diff -r 30dc3baf1ab9 -r b62fdb81ce6a client/annotviz/app/js/pianoroll.js
--- a/client/annotviz/app/js/pianoroll.js	Fri Jan 23 11:31:51 2015 +0100
+++ b/client/annotviz/app/js/pianoroll.js	Fri Jan 23 15:30:58 2015 +0100
@@ -107,7 +107,7 @@
                 }
             }
             else {
-                noteDuration = Date.now() - ts;
+                noteDuration = Math.abs(Date.now() - ts);
                 this.noteDict[channel][note] = { ts: ts, velocity: velocity, sessionTs: sessionTs};
             }
         }