From 1167bdec16c81bb99a190dde8ef192fcdb485d5b Mon Sep 17 00:00:00 2001
From: erikkaashoek <erik@kaashoek.com>
Date: Sat, 15 Aug 2020 14:39:51 +0200
Subject: [PATCH] Hyperbolic interpolation of tracking marker frequencies
 (OneOfEleven)

---
 plot.c    | 15 ++++++---------
 sa_core.c | 13 +++++++++++++
 2 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/plot.c b/plot.c
index 8dbbf28..51bc308 100644
--- a/plot.c
+++ b/plot.c
@@ -845,13 +845,10 @@ extern const char *unit_string[];
 static void trace_get_value_string(
     int t, char *buf, int len,
     int i, float coeff[POINTS_COUNT],
-    uint32_t freq[POINTS_COUNT],
-    int point_count,
-    int ri, int mtype)
+    int ri, int mtype,
+    uint32_t i_freq, uint32_t ref_freq)
 {
   (void) t;
-  (void)freq;
-  (void) point_count;
   float v;
   char buf2[16];
   char buf3[8];
@@ -864,17 +861,17 @@ static void trace_get_value_string(
     *ptr2++ = S_DELTA[0];
     unit_index = setting.unit+5;
     if (ri > i) {
-      dfreq = frequencies[ri] - frequencies[i];
+      dfreq = ref_freq - i_freq;
       ii = ri - i;
       *ptr2++ = '-';
     } else {
-      dfreq = frequencies[i] - frequencies[ri];
+      dfreq = i_freq - ref_freq;
       ii = i - ri;
       *ptr2++ = '+';
     }
     rlevel = value(coeff[ri]);
   } else {
-    dfreq = frequencies[i];
+    dfreq = i_freq;
   }
   if (FREQ_IS_CW()) {
     float t = ii*(setting.actual_sweep_time_us)/(sweep_points - 1);
@@ -2112,7 +2109,7 @@ static void cell_draw_marker_info(int x0, int y0)
 //        cell_drawstring_7x13(buf, xpos, ypos);
       trace_get_value_string(
           t, &buf[k], (sizeof buf) - k,
-          idx, measured[trace[t].channel], frequencies, sweep_points, ridx, markers[i].mtype);
+          idx, measured[trace[t].channel], ridx, markers[i].mtype,markers[i].frequency, markers[ref_marker].frequency);
       if (/* strlen(buf)*7> WIDTH/2 && */active > 1)
         cell_drawstring(buf, xpos, ypos);
       else
diff --git a/sa_core.c b/sa_core.c
index 3e512e3..442f3ca 100644
--- a/sa_core.c
+++ b/sa_core.c
@@ -2073,6 +2073,19 @@ sweep_again:                                // stay in sweep loop when output mo
         if (markers[m].enabled && markers[m].mtype & M_TRACKING) {   // Available marker found
           markers[m].index = max_index[i];
           markers[m].frequency = frequencies[markers[m].index];
+#if 1                                                        // Hyperbolic interpolation, can be removed to save memory
+          const int idx          = markers[m].index;
+          if (idx > 0 && idx < sweep_points-1)
+          {
+            const float y1         = actual_t[idx - 1];
+            const float y2         = actual_t[idx + 0];
+            const float y3         = actual_t[idx + 1];
+            const float d          = 0.5f * (y1 - y3) / ((y1 - (2 * y2) + y3) + 1e-12f);
+            //const float bin      = (float)idx + d;
+            const int32_t delta_Hz = abs((int64_t)frequencies[idx + 0] - frequencies[idx + 1]);
+            markers[m].frequency   += (int32_t)(delta_Hz * d);
+          }
+#endif
           m++;
           break;                          // Next maximum
         }