code: def spkt_welch_density(self, x, param=None):
          
          if param is None:
              param = [{'coeff': 2}, {'coeff': 5}, {'coeff': 8}]
          welch = feature_calculators.spkt_welch_density(x, param)
          logging.debug("spkt welch density by tsfresh calculated")
          return list(welch)
docs: As in tsfresh `spkt_welch_density <https://github.com/blue-yonder/tsfresh/blob/master/tsfresh/\
              feature_extraction/feature_calculators.py#L1162>`_ . This feature calculator estimates the cross power \
              spectral density of the time series x at different frequencies. To do so, the time series is first shifted \
              from the time domain to the frequency domain. \
              The feature calculators returns the power spectrum of the different frequencies.
              
              :param x: the time series to calculate the feature of
              :type x: pandas.Series
              :param param: contains dictionaries {"coeff": x} with x int
              :type param: list
              :return: the different feature values
              :rtype: pandas.Series