title: GPUImageMovieWriter - black frame caused by audioEncodingTarget
body: <p>I'm trying to record video using the GPUImage library. The recorded clip ends with a black frame. I know that is caused by <strong>audioEncodingTarget</strong> that is a very expensive operation. This has been discussed a lot, but i still don't find a solution.
  Here is my code:</p>
  
  <p><strong>GPUCameraRecorder class init</strong></p>
  
  <pre><code>    videoCamera = GPUImageVideoCamera(sessionPreset: AVCaptureSessionPresetiFrame960x540, cameraPosition: .Back)
      videoCamera.outputImageOrientation = .LandscapeRight;
      videoCamera.horizontallyMirrorFrontFacingCamera = true
  
      filter = GPUImageFilter()
      videoCamera.addTarget(filter)
  
      view = GPUImageView(frame: frame)
      view.fillMode = kGPUImageFillModePreserveAspectRatioAndFill
  
      movieWriter = GPUImageMovieWriter(movieURL: output, size: view.frame.size)
      movieWriter?.encodingLiveVideo = true
  
      filter?.addTarget(movieWriter!)
      filter?.addTarget(view as GPUImageView)
  
      videoCamera.audioEncodingTarget = self.movieWriter!
      videoCamera.startCameraCapture()
  </code></pre>
  
  <p><strong>start recording function</strong></p>
  
  <pre><code>func startRecording(){
      println("Start recording.")
      recording = true
  
      dispatch_async(dispatch_get_main_queue()) {
          var res = self.videoCamera.addAudioInputsAndOutputs()
          println(res)
  
          self.movieWriter!.startRecording()            
      }
  }
  </code></pre>
  
  <p><strong>end recording function</strong></p>
  
  <pre><code>func stopRecording(){
      println("Recording end.")
  
      dispatch_async(dispatch_get_main_queue()) {            
          self.filter?.removeTarget(self.movieWriter!)
          self.videoCamera.audioEncodingTarget = nil
          self.movieWriter!.finishRecording()
  
          self.putTorchOff()
          self.delegate?.recordingEnd()
  
          println("completed")
      }
   }
  </code></pre>
  
  <p>Any suggestion? Someone have find a way to make it work without black frames?</p>