diff --git a/include/engine/Engine.hpp b/include/engine/Engine.hpp
index dde89ec5..990e7d77 100644
--- a/include/engine/Engine.hpp
+++ b/include/engine/Engine.hpp
@@ -63,11 +63,13 @@ struct Engine {
 	/** Returns the number of stepBlock() calls since the Engine was created.
 	*/
 	int64_t getBlock();
-	/** Returns the number of audio samples since the Engine was created.
+	/** Returns the frame counter which increases every sample step.
+	Not necessarily monotonically increasing. Can be reset at any time.
 	*/
 	int64_t getFrame();
-	/** Sets the frame of the next stepBlock() call.
-	There is no reason to reset the frame in standalone Rack.
+	/** Sets the frame counter.
+	Useful for when the DAW playhead position jumps to a new position.
+	Rack plugins and standalone Rack should not call this function.
 	*/
 	void setFrame(int64_t frame);
 	/** Returns the frame when stepBlock() was last called.
diff --git a/include/midi.hpp b/include/midi.hpp
index 24178354..a9497e96 100644
--- a/include/midi.hpp
+++ b/include/midi.hpp
@@ -80,7 +80,7 @@ struct Message {
 		bytes[2] = value & 0x7f;
 	}
 
-	std::string toString();
+	std::string toString() const;
 };
 
 ////////////////////
diff --git a/src/core/MIDI_CC.cpp b/src/core/MIDI_CC.cpp
index bcd1dc90..5b9c409a 100644
--- a/src/core/MIDI_CC.cpp
+++ b/src/core/MIDI_CC.cpp
@@ -71,7 +71,7 @@ struct MIDI_CC : Module {
 
 	void process(const ProcessArgs& args) override {
 		while (!midiInput.queue.empty()) {
-			midi::Message& msg = midiInput.queue.front();
+			const midi::Message& msg = midiInput.queue.front();
 			// Don't process MIDI message until we've reached its frame.
 			if (msg.frame > args.frame)
 				break;
diff --git a/src/core/MIDI_CV.cpp b/src/core/MIDI_CV.cpp
index e7fa7912..22372f6e 100644
--- a/src/core/MIDI_CV.cpp
+++ b/src/core/MIDI_CV.cpp
@@ -122,7 +122,7 @@ struct MIDI_CV : Module {
 
 	void process(const ProcessArgs& args) override {
 		while (!midiInput.queue.empty()) {
-			midi::Message& msg = midiInput.queue.front();
+			const midi::Message& msg = midiInput.queue.front();
 			// Don't process MIDI message until we've reached its frame.
 			if (msg.frame > args.frame)
 				break;
@@ -184,8 +184,8 @@ struct MIDI_CV : Module {
 		outputs[CONTINUE_OUTPUT].setVoltage(continuePulse.process(args.sampleTime) ? 10.f : 0.f);
 	}
 
-	void processMessage(const midi::Message &msg) {
-		// DEBUG("MIDI: %01x %01x %02x %02x", msg.getStatus(), msg.getChannel(), msg.getNote(), msg.getValue());
+	void processMessage(const midi::Message& msg) {
+		// DEBUG("MIDI: %ld %s", msg.frame, msg.toString().c_str());
 
 		switch (msg.getStatus()) {
 			// note off
diff --git a/src/core/MIDI_Gate.cpp b/src/core/MIDI_Gate.cpp
index 725b34a3..50801bd5 100644
--- a/src/core/MIDI_Gate.cpp
+++ b/src/core/MIDI_Gate.cpp
@@ -65,7 +65,7 @@ struct MIDI_Gate : Module {
 
 	void process(const ProcessArgs& args) override {
 		while (!midiInput.queue.empty()) {
-			midi::Message& msg = midiInput.queue.front();
+			const midi::Message& msg = midiInput.queue.front();
 			// Don't process MIDI message until we've reached its frame.
 			if (msg.frame > args.frame)
 				break;
@@ -91,7 +91,7 @@ struct MIDI_Gate : Module {
 		}
 	}
 
-	void processMessage(const midi::Message &msg) {
+	void processMessage(const midi::Message& msg) {
 		switch (msg.getStatus()) {
 			// note off
 			case 0x8: {
diff --git a/src/core/MIDI_Map.cpp b/src/core/MIDI_Map.cpp
index b8d51fca..3ea2cfaf 100644
--- a/src/core/MIDI_Map.cpp
+++ b/src/core/MIDI_Map.cpp
@@ -83,7 +83,7 @@ struct MIDI_Map : Module {
 			return;
 
 		while (!midiInput.queue.empty()) {
-			midi::Message& msg = midiInput.queue.front();
+			const midi::Message& msg = midiInput.queue.front();
 			// Don't process MIDI message until we've reached its frame.
 			if (msg.frame > args.frame)
 				break;
@@ -130,7 +130,7 @@ struct MIDI_Map : Module {
 		}
 	}
 
-	void processMessage(const midi::Message &msg) {
+	void processMessage(const midi::Message& msg) {
 		// DEBUG("MIDI: %01x %01x %02x %02x", msg.getStatus(), msg.getChannel(), msg.getNote(), msg.getValue());
 
 		switch (msg.getStatus()) {
@@ -142,7 +142,7 @@ struct MIDI_Map : Module {
 		}
 	}
 
-	void processCC(const midi::Message &msg) {
+	void processCC(const midi::Message& msg) {
 		uint8_t cc = msg.getNote();
 		int8_t value = msg.getValue();
 		// Learn
diff --git a/src/midi.cpp b/src/midi.cpp
index c2dd1b78..8f205494 100644
--- a/src/midi.cpp
+++ b/src/midi.cpp
@@ -14,7 +14,7 @@ namespace midi {
 
 static std::vector<std::pair<int, Driver*>> drivers;
 
-std::string Message::toString() {
+std::string Message::toString() const {
 	std::string s;
 	for (size_t i = 0; i < bytes.size(); i++) {
 		if (i > 0)