[CM] make-bandpass xcoeffs

James Hearon j_hearon at hotmail.com
Wed Jun 29 10:51:48 PDT 2022

It seems make-bandpass doesn't have the mus-xcoeffs feature as general make-filter does, but I was trying to get the numbers to do some comparisons all in one definstrument, define, or let.  It seems calling the make-bandpass function as a generator and trying to place the output into a vector is the wrong approach, or maybe I have the inp variable wrong?  Also it seems one can use a bandpass or fir-filter call interchangeably?

How can I best get accurate xcoeffs numbers in-line, so to speak, from the make-bandpass gen?

(define (gentest amp flo1 fhi1 order)
 (let ((flt1 (make-bandpass (hz->radians flo1) (hz->radians fhi1) order)) ;flow, fhigh, order
         (v (make-float-vector order))
   (do ((l 0 (+ l 1)))
       ((= l order))
(float-vector-set! v l (bandpass flt1 l))  ;or (fir-filter flt1 l)
(format #t "~%l: ~F bandpass: ~0,6F v: ~0,6F" l (bandpass flt1 l) (v l))
    ) ))

(with-sound (:srate 48000 :channels 2 :header-type mus-riff )
           (gentest .5 200 400 3 )

As a test, I modified make-bandpass from dsp.scm to be able to print the coeffs which makes me believe the float-vector-set! approach above doesn't give the coeffs I was hoping to see.

(define (mymake-bandpass flo fhi order)
   (let* (
          (len order)
          (arrlen (+ 1 (* 2 len)))
          (arr (make-float-vector arrlen))
(do ((i (- len) (+ i 1))) ;-len to +len
   ((= i len)
    (make-fir-filter arrlen arr)) ;order xcoeffs
 (let* ((k (+ i len))
(denom (* pi i))
(num (- (sin (* fhi i)) (sin (* flo i)))))
   (set! (arr k)
 (if (= i 0)
     (/ (- fhi flo) pi)
     (* (/ num denom)
(+ .54 (* .46 (cos (/ (* i pi) len)))))))))
   (do ((l 0 (+ l 1)))
       ((= l arrlen))
(format #t "~%l: ~F  xcoeffs: ~0,6F" l (arr l))
    )) )

(mymake-bandpass (hz->radians 200)  (hz->radians 400) 3)

Thank you for any suggestions,
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