[refactor](trx-backend-soapysdr): auto-calculate FIR taps from bandwidth

Replace the static fir_taps parameter with auto_taps(cutoff_norm) which
computes ceil(3.32 / cutoff_norm).clamp(63, 16383). This ensures the
filter transition band equals one passband width regardless of SDR sample
rate, giving correct image rejection when the user sets audio_bandwidth_hz.

At 912 ksps with 3 kHz audio bandwidth this yields ~2018 taps instead
of the previous hardcoded 64, eliminating the 114 kHz stopband gap that
caused adjacent-band signals to alias into the audio output.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-server/trx-backend/trx-backend-soapysdr/src/dsp.rs

@@ -147,7 +147,7 @@ impl SdrPipeline {
         wfm_deemphasis_us: u32,
         wfm_stereo: bool,
         squelch_cfg: VirtualSquelchConfig,
-        channels: &[(f64, RigMode, u32, usize)],
+        channels: &[(f64, RigMode, u32)],
     ) -> Self {
         const IQ_BROADCAST_CAPACITY: usize = 64;
         let (iq_tx, _iq_rx) = broadcast::channel::<Vec<Complex<f32>>>(IQ_BROADCAST_CAPACITY);
@@ -158,7 +158,7 @@ impl SdrPipeline {
         let mut iq_senders = Vec::with_capacity(channels.len());
         let mut channel_dsps_vec: Vec<Arc<Mutex<ChannelDsp>>> = Vec::with_capacity(channels.len());
 
-        for (channel_idx, &(channel_if_hz, ref mode, audio_bandwidth_hz, fir_taps)) in
+        for (channel_idx, &(channel_if_hz, ref mode, audio_bandwidth_hz)) in
             channels.iter().enumerate()
         {
             let (pcm_tx, _pcm_rx) = broadcast::channel::<Vec<f32>>(PCM_BROADCAST_CAPACITY);
@@ -178,7 +178,6 @@ impl SdrPipeline {
                 audio_bandwidth_hz,
                 wfm_deemphasis_us,
                 wfm_stereo,
-                fir_taps,
                 false,
                 channel_squelch_cfg,
                 pcm_tx.clone(),
@@ -250,7 +249,6 @@ impl SdrPipeline {
         channel_if_hz: f64,
         mode: &RigMode,
         bandwidth_hz: u32,
-        fir_taps: usize,
     ) -> (broadcast::Sender<Vec<f32>>, broadcast::Sender<Vec<Complex<f32>>>) {
         const PCM_BROADCAST_CAPACITY: usize = 32;
         const IQ_BROADCAST_CAPACITY: usize = 64;
@@ -266,7 +264,6 @@ impl SdrPipeline {
             bandwidth_hz,
             self.wfm_deemphasis_us,
             self.wfm_stereo,
-            fir_taps.max(1),
             false,
             VirtualSquelchConfig::default(),
             pcm_tx.clone(),
@@ -558,7 +555,7 @@ mod tests {
             75,
             true,
             VirtualSquelchConfig::default(),
-            &[(200_000.0, RigMode::USB, 3000, 64)],
+            &[(200_000.0, RigMode::USB, 3000)],
         );
         assert_eq!(pipeline.pcm_senders.len(), 1);
         assert_eq!(pipeline.channel_dsps.read().unwrap().len(), 1);

src/trx-server/trx-backend/trx-backend-soapysdr/src/dsp/channel.rs

@@ -160,6 +160,18 @@ fn default_bandwidth_for_mode(mode: &RigMode) -> u32 {
     }
 }
 
+/// Calculate the FIR tap count automatically from the normalised cutoff frequency.
+///
+/// Uses the Hann-windowed sinc rule-of-thumb: taps = ceil(3.32 / cutoff_norm),
+/// clamped to [63, 16383].  This gives enough taps so the filter transition band
+/// equals one passband width (image rejection starts at audio_bandwidth_hz).
+fn auto_taps(cutoff_norm: f32) -> usize {
+    if cutoff_norm <= 0.0 {
+        return 63;
+    }
+    ((3.32 / cutoff_norm).ceil() as usize).clamp(63, 16383)
+}
+
 /// Per-channel DSP state: mixer, FFT-FIR, decimator, demodulator, frame accumulator.
 pub struct ChannelDsp {
     pub channel_if_hz: f64,
@@ -169,7 +181,6 @@ pub struct ChannelDsp {
     sdr_sample_rate: u32,
     audio_sample_rate: u32,
     audio_bandwidth_hz: u32,
-    fir_taps: usize,
     wfm_deemphasis_us: u32,
     wfm_stereo: bool,
     wfm_denoise: WfmDenoiseLevel,
@@ -254,7 +265,7 @@ impl ChannelDsp {
         } else {
             (cutoff_hz / self.sdr_sample_rate as f32).min(0.499)
         };
-        self.lpf_iq = BlockFirFilterPair::new(cutoff_norm, ssb_shift_norm(&self.mode, cutoff_norm), self.fir_taps, IQ_BLOCK_SIZE);
+        self.lpf_iq = BlockFirFilterPair::new(cutoff_norm, ssb_shift_norm(&self.mode, cutoff_norm), auto_taps(cutoff_norm), IQ_BLOCK_SIZE);
         let rate_changed = self.decim_factor != next_decim_factor;
         self.decim_factor = next_decim_factor;
         self.decim_counter = 0;
@@ -297,7 +308,6 @@ impl ChannelDsp {
         audio_bandwidth_hz: u32,
         wfm_deemphasis_us: u32,
         wfm_stereo: bool,
-        fir_taps: usize,
         force_mono_pcm: bool,
         squelch_cfg: VirtualSquelchConfig,
         pcm_tx: broadcast::Sender<Vec<f32>>,
@@ -311,7 +321,6 @@ impl ChannelDsp {
             (audio_sample_rate as usize * frame_duration_ms as usize * output_channels) / 1000
         };
 
-        let taps = fir_taps.max(1);
         let (decim_factor, channel_sample_rate) =
             Self::pipeline_rates(mode, sdr_sample_rate, audio_sample_rate, audio_bandwidth_hz);
         let cutoff_hz = audio_bandwidth_hz.min(channel_sample_rate.saturating_sub(1)) as f32 / 2.0;
@@ -331,11 +340,10 @@ impl ChannelDsp {
             channel_if_hz,
             demodulator: Demodulator::for_mode(mode),
             mode: mode.clone(),
-            lpf_iq: BlockFirFilterPair::new(cutoff_norm, ssb_shift_norm(mode, cutoff_norm), taps, IQ_BLOCK_SIZE),
+            lpf_iq: BlockFirFilterPair::new(cutoff_norm, ssb_shift_norm(mode, cutoff_norm), auto_taps(cutoff_norm), IQ_BLOCK_SIZE),
             sdr_sample_rate,
             audio_sample_rate,
             audio_bandwidth_hz,
-            fir_taps: taps,
             wfm_deemphasis_us,
             wfm_stereo,
             wfm_denoise: WfmDenoiseLevel::Auto,
@@ -406,9 +414,8 @@ impl ChannelDsp {
         self.rebuild_filters(true);
     }
 
-    pub fn set_filter(&mut self, bandwidth_hz: u32, taps: usize) {
+    pub fn set_filter(&mut self, bandwidth_hz: u32) {
         self.audio_bandwidth_hz = Self::clamp_bandwidth_for_mode(&self.mode, bandwidth_hz);
-        self.fir_taps = taps.max(1);
         self.rebuild_filters(false);
     }
 
@@ -633,7 +640,6 @@ mod tests {
             3000,
             75,
             true,
-            31,
             false,
             VirtualSquelchConfig::default(),
             pcm_tx,
@@ -657,7 +663,6 @@ mod tests {
             3000,
             75,
             true,
-            31,
             false,
             VirtualSquelchConfig::default(),
             pcm_tx,

src/trx-server/trx-backend/trx-backend-soapysdr/src/lib.rs

@@ -33,7 +33,6 @@ pub struct SoapySdrRig {
     primary_channel_idx: usize,
     /// Current filter state of the primary channel (for filter_controls support).
     bandwidth_hz: u32,
-    fir_taps: u32,
     /// Shared spectrum magnitude buffer populated by the IQ read loop.
     spectrum_buf: Arc<Mutex<Option<Vec<f32>>>>,
     /// How many Hz below the dial frequency the SDR hardware is actually tuned.
@@ -89,7 +88,7 @@ impl SoapySdrRig {
     /// - `args`: SoapySDR device args string (e.g. `"driver=rtlsdr"`).
     ///   Opens a real hardware device via SoapySDR.
     /// - `channels`: per-channel tuples of
-    ///   `(channel_if_hz, initial_mode, audio_bandwidth_hz, fir_taps)`.
+    ///   `(channel_if_hz, initial_mode, audio_bandwidth_hz)`.
     /// - `gain_mode`: `"auto"` or `"manual"`.
     /// - `gain_db`: gain in dB; used when `gain_mode == "manual"`.
     /// - `max_gain_db`: optional hard ceiling for the applied hardware gain.
@@ -111,7 +110,7 @@ impl SoapySdrRig {
     #[allow(clippy::too_many_arguments)]
     pub fn new_with_config(
         args: &str,
-        channels: &[(f64, RigMode, u32, usize)],
+        channels: &[(f64, RigMode, u32)],
         gain_mode: &str,
         gain_db: f64,
         max_gain_db: Option<f64>,
@@ -178,13 +177,11 @@ impl SoapySdrRig {
             (initial_freq.hz as i64 - hardware_center_hz) as f64,
             RigMode::FM,
             25_000,
-            96,
         ));
         all_channels.push((
             (initial_freq.hz as i64 + AIS_CHANNEL_SPACING_HZ - hardware_center_hz) as f64,
             RigMode::FM,
             25_000,
-            96,
         ));
         let block_ms = if sdr_sample_rate == 0 {
             0.0
@@ -259,10 +256,10 @@ impl SoapySdrRig {
         };
 
         // Initialise filter state from primary channel config (index 0), or defaults.
-        let (bandwidth_hz, fir_taps) = channels
+        let bandwidth_hz = channels
             .first()
-            .map(|&(_, _, bw, taps)| (bw, taps as u32))
+            .map(|&(_, _, bw)| bw)
-            .unwrap_or((3000, 64));
+            .unwrap_or(3000);
 
         let spectrum_buf = pipeline.spectrum_buf.clone();
         let retune_cmd = pipeline.retune_cmd.clone();
@@ -286,7 +283,6 @@ impl SoapySdrRig {
             pipeline,
             primary_channel_idx: 0,
             bandwidth_hz,
-            fir_taps,
             spectrum_buf,
             center_offset_hz,
             center_hz: hardware_center_hz,
@@ -365,7 +361,7 @@ impl SoapySdrRig {
                 dsp_arc
                     .lock()
                     .unwrap()
-                    .set_filter(self.bandwidth_hz, self.fir_taps as usize);
+                    .set_filter(self.bandwidth_hz);
             }
         }
     }
@@ -537,7 +533,7 @@ impl RigCat for SoapySdrRig {
                 if let Some(dsp_arc) = dsps.get(self.primary_channel_idx) {
                     let mut dsp = dsp_arc.lock().unwrap();
                     dsp.set_mode(&mode);
-                    dsp.set_filter(self.bandwidth_hz, self.fir_taps as usize);
+                    dsp.set_filter(self.bandwidth_hz);
                 }
             }
             self.apply_ais_channel_activity();
@@ -755,28 +751,7 @@ impl RigCat for SoapySdrRig {
                     dsp_arc
                         .lock()
                         .unwrap()
-                        .set_filter(bandwidth_hz, self.fir_taps as usize);
+                        .set_filter(bandwidth_hz);
-                }
-            }
-            self.apply_ais_channel_filters();
-            Ok(())
-        })
-    }
-
-    fn set_fir_taps<'a>(
-        &'a mut self,
-        taps: u32,
-    ) -> std::pin::Pin<Box<dyn std::future::Future<Output = DynResult<()>> + Send + 'a>> {
-        Box::pin(async move {
-            tracing::debug!("SoapySdrRig: set_fir_taps -> {}", taps);
-            self.fir_taps = taps;
-            {
-                let dsps = self.pipeline.channel_dsps.read().unwrap();
-                if let Some(dsp_arc) = dsps.get(self.primary_channel_idx) {
-                    dsp_arc
-                        .lock()
-                        .unwrap()
-                        .set_filter(self.bandwidth_hz, taps as usize);
                 }
             }
             self.apply_ais_channel_filters();
@@ -827,7 +802,6 @@ impl RigCat for SoapySdrRig {
             .unwrap_or(false);
         Some(RigFilterState {
             bandwidth_hz: self.bandwidth_hz,
-            fir_taps: self.fir_taps,
             cw_center_hz: 700,
             sdr_gain_db: Some(
                 self.max_gain_db

src/trx-server/trx-backend/trx-backend-soapysdr/src/vchan_impl.rs

@@ -39,8 +39,6 @@ use trx_core::vchan::{VChanError, VChannelInfo, VirtualChannelManager};
 // Default DSP parameters for virtual channels
 // ---------------------------------------------------------------------------
 
-const DEFAULT_FIR_TAPS: usize = 64;
-
 fn default_bandwidth_hz(mode: &RigMode) -> u32 {
     match mode {
         RigMode::CW | RigMode::CWR => 500,
@@ -181,7 +179,7 @@ impl SdrVirtualChannelManager {
         let bandwidth_hz = default_bandwidth_hz(mode);
         let (pcm_tx, iq_tx) =
             self.pipeline
-                .add_virtual_channel(if_hz as f64, mode, bandwidth_hz, DEFAULT_FIR_TAPS);
+                .add_virtual_channel(if_hz as f64, mode, bandwidth_hz);
 
         let pipeline_slot = self
             .pipeline
@@ -344,7 +342,7 @@ impl VirtualChannelManager for SdrVirtualChannelManager {
 
         let dsps = self.pipeline.channel_dsps.read().unwrap();
         if let Some(dsp_arc) = dsps.get(ch.pipeline_slot) {
-            dsp_arc.lock().unwrap().set_filter(bandwidth_hz, DEFAULT_FIR_TAPS);
+            dsp_arc.lock().unwrap().set_filter(bandwidth_hz);
         }
         Ok(())
     }
@@ -436,7 +434,7 @@ mod tests {
             75,
             true,
             VirtualSquelchConfig::default(),
-            &[(0.0, RigMode::USB, 3_000, 64)],
+            &[(0.0, RigMode::USB, 3_000)],
         ))
     }