 
     private var _reader: AVAssetReader!
     private var _output: AVAssetReaderOutput!
-    private var _duration: CMTime!
+    private var _fragmentDuration: CMTime!
+    private var _frameInterval: CMTime = kCMTimeIndefinite
     private var _lastPresentationTimestamp: CMTime = kCMTimeZero
 
     /**

    /**
    アセットの再生時間を返す
    */
    var duration: CMTime { return _duration }

    /**
    再生開始位置を加味した、現在のPTSを返す

    let rate: Float
    let startTime: CMTime
    let endTime: CMTime
    let preferredTransform: CGAffineTransform

    init!(asset:AVAsset, rate:Float=1.0, startTime:CMTime=kCMTimeZero, var endTime:CMTime=kCMTimePositiveInfinity) {
