def _interp_kernel_ft(norm_freqs, interp):
    """Scaled FT of a one-dimensional interpolation kernel.

    For normalized frequencies ``-1/2 <= xi <= 1/2``, this
    function returns::

        sinc(pi * xi)**k / sqrt(2 * pi)

    where ``k=1`` for 'nearest' and ``k=2`` for 'linear' interpolation.

    Parameters
    ----------
    norm_freqs : `numpy.ndarray`
        Normalized frequencies between -1/2 and 1/2
    interp : {'nearest', 'linear'}
        Type of interpolation kernel

    Returns
    -------
    ker_ft : `numpy.ndarray`
        Values of the kernel FT at the given frequencies
    """
    # Numpy's sinc(x) is equal to the 'math' sinc(pi * x)
    ker_ft = np.sinc(norm_freqs)
    interp_ = str(interp).lower()
    if interp_ == 'nearest':
        pass
    elif interp_ == 'linear':
        ker_ft *= ker_ft
    else:
        raise ValueError("`interp` '{}' not understood".format(interp))

    ker_ft /= np.sqrt(2 * np.pi)
    return ker_ft