diff --git a/dsp/fn529fe0.cpp b/dsp/fn529fe0.cpp
index a60b7b6..e738a95 100644
--- a/dsp/fn529fe0.cpp
+++ b/dsp/fn529fe0.cpp
@@ -208,6 +208,25 @@ static inline void iir4_bidir_340510(float* x, size_t nbin) {
     acc = 0.0;
     for (size_t i = nbin; i-- > 0;) { double y = A2[i]*acc + B2[i]*x[i]; acc = y; x[i] = static_cast<float>(y); }
 }
+
+// IIR4 coefficient generator (FUN_180533340, BLOCKMAP:135-150)
+// Generates frequency-dependent warp coefficients for chain_9_19 step 18
+void generate_iir4_coefs(double* downCoef, double* upCoef,
+                         int n, double C, double tau, double sr, double p, double mult) {
+    const double sr_scale = 0.9994880557060242;   // DAT_1824c3d8c (live)
+    const double exp_scale = 0.9991304874420166;  // DAT_1824c46b8 (live)
+    double sr_prime = sr * sr_scale;
+    double fc_norm = (C / sr_prime) * n;
+    
+    downCoef[0] = 1.0;
+    upCoef[0] = 0.0;
+    for (int i = 1; i < n; i++) {
+        double g = (i <= fc_norm) ? (fc_norm / i) : std::pow(fc_norm / i, p);
+        double c = 1.0 / (g * tau / mult + 1.0);
+        upCoef[i] = std::exp(c * g * tau * exp_scale);
+        downCoef[i] = 1.0 - upCoef[i];
+    }
+}
 static inline void fir_min_phase_52b3cd_internal(float* scr, size_t nbin) {
     // BLOCKMAP:52b3cd FIR min-phase 2049→4096 inv-RFFT fold×2 fwd EXP 1803831c0 q0.80
     // Real RFFT pipeline validated cascade_sim.py fir_kernel 0.0065dB. Gate RT_FIR=1
diff --git a/dsp/fn529fe0.hpp b/dsp/fn529fe0.hpp
index eb9ba2c..833ef77 100644
--- a/dsp/fn529fe0.hpp
+++ b/dsp/fn529fe0.hpp
@@ -67,8 +67,10 @@ void cascade_detect(
     bool is_magnitude = false      // true = input_data is already |z|, skip Phase 1
 );
 
-// FIR min-phase 52b3cd 2049→4096 (BLOCKMAP:760) — exposed for VLAW path
-void fir_min_phase_52b3cd(float* scr, size_t nbin);
+// IIR4 coefficient generator (FUN_180533340, BLOCKMAP:135-150)
+// Generates frequency-dependent warp coefficients for chain_9_19 step 18
+void generate_iir4_coefs(double* downCoef, double* upCoef,
+                         int n, double C, double tau, double sr, double p, double mult);
 
 // Main chain 9–19 (BLOCKMAP:620) — DIVIDE/FMA/EXP/FIR proxy (1c)
 void chain_9_19(float* bands, float* tmp6f8, float* accVec,
diff --git a/dsp/framed_model.cpp b/dsp/framed_model.cpp
index e26a043..1544536 100644
--- a/dsp/framed_model.cpp
+++ b/dsp/framed_model.cpp
@@ -409,43 +409,27 @@ static void process_band_structural(
     }
 
     // RT_CASC=1: chain_9_19 structural path (BLOCKMAP:620-644)
-    // Replaces warp + IIR3 with calibrated chain including IIR4×2 + FIR min-phase
+    // Input: raw_level (am/res*scale, 0-17.6 mean=0.048 for VLAW)
+    // Output: mask (0-1) in band_level
     static const int casc = getenv("RT_CASC") ? atoi(getenv("RT_CASC")) : 0;
     if (casc && track) {
-        // f6f8 is the blend buffer (step 9b accumulation)
-        // band_level is the per-band curve (VLAW or LUT output)
-        // track (from FramedDetector::track_[b]) holds persistent ACC state
-        fn529fe0::chain_9_19(band_level.data(), f6f8.data(), track,
+        fn529fe0::chain_9_19(raw_level.data(), f6f8.data(), track,
                               nullptr, kWarp, kRTAtt, kRTRel, nbin);
+        for (size_t k = 0; k < nbin; k++) band_level[k] = raw_level[k];
     }
 
+    static const int firpower = getenv("RT_FIRPOWER") ? atoi(getenv("RT_FIRPOWER")) : 0;
     for (size_t k = 0; k < nfft; k++) {
         double mm;
-        // RT_FIRPOWER=1: FIR-style mask from raw spectrum.
-        // Plugin's actual pipeline (52b550-52b8bb):
-        //   1. scratch = log(raw_spectrum)
-        //   2. FIR = exp(0.984 × scratch) = raw^0.984
-        //   3. FIR *= hann_window (freq-domain)
-        //   4. FIR *= 0x540888 (scalar)
-        //   5. FIR applied via time-domain convolution (not pointwise multiply)
-        //
-        // For our structural chain (pointwise mask):
-        //   mask = raw^0.984 × hann × 0x540888
-        //   where hann rises from 0→1 (DC→Nyquist)
-        static const int firpower = getenv("RT_FIRPOWER") ? atoi(getenv("RT_FIRPOWER")) : 0;
-        if (vlaw) {
+        size_t idx = (k < nbin) ? k : nfft - 1 - k;
+        if (casc && track) {
+            mm = static_cast<double>(band_level[idx]);
+        } else if (vlaw) {
             mm = static_cast<double>(band_level[k]);
-            // Signal spectral.cpp that power law is already applied (skip in FIRCONV=3)
-            if (firconv3 == 3) {
-                setenv("RT_FIRCONV3_APPLIED", "1", 1);
-            }
+            if (firconv3 == 3) setenv("RT_FIRCONV3_APPLIED", "1", 1);
         } else if (firpower) {
             double raw = static_cast<double>(raw_level[k]);
-            if (raw > 1e-12) {
-                mm = std::pow(raw, 0.984);
-            } else {
-                mm = 1.0;
-            }
+            mm = (raw > 1e-12) ? std::pow(raw, 0.984) : 1.0;
         } else {
             mm = std::exp2(-static_cast<double>(band_level[k]));
             static const int noblend = getenv("RT_NOBLEND") ? atoi(getenv("RT_NOBLEND")) : 0;