code: def fft_spectrum(frames, fft_points=512):
      
      SPECTRUM_VECTOR = np.fft.rfft(frames, n=fft_points, axis=-1, norm=None)
      return np.absolute(SPECTRUM_VECTOR)
docs: This function computes the one-dimensional n-point discrete Fourier
      Transform (DFT) of a real-valued array by means of an efficient algorithm
      called the Fast Fourier Transform (FFT). Please refer to
      https://docs.scipy.org/doc/numpy/reference/generated/numpy.fft.rfft.html
      for further details.
  
      Args:
          frames (array): The frame array in which each row is a frame.
          fft_points (int): The length of FFT. If fft_length is greater than frame_len, the frames will be zero-padded.
  
      Returns:
              array: The fft spectrum.
              If frames is an num_frames x sample_per_frame matrix, output
              will be num_frames x FFT_LENGTH.