diff --git a/include/media/IAudioTrack.h b/include/media/IAudioTrack.h
index b83e552..b346722 100644
--- a/include/media/IAudioTrack.h
+++ b/include/media/IAudioTrack.h
@@ -40,8 +40,9 @@
 
     /* After it's created the track is not active. Call start() to
      * make it active. If set, the callback will start being called.
+     * tid identifies the client callback thread, or 0 if not needed.
      */
-    virtual status_t    start() = 0;
+    virtual status_t    start(pid_t tid) = 0;
 
     /* Stop a track. If set, the callback will cease being called and
      * obtainBuffer will return an error. Buffers that are already released