diff --git a/src/transcriber.py b/src/transcriber.py
index a70b411..8071c44 100644
--- a/src/transcriber.py
+++ b/src/transcriber.py
@@ -155,7 +155,7 @@ def format_cents_deviation(freq):
 
 def format_dim_diff(dim_diff, ref):
     """Format dimensional difference markup."""
-    if dim_diff is None or ref is None or ref < 0:
+    if dim_diff is None or ref is None or ref < 0 or dim_diff == 0:
         return ""
 
     diff_str = str(abs(dim_diff))
@@ -164,7 +164,8 @@ def format_dim_diff(dim_diff, ref):
     elif dim_diff < 0:
         diff_str += "↓"
 
-    ref_name = ["III", "II", "I"][ref] if 0 <= ref <= 2 else ""
+    ref_names = ["IV", "III", "II", "I"]
+    ref_name = ref_names[ref] if 0 <= ref <= 3 else ""
 
     return f'_\\markup {{ \\lower #3 \\pad-markup #0.2 \\concat{{ "{ref_name}"\\normal-size-super " {diff_str}" }} }}'
 
@@ -332,6 +333,60 @@ def generate_parts(music_data, name, output_dir="lilypond"):
         print(f"Generated: {part_file}")
 
 
+def _is_adjacent(hs1: tuple, hs2: tuple) -> bool:
+    """Check if two hs_arrays are adjacent (differ by ±1 in exactly one dimension, excluding dim 0)."""
+    diff_count = 0
+    for i in range(1, len(hs1)):
+        diff = abs(hs1[i] - hs2[i])
+        if diff > 1:
+            return False
+        if diff == 1:
+            diff_count += 1
+    return diff_count == 1
+
+
+def _compute_dim_diff(current: tuple, prev: tuple) -> int:
+    """Compute dim_diff between two hs_arrays. Returns prime * direction."""
+    primes = [3, 5, 7]
+    for i in range(1, 4):
+        diff = current[i] - prev[i]
+        if diff == 1:
+            return primes[i - 1]
+        if diff == -1:
+            return -primes[i - 1]
+    return 0
+
+
+def _find_ref_and_dim_diff(
+    current_hs: tuple, prev_chord: list, staying_voices: list
+) -> tuple[int, int]:
+    """Find ref (staying voice index) and dim_diff for a changed pitch.
+
+    Args:
+        current_hs: hs_array of current pitch
+        prev_chord: list of hs_arrays from previous chord
+        staying_voices: indices of voices that stay
+
+    Returns:
+        (ref, dim_diff) tuple
+    """
+    if not staying_voices:
+        return -1, 0
+
+    adjacent = []
+    for idx in staying_voices:
+        prev_hs = prev_chord[idx]
+        if _is_adjacent(current_hs, prev_hs):
+            dim_diff = _compute_dim_diff(current_hs, prev_hs)
+            adjacent.append((idx, dim_diff))
+
+    if not adjacent:
+        return -1, 0
+
+    adjacent.sort(key=lambda x: abs(x[1]))
+    return adjacent[0]
+
+
 def output_chords_to_music_data(chords, fundamental=55, chord_duration=4):
     """Convert output_chords.json format to generic music data.
 
@@ -350,20 +405,41 @@ def output_chords_to_music_data(chords, fundamental=55, chord_duration=4):
 
     music_data = [[] for _ in range(num_voices)]
 
+    prev_chord = None
     for chord in chords:
+        current_hs = [tuple(p["hs_array"]) for p in chord]
+
+        if prev_chord is None:
+            staying_voices = []
+        else:
+            staying_voices = [
+                i for i in range(num_voices) if current_hs[i] == prev_chord[i]
+            ]
+
         for voice_idx, pitch in enumerate(chord):
             if voice_idx >= num_voices:
                 break
 
             frac = Fraction(pitch["fraction"])
             freq = fundamental * float(frac)
+            current_hs_array = current_hs[voice_idx]
 
-            ref = None
-            dim_diff = 0
+            if prev_chord is None:
+                ref = -1
+                dim_diff = 0
+            elif current_hs_array == prev_chord[voice_idx]:
+                ref = -1
+                dim_diff = 0
+            else:
+                ref, dim_diff = _find_ref_and_dim_diff(
+                    current_hs_array, prev_chord, staying_voices
+                )
 
             event = [freq, chord_duration, ref, dim_diff]
             music_data[voice_idx].append(event)
 
+        prev_chord = current_hs
+
     return music_data