# scipy.signal.zoom_fft

scipy.signal.zoom_fft(x, fn, m=None, *, fs=2, endpoint=False, axis=-1)

  Compute the DFT of x only for frequencies in range fn.

  Parameters:
    x array

      The signal to transform.

    fn array_like

      A length-2 sequence [f1, f2] giving the frequency range, or a scalar, for which the range [0, fn] is assumed.

    m int, optional

      The number of points to evaluate. The default is the length of x.

    fs float, optional

      The sampling frequency. If `fs=10` represented 10 kHz, for example, then f1 and f2 would also be given in kHz. The default sampling frequency is 2, so f1 and f2 should be in the range [0, 1] to keep the transform below the Nyquist frequency.

    endpoint bool, optional

      If True, f2 is the last sample. Otherwise, it is not included. Default is False.

    axis int, optional

      Axis over which to compute the FFT. If not given, the last axis is used.

  Returns:
    out ndarray

      The transformed signal. The Fourier transform will be calculated at the points f1, f1+df, f1+2df, …, f2, where df=(f2-f1)/m.

  See also

  `ZoomFFT`

    Class that creates a callable partial FFT function.

  Notes

  The defaults are chosen such that `signal.zoom_fft(x, 2)` is equivalent to `fft.fft(x)` and, if `m > len(x)`, that `signal.zoom_fft(x, 2, m)` is equivalent to `fft.fft(x, m)`.

  To graph the magnitude of the resulting transform, use:

      plot(linspace(f1, f2, m, endpoint=False), abs(zoom_fft(x, [f1, f2], m)))

  If the transform needs to be repeated, use `ZoomFFT` to construct a specialized transform function which can be reused without recomputing constants.

  Examples

  To plot the transform results use something like the following:

      >>> import numpy as np
      >>> from scipy.signal import zoom_fft
      >>> t = np.linspace(0, 1, 1021)
      >>> x = np.cos(2*np.pi*15*t) + np.sin(2*np.pi*17*t)
      >>> f1, f2 = 5, 27
      >>> X = zoom_fft(x, [f1, f2], len(x), fs=1021)
      >>> f = np.linspace(f1, f2, len(x))
      >>> import matplotlib.pyplot as plt
      >>> plt.plot(f, 20*np.log10(np.abs(X)))
      >>> plt.show()