title: iOS Capture Video from Camera and Mixing with audio file in real time
body: <p>I am trying to capture video from iPhone camera and save the movie mixed with an audio file.
  I can capture the video with the audio (from mic) with no problems. What I want to do is capture the video but instead of mic audio, use a music track (a .caf file). 
  I am capturing the video with AVAssetWriter. I've tried to set up an AVAssetReader to read the audio file, but I couldn't make it work with the AVAssetWriter (maybe because the decoding of audio happens real fast).
  Also, I don't want to save the movie without audio and mix it afterwards with an AVAssetExportSession. It would be to slow for my purpose.
  Any Ideas ? Thanks in advance.</p>