<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def start_recording(self, output_file):
 """ Starts recording to a given output video file. Parameters output_file : :obj:`str` filename to write video to """

        if not self._started:
            raise Exception("Must start the video recorder first by calling .start()!")
        if self._recording:
            raise Exception("Cannot record a video while one is already recording!")
        self._recording = True
        self._cmd_q.put(('start', output_file))