Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'type') and self.type is not None:
            _dict['type'] = self.type
        if hasattr(self, 'codec') and self.codec is not None:
            _dict['codec'] = self.codec
        if hasattr(self, 'frequency') and self.frequency is not None:
            _dict['frequency'] = self.frequency
        if hasattr(self, 'compression') and self.compression is not None:
            _dict['compression'] = self.compression
        return _dict