Add sliding-frequency audio receiver tuning

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};