title: Drawing waveform with AVAssetReader
body: <p>I reading song from iPod library using assetUrl (in code it named audioUrl)
  I can play it many ways, I can cut it, I can make some precessing with this but...
  I really don't understand what I gonna do with this CMSampleBufferRef to get data for drawing waveform! I need info about peak values, how I can get it this (maybe another) way? </p>
  
  <pre><code>    AVAssetTrack * songTrack = [audioUrl.tracks objectAtIndex:0];
      AVAssetReaderTrackOutput * output = [[AVAssetReaderTrackOutput alloc] initWithTrack:songTrack outputSettings:nil];
      [reader addOutput:output];
      [output release];
  
      NSMutableData * fullSongData = [[NSMutableData alloc] init];
      [reader startReading];
  
      while (reader.status == AVAssetReaderStatusReading){
  
          AVAssetReaderTrackOutput * trackOutput = 
          (AVAssetReaderTrackOutput *)[reader.outputs objectAtIndex:0];
  
          CMSampleBufferRef sampleBufferRef = [trackOutput copyNextSampleBuffer];
  
          if (sampleBufferRef){/* what I gonna do with this? */}
  </code></pre>
  
  <p>Please help me! </p>