Hi, using Winamp 5.51 (MP4/LC-AAC Encoder 1.27 - 112kbps). the created file contains three trak atoms whereas encoding the same file using iTunes or Facc only creates one trak atom, I thought a simple audio file would only ever contain one trak atom, could anyone explain this please.

The other two tracks are empty tracks for the object descriptors and BIFS (scene stuff), not audio tracks. They are essential empty, but the two tracks are required for ISMA compatibility.

Thanks, I looked at ISMA (Internet Streaming Media Alliance) on wikipedia and it seems primarly conerned with video codecs, and BIFS (Binary Format for Scenes ) seems to be do with visualization - I dont understand why these are in a audio file.

atoms -> container -> MP4 -> multimedia container -> Audio/Video (+others)

i.e. I guess this implies that a video player could search for those. I don't really know if they are required or not, as benski suggests.

So you are saying that because the mp4 container formats supports audio and video, that the file should be readable in some sense by a video player even if it contains no video. Whats the best way to identify that this file is actually an audio file, without falsely matching video files. Currently my first check was that file should only contain one trak atom which obviously fails in this case.