# 5.5.1.2. eqcorrscan.utils.correlate.fftw_normxcorr

`eqcorrscan.utils.correlate.``fftw_normxcorr`(templates, stream, pads, threaded=False, *args, **kwargs)

  Normalised cross-correlation using the fftw library.

  Internally this function used double precision numbers, which is definitely required for seismic data. Cross-correlations are computed as the inverse fft of the dot product of the ffts of the stream and the reversed, normalised, templates. The cross-correlation is then normalised using the running mean and standard deviation (not using the N-1 correction) of the stream and the sums of the normalised templates.

  This python fucntion wraps the C-library written by C. Chamberlain for this purpose.

  Parameters: | templates (np.ndarray) 2D Array of templates stream (np.ndarray) 1D array of continuous data pads (list) List of ints of pad lengths in the same order as templates threaded (bool) Whether to use the threaded routine or not - note openMP and python multiprocessing don’t seem to play nice for this.
  Returns: | np.ndarray of cross-correlations
  Returns: | np.ndarray channels used