[fix](trx-ftx): align FT2 decoder with Fortran reference thresholds

Remove normalize_llr which was undoing the scalefac=2.83 scaling,
causing LLRs to be 2.83x too small for the BP+OSD decoder. Align
sync thresholds with reference: coarse 0.50->0.40, decode 0.65->0.55,
sync quality 10->9, maxosd 3->4. Revert norm_sqr back to norm in
bitmetrics since the metric difference is nonlinear.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/decoders/trx-ftx/src/decode.rs

@@ -521,32 +521,6 @@ fn ft2_extract_logl_seq(
     }
 }
 
-/// Normalize LLR array by dividing by standard deviation, then optionally
-/// scaling to a target variance.
-///
-/// If `target_variance` is `Some(v)`, the output has variance ≈ v.
-/// If `None`, the output has unit variance (σ = 1).
-pub(crate) fn normalize_llr(log174: &mut [f32; FTX_LDPC_N], target_variance: Option<f32>) {
-    let mut sum = 0.0f32;
-    let mut sum2 = 0.0f32;
-    for &v in log174.iter() {
-        sum += v;
-        sum2 += v * v;
-    }
-    let inv_n = 1.0 / FTX_LDPC_N as f32;
-    let variance = (sum2 - sum * sum * inv_n) * inv_n;
-    if variance <= 1e-12 {
-        return;
-    }
-    let scale = match target_variance {
-        Some(tv) => (tv / variance).sqrt(),
-        None => 1.0 / variance.sqrt(),
-    };
-    for v in log174.iter_mut() {
-        *v *= scale;
-    }
-}
-
 /// Verify CRC of a 174-bit plaintext and build an FtxMessage.
 ///
 /// `plain174`: decoded LDPC codeword (174 bits, each 0 or 1).

src/decoders/trx-ftx/src/ft2/bitmetrics.rs

@@ -134,7 +134,7 @@ impl BitMetricsWorkspace {
                         }
                         _ => Complex32::new(0.0, 0.0),
                     };
-                    let coherent = sum.norm_sqr();
+                    let coherent = sum.norm();
 
                     for ib in 0..=ibmax {
                         if ((i >> (ibmax - ib)) & 1) != 0 {

src/decoders/trx-ftx/src/ft2/mod.rs

@@ -16,7 +16,7 @@ pub mod sync;
 use num_complex::Complex32;
 use realfft::RealFftPlanner;
 
-use crate::decode::{normalize_llr, verify_crc_and_build_message, FtxMessage};
+use crate::decode::{verify_crc_and_build_message, FtxMessage};
 use crate::protocol::*;
 
 use bitmetrics::BitMetricsWorkspace;
@@ -428,7 +428,7 @@ impl Ft2Pipeline {
                 idf += 3;
             }
 
-            if best_score < 0.50 {
+            if best_score < 0.40 {
                 continue;
             }
 
@@ -452,7 +452,7 @@ impl Ft2Pipeline {
                 }
             }
 
-            if best_score < 0.50 {
+            if best_score < 0.40 {
                 continue;
             }
 
@@ -516,7 +516,7 @@ impl Ft2Pipeline {
             }
         }
 
-        if best_score < 0.65 {
+        if best_score < 0.55 {
             return None;
         }
 
@@ -575,7 +575,7 @@ impl Ft2Pipeline {
                 0
             };
         }
-        if sync_qual < 10 {
+        if sync_qual < 9 {
             return None;
         }
 
@@ -628,7 +628,6 @@ impl Ft2Pipeline {
                 break;
             }
             let mut log174 = llr_passes[pass];
-            normalize_llr(&mut log174, None);
 
             let mut message91 = [0u8; FTX_LDPC_K];
             let mut cw = [0u8; FTX_LDPC_N];
@@ -639,7 +638,7 @@ impl Ft2Pipeline {
             osd::ft2_decode174_91_osd(
                 &mut log174,
                 FTX_LDPC_K,
-                3,
+                4,
                 3,
                 &mut apmask,
                 &mut message91,