def get_active_length(self):
        """
        Return the maximum active length (i.e., without trailing silence) among
        the pianorolls of all tracks. The unit is time step.

        Returns
        -------
        active_length : int
            The maximum active length (i.e., without trailing silence) among the
            pianorolls of all tracks. The unit is time step.

        """
        active_length = 0
        for track in self.tracks:
            now_length = track.get_active_length()
            if active_length < track.get_active_length():
                active_length = now_length
        return active_length