[CM] make-bandpass

[bil at ccrma.Stanford.EDU]

If the bandpass generator is getting no input, it returns
0.0.  Otherwise it returns the output of the bandpass
filter (normally audio samples).  mus-xcoeffs is an
array of filter coefficients -- these are convolved
with the input to produce the output, so

> ;(float-vector-set! v l (bandpass b l)) ;what is output here?

The output of (bandpass b 0) then (bandpass b 1).

> (float-vector-set! v l ((mus-xcoeffs b) l)) ;filter magnitudes

The first and second filter coefficients.

> (flt3 (lambda (y) (+ (bandpass flt1 y) (bandpass flt2 y))))

this adds the output of two bandpass generators.  It is not
the same as adding the two generator's filter coefficients.

Julius Smith has written a very good introduction to filters,
available for free online I think.