(define (ring-mod carrier-pitch modulator-pitch)
  (let ((carrier-frequency (hertz carrier-pitch))
        (modulator-frequency (hertz modulator-pitch)))
    (list (keynum (+ carrier-frequency modulator-frequency) :hz)
          (keynum (- carrier-frequency modulator-frequency) :hz))))

(define (ring-mod-list carrier-pitches modulator-pitches)
  (if (and carrier-pitches modulator-pitches)
    (cons (ring-mod (car carrier-pitches) (car modulator-pitches))
          (ring-mod-list (cdr carrier-pitches) (cdr modulator-pitches)))))

(define (make-glissando start-frequency end-frequency steps)
  (cond ((<= steps 0) nil)
        ((= steps 1) (list (keynum end-frequency :hz)))
        (t (cons (keynum start-frequency :hz)
                 (make-glissando (+ start-frequency (/ (- end-frequency start-frequency) (- steps 1)))
                                 end-frequency
                                 (- steps 1))))))

(ring-mod-list (make-glissando 440 880 13) (make-glissando 330 220 13))