sos in a0 a1 a2 b1 b2

Second order filter section (biquad).  A standard second order
filter section. Filter coefficients are given directly rather than
calculated for you.

Same as TwoPole

> let { theta = line KR (0.2 * pi) pi 5 RemoveSynth
>     ; rho = line KR 0.6 0.99 5 RemoveSynth
>     ; b1 = 2 * rho * cos theta
>     ; b2 = - (rho * rho) }
> in audition (out 0 (sos (lfSaw AR 200 0 * 0.1) 1 0 0 b1 b2))