code: def fft_mean_coeff(signal, fs, nfreq=256):
      """Computes the mean value of each spectrogram frequency.
  
      nfreq can not be higher than half signal length plus one.
      When it does, it is automatically set to half signal length plus one.
  
      Feature computational cost: 1
  
      Parameters
      ----------
      signal : nd-array
          Input from which fft mean coefficients are computed
      fs : float
          Sampling frequency
      nfreq : int
          The number of frequencies
  
      Returns
      -------
      nd-array
          The mean value of each spectrogram frequency
  
      """
      if nfreq > len(signal) // 2 + 1:
          nfreq = len(signal) // 2 + 1
  
      fmag_mean = scipy.signal.spectrogram(
          signal, fs, nperseg=nfreq * 2 - 2
      )[2].mean(1)
  
      return tuple(fmag_mean)
  
imports: import scipy.signal
  from tsfel.feature_extraction.features_utils import *
label_desc: Use image node_id 54 for calling a global function with example usage: fft_mean_coeff(signal, fs, nfreq) and returns: tuple