The following matlab example expands the filter

B=[1 0 0 0 0 1]; A=[1 0 0 0 0 .9]; [sos,g] = tf2sos(B,A) sos = 1.00000 0.61803 1.00000 1.00000 0.60515 0.95873 1.00000 -1.61803 1.00000 1.00000 -1.58430 0.95873 1.00000 1.00000 -0.00000 1.00000 0.97915 -0.00000 g = 1The

Note that the first two sections are second-order, while the third is first-order (when coefficients are rounded to five digits of precision after the decimal point).

In addition to
`tf2sos`,
`tf2zp`, and
`zp2sos` discussed above, there are also functions
`sos2zp` and
`sos2tf`, which do the obvious conversion in both Matlab
and Octave.^{10.4} The
`sos2tf` function can be used to check that the
second-order factorization is accurate:

% Pathological "clustered roots" example: [B,A] = zp2tf(ones(10,1),0.9*ones(10,1),1); [sos,g] = tf2sos(B,A); [Bh,Ah] = sos2tf(sos,g); format long; disp(sprintf('Relative L2 numerator error: %g',... norm(Bh-B)/norm(B))); % Relative L2 numerator error: 1.26558e-15 disp(sprintf('Relative L2 denominator error: %g',... norm(Ah-A)/norm(A))); % Relative L2 denominator error: 1.65594e-15Thus, in this test, the original direct-form filter is compared with one created from the second-order sections. Such checking should be done for high-order filters, or filters having many poles and/or zeros close together, because the polynomial factorization used to find the poles and zeros can fail numerically. Moreover, the stability of the factors should be checked individually.

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