From e64c4076289669717efca4430bce0e6fd578ee6a Mon Sep 17 00:00:00 2001
From: f4exb <f4exb06@gmail.com>
Date: Sun, 8 Mar 2026 23:33:25 +0100
Subject: [PATCH] FT4 demod: added a subtract_edge_symbols parameter to take
 the ramp up/down symbol (for completeness but no significant improvement)

---
 ft8/ft4.cpp           | 46 +++++++++++++++++++++++++++++++++++++++++++
 ft8/ft4.h             |  4 ++++
 sdrbench/test_ft4.cpp |  5 +++--
 3 files changed, 53 insertions(+), 2 deletions(-)

diff --git a/ft8/ft4.cpp b/ft8/ft4.cpp
index bc31ed504..d3485be2a 100644
--- a/ft8/ft4.cpp
+++ b/ft8/ft4.cpp
@@ -3443,6 +3443,29 @@ void FT4::subtract(
         ramp = 1;
     }
 
+    if (params.subtract_edge_symbols)
+    {
+        int sym = bin0 + re103[0];
+        float phase = phases[0];
+        float amp = amps[0];
+        float hz = sym * bin_hz;
+        float dtheta = 2 * M_PI / (rate_ / hz); // advance per sample
+
+        for (int jj = 0; jj < ramp; jj++)
+        {
+            int iii = off0 - ramp + jj;
+
+            if (iii < 0 || iii >= (int)moved.size()) {
+                continue;
+            }
+
+            float theta = phase + (jj - ramp) * dtheta;
+            float x = amp * cos(theta);
+            x *= jj / (float)ramp;
+            moved[iii] -= x;
+        }
+    }
+
     // initial ramp part of first symbol.
     {
         int sym = bin0 + re103[0];
@@ -3550,6 +3573,29 @@ void FT4::subtract(
         }
     }
 
+    if (params.subtract_edge_symbols)
+    {
+        int sym = bin0 + re103[102];
+        float phase = phases[102];
+        float amp = amps[102];
+        float hz = sym * bin_hz;
+        float dtheta = 2 * M_PI / (rate_ / hz); // advance per sample
+
+        for (int jj = 0; jj < ramp; jj++)
+        {
+            int iii = off0 + block * 103 + jj;
+
+            if (iii < 0 || iii >= (int)moved.size()) {
+                continue;
+            }
+
+            float theta = phase + (block + jj) * dtheta;
+            float x = amp * cos(theta);
+            x *= 1.0 - (jj / (float)ramp);
+            moved[iii] -= x;
+        }
+    }
+
     nsamples_ = fftEngine_->hilbert_shift(moved, -diff0, -diff1, rate_);
 }
 
diff --git a/ft8/ft4.h b/ft8/ft4.h
index 5691bcea2..0397b0931 100644
--- a/ft8/ft4.h
+++ b/ft8/ft4.h
@@ -86,6 +86,7 @@ public:
     int second_count;
     int soft_phase_win;
     float subtract_ramp;
+    int subtract_edge_symbols; // model one extra tapered symbol at frame start/end during subtraction (does not yield significant improvement)
     int soft_ones;
     int soft_pairs;
     int soft_triples;
@@ -155,6 +156,7 @@ public:
         second_count = 3;
         soft_phase_win = 2;
         subtract_ramp = 0.11;
+        subtract_edge_symbols = 0;
         soft_ones = 2;
         soft_pairs = 1;
         soft_triples = 1;
@@ -180,6 +182,7 @@ public:
     int use_osd;
     int osd_depth;
     int osd_ldpc_thresh;
+    int subtract_edge_symbols;
     int max_candidates;
 
     FT4ParamsLight() :
@@ -188,6 +191,7 @@ public:
         use_osd(1),
         osd_depth(0),
         osd_ldpc_thresh(70),
+        subtract_edge_symbols(0),
         max_candidates(96)
     {}
 };
diff --git a/sdrbench/test_ft4.cpp b/sdrbench/test_ft4.cpp
index 8f5202523..4d6cd4603 100644
--- a/sdrbench/test_ft4.cpp
+++ b/sdrbench/test_ft4.cpp
@@ -219,8 +219,9 @@ void MainBench::testFT4(const QString& wavFile, const QString& argsStr)
     FT8::FT4Decoder decoder;
     decoder.getParams().nthreads = nthreads;
     decoder.getParams().use_osd = 1;
-    decoder.getParams().osd_depth = 6;
-    decoder.getParams().osd_ldpc_thresh = 77;  // Require at least 77/83 correct LDPC bits before OSD
+    decoder.getParams().osd_depth = 3; // OSD candidates must be in top 3 to be considered
+    decoder.getParams().osd_ldpc_thresh = 75;  // Require at least 75/83 correct LDPC bits before OSD
+    decoder.getParams().subtract_edge_symbols = 0; // Experimental: model pre/post ramp symbols in subtraction
 
     decoder.entry(
         samples.data(),