def _coeffsBA(f, samplerate):
        """ return 12db/oct low pass biquad coeffs for given frequency """
        w0 = 2 * pi * f / samplerate
        alpha = sin(w0) / 2 / 0.7071067811865476 # Butterworth characteristic, Q = 0.707...
        cs = cos(w0)
        norm = 1 / (1 + alpha)
        b0 = (1 - cs) / 2 * norm
        b1 = (1 - cs) * norm
        b2 = b0
        a1 = -2 * cs * norm
        a2 = (1 - alpha) * norm
        return (b0, b1, b2), (1, a1, a2)