diff --git a/libs/ardour/ardour/midi_state_tracker.h b/libs/ardour/ardour/midi_state_tracker.h
index 984baf8aeb..15075d7b98 100644
--- a/libs/ardour/ardour/midi_state_tracker.h
+++ b/libs/ardour/ardour/midi_state_tracker.h
@@ -60,7 +60,7 @@ public:
 
 	bool empty() const { return _on == 0; }
 	uint16_t on() const { return _on; }
-	bool active (uint8_t note, uint8_t channel) {
+	bool active (uint8_t note, uint8_t channel) const {
 		return _active_notes[(channel*128)+note] > 0;
 	}
 
@@ -89,6 +89,7 @@ class LIBARDOUR_API MidiStateTracker : public MidiNoteTracker
 
 	void flush (MidiBuffer&, samplepos_t, bool reset);
 	void resolve_state (Evoral::EventSink<samplepos_t>&, Evoral::EventList<samplepos_t> const&, samplepos_t time, bool reset = true);
+	void resolve_diff (MidiStateTracker const& other, Evoral::EventSink<samplepos_t>&, samplepos_t time, bool reset = true);
 
   private:
 	uint8_t  program[16];
diff --git a/libs/ardour/midi_state_tracker.cc b/libs/ardour/midi_state_tracker.cc
index e72c48d1b8..b839c79845 100644
--- a/libs/ardour/midi_state_tracker.cc
+++ b/libs/ardour/midi_state_tracker.cc
@@ -514,3 +514,56 @@ MidiStateTracker::resolve_state (Evoral::EventSink<samplepos_t>& dst, Evoral::Ev
 		}
 	}
 }
+
+void
+MidiStateTracker::resolve_diff (MidiStateTracker const & other, Evoral::EventSink<samplepos_t>& dst, samplepos_t time, bool reset)
+{
+	/* This fills @param dst with the messages required to get the MIDI
+	 * receiver's state (assumed to match ours) into the condition
+	 * indicated by @param other
+	 */
+
+	uint8_t      buf[3];
+
+
+	for (int channel = 0; channel < 16; ++channel) {
+
+		for (int n = 0; n < 128; ++n) {
+
+			bool on;
+
+			if ((on = active (n, channel)) != other.active (n, channel)) {
+				buf[0] = (on ? MIDI_CMD_NOTE_OFF : MIDI_CMD_NOTE_ON) | channel;
+				buf[1] = n;
+				buf[2] = 64; /* not good, needs nuance */
+				dst.write (time, Evoral::MIDI_EVENT, 3, buf);
+			}
+
+			if (control[channel][n] != other.control[channel][n]) {
+				buf[0] = MIDI_CMD_CONTROL | channel;
+				buf[1] = n;
+				buf[2] = control[channel][n];
+				dst.write (time, Evoral::MIDI_EVENT, 3, buf);
+			}
+		}
+
+		if (program[channel] != other.program[channel]) {
+			buf[0] = MIDI_CMD_PGM_CHANGE | channel;
+			buf[1] = other.program[channel];
+			dst.write (time, Evoral::MIDI_EVENT, 2, buf);
+		}
+
+
+		if (pressure[channel] != other.pressure[channel]) {
+			buf[0] = MIDI_CMD_CHANNEL_PRESSURE | channel;
+			buf[1] = other.pressure[channel];;
+			dst.write (0, Evoral::MIDI_EVENT, 2, buf);
+		}
+
+		if (bender[channel] != other.bender[channel]) {
+			buf[0] = MIDI_CMD_BENDER | channel;
+			buf[1] = other.bender[channel];
+			dst.write (0, Evoral::MIDI_EVENT, 2, buf);
+		}
+	}
+}