[Pd] Pd's biquad

[Chuckk Hubbard]

Hi Chuck.  Thanks for the rundown.  This mostly makes sense to me, except this:

> w(n)-fb1*w(n-1)-fb2*w(n-2)=x(n)
> ff1*w(n)+ff2*w(n-1)+ff3*w(n-2)=y(n)
> so,
>
> y(n)-fb1*y(n-1)-fb2*y(n-2)=ff1*x(n)+ff2*x(n-1)+ff3*x(n-2)

I can't seem to connect them.  The dummy-variable equations seem
unnecessarily dense.
But, so there's nothing special about a biquad filter; just a
2nd-order IIR filter.
-Chuckk