def _unkown_type(self, uridecodebin, decodebin, caps):
        """The callback for decodebin's "unknown-type" signal.
        """
        # This is called *before* the stream becomes ready when the
        # file can't be read.
        streaminfo = caps.to_string()
        if not streaminfo.startswith('audio/'):
            # Ignore non-audio (e.g., video) decode errors.
            return
        self.read_exc = UnknownTypeError(streaminfo)
        self.ready_sem.release()
The callback for decodebin's "unknown-type" signal.