diff --git a/libs/ardour/ardour/automation_control.h b/libs/ardour/ardour/automation_control.h
index 245c70b915..99252f2552 100644
--- a/libs/ardour/ardour/automation_control.h
+++ b/libs/ardour/ardour/automation_control.h
@@ -113,8 +113,8 @@ public:
 	double normal()  const { return _desc.normal; }
 	bool   toggled() const { return _desc.toggled; }
 
-	double internal_to_interface (double i) const;
-	double interface_to_internal (double i) const;
+	double internal_to_interface (double, bool rotary = false) const;
+	double interface_to_internal (double, bool rotary = false) const;
 
 	virtual std::string get_user_string() const;
 
diff --git a/libs/ardour/ardour/parameter_descriptor.h b/libs/ardour/ardour/parameter_descriptor.h
index e0df2f9537..e531133b15 100644
--- a/libs/ardour/ardour/parameter_descriptor.h
+++ b/libs/ardour/ardour/parameter_descriptor.h
@@ -61,8 +61,12 @@ struct LIBARDOUR_API ParameterDescriptor : public Evoral::ParameterDescriptor
 	 * interface value, using settings from Evoral::ParameterDescriptor.
 	 *
 	 * default for AutomationControl::internal_to_interface ();
+	 *
+	 * @param v the control-value to convert
+	 * @param rotary set to true if the GUI control is a rotary knob
+	 * @return interface value in range 0..1
 	 */
-	float to_interface (float) const;
+	float to_interface (float v, bool rotary = false) const;
 
 	/** normalized [0..1] to control-value range
 	 *
@@ -70,8 +74,12 @@ struct LIBARDOUR_API ParameterDescriptor : public Evoral::ParameterDescriptor
 	 * using settings from Evoral::ParameterDescriptor.
 	 *
 	 * default for AutomationControl::interface_to_internal ();
+	 *
+	 * @param v the value in range 0..1 to on convert
+	 * @param rotary set to true if the GUI control is a rotary knob
+	 * @return control-value in range lower..upper
 	 */
-	float from_interface (float) const;
+	float from_interface (float v, bool rotary = false) const;
 
 	bool  is_linear () const;
 	float compute_delta (float from, float to) const;
diff --git a/libs/ardour/automation_control.cc b/libs/ardour/automation_control.cc
index b8d5226b29..eda77f64b2 100644
--- a/libs/ardour/automation_control.cc
+++ b/libs/ardour/automation_control.cc
@@ -337,23 +337,23 @@ AutomationControl::commit_transaction (bool did_write)
 
 /* take control-value and return UI range [0..1] */
 double
-AutomationControl::internal_to_interface (double val) const
+AutomationControl::internal_to_interface (double val, bool rotary) const
 {
 	// XXX maybe optimize. _desc.from_interface() has
 	// a switch-statement depending on AutomationType.
-	return _desc.to_interface (val);
+	return _desc.to_interface (val, rotary);
 }
 
 /* map GUI range [0..1] to control-value */
 double
-AutomationControl::interface_to_internal (double val) const
+AutomationControl::interface_to_internal (double val, bool rotary) const
 {
 	if (!isfinite_local (val)) {
 		assert (0);
 		val = 0;
 	}
 	// XXX maybe optimize. see above.
-	return _desc.from_interface (val);
+	return _desc.from_interface (val, rotary);
 }
 
 std::string
diff --git a/libs/ardour/parameter_descriptor.cc b/libs/ardour/parameter_descriptor.cc
index c86be207f2..1269d69d1a 100644
--- a/libs/ardour/parameter_descriptor.cc
+++ b/libs/ardour/parameter_descriptor.cc
@@ -306,7 +306,7 @@ ParameterDescriptor::midi_note_num (const std::string& name)
 }
 
 float
-ParameterDescriptor::to_interface (float val) const
+ParameterDescriptor::to_interface (float val, bool rotary) const
 {
 	val = std::min (upper, std::max (lower, val));
 	switch(type) {
@@ -323,6 +323,12 @@ ParameterDescriptor::to_interface (float val) const
 			}
 			break;
 		case PanAzimuthAutomation:
+			if (rotary) {
+				val = val;
+			} else {
+				val = 1.0 - val;
+			}
+			break;
 		case PanElevationAutomation:
 			val = val;
 			break;
@@ -356,7 +362,7 @@ ParameterDescriptor::to_interface (float val) const
 }
 
 float
-ParameterDescriptor::from_interface (float val) const
+ParameterDescriptor::from_interface (float val, bool rotary) const
 {
 	val = std::max (0.f, std::min (1.f, val));
 
@@ -374,6 +380,12 @@ ParameterDescriptor::from_interface (float val) const
 			}
 			break;
 		case PanAzimuthAutomation:
+			if (rotary) {
+				val = val;
+			} else {
+				val = 1.0 - val;
+			}
+			break;
 		case PanElevationAutomation:
 			 val = val;
 			break;
diff --git a/libs/pbd/pbd/controllable.h b/libs/pbd/pbd/controllable.h
index 62127fb048..a83a42bcb3 100644
--- a/libs/pbd/pbd/controllable.h
+++ b/libs/pbd/pbd/controllable.h
@@ -119,8 +119,15 @@ public:
 	virtual double get_save_value () const { return get_value(); }
 
 	/** Conversions between `internal', 'interface', and 'user' values */
-	virtual double internal_to_interface (double i) const {return  (i-lower())/(upper() - lower());}  //by default, the interface range is just a linear interpolation between lower and upper values
-	virtual double interface_to_internal (double i) const {return lower() + i*(upper() - lower());}
+	virtual double internal_to_interface (double i, bool rotary = false) const {
+		/* by default, the interface range is just a linear
+		 * interpolation between lower and upper values */
+		return  (i-lower())/(upper() - lower());
+	}
+
+	virtual double interface_to_internal (double i, bool rotary = false) const {
+		return lower() + i*(upper() - lower());
+	}
 
 	/** Get and Set `interface' value  (typically, fraction of knob travel) */
 	virtual float get_interface() const { return (internal_to_interface(get_value())); }