[Pd] Pd's biquad

[Charles Henry]

> I got it now.  I don't understand why both expressions are useful, but
> I see that they're the same.

Exactly.  There's really no reason to use one or the other.  The
two-equation form is the canonical representation, but the equations
can be written as a single equation, which sometimes appears in
textbooks, too.  The order of calculating the IIR part and the FIR
part can be exchanged.  But that's all just computations.  It's not
really useful to have the distinction unless you're actually
programming the filter, and trying to optimize your code.  It's enough
to see that there are a few different formulations of the same thing.

A few other functions that you can make with these are integrators and
differentiators.  Not just simple ones, we can actually make
integrators/differentiators of higher precision, by using several of
them.  Since all polynomials over the real number field can be
decomposed into first/second order factors, we can build filters of
arbitrary order (if we know how to factor them).  Beyond a certain
point, it becomes more efficient to use fft rather than biquads.