;;; various even order analog filters, based primarily on Anders Johansson's (GPL'd) code
;;;
;;; butterworth-lowpass|highpass|bandstop|bandpass
;;; chebyshev-lowpass|highpass|bandstop|bandpass
;;; inverse-chebyshev-lowpass|highpass|bandstop|bandpass
;;;
;;; if GSL included in Snd:
;;; bessel-lowpass|highpass|bandstop|bandpass
;;; elliptic-lowpass|highpass|bandstop|bandpass
;;;
;;; build Snd with gsl for best results

(provide 'snd-analog-filter.scm)


(define* (analog->digital n num den fz)
  (let ((g 1.0)
	(Q 1.0)
	(wc (tan (* pi fz)))
	(c (make-vct (* 2 n))))

    (define (cascade->canonical A)
      "(cascade->canonical A) converts cascade filter coeffs to canonical form"
      ;; from Orfanidis "Introduction to Signal Processing"

      (define (conv M h L x y)
	;; x * h -> y
	(do ((n 0 (+ n 1)))
	    ((= n (+ L M)))
	  (set! (y n) 0.0)
	  (do ((m (max 0 (- n (+ 1 L))) (+ 1 m)))
	      ((> m (min n M)))
	    (set! (y n) (+ (y n) (* (h m) (x (- n m))))))))

      (let* ((K (length A))
	     (d (make-vct (+ 1 (* 2 K))))
	     (a1 (make-vct (+ 1 (* 2 K)))))
	(set! (a1 0) 1.0)
	(do ((i 0 (+ 1 i)))
	    ((= i K))
	  (conv 2 (A i) (+ 1 (* 2 i)) a1 d)
	  (do ((j 0 (+ 1 j)))
	      ((= j (+ 3 (* 2 i))))
	    (set! (a1 j) (d j))))
	a1))

    (do ((i 0 (+ i 2))
	 (j 0 (+ j 3))
	 (k 0 (+ k 4)))
	((>= i n))
      (let* ((nt0 (/ (num (+ j 0)) (* wc wc)))
	    (nt1 (/ (num (+ j 1)) wc))
	    (nt2 (num (+ j 2)))
	    (dt0 (/ (den (+ j 0)) (* wc wc)))
	    (dt1 (/ (den (+ j 1)) (* wc Q)))
	    (dt2 (den (+ j 2)))
	    (kd (+ dt0 dt1 dt2))
	    (kn (+ nt0 nt1 nt2)))
	(set! (c    k   ) (/ (- (* 2.0 dt2) (* 2.0 dt0)) kd))
	(set! (c (+ k 1)) (/ (+ dt0 (- dt1) dt2) kd))
	(set! (c (+ k 2)) (/ (- (* 2.0 nt2) (* 2.0 nt0)) kn))
	(set! (c (+ k 3)) (/ (+ nt0 (- nt1) nt2) kn))
	(set! g (* g (/ kn kd)))))

    (let ((a '())
	  (b '()))
      (do ((i 0 (+ i 2))
	   (k 0 (+ k 4))) ; c
	  ((>= i n))
	(set! a (cons (vct (c (+ k 3)) (c (+ k 2)) (c (+ k 3))) a))
	(set! b (cons (vct 1.0 (c k) (c (+ k 1))) b)))

      (list (vct-scale! (cascade->canonical a) g) ; scale entire numerator because this is the convolved form
	    (cascade->canonical b)))))

(define (prototype->highpass n num den)
  (let ((g 1.0)
	(numt (make-vct (length num)))
	(dent (make-vct (length den))))
    (do ((k 0 (+ k 2))
	 (i 0 (+ i 3)))
	((>= k n)) 
      (set! g (* g (/ (num (+ i 2)) (den (+ i 2)))))
      (set! (numt    i   ) 1.0)
      (set! (numt (+ i 1)) (/ (num (+ i 1)) (num (+ i 2))))
      (set! (numt (+ i 2)) (/ (num i) (num (+ i 2))))
      (set! (dent    i   ) 1.0)
      (set! (dent (+ i 1)) (/ (den (+ i 1)) (den (+ i 2))))
      (set! (dent (+ i 2)) (/ (den i) (den (+ i 2)))))
    (set! (numt 0) g)
    (list numt dent)))



;;; ---------------- Butterworth ----------------

(define (butterworth-prototype n)
  (let* ((len (/ (* n 3) 2))
	 (num (make-vct len))
	 (den (make-vct len)))
    (do ((w 1 (+ w 2))
	 (j 0 (+ j 3)))
	((>= w n))
      (set! (num j) 0.0)
      (set! (num (+ j 1)) 0.0)
      (set! (num (+ j 2)) 1.0)
      (set! (den j) 1.0)
      (set! (den (+ j 1)) (* 2.0 (cos (/ (* w pi) (* 2.0 n)))))
      (set! (den (+ j 2)) 1.0))
    (list num den)))

(define (make-butterworth-lowpass n fc)
  "(make-butterworth-lowpass n fc) returns a lowpass Buttterworth filter; n = order, fc = cutoff \
freq (srate = 1.0): (make-butterworth-lowpass 8 .1)"
  ;; identical to make-butter-lp except for fc (freq->1.0) fixup
  (if (odd? n) (set! n (+ n 1)))
  (let* ((proto (butterworth-prototype n))
	 (coeffs (analog->digital n (car proto) (cadr proto) fc)))
    (make-filter :xcoeffs (car coeffs) :ycoeffs (cadr coeffs))))

(define (make-butterworth-highpass n fc) 
  "(make-butterworth-highpass n fc) returns a highpass Butterworth filter; n = order, fc = cutoff \
freq (srate = 1.0): (make-butterworth-highpass 8 .1)"
  (if (odd? n) (set! n (+ n 1)))
  (let* ((proto (butterworth-prototype n))
	 (hproto (prototype->highpass n (car proto) (cadr proto)))
	 (coeffs (analog->digital n (car hproto) (cadr hproto) fc)))
    (make-filter :xcoeffs (car coeffs) :ycoeffs (cadr coeffs))))

(define (make-butterworth-bandpass n fl fh)
  "(make-butterworth-bandpass n fl fh) returns a bandpass Butterworth filter; n = order, fl and fh \
are (1.0-based) edge freqs: (make-butterworth-bandpass 4 .1 .2)"
  (if (odd? n) (set! n (+ n 1)))
  (let ((lp (make-butterworth-lowpass n fh))
	(hp (make-butterworth-highpass n fl)))
    (lambda (y) 
      (filter lp (filter hp y)))))

(define (make-butterworth-bandstop n fl fh)
  "(make-butterworth-bandstop n fl fh) returns a bandstop Butterworth filter; n = order, fl and fh \
are (1.0-based) edge freqs: (make-butterworth-bandstop 4 .1 .2)"
  (if (odd? n) (set! n (+ n 1)))
  (let ((lp (make-butterworth-lowpass n fl))
	(hp (make-butterworth-highpass n fh)))
    (lambda (y) 
      (+ (filter lp y) (filter hp y)))))



;;; ---------------- Chebyshev ---------------- 

(define* (chebyshev-prototype n (ripple 1.0)) ; ripple in dB (positive)
  (let* ((e (sqrt (- (expt 10.0 (* 0.1 ripple)) 1.0)))
	 (v0 (/ (asinh (/ 1.0 e)) n))
	 (len (/ (* n 3) 2))
	 (num (make-vct len))
	 (den (make-vct len)))
    (do ((l 1.0 (+ l 2.0))
	 (j 0 (+ j 3)))
	((>= l n))
      (let ((u (- (* (sinh v0) (sin (/ (* l pi) (* 2.0 n))))))
	    (w (* (cosh v0) (cos (/ (* l pi) (* 2.0 n))))))
      (set! (num    j   ) 0.0)
      (set! (num (+ j 1)) 0.0)
      (set! (num (+ j 2)) 1.0)
      (set! (den    j   ) 1.0)
      (set! (den (+ j 1)) (* -2.0 u))
      (set! (den (+ j 2)) (+ (* u u) (* w w)))))
    (set! (num 2) (/ (expt 2.0 (- 2 n))
		     (expt 3.2 (/ (log ripple) (log 10.0))))) ; whatever works...
    (list num den)))

(define* (make-chebyshev-lowpass n fc (ripple 1.0))
  "(make-chebyshev-lowpass n fc (ripple 1.0)) returns a lowpass Chebyshev filter; n = order, \
fc = cutoff freq (srate = 1.0): (make-chebyshev-lowpass 8 .1)"
  (if (odd? n) (set! n (+ n 1)))
  (let* ((proto (chebyshev-prototype n ripple))
	 (coeffs (analog->digital n (car proto) (cadr proto) fc)))
    (make-filter :xcoeffs (car coeffs) :ycoeffs (cadr coeffs))))

(define* (make-chebyshev-highpass n fc (ripple 1.0))
  "(make-chebyshev-highpass n fc (ripple 1.0)) returns a highpass Chebyshev filter; n = order, \
fc = cutoff freq (srate = 1.0): (make-chebyshev-highpass 8 .1 .01)"
  (if (odd? n) (set! n (+ n 1)))
  (let* ((proto (chebyshev-prototype n ripple))
	 (hproto (prototype->highpass n (car proto) (cadr proto)))
	 (coeffs (analog->digital n (car hproto) (cadr hproto) fc)))
    (make-filter :xcoeffs (car coeffs) :ycoeffs (cadr coeffs))))

(define* (make-chebyshev-bandpass n fl fh (ripple 1.0))
  "(make-chebyshev-bandpass n fl fh (ripple 1.0)) returns a bandpass Chebyshev filter; n = order, \
fl and fh = edge freqs (srate = 1.0): (make-chebyshev-bandpass 4 .1 .2)"
  (if (odd? n) (set! n (+ n 1)))
  (let ((lp (make-chebyshev-lowpass n fh ripple))
	(hp (make-chebyshev-highpass n fl ripple)))
    (lambda (y) 
      (filter lp (filter hp y)))))

(define* (make-chebyshev-bandstop n fl fh (ripple 1.0))
  "(make-chebyshev-bandstop n fl fh (ripple 1.0)) returns a bandstop Chebyshev filter; n = order, \
fl and fh = edge freqs (srate = 1.0): (make-chebyshev-bandstop 8 .1 .4 .01)"
  (if (odd? n) (set! n (+ n 1)))
  (let ((lp (make-chebyshev-lowpass n fl ripple))
	(hp (make-chebyshev-highpass n fh ripple)))
    (lambda (y) 
      (+ (filter lp y) (filter hp y)))))



;;; ---------------- inverse Chebyshev ---------------- 

(define* (inverse-chebyshev-prototype n (loss-dB 60.0)) ; stopband loss
  (let* ((e (sqrt (/ 1.0 (- (expt 10.0 (* 0.1 loss-dB)) 1.0))))
	 (v0 (/ (asinh (/ 1.0 e)) n))
	 (len (/ (* n 3) 2))
	 (num (make-vct len))
	 (den (make-vct len)))
    (let ((pl 0.0))
      (do ((l 1.0 (+ l 2.0))
	   (j 0 (+ j 3)))
	  ((>= l n))
	(let ((u (- (* (sinh v0) (sin (/ (* l pi) (* 2.0 n))))))
	      (w (* (cosh v0) (cos (/ (* l pi) (* 2.0 n)))))
	      (t (/ 1.0 (sin (/ (* (+ l pl) pi) (* 2.0 n))))))
	  (set! (num    j   ) 1.0)
	  (set! (num (+ j 1)) 0.0)
	  (set! (num (+ j 2)) (* t t))
	  (set! (den    j   ) 1.0)
	  (set! (den (+ j 1)) (/ (* -2.0 u) (+ (* u u) (* w w))))
	  (set! (den (+ j 2)) (/ 1.0 (+ (* u u) (* w w)))))))
    (list num den
	  (expt 1.122 (- loss-dB))))) ; argh

(define* (make-inverse-chebyshev-lowpass n fc (loss-dB 60.0))
  "(make-inverse-chebyshev-lowpass n fc (loss-dB 60.0)) returns a lowpass inverse-Chebyshev filter; n = order, \
fc = cutoff freq (srate = 1.0): (make-inverse-chebyshev-lowpass 10 .4 120)"
  (if (odd? n) (set! n (+ n 1)))
  (let* ((proto (inverse-chebyshev-prototype n loss-dB))
	 (coeffs (analog->digital n (car proto) (cadr proto) fc)))
    (make-filter :xcoeffs (vct-scale! (car coeffs) (caddr proto)) :ycoeffs (cadr coeffs))))

(define* (make-inverse-chebyshev-highpass n fc (loss-dB 60.0))
  "(make-inverse-chebyshev-highpass n fc (loss-dB 60.0)) returns a highpass inverse-Chebyshev filter; n = order, \
fc = cutoff freq (srate = 1.0): (make-inverse-chebyshev-highpass 10 .1 120)"
  (if (odd? n) (set! n (+ n 1)))
  (let* ((proto (inverse-chebyshev-prototype n loss-dB))
	 (hproto (prototype->highpass n (car proto) (cadr proto)))
	 (coeffs (analog->digital n (car hproto) (cadr hproto) fc)))
    (make-filter :xcoeffs (vct-scale! (car coeffs) (caddr proto)) :ycoeffs (cadr coeffs))))

(define* (make-inverse-chebyshev-bandpass n fl fh (loss-dB 60.0))
  "(make-inverse-chebyshev-bandpass n fl fh (loss-dB 60.0)) returns a bandpass inverse-Chebyshev filter; n = order, \
fl and fh are edge freqs (srate=1.0): (make-inverse-chebyshev-bandpass 8 .1 .4)"
  (if (odd? n) (set! n (+ n 1)))
  (let ((lp (make-inverse-chebyshev-lowpass n fh loss-dB))
	(hp (make-inverse-chebyshev-highpass n fl loss-dB)))
    (lambda (y) (filter lp (filter hp y)))))

(define* (make-inverse-chebyshev-bandstop n fl fh (loss-dB 60.0))
  "(make-inverse-chebyshev-bandstop n fl fh (loss-dB 60.0)) returns a bandstop inverse-Chebyshev filter; n = order, \
fl and fh are edge freqs (srate=1.0): (make-inverse-chebyshev-bandstop 8 .1 .4 90)"
  (if (odd? n) (set! n (+ n 1)))
  (let ((lp (make-inverse-chebyshev-lowpass n fl loss-dB))
	(hp (make-inverse-chebyshev-highpass n fh loss-dB)))
    (lambda (y) (+ (filter lp y) (filter hp y)))))



;;; ---------------- Bessel (-Thompson) ---------------- 

(define (bessel-prototype n)

  (define (fact n)
    (let ((x 1))
      (do ((i 2 (+ 1 i)))
	  ((> i n))
	(set! x (* x i)))
      x))
; this form overflows if we don't have bignums
;  (define (bessel-i n)
;    (let ((cs (make-vct (+ n 1))))
;      (do ((i 0 (+ 1 i)))
;	  ((> i n))
;	(set! (cs i) (/ (fact (- (* 2 n) i))
;			(* (expt 2 (- n i))
;			   (fact i)
;			   (fact (- n i))))))
;      cs))
  (define (bessel-i n)
    (let ((cs (make-vct (+ n 1))))
      (do ((i 0 (+ 1 i)))
	  ((> i n))
	(let ((val (/ 1.0 (* (fact i) (expt 2 (- n i))))))
	  (do ((k 1 (+ k 1))
	       (f (+ 1 (- n i)) (+ 1 f)))
	      ((> k n))
	    (set! val (* val f)))
	  (set! (cs i) val)))
      cs))

  (let* ((len (/ (* n 3) 2))
	 (num (make-vct len))
	 (den (make-vct len))
	 (b2 (bessel-i n)))
    (let ((p (gsl-roots (vct->vector b2))))
      (do ((i 0 (+ 1 i)))
	  ((= i n))
	(set! (p i) (/ (p i) (expt (b2 0) (/ 1.0 n)))))
      (do ((j 0 (+ j 3))
	   (i 0 (+ i 2)))
	  ((>= i n))
	(set! (num    j   ) 0.0)
	(set! (num (+ j 1)) 0.0)
	(set! (num (+ j 2)) 1.0)
	(set! (den    j   ) 1.0)
	(set! (den (+ j 1)) (* -2.0 (real-part (p i))))
	(set! (den (+ j 2)) (real-part (* (p i) (p (+ i 1)))))))
    (list num den)))

(define (make-bessel-lowpass n fc)
  "(make-bessel-lowpass n fc) returns a lowpass Bessel filter; n = order, fc = cutoff freq (srate = 1.0): (make-bessel-lowpass 4 .1)"
  (if (odd? n) (set! n (+ n 1)))
  (let* ((proto (bessel-prototype n))
	 (coeffs (analog->digital n (car proto) (cadr proto) fc)))
    (make-filter :xcoeffs (car coeffs) :ycoeffs (cadr coeffs))))

(define* (make-bessel-highpass n fc)
  "(make-bessel-highpass n fc) returns a highpass Bessel filter; n = order, fc = cutoff freq (srate = 1.0): (make-bessel-highpass 8 .1)"
  (if (odd? n) (set! n (+ n 1)))
  (let* ((proto (bessel-prototype n))
	 (hproto (prototype->highpass n (car proto) (cadr proto)))
	 (coeffs (analog->digital n (car hproto) (cadr hproto) fc)))
    (make-filter :xcoeffs (car coeffs) :ycoeffs (cadr coeffs))))

(define* (make-bessel-bandpass n fl fh)
  "(make-bessel-bandpass n fl fh) returns a bandpass Bessel filter; n = order, fl and fh are edge freqs (srate=1.0): (make-bessel-bandpass 4 .1 .2)"
  (if (odd? n) (set! n (+ n 1)))
  (let ((lp (make-bessel-lowpass n fh))
	(hp (make-bessel-highpass n fl)))
    (lambda (y) 
      (filter lp (filter hp y)))))

(define* (make-bessel-bandstop n fl fh)
  "(make-bessel-bandstop n fl fh) returns a bandstop Bessel filter; n = order, fl and fh are edge freqs (srate=1.0): (make-bessel-bandstop 8 .1 .2)"
  (if (odd? n) (set! n (+ n 1)))
  (let ((lp (make-bessel-lowpass n fl))
	(hp (make-bessel-highpass n fh)))
    (lambda (y) 
      (+ (filter lp y) (filter hp y)))))



;;; ---------------- Elliptic ---------------- 

(define* (elliptic-prototype n (ripple 1.0) (loss-dB 60.0))

  (define* (minimize-function f xmin xmax arg1 arg2)
    (let* ((n 20)
	   (x (make-vct n))
	   (fx (f xmin arg1 arg2)))
      (do ((i 0 (+ 1 i)))
	  ((= i n))
	(let ((step (/ (- xmax xmin) (- n 1.0))))
	  (do ((j 0 (+ 1 j))
	       (s xmin (+ s step)))
	      ((= j (- n 1)))
	    (set! (x j) s))
	  (set! (x (- n 1)) xmax))
	(do ((j 0 (+ 1 j)))
	    ((= j n))
	  (let ((ft (f (x j) arg1 arg2)))
	    (if (< ft fx)
		(begin
		  (set! fx ft)
		  (set! xmax (if (< j (- n 1)) (x (+ 1 j)) (x (- n 1))))
		  (set! xmin (if (> j 0) (x (- j 1)) (x 0))))))))
      (/ (+ xmax xmin) 2.0)))

  (define (findm m arg1 arg2)
    (abs (- (/ (gsl-ellipk m) (gsl-ellipk (- 1.0 m))) arg1)))

  (define (findv u arg1 arg2)
    (let ((vals (gsl-ellipj u arg1)))
      (abs (- arg2 (/ (car vals) (cadr vals))))))
  
  (let* ((e (sqrt (- (expt 10.0 (* 0.1 ripple)) 1.0)))
	 (k1 (/ e (sqrt (- (expt 10.0 (* 0.1 loss-dB)) 1.0))))
	 (k1p (sqrt (- 1.0 (* k1 k1))))
	 (kr 0.0)
	 (m 0.0)
	 (k 0.0)
	 (len (/ (* n 3) 2))
	 (num (make-vct len))
	 (den (make-vct len))
	 (g 1.0)
	 (eps 0.0000001))
    (if (> (abs (- 1.0 (* k1p k1p))) eps)
	(set! kr (* n (/ (gsl-ellipk (* k1 k1)) (gsl-ellipk (* k1p k1p))))))
    (set! m (minimize-function findm 0.001 0.999 kr))
    (set! k (gsl-ellipk m))
    (let ((cv (make-vct (floor (* 0.5 3 (+ n 1))))))
      (do ((i 0 (+ i 2))
	   (j 0 (+ j 3)))
	  ((>= i n))
	(let* ((vals (gsl-ellipj (/ (* (+ i 1) k) (* 1.0 n)) m))
	       (sn (car vals))
	       (cn (cadr vals))
	       (dn (caddr vals)))
	  (set! (cv    j   ) sn)
	  (set! (cv (+ j 1)) cn)
	  (set! (cv (+ j 2)) dn)
	  (let* ((z (/ 0.0-i (* (sqrt m) sn)))
		 (pz (real-part (* z (make-rectangular (real-part z) (- (imag-part z)))))))
	    (set! g (/ g pz))
	    (set! (num    j   ) 1.0)
	    (set! (num (+ j 1)) (* -2.0 (real-part z)))
	    (set! (num (+ j 2)) pz))))
      (let* ((optarg0 (* k1p k1p))
	     (optarg1 (/ 1.0 e))
	     (minf (minimize-function findv 0.0 (/ 1.0 e) optarg0 optarg1))
	     (v0 (/ (* k minf)
		    (* n (gsl-ellipk (* k k1)))))
	     (vals (gsl-ellipj v0 (- 1.0 m)))
	     (sn (car vals))
	     (cn (cadr vals))
	     (dn (caddr vals)))
	(do ((i 0 (+ i 2))
	     (j 0 (+ j 3)))
	    ((>= i n))
	  (let ((p (/ (- (+ (* (cv (+ j 1)) (cv (+ j 2)) sn cn)
			    (* 0.0+i (cv j) dn)))
		      (- 1.0 (* (cv (+ j 2)) sn
				(cv (+ j 2)) sn)))))
	    (let ((pp (real-part (* p (make-rectangular (real-part p) (- (imag-part p)))))))
	      (set! g (* g pp))
	      (set! (den    j   ) 1.0)
	      (set! (den (+ j 1)) (* -2.0 (real-part p)))
	      (set! (den (+ j 2)) pp))))))
    (set! g (abs (/ g (sqrt (+ 1.0 (* e e))))))
    (list num den g)))

(define* (make-elliptic-lowpass n fc (ripple 1.0) (loss-dB 60.0))
  "(make-elliptic-lowpass n fc (ripple 1.0) (loss-dB 60.0)) returns a lowpass elliptic filter; n = order, \
fc = cutoff freq (srate = 1.0): (make-elliptic-lowpass 8 .25 .01 90)"
  (if (odd? n) (set! n (+ n 1)))
  (let* ((proto (elliptic-prototype n ripple loss-dB))
	 (coeffs (analog->digital n (car proto) (cadr proto) fc)))
    (make-filter :xcoeffs (vct-scale! (car coeffs) (caddr proto)) :ycoeffs (cadr coeffs))))

(define* (make-elliptic-highpass n fc (ripple 1.0) (loss-dB 60.0))
  "(make-elliptic-highpass n fc (ripple 1.0) (loss-dB 60.0)) returns a highpass elliptic filter; n = order, \
fc = cutoff freq (srate = 1.0): (make-elliptic-highpass 8 .25 .01 90)"
  (if (odd? n) (set! n (+ n 1)))
  (let* ((proto (elliptic-prototype n ripple loss-dB))
	 (hproto (prototype->highpass n (car proto) (cadr proto)))
	 (coeffs (analog->digital n (car hproto) (cadr hproto) fc)))
    (make-filter :xcoeffs (vct-scale! (car coeffs) (caddr proto)) :ycoeffs (cadr coeffs))))

(define* (make-elliptic-bandpass n fl fh (ripple 1.0) (loss-dB 60.0))
  "(make-elliptic-bandpass n fl fh (ripple 1.0) (loss-dB 60.0)) returns a bandpass elliptic filter; n = order, \
fl and fh are edge freqs (srate=1.0): (make-elliptic-bandpass 6 .1 .2 .1 90)"
  (if (odd? n) (set! n (+ n 1)))
  (let ((lp (make-elliptic-lowpass n fh ripple loss-dB))
	(hp (make-elliptic-highpass n fl ripple loss-dB)))
    (lambda (y) 
      (filter lp (filter hp y)))))

(define* (make-elliptic-bandstop n fl fh (ripple 1.0) (loss-dB 60.0))
  "(make-elliptic-bandstop n fl fh (ripple 1.0) (loss-dB 60.0)) returns a bandstop elliptic filter; n = order, \
fl and fh are edge freqs (srate=1.0): (make-elliptic-bandstop 6 .1 .2 .1 90)"
  (if (odd? n) (set! n (+ n 1)))
  (let ((lp (make-elliptic-lowpass n fl ripple loss-dB))
	(hp (make-elliptic-highpass n fh ripple loss-dB)))
    (lambda (y) 
      (+ (filter lp y) (filter hp y)))))

