diff --git a/events.py b/events.py
index bae41e7..7f18c7c 100644
--- a/events.py
+++ b/events.py
@@ -89,12 +89,15 @@ class Playback(Event):
         self._gain = 10**(gain/10) # Convert from dB to linear gain
         self._fadecurve = None
         self._fadeframe = None
+        if (tempo is not None) != (beats is not None):
+            raise ValueError("Playback object must have both tempo and beats, or neither.")
 
     def __repr__(self):
         return '{}(at={}, media={}, device={}, tempo={}, beats={}, leadin={}, loop={}, gain={})'.format(type(self).__name__, self._at, self._media, self._device, self._tempo, self._beats, self._leadin, self._loop, self._gain)
 
     def prepare(self):
-        self._at._timings.register_tempo(self._tempo, self._beats, self._at)
+        if self._tempo is not None:
+            self._at._timings.register_tempo(self._tempo, self._beats, self._at)
 
 #    def start(self):
 #        print("Start playback")
@@ -183,12 +186,12 @@ class Playback(Event):
         # We really should check that the media is the sampe shape as the
         # existing playback. But for now, let's trust the user. *cough*
 
-    def notify(self, message, fadetime=10, db=30, media=None, delay=False, setmark=None, *args, **kwargs):
+    def notify(self, message, fadetime=10, db=30, media=None, delay=False, loop=False, setmark=None, *args, **kwargs):
         if message == 'fadeout':
             self._fadecurve = np.logspace(0, -db/10, fadetime * self._fs)
             self._fadeframe = 0
         elif message == 'replaceplayback':
-            self.replace(media, delay, setmark=setmark)
+            self.replace(media, delay, loop=loop, setmark=setmark)
         elif message == 'endloop':
             self._loop = False