diff --git a/libs/ardour/ardour/graph.h b/libs/ardour/ardour/graph.h
index 2725c5372b..f7a84fce95 100644
--- a/libs/ardour/ardour/graph.h
+++ b/libs/ardour/ardour/graph.h
@@ -41,6 +41,7 @@
 
 namespace ARDOUR
 {
+class ProcessNode;
 class GraphNode;
 class Graph;
 
@@ -82,7 +83,7 @@ public:
 	uint32_t n_threads () const;
 
 	/* called by GraphNode */
-	void trigger (GraphNode* n);
+	void trigger (ProcessNode* n);
 	void reached_terminal_node ();
 
 	/* called by virtual GraphNode::process() */
@@ -101,8 +102,8 @@ private:
 
 	void helper_thread ();
 
-	PBD::MPMCQueue<GraphNode*> _trigger_queue;      ///< nodes that can be processed
-	GATOMIC_QUAL guint         _trigger_queue_size; ///< number of entries in trigger-queue
+	PBD::MPMCQueue<ProcessNode*> _trigger_queue;      ///< nodes that can be processed
+	GATOMIC_QUAL guint           _trigger_queue_size; ///< number of entries in trigger-queue
 
 	/** Start worker threads */
 	PBD::Semaphore _execution_sem;
diff --git a/libs/ardour/ardour/graphnode.h b/libs/ardour/ardour/graphnode.h
index 673fce1d4c..2457e3e2ba 100644
--- a/libs/ardour/ardour/graphnode.h
+++ b/libs/ardour/ardour/graphnode.h
@@ -42,6 +42,14 @@ typedef boost::shared_ptr<GraphNode> node_ptr_t;
 typedef std::set<node_ptr_t>         node_set_t;
 typedef std::list<node_ptr_t>        node_list_t;
 
+class LIBARDOUR_API ProcessNode
+{
+public:
+	virtual ~ProcessNode() {}
+	virtual void prep (GraphChain const*) = 0;
+	virtual void run (GraphChain const*) = 0;
+};
+
 class LIBARDOUR_API GraphActivision
 {
 public:
@@ -64,15 +72,14 @@ protected:
 };
 
 /** A node on our processing graph, ie a Route */
-class LIBARDOUR_API GraphNode : public GraphActivision
+class LIBARDOUR_API GraphNode : public ProcessNode, public GraphActivision
 {
 public:
 	GraphNode (boost::shared_ptr<Graph> Graph);
 
 	/* API used by Graph */
 	void prep (GraphChain const*);
-	void trigger ();
-	void run (GraphChain const* chain);
+	void run (GraphChain const*);
 
 	/* API used to sort Nodes and create GraphChain */
 	virtual std::string graph_node_name () const = 0;
@@ -80,6 +87,7 @@ public:
 	virtual bool direct_feeds_according_to_reality (boost::shared_ptr<GraphNode>, bool* via_send_only = 0) = 0;
 
 protected:
+	void trigger ();
 	virtual void process () = 0;
 
 	boost::shared_ptr<Graph> _graph;
diff --git a/libs/ardour/graph.cc b/libs/ardour/graph.cc
index 38d16ef912..e71942a978 100644
--- a/libs/ardour/graph.cc
+++ b/libs/ardour/graph.cc
@@ -226,7 +226,7 @@ Graph::prep ()
 }
 
 void
-Graph::trigger (GraphNode* n)
+Graph::trigger (ProcessNode* n)
 {
 	g_atomic_int_inc (&_trigger_queue_size);
 	_trigger_queue.push_back (n);
@@ -274,7 +274,7 @@ Graph::reached_terminal_node ()
 		 *  - Reset terminal reference count
 		 *  - queue initial nodes
 		 */
-		prep (); // XXX
+		prep ();
 
 		if (_graph_empty && !g_atomic_int_get (&_terminate)) {
 			goto again;
@@ -287,7 +287,7 @@ Graph::reached_terminal_node ()
 void
 Graph::run_one ()
 {
-	GraphNode* to_run = NULL;
+	ProcessNode* to_run = NULL;
 
 	if (g_atomic_int_get (&_terminate)) {
 		return;