tf2sos This function converts direct-form filter coefficients to series second-order sections. [sos] = tf2sos (b, a) [sos, g] = tf2sos (b, a) b: matrix of real numbers a: matrix of real numbers This is an Octave function. This function converts direct-form filter coefficients to series second-order sections. The input parameters b and a are vectors specifying the digital filter H(z) = B(z)/A(z). The output is the sos matrix and the overall gain. If there is only one output argument, the overall filter gain is applied to the first second-order section in the sos matrix.