Add selectable RTA filter characteristics

src/audio.rs

@@ -21,9 +21,9 @@ use crate::model::{RtaFrame, SpectroFrame, WaveEnvFrame};
 use crate::ppm::{PpmDetector, PpmStandard};
 #[cfg(target_os = "linux")]
 use crate::rta::{
-    design_fractional_octave_band, exact_fractional_octave_center, fractional_octave_edges,
+    design_fractional_octave_band, design_legacy_repeated_band, exact_fractional_octave_center,
-    integrate_power, normalize_weighting, weighting_power_correction, weighting_reference_db,
+    fractional_octave_edges, integrate_power, normalize_weighting, weighting_power_correction,
-    FrequencyWeighting, StereoCascade, RTW_THIRD_OCTAVE_CENTERS,
+    weighting_reference_db, FrequencyWeighting, StereoCascade, RTW_THIRD_OCTAVE_CENTERS,
 };
 use crate::state::NativeWavRecorder;
 use crate::{
@@ -1329,8 +1329,9 @@ fn build_meter_frame(
 #[cfg(target_os = "linux")]
 fn ensure_rta_state(state: &mut PpmState, sample_rate: u32, config: &PhoenixRtaConfig) {
     let signature = format!(
-        "{}|{}|{}|{}|{}|{}|{}|{}",
+        "{}|{}|{}|{}|{}|{}|{}|{}|{}",
         normalize_engine(&config.engine),
+        normalize_filterbank(&config.filterbank),
         normalize_layout(&config.layout),
         normalize_bpo_key(&config.bpo),
         normalize_freq_range(&config.freq_range),
@@ -1439,9 +1440,9 @@ fn build_active_rta_bands(
 #[cfg(target_os = "linux")]
 fn create_rta_bank(sample_rate: u32, config: &PhoenixRtaConfig) -> RtaBank {
     let (active_layout, bpo_key, active_bands) = build_active_rta_bands(sample_rate, config);
-    // The RTW profile uses a third-order Butterworth prototype, resulting in
+    // In Butterworth mode the RTW profile uses a third-order prototype,
-    // a sixth-order bandpass. Phoenix extension profiles retain selectable
+    // resulting in a sixth-order bandpass. Phoenix extension profiles retain
-    // even total orders.
+    // selectable even total orders.
     let prototype_order = if active_layout == "rtw" {
         3
     } else {
@@ -1461,13 +1462,22 @@ fn create_rta_bank(sample_rate: u32, config: &PhoenixRtaConfig) -> RtaBank {
         .unwrap_or(20_000.0);
     let mut filters = Vec::with_capacity(active_bands.len());
     for band in &active_bands {
-        let coefficients = design_fractional_octave_band(
+        let coefficients = if normalize_filterbank(&config.filterbank) == "legacy" {
-            band.measurement_center,
+            design_legacy_repeated_band(
-            band.f_lo,
+                band.center,
-            band.f_hi,
+                sample_rate,
-            sample_rate,
+                bpo_value(bpo_key),
-            prototype_order,
+                config.order.clamp(2, 8) as usize,
-        );
+            )
+        } else {
+            design_fractional_octave_band(
+                band.measurement_center,
+                band.f_lo,
+                band.f_hi,
+                sample_rate,
+                prototype_order,
+            )
+        };
         filters.push(StereoCascade::new(coefficients));
     }
     let len = active_bands.len();
@@ -1739,6 +1749,7 @@ fn finalize_spectro_state(state: &mut SpectroState, sample_rate: u32) -> bool {
 fn build_iir_rta_frame(bank: &RtaBank, sample_rate: u32, config: &PhoenixRtaConfig) -> RtaFrame {
     RtaFrame {
         engine: "iir".to_string(),
+        filterbank: normalize_filterbank(&config.filterbank).to_string(),
         bands_avg: bank.levels.clone(),
         bands_peak: bank.peaks.clone(),
         centers: bank.centers.clone(),
@@ -1759,6 +1770,7 @@ fn build_fft_rta_frame(
 ) -> RtaFrame {
     RtaFrame {
         engine: "fft".to_string(),
+        filterbank: "none".to_string(),
         bands_avg: state.levels.clone(),
         bands_peak: state.peaks.clone(),
         centers: state.centers.clone(),
@@ -1803,6 +1815,14 @@ fn normalize_engine(value: &str) -> &'static str {
     }
 }
 
+#[cfg(target_os = "linux")]
+fn normalize_filterbank(value: &str) -> &'static str {
+    match value.trim().to_ascii_lowercase().as_str() {
+        "butterworth" | "butter" => "butterworth",
+        _ => "legacy",
+    }
+}
+
 #[cfg(target_os = "linux")]
 fn normalize_fft_size(value: u32) -> usize {
     match value {

src/model.rs

@@ -19,6 +19,7 @@ pub struct ServiceStatus {
 #[derive(Clone, Debug, Serialize)]
 pub struct RtaFrame {
     pub engine: String,
+    pub filterbank: String,
     pub bands_avg: Vec<f32>,
     pub bands_peak: Vec<f32>,
     pub centers: Vec<f32>,
@@ -51,6 +52,7 @@ pub struct WaveEnvFrame {
 #[serde(default, rename_all = "camelCase")]
 pub struct PhoenixRtaConfig {
     pub engine: String,
+    pub filterbank: String,
     pub fft_size: u32,
     pub mono_input: bool,
     pub lr_fractional_delay_enabled: bool,
@@ -81,6 +83,7 @@ impl Default for PhoenixRtaConfig {
     fn default() -> Self {
         Self {
             engine: "iir".to_string(),
+            filterbank: "legacy".to_string(),
             fft_size: 8192,
             mono_input: false,
             lr_fractional_delay_enabled: true,

src/rta.rs

@@ -1,8 +1,7 @@
 //! Realtime-analyzer filter primitives.
 //!
-//! Fractional-octave bands are designed as complete Butterworth bandpasses:
+//! Provides both the original repeated-RBJ Phoenix characteristic and complete
-//! an analog low-pass prototype is transformed to a bandpass, pre-warped and
+//! Butterworth fractional-octave bandpasses for controlled comparison.
-//! mapped with the bilinear transform, then emitted as distinct SOS sections.
 
 pub const RTW_THIRD_OCTAVE_CENTERS: &[f32] = &[
     20.0, 25.0, 31.5, 40.0, 50.0, 63.0, 80.0, 100.0, 125.0, 160.0, 200.0, 250.0, 315.0, 400.0,
@@ -155,6 +154,35 @@ pub fn design_fractional_octave_band(
     sections
 }
 
+/// Reproduces the original Phoenix analyzer characteristic: one constant-peak
+/// RBJ bandpass is repeated for every second order of the requested cascade.
+/// It is retained for comparison with existing Phoenix/RTW captures and is
+/// not presented as a filter implementation prescribed by IEC 61260.
+pub fn design_legacy_repeated_band(
+    center_hz: f32,
+    sample_rate: u32,
+    bands_per_octave: usize,
+    total_order: usize,
+) -> Vec<BiquadCoeffs> {
+    let fs = f64::from(sample_rate.max(8_000));
+    let center = f64::from(center_hz).clamp(0.01, fs * 0.499_9);
+    let bpo = bands_per_octave.max(1) as f64;
+    let upper = 2.0f64.powf(1.0 / (2.0 * bpo));
+    let lower = 2.0f64.powf(-1.0 / (2.0 * bpo));
+    let q = 1.0 / (upper - lower);
+    let omega = 2.0 * std::f64::consts::PI * center / fs;
+    let alpha = omega.sin() / (2.0 * q);
+    let a0 = 1.0 + alpha;
+    let section = BiquadCoeffs {
+        b0: alpha / a0,
+        b1: 0.0,
+        b2: -alpha / a0,
+        a1: -2.0 * omega.cos() / a0,
+        a2: (1.0 - alpha) / a0,
+    };
+    vec![section; (total_order.clamp(2, 8) / 2).max(1)]
+}
+
 pub fn cascade_magnitude(sections: &[BiquadCoeffs], freq_hz: f32, sample_rate: u32) -> f64 {
     let omega = 2.0 * std::f64::consts::PI * f64::from(freq_hz) / f64::from(sample_rate.max(8_000));
     let z1 = Complex::new(omega.cos(), -omega.sin());
@@ -396,6 +424,27 @@ mod tests {
         assert!(((lower * upper).sqrt() - center).abs() < 0.001);
     }
 
+    #[test]
+    fn legacy_sixth_order_repeats_three_identical_sections() {
+        let sections = design_legacy_repeated_band(1_000.0, 48_000, 6, 6);
+        assert_eq!(sections.len(), 3);
+        assert_eq!(sections[0].b0, sections[1].b0);
+        assert_eq!(sections[1].a2, sections[2].a2);
+        assert!(cascade_db(&sections, 1_000.0, 48_000).abs() < 0.001);
+    }
+
+    #[test]
+    fn legacy_and_butterworth_characteristics_remain_distinct() {
+        let factor = 2.0f32.powf(1.0 / 12.0);
+        let legacy = design_legacy_repeated_band(1_000.0, 48_000, 6, 6);
+        let butterworth =
+            design_fractional_octave_band(1_000.0, 1_000.0 / factor, 1_000.0 * factor, 48_000, 3);
+        let legacy_lower = cascade_db(&legacy, 1_000.0 / factor, 48_000);
+        let butterworth_lower = cascade_db(&butterworth, 1_000.0 / factor, 48_000);
+        assert!(legacy_lower < -8.5);
+        assert!((butterworth_lower + 3.0103).abs() < 0.08);
+    }
+
     #[test]
     fn neighboring_third_octave_centers_are_suppressed() {
         let factor = 2.0f32.powf(1.0 / 6.0);

src/state.rs

@@ -321,6 +321,10 @@ fn normalize_rta_config(mut config: PhoenixRtaConfig) -> PhoenixRtaConfig {
         "fft" => "fft".to_string(),
         _ => "iir".to_string(),
     };
+    config.filterbank = match config.filterbank.trim().to_ascii_lowercase().as_str() {
+        "butterworth" | "butter" => "butterworth".to_string(),
+        _ => "legacy".to_string(),
+    };
     config.fft_size = match config.fft_size {
         2048 | 4096 | 8192 | 16384 => config.fft_size,
         n if n < 3072 => 2048,
@@ -721,6 +725,7 @@ mod tests {
             ..PhoenixRtaConfig::default()
         });
         assert_eq!(config.engine, "iir");
+        assert_eq!(config.filterbank, "legacy");
         assert_eq!(config.bpo, "1_12");
         assert_eq!(config.order, 6);
         assert_eq!(config.freq_range, "norm");
@@ -730,6 +735,7 @@ mod tests {
     fn phoenix_profile_retains_explicit_extensions() {
         let config = normalize_rta_config(PhoenixRtaConfig {
             engine: "fft".to_string(),
+            filterbank: "butterworth".to_string(),
             bpo: "1_12".to_string(),
             order: 8,
             freq_range: "lf".to_string(),
@@ -738,6 +744,7 @@ mod tests {
             ..PhoenixRtaConfig::default()
         });
         assert_eq!(config.engine, "fft");
+        assert_eq!(config.filterbank, "butterworth");
         assert_eq!(config.bpo, "1_12");
         assert_eq!(config.order, 8);
         assert_eq!(config.freq_range, "lf");

www/core/audio.js

@@ -516,6 +516,7 @@ function buildPhoenixMeterPacket(frame) {
     xyR,
     rta: rta ? {
       engine: String(rta?.engine || 'iir'),
+      filterbank: String(rta?.filterbank || 'legacy'),
       bands_avg: Array.isArray(rta?.bands_avg) ? rta.bands_avg : (Array.isArray(rta?.bands) ? rta.bands : []),
       bands_peak: Array.isArray(rta?.bands_peak) ? rta.bands_peak : [],
       bands: Array.isArray(rta?.bands) ? rta.bands : (Array.isArray(rta?.bands_avg) ? rta.bands_avg : []),
@@ -871,6 +872,7 @@ export function buildRtaRuntimeConfig(CONFIG = {}) {
   const runtimeBpoMode = bpoMode;
   return {
     engine: rtwProfile ? 'iir' : (CONFIG.RTA_ENGINE || 'iir'),
+    filterbank: CONFIG.RTA_IIR_FILTERBANK === 'butterworth' ? 'butterworth' : 'legacy',
     fftSize: CONFIG.FFT_SIZE || 4096,
     monoInput: !!CONFIG.MONO_INPUT,
     lrFractionalDelayEnabled: !!CONFIG.LR_FRACTIONAL_DELAY_ENABLED,

www/core/config.js

@@ -50,6 +50,7 @@ const CONFIG = {
   REALTIME_BAR_HOLD_MS: 800,
   REALTIME_BAR_DECAY_DB_PER_S: 20,
   RTA_ENGINE: 'iir',             // 'fft' | 'iir'
+  RTA_IIR_FILTERBANK: 'legacy',  // 'legacy' | 'butterworth'
   RTA_FREQ_RANGE: 'norm',        // 'norm' | 'lf'
   RTA_BPO_MODE: '1_3',
   RTA_WEIGHTING: 'z',            // 'z' | 'a' | 'c'
@@ -784,6 +785,9 @@ function loadConfig(opts = {}) {
     if (typeof saved.PPM_DIN_DECAY_DB_PER_S === 'number') CONFIG.PPM_DIN_DECAY_DB_PER_S = saved.PPM_DIN_DECAY_DB_PER_S;
     if (typeof saved.PPM_DIN_HOLD_MS === 'number') CONFIG.PPM_DIN_HOLD_MS = saved.PPM_DIN_HOLD_MS;
     if (CONFIG.RTA_BAR_LAYOUT === 'classic') CONFIG.RTA_BAR_LAYOUT = 'rtw';
+    CONFIG.RTA_IIR_FILTERBANK = CONFIG.RTA_IIR_FILTERBANK === 'butterworth'
+      ? 'butterworth'
+      : 'legacy';
     if (CONFIG.RTA_BAR_LAYOUT === 'rtw') {
       CONFIG.RTA_ENGINE = 'iir';
       CONFIG.RTA_IIR_ORDER = 6;

www/index.html

@@ -303,6 +303,14 @@
             </select>
             <small>FFT (fix) oder IIR-Filterbank</small>
           </div>
+          <div class="opt">
+            <label>IIR-Filtercharakteristik</label>
+            <select id="opt_rtaFilterbank" style="width:230px">
+              <option value="legacy" selected>Legacy (Phoenix/RTW-Vergleich)</option>
+              <option value="butterworth">Butterworth 6. Ordnung</option>
+            </select>
+            <small>Beide Varianten sind IIR. Legacy bildet die bisherige Anzeige nach; Butterworth ist die neue mathematische Konstruktion.</small>
+          </div>
           <div class="opt">
             <label>Bänder pro Oktave</label>
             <select id="opt_rtaBpo" style="width:150px">
@@ -1258,7 +1266,7 @@
 				                  <li><strong>Browserlast:</strong> Single-Slot-Puffer und Sequenzprüfung für Mess-, Spektrogramm- und Visualisierungsdaten ergänzt. Große Rohsamplefelder werden nicht mehr in jedem JSON-Messpaket wiederholt.</li>
 				                  <li><strong>Spektrogramm:</strong> Langzeitstillstand und stotterndes Nachholen beseitigt. Inkrementelles Spaltenzeichnen, Worker-ACK, Watchdog/Neustart und begrenztes Überspringen veralteter Spalten ergänzt. 0,5×, 1×, 2×, 4× und 6× besitzen nun eine feste, FFT- und DPI-unabhängige Zeitbasis.</li>
 				                  <li><strong>DIN-/EBU-PPM:</strong> Blockunabhängige Quasi-Peak-Detektoren mit bandbegrenzter Interpolation, korrekter Attack- und Rücklaufballistik sowie automatischen Tonburst-, Frequenzgang- und Polaritätstests implementiert. Eine zweite Browser-Anstiegsballistik wurde entfernt.</li>
-				                  <li><strong>RTW-RTA:</strong> RTW-Profil verbindlich auf die IIR-Oktavteilband-Filterbank festgelegt. Die Filterbank verwendet vollständige Butterworth-Bandpässe sechster Ordnung aus unterschiedlichen SOS-Sektionen, interne f64-Rechnung sowie exakte IEC-Basis-10-Mitten und -Bandkanten bei vertrauten Nominalbeschriftungen.</li>
+				                  <li><strong>RTW-RTA:</strong> RTW-Profil verbindlich auf die IIR-Oktavteilband-Filterbank festgelegt. Bis zur Vergleichsmessung am realen RTW-Gerät sind die bisherige Phoenix/RTW-Charakteristik und vollständige Butterworth-Bandpässe sechster Ordnung getrennt auswählbar.</li>
 				                  <li><strong>RTA-Bewertung und Integration:</strong> A-, C- und Z-Bewertung sampleweise vor der Filterbank ergänzt. Fast, Medium, Slow, Impulse, Average und Peak rechnen im Leistungsbereich und unabhängig von der ALSA-Periodengröße.</li>
 				                  <li><strong>RTA-Auflösung:</strong> Umschaltung zwischen 1/3, 1/6 und 1/12 repariert. Alle drei Auflösungen bleiben im RTW-Profil aktiv und verwenden dessen IIR-Filterbank, Darstellung und Ballistik.</li>
 				                  <li><strong>Hardwareparameter:</strong> Tatsächlich von ALSA ausgehandelte Samplerate, Perioden- und Puffergröße werden nun durch DSP, Aufnahme, Status und Browser verwendet; die feste 48-kHz-Annahme wurde entfernt.</li>

www/ui/options.js

@@ -278,6 +278,7 @@ function syncUI() {
     ['opt_barThin', CONFIG.METER_BAR_THIN, 'val_barThin', (v)=>Number(v).toFixed(2)],
     ['opt_headerTextColor', CONFIG.HEADER_TEXT_COLOR || '#ffe066'],
     ['opt_rtaEngine', CONFIG.RTA_ENGINE || 'iir'],
+    ['opt_rtaFilterbank', CONFIG.RTA_IIR_FILTERBANK || 'legacy'],
     ['opt_rtaBpo', CONFIG.RTA_BPO_MODE || '1_6'],
     ['opt_rtaOrder', CONFIG.RTA_IIR_ORDER ?? 4],
     ['opt_rtaDisplayGainFft', CONFIG.RTA_DISPLAY_GAIN_FFT_DB ?? 12, 'val_rtaDisplayGainFft', (v)=>`${v} dB`],
@@ -1074,6 +1075,10 @@ function wireHandlers(env) {
     CONFIG.RTA_ENGINE = CONFIG.RTA_BAR_LAYOUT === 'rtw' ? 'iir' : ((v === 'iir') ? 'iir' : 'fft');
     return CONFIG.RTA_ENGINE;
   });
+  h('opt_rtaFilterbank', v => {
+    CONFIG.RTA_IIR_FILTERBANK = v === 'butterworth' ? 'butterworth' : 'legacy';
+    return CONFIG.RTA_IIR_FILTERBANK;
+  });
   h('opt_rtaBpo', v => {
     applyRtaBpoSelection(v);
     const layoutControl = E('opt_rtaLayout');