+        }
+
+        const auto center =
+            *reinterpret_cast<const uint32_t*>(&samples_[samples_head_ + taps_count / 2]);
+        real += int16_t(center) * taps_[taps_count / 2];
+        imag += int16_t(center >> 16) * taps_[taps_count / 2];
+
+        const auto real_s16 =
+            __SSAT((real + (real >= 0 ? 32768 : -32768)) / 65536, 16);
+        const auto imag_s16 =
+            __SSAT((imag + (imag >= 0 ? 32768 : -32768)) / 65536, 16);
+        *(dst_p++) = __PKHBT(real_s16, imag_s16, 16);
+    }
+
+    return {
+        dst.p,
+        src.count / decimation_factor,
+        src.sampling_rate / decimation_factor};
+}
+
 // FIRC16xR16x32Decim8 ////////////////////////////////////////////////////
 
 void FIRC16xR16x32Decim8::configure(
@@ -625,10 +685,13 @@ buffer_s16_t FIR64AndDecimateBy2Real::execute(
 void FIRAndDecimateComplex::configure_common(
     const size_t taps_count,
     const size_t decimation_factor) {
-    samples_ = std::make_unique<samples_t>(taps_count);
+    /* Mirror the delay line so a convolution always sees one contiguous
+     * taps_count window, even when the logical head wraps. */
+    samples_ = std::make_unique<samples_t>(taps_count * 2);
     taps_reversed_ = std::make_unique<taps_t>(taps_count);
     taps_count_ = taps_count;
     decimation_factor_ = decimation_factor;
+    samples_head_ = 0;
 }
 
 buffer_c16_t FIRAndDecimateComplex::execute(
@@ -647,15 +710,18 @@ buffer_c16_t FIRAndDecimateComplex::execute(
     const void* src_p = src.p;
     size_t outer_count = output_samples;
     while (outer_count > 0) {
-        /* Put new samples into delay buffer */
-        void* z_new_p = &samples_[taps_count_ - decimation_factor_];
+        /* Put new samples into both halves of the mirrored ring. */
         for (size_t i = 0; i < decimation_factor_; i++) {
-            *__SIMD32(z_new_p)++ = *__SIMD32(src_p)++;
+            const uint32_t sample = *__SIMD32(src_p)++;
+            *reinterpret_cast<uint32_t*>(&samples_[samples_head_]) = sample;
+            *reinterpret_cast<uint32_t*>(&samples_[samples_head_ + taps_count_]) = sample;
+            if (++samples_head_ == taps_count_)
+                samples_head_ = 0;
         }
 
         size_t loop_count = taps_count_ / 8;
         void* t_p = &taps_reversed_[0];
-        void* z_p = &samples_[0];
+        void* z_p = &samples_[samples_head_];
 
         int64_t t_real = 0;
         int64_t t_imag = 0;
@@ -712,27 +778,6 @@ buffer_c16_t FIRAndDecimateComplex::execute(
             i_sat,
             16);
 
-        /* Shift sample buffer left/down by decimation factor. */
-        const size_t unroll_factor = 4;
-        size_t shift_count = (taps_count_ - decimation_factor_) / unroll_factor;
-
-        void* t = &samples_[0];
-        const void* s = &samples_[decimation_factor_];
-
-        while (shift_count > 0) {
-            *__SIMD32(t)++ = *__SIMD32(s)++;
-            *__SIMD32(t)++ = *__SIMD32(s)++;
-            *__SIMD32(t)++ = *__SIMD32(s)++;
-            *__SIMD32(t)++ = *__SIMD32(s)++;
-            shift_count--;
-        }
-
-        shift_count = (taps_count_ - decimation_factor_) % unroll_factor;
-        while (shift_count > 0) {
-            *__SIMD32(t)++ = *__SIMD32(s)++;
-            shift_count--;
-        }
-
         outer_count--;
     }
 

firmware/baseband/dsp_decimate.hpp

@@ -170,6 +170,24 @@ class FIRC16xR16x16Decim2 {
     int32_t output_scale = 0;
 };
 
+class FIRC16xR16x63HalfbandDecim2 {
+   public:
+    static constexpr size_t taps_count = 63;
+    static constexpr size_t decimation_factor = 2;
+
+    void configure(const std::array<int16_t, taps_count>& taps);
+    void reset();
+
+    buffer_c16_t execute(
+        const buffer_c16_t& src,
+        const buffer_c16_t& dst);
+
+   private:
+    alignas(4) std::array<complex16_t, taps_count * 2> samples_{};
+    alignas(4) std::array<int16_t, taps_count> taps_{};
+    size_t samples_head_{0};
+};
+
 class FIRC16xR16x32Decim8 {
    public:
     static constexpr size_t taps_count = 32;
@@ -210,6 +228,23 @@ class FIRAndDecimateComplex {
         configure(taps.data(), taps.size(), decimation_factor);
     }
 
+    template <size_t N>
+    void configure(
+        const std::array<int16_t, N>& taps,
+        const size_t decimation_factor) {
+        configure_common(N, decimation_factor);
+        for (size_t i = 0; i < N; ++i) {
+            taps_reversed_[i] = {taps[N - 1 - i], 0};
+        }
+    }
+
+    template <size_t N>
+    void set_taps(const std::array<complex16_t, N>& taps) {
+        if (N == taps_count_) {
+            std::reverse_copy(taps.begin(), taps.end(), &taps_reversed_[0]);
+        }
+    }
+
     buffer_c16_t execute(
         const buffer_c16_t& src,
         const buffer_c16_t& dst);
@@ -221,6 +256,7 @@ class FIRAndDecimateComplex {
     std::unique_ptr<taps_t> taps_reversed_{};
     size_t taps_count_{0};
     size_t decimation_factor_{1};
+    size_t samples_head_{0};
 
     template <typename T>
     void configure(

firmware/baseband/dsp_frequency_xlator.hpp

@@ -0,0 +1,232 @@
+/*
+ * Copyright (C) 2026
+ *
+ * This file is part of PortaPack.
+ */
+
+#ifndef __DSP_FREQUENCY_XLATOR_H__
+#define __DSP_FREQUENCY_XLATOR_H__
+
+#include "dsp_decimate.hpp"
+#include "dsp_types.hpp"
+
+#include <array>
+#include <cstdint>
+
+namespace dsp {
+
+/* Fixed-point complex mixer for the Audio RX channelizer. */
+class FrequencyTranslator {
+   public:
+    FrequencyTranslator() {
+        for (size_t i = 0; i < oscillator_q15_.size(); ++i) {
+            oscillator_q15_[i] =
+                static_cast<uint16_t>(sine_q15_[static_cast<uint8_t>(i + 64)]) |
+                (static_cast<uint32_t>(
+                     static_cast<uint16_t>(sine_q15_[i]))
+                 << 16);
+        }
+    }
+
+    void set_sample_rate(const uint32_t sampling_rate) {
+        sampling_rate_ = sampling_rate;
+        update_phase_increment();
+    }
+
+    void set_frequency(const int32_t frequency) {
+        frequency_ = frequency;
+        update_phase_increment();
+    }
+
+    buffer_c16_t execute(const buffer_c16_t& src, const buffer_c16_t& dst) {
+        auto phase = phase_;
+        for (size_t i = 0; i < src.count; ++i) {
+            const uint8_t index = phase >> 24;
+            const uint8_t next = index + 1;
+            const int32_t fraction = (phase >> 16) & 0xff;
+            const uint32_t oscillator_first = oscillator_q15_[index];
+            const uint32_t oscillator_next = oscillator_q15_[next];
+            const int32_t sine_first =
+                static_cast<int16_t>(oscillator_first >> 16);
+            const int32_t cosine_first =
+                static_cast<int16_t>(oscillator_first);
+            const int32_t sine =
+                sine_first +
+                (((static_cast<int16_t>(oscillator_next >> 16) -
+                   sine_first) *
+                  fraction) >>
+                 8);
+            const int32_t cosine =
+                cosine_first +
+                (((static_cast<int16_t>(oscillator_next) -
+                   cosine_first) *
+                  fraction) >>
+                 8);
+            const uint32_t oscillator =
+                static_cast<uint16_t>(cosine) |
+                (static_cast<uint32_t>(
+                     static_cast<uint16_t>(sine))
+                 << 16);
+            const uint32_t sample =
+                *reinterpret_cast<const uint32_t*>(&src.p[i]);
+
+            /* Two packed dual-16-bit multiplies implement
+             * (I+jQ) * (cos-j sin). */
+            const int32_t out_i =
+                rounded_shift(__SMUAD(sample, oscillator), 15);
+            const int32_t out_q =
+                rounded_shift(__SMUSDX(oscillator, sample), 15);
+            *reinterpret_cast<uint32_t*>(&dst.p[i]) =
+                __PKHBT(__SSAT(out_i, 16), __SSAT(out_q, 16), 16);
+            phase += phase_increment_;
+        }
+        phase_ = phase;
+        return {dst.p, src.count, src.sampling_rate};
+    }
+
+   private:
+    friend class FrequencyTranslatingDecimator32By8;
+
+    static constexpr int32_t rounded_shift(
+        const int32_t value,
+        const uint32_t bits) {
+        const int32_t rounding = int32_t{1} << (bits - 1);
+        return value >= 0
+                   ? (value + rounding) >> bits
+                   : -((-value + rounding) >> bits);
+    }
+
+    void update_phase_increment() {
+        if (sampling_rate_) {
+            phase_increment_ = static_cast<uint32_t>(
+                (static_cast<int64_t>(frequency_) * (int64_t{1} << 32)) /
+                sampling_rate_);
+        }
+    }
+
+    static constexpr std::array<int16_t, 256> sine_q15_{{
+        0, 804, 1608, 2410, 3212, 4011, 4808, 5602, 6393, 7179, 7962, 8739, 9512, 10278, 11039, 11793,
+        12539, 13279, 14010, 14732, 15446, 16151, 16846, 17530, 18204, 18868, 19519, 20159, 20787, 21403, 22005, 22594,
+        23170, 23731, 24279, 24811, 25329, 25832, 26319, 26790, 27245, 27683, 28105, 28510, 28898, 29268, 29621, 29956,
+        30273, 30571, 30852, 31113, 31356, 31580, 31785, 31971, 32137, 32285, 32412, 32521, 32609, 32678, 32728, 32757,
+        32767, 32757, 32728, 32678, 32609, 32521, 32412, 32285, 32137, 31971, 31785, 31580, 31356, 31113, 30852, 30571,
+        30273, 29956, 29621, 29268, 28898, 28510, 28105, 27683, 27245, 26790, 26319, 25832, 25329, 24811, 24279, 23731,
+        23170, 22594, 22005, 21403, 20787, 20159, 19519, 18868, 18204, 17530, 16846, 16151, 15446, 14732, 14010, 13279,
+        12539, 11793, 11039, 10278, 9512, 8739, 7962, 7179, 6393, 5602, 4808, 4011, 3212, 2410, 1608, 804,
+        0, -804, -1608, -2410, -3212, -4011, -4808, -5602, -6393, -7179, -7962, -8739, -9512, -10278, -11039, -11793,
+        -12539, -13279, -14010, -14732, -15446, -16151, -16846, -17530, -18204, -18868, -19519, -20159, -20787, -21403, -22005, -22594,
+        -23170, -23731, -24279, -24811, -25329, -25832, -26319, -26790, -27245, -27683, -28105, -28510, -28898, -29268, -29621, -29956,
+        -30273, -30571, -30852, -31113, -31356, -31580, -31785, -31971, -32137, -32285, -32412, -32521, -32609, -32678, -32728, -32757,
+        -32767, -32757, -32728, -32678, -32609, -32521, -32412, -32285, -32137, -31971, -31785, -31580, -31356, -31113, -30852, -30571,
+        -30273, -29956, -29621, -29268, -28898, -28510, -28105, -27683, -27245, -26790, -26319, -25832, -25329, -24811, -24279, -23731,
+        -23170, -22594, -22005, -21403, -20787, -20159, -19519, -18868, -18204, -17530, -16846, -16151, -15446, -14732, -14010, -13279,
+        -12539, -11793, -11039, -10278, -9512, -8739, -7962, -7179, -6393, -5602, -4808, -4011, -3212, -2410, -1608, -804,
+    }};
+
+    std::array<uint32_t, 256> oscillator_q15_{};
+    uint32_t phase_{0};
+    uint32_t phase_increment_{0};
+    uint32_t sampling_rate_{192000};
+    int32_t frequency_{0};
+};
+
+/*
+ * Frequency-translating 32-tap FIR decimator.  Frequency translation is
+ * split between coefficients modulated when tuning changes and a cheap
+ * output-rate phase rotation.  This avoids running an NCO at the 384kHz
+ * input rate.
+ */
+class FrequencyTranslatingDecimator32By8 {
+   public:
+    static constexpr size_t decimation_factor = 8;
+    static constexpr size_t taps_count = 32;
+
+    void configure(
+        const std::array<int16_t, taps_count>& taps,
+        const uint32_t input_sampling_rate) {
+        taps_ = taps;
+        input_sampling_rate_ = input_sampling_rate;
+        decimator_.configure(complex_taps_, decimation_factor);
+        output_xlator_.set_sample_rate(input_sampling_rate / decimation_factor);
+        update_taps();
+    }
+
+    void set_frequency(const int32_t frequency) {
+        frequency_ = frequency;
+        output_xlator_.set_frequency(frequency);
+        update_taps();
+    }
+
+    buffer_c16_t execute(
+        const buffer_c16_t& src,
+        const buffer_c16_t& dst) {
+        const auto filtered = decimator_.execute(src, dst);
+        return output_xlator_.execute(filtered, dst);
+    }
+
+   private:
+    static void oscillator(
+        const uint32_t phase,
+        int32_t& sine,
+        int32_t& cosine) {
+        const uint8_t index = phase >> 24;
+        const uint8_t next = index + 1;
+        const uint8_t cosine_index = index + 64;
+        const uint8_t cosine_next = cosine_index + 1;
+        const int32_t fraction = (phase >> 16) & 0xff;
+        const int32_t sine_first = FrequencyTranslator::sine_q15_[index];
+        const int32_t cosine_first = FrequencyTranslator::sine_q15_[cosine_index];
+        sine = sine_first +
+               (((FrequencyTranslator::sine_q15_[next] - sine_first) * fraction) >> 8);
+        cosine = cosine_first +
+                 (((FrequencyTranslator::sine_q15_[cosine_next] - cosine_first) * fraction) >> 8);
+    }
+
+    void update_taps() {
+        if (!input_sampling_rate_)
+            return;
+
+        const uint32_t tap_phase_increment = static_cast<uint32_t>(
+            (static_cast<int64_t>(frequency_) * (int64_t{1} << 32)) /
+            input_sampling_rate_);
+
+        /* Centre the modulation on the FIR midpoint.  Besides changing
+         * only a constant output phase, this makes the two coefficients
+         * in each symmetric pair complex conjugates.  That property is
+         * important after quantization: starting at tap zero accumulated
+         * a one-sided phase and rounding error across the whole filter. */
+        uint32_t phase = static_cast<uint32_t>(
+            -((static_cast<int64_t>(
+                    static_cast<int32_t>(tap_phase_increment)) *
+                static_cast<int64_t>(taps_count - 1)) /
+              2));
+        for (size_t i = 0; i < taps_count; ++i) {
+            int32_t sine;
+            int32_t cosine;
+            oscillator(phase, sine, cosine);
+            /* FIRAndDecimateComplex uses Q16 coefficients; the source
+             * real-tap filters use Q15 coefficients. */
+            const int32_t tap = taps_[i];
+            complex_taps_[i] = {
+                static_cast<int16_t>(
+                    FrequencyTranslator::rounded_shift(
+                        tap * cosine, 14)),
+                static_cast<int16_t>(
+                    FrequencyTranslator::rounded_shift(
+                        tap * sine, 14))};
+            phase += tap_phase_increment;
+        }
+        decimator_.set_taps(complex_taps_);
+    }
+
+    std::array<int16_t, taps_count> taps_{};
+    std::array<complex16_t, taps_count> complex_taps_{};
+    decimate::FIRAndDecimateComplex decimator_{};
+    FrequencyTranslator output_xlator_{};
+    uint32_t input_sampling_rate_{0};
+    int32_t frequency_{0};
+};
+
+} /* namespace dsp */
+
+#endif /*__DSP_FREQUENCY_XLATOR_H__*/

firmware/baseband/proc_am_audio.cpp

@@ -45,10 +45,26 @@ void NarrowbandAMAudio::execute(const buffer_c8_t& buffer) {
     }
 
     const auto decim_0_out = decim_0.execute(buffer, dst_buffer);
-    const auto decim_1_out = decim_1.execute(decim_0_out, dst_buffer);
+    const auto audio_decim_0_out = audio_decim_0.execute(decim_0_out, dst_buffer);
 
-    channel_spectrum.feed(decim_1_out, channel_filter_low_f, channel_filter_high_f, channel_filter_transition);
+    spectrum_samples += decim_0_out.count;
+    if (!spectrum_capture_active &&
+        spectrum_samples >= spectrum_interval_samples) {
+        spectrum_samples -= spectrum_interval_samples;
+        channel_spectrum.start_filtered_capture(spectrum_zoom_x2 ? 4 : 2);
+        spectrum_capture_active = true;
+    }
 
+    if (spectrum_capture_active &&
+        channel_spectrum.feed_filtered(
+            audio_decim_0_out,
+            channel_filter_low_f,
+            channel_filter_high_f,
+            channel_filter_transition)) {
+        spectrum_capture_active = false;
+    }
+
+    const auto decim_1_out = translating_decim_1.execute(audio_decim_0_out, dst_buffer);
     const auto decim_2_out = decim_2.execute(decim_1_out, dst_buffer);
     const auto channel_out = channel_filter.execute(decim_2_out, dst_buffer);
 
@@ -97,6 +113,10 @@ void NarrowbandAMAudio::on_message(const Message* const message) {
             capture_config(*reinterpret_cast<const CaptureConfigMessage*>(message));
             break;
 
+        case Message::ID::AudioDDCConfig:
+            ddc_config(*reinterpret_cast<const AudioDDCConfigMessage*>(message));
+            break;
+
         default:
             break;
     }
@@ -106,17 +126,19 @@ void NarrowbandAMAudio::configure(const AMConfigureMessage& message) {
     constexpr size_t decim_0_input_fs = baseband_fs;
     constexpr size_t decim_0_output_fs = decim_0_input_fs / decim_0.decimation_factor;
 
-    constexpr size_t decim_1_input_fs = decim_0_output_fs;
-    constexpr size_t decim_1_output_fs = decim_1_input_fs / decim_1.decimation_factor;
-
+    constexpr size_t audio_decim_0_output_fs = decim_0_output_fs / 2;
+    constexpr size_t decim_1_output_fs =
+        audio_decim_0_output_fs / translating_decim_1.decimation_factor;
     constexpr size_t decim_2_input_fs = decim_1_output_fs;
     constexpr size_t decim_2_output_fs = decim_2_input_fs / decim_2_decimation_factor;
 
     constexpr size_t channel_filter_input_fs = decim_2_output_fs;
     // const size_t channel_filter_output_fs = channel_filter_input_fs / channel_filter_decimation_factor;
 
-    decim_0.configure(message.decim_0_filter.taps);
-    decim_1.configure(message.decim_1_filter.taps);
+    decim_0.configure(message.decim_0_filter.taps, 33554432);
+    audio_decim_0.configure(taps_audio_wide_halfband_0.taps);
+    translating_decim_1.configure(
+        message.decim_1_filter.taps, audio_decim_0_output_fs);
     decim_2.configure(message.decim_2_filter.taps, decim_2_decimation_factor);
     channel_filter.configure(message.channel_filter.taps, channel_filter_decimation_factor);
     channel_filter_low_f = message.channel_filter.low_frequency_normalized * channel_filter_input_fs;
@@ -124,12 +146,20 @@ void NarrowbandAMAudio::configure(const AMConfigureMessage& message) {
     channel_filter_transition = message.channel_filter.transition_normalized * channel_filter_input_fs;
 
     modulation_ssb = (int)message.modulation;  // now sending by message , 3 types of AM demod :   enum class Modulation : int32_t {DSB = 0, SSB = 1, SSB_FM = 2}
-    channel_spectrum.set_decimation_factor(message.channel_spectrum_decimation_factor);
+    spectrum_zoom_x2 = message.channel_spectrum_decimation_factor == 2;
+    channel_spectrum.set_decimation_factor(1);
+    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).
 
     configured = true;
 }
 
+void NarrowbandAMAudio::ddc_config(const AudioDDCConfigMessage& message) {
+    translating_decim_1.set_frequency(message.frequency);
+    channel_spectrum.set_channel_filter_offset(message.frequency);
+}
+
 void NarrowbandAMAudio::capture_config(const CaptureConfigMessage& message) {
     if (message.config) {
         audio_output.set_stream(std::make_unique<StreamInput>(message.config));

firmware/baseband/proc_am_audio.hpp

@@ -28,6 +28,7 @@
 
 #include "dsp_decimate.hpp"
 #include "dsp_demodulate.hpp"
+#include "dsp_frequency_xlator.hpp"
 #include "audio_compressor.hpp"
 
 #include "audio_output.hpp"
@@ -44,6 +45,7 @@ class NarrowbandAMAudio : public BasebandProcessor {
 
    private:
     static constexpr size_t baseband_fs = 3072000;
+    static constexpr auto spectrum_rate_hz = 30.0f;
     static constexpr size_t decim_2_decimation_factor = 4;
     static constexpr size_t channel_filter_decimation_factor = 1;
 
@@ -56,14 +58,19 @@ class NarrowbandAMAudio : public BasebandProcessor {
         audio.data(),
         audio.size()};
 
-    dsp::decimate::FIRC8xR16x24FS4Decim8 decim_0{};
-    dsp::decimate::FIRC16xR16x32Decim8 decim_1{};
+    dsp::decimate::FIRC8xR16x24FS4Decim4 decim_0{};
+    dsp::decimate::FIRC16xR16x16Decim2 audio_decim_0{};
+    dsp::FrequencyTranslatingDecimator32By8 translating_decim_1{};
     dsp::decimate::FIRAndDecimateComplex decim_2{};
     dsp::decimate::FIRAndDecimateComplex channel_filter{};
     int32_t channel_filter_low_f = 0;
     int32_t channel_filter_high_f = 0;
     int32_t channel_filter_transition = 0;
     bool configured{false};
+    size_t spectrum_interval_samples{0};
+    size_t spectrum_samples{0};
+    bool spectrum_capture_active{false};
+    bool spectrum_zoom_x2{false};
 
     // bool modulation_ssb = false;  // Origianlly we only had 2 AM demod types {DSB = 0, SSB = 1} , and we could handle it with bool var , 1 bit.
     int8_t modulation_ssb = 0;  // Now we have 3 AM demod types we will send now index integer  {DSB = 0, SSB = 1, SSB_FM = 2}
@@ -86,6 +93,7 @@ class NarrowbandAMAudio : public BasebandProcessor {
 #endif
 
     void configure(const AMConfigureMessage& message);
+    void ddc_config(const AudioDDCConfigMessage& message);
     void capture_config(const CaptureConfigMessage& message);
 
     buffer_f32_t demodulate(const buffer_c16_t& channel);

firmware/baseband/proc_nfm_audio.cpp

@@ -51,10 +51,26 @@ void NarrowbandFMAudio::execute(const buffer_c8_t& buffer) {
     }
 
     const auto decim_0_out = decim_0.execute(buffer, dst_buffer);
-    const auto decim_1_out = decim_1.execute(decim_0_out, dst_buffer);
+    const auto audio_decim_0_out = audio_decim_0.execute(decim_0_out, dst_buffer);
 
-    channel_spectrum.feed(decim_1_out, channel_filter_low_f, channel_filter_high_f, channel_filter_transition);
+    spectrum_samples += decim_0_out.count;
+    if (!spectrum_capture_active &&
+        spectrum_samples >= spectrum_interval_samples) {
+        spectrum_samples -= spectrum_interval_samples;
+        channel_spectrum.start_filtered_capture(2);
+        spectrum_capture_active = true;
+    }
 
+    if (spectrum_capture_active &&
+        channel_spectrum.feed_filtered(
+            audio_decim_0_out,
+            channel_filter_low_f,
+            channel_filter_high_f,
+            channel_filter_transition)) {
+        spectrum_capture_active = false;
+    }
+
+    const auto decim_1_out = translating_decim_1.execute(audio_decim_0_out, dst_buffer);
     const auto channel_out = channel_filter.execute(decim_1_out, dst_buffer);
 
     feed_channel_stats(channel_out);
@@ -141,6 +157,10 @@ void NarrowbandFMAudio::on_message(const Message* const message) {
             pitch_rssi_config(*reinterpret_cast<const PitchRSSIConfigureMessage*>(message));
             break;
 
+        case Message::ID::AudioDDCConfig:
+            ddc_config(*reinterpret_cast<const AudioDDCConfigMessage*>(message));
+            break;
+
         default:
             break;
     }
@@ -150,22 +170,26 @@ void NarrowbandFMAudio::configure(const NBFMConfigureMessage& message) {
     constexpr size_t decim_0_input_fs = baseband_fs;
     constexpr size_t decim_0_output_fs = decim_0_input_fs / decim_0.decimation_factor;
 
-    constexpr size_t decim_1_input_fs = decim_0_output_fs;
-    constexpr size_t decim_1_output_fs = decim_1_input_fs / decim_1.decimation_factor;
-
+    constexpr size_t audio_decim_0_output_fs = decim_0_output_fs / 2;
+    constexpr size_t decim_1_output_fs =
+        audio_decim_0_output_fs / translating_decim_1.decimation_factor;
     constexpr size_t channel_filter_input_fs = decim_1_output_fs;
     const size_t channel_filter_output_fs = channel_filter_input_fs / message.channel_decimation;
 
     const size_t demod_input_fs = channel_filter_output_fs;
 
-    decim_0.configure(message.decim_0_filter.taps);
-    decim_1.configure(message.decim_1_filter.taps);
+    decim_0.configure(message.decim_0_filter.taps, 33554432);
+    audio_decim_0.configure(taps_audio_wide_halfband_0.taps);
+    translating_decim_1.configure(
+        message.decim_1_filter.taps, audio_decim_0_output_fs);
     channel_filter.configure(message.channel_filter.taps, message.channel_decimation);
     demod.configure(demod_input_fs, message.deviation);
     channel_filter_low_f = message.channel_filter.low_frequency_normalized * channel_filter_input_fs;
     channel_filter_high_f = message.channel_filter.high_frequency_normalized * channel_filter_input_fs;
     channel_filter_transition = message.channel_filter.transition_normalized * channel_filter_input_fs;
-    channel_spectrum.set_decimation_factor(1.0f);
+    channel_spectrum.set_decimation_factor(1);
+    spectrum_interval_samples =
+        decim_0_output_fs / spectrum_rate_hz;
     audio_output.configure(message.audio_hpf_config, message.audio_deemph_config, (float)message.squelch_level / 100.0);
 
     hpf.configure(audio_24k_hpf_30hz_config);
@@ -174,6 +198,11 @@ void NarrowbandFMAudio::configure(const NBFMConfigureMessage& message) {
     configured = true;
 }
 
+void NarrowbandFMAudio::ddc_config(const AudioDDCConfigMessage& message) {
+    translating_decim_1.set_frequency(message.frequency);
+    channel_spectrum.set_channel_filter_offset(message.frequency);
+}
+
 void NarrowbandFMAudio::pitch_rssi_config(const PitchRSSIConfigureMessage& message) {
     pitch_rssi_enabled = message.enabled;
     tone_delta = (message.rssi + 1000) * ((1ULL << 32) / 24000);

firmware/baseband/proc_nfm_audio.hpp

@@ -29,6 +29,7 @@
 
 #include "dsp_decimate.hpp"
 #include "dsp_demodulate.hpp"
+#include "dsp_frequency_xlator.hpp"
 #include "dsp_iir.hpp"
 
 #include "audio_output.hpp"
@@ -50,6 +51,7 @@ class NarrowbandFMAudio : public BasebandProcessor {
 
    private:
     static constexpr size_t baseband_fs = 3072000;
+    static constexpr auto spectrum_rate_hz = 30.0f;
 
     std::array<complex16_t, 512> dst{};
     const buffer_c16_t dst_buffer{
@@ -69,8 +71,9 @@ class NarrowbandFMAudio : public BasebandProcessor {
         (int16_t*)tone.data(),
         sizeof(tone) / sizeof(int16_t)};
 
-    dsp::decimate::FIRC8xR16x24FS4Decim8 decim_0{};
-    dsp::decimate::FIRC16xR16x32Decim8 decim_1{};
+    dsp::decimate::FIRC8xR16x24FS4Decim4 decim_0{};
+    dsp::decimate::FIRC16xR16x16Decim2 audio_decim_0{};
+    dsp::FrequencyTranslatingDecimator32By8 translating_decim_1{};
     dsp::decimate::FIRAndDecimateComplex channel_filter{};
     int32_t channel_filter_low_f = 0;
     int32_t channel_filter_high_f = 0;
@@ -85,6 +88,9 @@ class NarrowbandFMAudio : public BasebandProcessor {
     AudioOutput audio_output{};
 
     SpectrumCollector channel_spectrum{};
+    size_t spectrum_interval_samples{0};
+    size_t spectrum_samples{0};
+    bool spectrum_capture_active{false};
 
     uint32_t tone_phase{0};
     uint32_t tone_delta{0};
@@ -113,6 +119,7 @@ class NarrowbandFMAudio : public BasebandProcessor {
     void pitch_rssi_config(const PitchRSSIConfigureMessage& message);
     void configure(const NBFMConfigureMessage& message);
     void capture_config(const CaptureConfigMessage& message);
+    void ddc_config(const AudioDDCConfigMessage& message);
 };
 
 #endif /*__PROC_NFM_AUDIO_H__*/

firmware/baseband/spectrum_collector.cpp

@@ -22,6 +22,7 @@
 #include "spectrum_collector.hpp"
 
 #include "dsp_fft.hpp"
+#include "dsp_fir_taps.hpp"
 
 #include "utility.hpp"
 #include "event_m4.hpp"
@@ -73,7 +74,7 @@ void SpectrumCollector::set_decimation_factor(
  * perform the deferred task on the buffer of data we prepared.
  */
 
-void SpectrumCollector::feed(
+bool SpectrumCollector::feed(
     const buffer_c16_t& channel,
     const int32_t filter_low_frequency,
     const int32_t filter_high_frequency,
@@ -83,11 +84,50 @@ void SpectrumCollector::feed(
     channel_filter_high_frequency = filter_high_frequency;
     channel_filter_transition = filter_transition;
 
+    bool block_completed = false;
     channel_spectrum_decimator.feed(
         channel,
-        [this](const buffer_c16_t& data) {
+        [this, &block_completed](const buffer_c16_t& data) {
             this->post_message(data);
+            block_completed = true;
         });
+    return block_completed;
+}
+
+void SpectrumCollector::start_filtered_capture(
+    const size_t decimation_factor) {
+    filtered_capture_decimation_ = decimation_factor;
+    filtered_capture_count_ = 0;
+    filtered_capture_ready_ = false;
+}
+
+bool SpectrumCollector::feed_filtered(
+    const buffer_c16_t& channel,
+    const int32_t filter_low_frequency,
+    const int32_t filter_high_frequency,
+    const int32_t filter_transition) {
+    channel_filter_low_frequency = filter_low_frequency;
+    channel_filter_high_frequency = filter_high_frequency;
+    channel_filter_transition = filter_transition;
+
+    const size_t required_samples = 256 * filtered_capture_decimation_;
+    const size_t copy_count = std::min(
+        channel.count, required_samples - filtered_capture_count_);
+    std::copy_n(
+        channel.p,
+        copy_count,
+        filtered_capture_.begin() + filtered_capture_count_);
+    filtered_capture_count_ += copy_count;
+
+    if (filtered_capture_count_ == required_samples) {
+        channel_spectrum_sampling_rate =
+            channel.sampling_rate / filtered_capture_decimation_;
+        filtered_capture_ready_ = true;
+        channel_spectrum_request_update = true;
+        EventDispatcher::events_flag(EVT_MASK_SPECTRUM);
+        return true;
+    }
+    return false;
 }
 
 void SpectrumCollector::post_message(const buffer_c16_t& data) {
@@ -131,11 +171,37 @@ static typename T::value_type spectrum_window_blackman_3(const T& s, const size_
 void SpectrumCollector::update() {
     // Called from idle thread (after EVT_MASK_SPECTRUM is flagged)
     if (streaming && channel_spectrum_request_update) {
+        if (filtered_capture_ready_) {
+            filtered_decim_0_.configure(taps_audio_spectrum_halfband.taps);
+            const buffer_c16_t capture{
+                filtered_capture_.data(),
+                256 * filtered_capture_decimation_,
+                channel_spectrum_sampling_rate * filtered_capture_decimation_};
+            const buffer_c16_t stage_0{
+                filtered_stage_0_.data(),
+                filtered_stage_0_.size()};
+            const auto filtered = filtered_decim_0_.execute(capture, stage_0);
+
+            if (filtered_capture_decimation_ == 4) {
+                filtered_decim_1_.configure(taps_audio_spectrum_halfband.taps);
+                const buffer_c16_t stage_1{
+                    filtered_stage_1_.data(),
+                    filtered_stage_1_.size()};
+                const auto zoom_filtered =
+                    filtered_decim_1_.execute(filtered, stage_1);
+                fft_swap(zoom_filtered, channel_spectrum);
+            } else {
+                fft_swap(filtered, channel_spectrum);
+            }
+            filtered_capture_ready_ = false;
+        }
+
         /* Decimated buffer is full. Compute spectrum. */
         fft_c_preswapped(channel_spectrum, 0, 8);
 
         ChannelSpectrum spectrum;
         spectrum.sampling_rate = channel_spectrum_sampling_rate;
+        spectrum.channel_filter_offset = channel_filter_offset;
         spectrum.channel_filter_low_frequency = channel_filter_low_frequency;
         spectrum.channel_filter_high_frequency = channel_filter_high_frequency;
         spectrum.channel_filter_transition = channel_filter_transition;

firmware/baseband/spectrum_collector.hpp

@@ -29,6 +29,7 @@
 #include "complex.hpp"
 
 #include "block_decimator.hpp"
+#include "dsp_decimate.hpp"
 
 #include <cstdint>
 #include <array>
@@ -40,8 +41,18 @@ class SpectrumCollector {
     void on_message(const Message* const message);
 
     void set_decimation_factor(const size_t decimation_factor);
+    void set_channel_filter_offset(const int32_t offset) {
+        channel_filter_offset = offset;
+    }
 
-    void feed(
+    bool feed(
+        const buffer_c16_t& channel,
+        const int32_t filter_low_frequency,
+        const int32_t filter_high_frequency,
+        const int32_t filter_transition);
+
+    void start_filtered_capture(const size_t decimation_factor);
+    bool feed_filtered(
         const buffer_c16_t& channel,
         const int32_t filter_low_frequency,
         const int32_t filter_high_frequency,
@@ -55,10 +66,19 @@ class SpectrumCollector {
     volatile bool channel_spectrum_request_update{false};
     bool streaming{false};
     std::array<std::complex<float>, 256> channel_spectrum{};
+    std::array<complex16_t, 1024> filtered_capture_{};
+    std::array<complex16_t, 512> filtered_stage_0_{};
+    std::array<complex16_t, 256> filtered_stage_1_{};
+    dsp::decimate::FIRC16xR16x63HalfbandDecim2 filtered_decim_0_{};
+    dsp::decimate::FIRC16xR16x63HalfbandDecim2 filtered_decim_1_{};
+    size_t filtered_capture_count_{0};
+    size_t filtered_capture_decimation_{1};
+    bool filtered_capture_ready_{false};
     uint32_t channel_spectrum_sampling_rate{0};
     int32_t channel_filter_low_frequency{0};
     int32_t channel_filter_high_frequency{0};
     int32_t channel_filter_transition{0};
+    int32_t channel_filter_offset{0};
 
     void post_message(const buffer_c16_t& data);
 

firmware/common/dsp_fir_taps.hpp

@@ -44,6 +44,42 @@ struct fir_taps_complex {
     std::array<complex16_t, N> taps;
 };
 
+/*
+ * 768kHz -> 384kHz half-band prefilter. The broad 80...304kHz
+ * transition makes this stage inexpensive while protecting the useful band.
+ */
+constexpr fir_taps_real<16> taps_audio_wide_halfband_0{
+    .low_frequency_normalized = -80000.0f / 768000.0f,
+    .high_frequency_normalized = 80000.0f / 768000.0f,
+    .transition_normalized = 224000.0f / 768000.0f,
+    .taps = {{
+        -171, 0, 1144, 0, -4481, 0, 19892, 32767,
+        19892, 0, -4481, 0, 1144, 0, -171, 0,
+    }},
+};
+
+/*
+ * Spectrum capture anti-alias half-band filter. It is run only while a
+ * contiguous 256-sample FFT frame is being collected:
+ *   384kHz -> 192kHz (Zoom x1)
+ *   192kHz ->  96kHz (additional stage for Zoom x2)
+ */
+constexpr fir_taps_real<63> taps_audio_spectrum_halfband{
+    .low_frequency_normalized = -0.23f,
+    .high_frequency_normalized = 0.23f,
+    .transition_normalized = 0.04f,
+    .taps = {{
+        -15, 0, 37, 0, -70, 0, 117, 0,
+        -184, 0, 274, 0, -393, 0, 548, 0,
+        -751, 0, 1018, 0, -1374, 0, 1872, 0,
+        -2622, 0, 3910, 0, -6794, 0, 20812, 32767,
+        20812, 0, -6794, 0, 3910, 0, -2622, 0,
+        1872, 0, -1374, 0, 1018, 0, -751, 0,
+        548, 0, -393, 0, 274, 0, -184, 0,
+        117, 0, -70, 0, 37, 0, -16,
+    }},
+};
+
 // NBFM 16K0F3E emission type /////////////////////////////////////////////
 
 // IFIR image-reject filter: fs=3072000, pass=8000, stop=344000, decim=8, fout=384000

firmware/common/message.hpp

@@ -170,6 +170,7 @@ class Message {
         HunterStop = 112,
         TetraBsch = 113,
         TetraDnb = 114,
+        AudioDDCConfig = 115,
         MAX
     };
 
@@ -310,6 +311,16 @@ class SpectrumStreamingConfigMessage : public Message {
     Mode mode{Mode::Stopped};
 };
 
+class AudioDDCConfigMessage : public Message {
+   public:
+    constexpr AudioDDCConfigMessage(int32_t frequency)
+        : Message{ID::AudioDDCConfig},
+          frequency{frequency} {
+    }
+
+    int32_t frequency{0};
+};
+
 class WidebandSpectrumConfigMessage : public Message {
    public:
     constexpr WidebandSpectrumConfigMessage(
@@ -357,6 +368,7 @@ class AudioSpectrumMessage : public Message {
 struct ChannelSpectrum {
     std::array<uint8_t, 256> db{{0}};
     uint32_t sampling_rate{0};
+    int32_t channel_filter_offset{0};
     int32_t channel_filter_low_frequency{0};
     int32_t channel_filter_high_frequency{0};
     int32_t channel_filter_transition{0};