--- a/modulator.c	Thu Feb 27 13:58:37 2025 +1030
+++ b/modulator.c	Thu Feb 27 15:19:32 2025 +1030
@@ -188,8 +188,10 @@
     nsamples = shapesamples * 2 * ncode + codegap * (ncode - 1) + slew1 + slew2 + 1;
   }
 
-  // Number of steps per samples in the pulse shape
-  stepsize = qdiv(qint(shapelen), qint(shapesamples));
+  // How far to advance the shape pointer for each sample point
+  // Needs to be 1 less than the shape length otherwise we overflow
+  // at the end of the pulse up.
+  stepsize = qdiv(qint(shapelen - 1), qint(shapesamples));
   qsprint(stepsize, tmps, sizeof(tmps));
   printf("shapelen = %d shapesamples = %lu nsamples = %lu stepsize = %s\n", shapelen, shapesamples, nsamples, tmps);
 
@@ -213,7 +215,7 @@
   // Mask start is 0 because we use 8 bit samples
   cfg = interp_default_config();
   interp_config_set_shift(&cfg, QBITS);
-  interp_config_set_mask(&cfg, 0, 32 - QBITS);
+  interp_config_set_mask(&cfg, 0, 32 - QBITS  - 1);
   interp_config_set_blend(&cfg, true);
   interp_set_config(interp0, 0, &cfg);
 
@@ -287,6 +289,7 @@
     for (uint16_t i = 0; i < shapesamples; i++) {
       ctrl[idx] = ctrltmp;
       if (c == 0) {
+	// First code bit
 	// Get sample pair
 	uint8_t *sample_pair = (uint8_t *) interp0->peek[2];
 	// Ask lane 1 for a LERP, using the lane 0 accumulator
@@ -304,6 +307,7 @@
     }
     if (c == 0)
       bit1stopup = idx - 1;
+
     // Pulse down
     // Since the pulse is symmetrical just copy the up slope in reverse
     // XXX: if we had asymmetrical predistortion this wouldn't be true