[CM] arpeggio in snd

[Juan Reyes]

Now that we are in the mood for Snd's instruments here is an example of
Guile code for Risset's arpeggio documented on Dodge and Jerse's
Computer Music pages 98~102.

  --* Juan

(define arpeggio
  (lambda* (beg dur freq amp
                #:key
                (ampenv '(0 0 0.5 1 1 0))
                (offset 1.0))
      (let* ((start (floor (* beg (srate))))
             (end (+ start (floor (* dur (srate)))))
             (waveshbank (make-vector 12))
             (amp-env (make-env :envelope ampenv
                                :scaler (* amp .25)
                                :start start  
                                :end end))
             (output (make-vct end)))
        (do ((i 0 (1+ i)))
          ((= i 12))
          (vector-set! waveshbank i (make-waveshape
                :frequency (+ freq (* offset (1+ (* i 0.1))))
                :partials '(1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9))
                       ))
        (do ((i 0 (1+ i)))
            ((= i end))
          (let ((wvsum 0.0))
            (do ((j 0 (1+ j)))
                ((= j 12))
              (set! wvsum
               (+ wvsum (waveshape (vector-ref waveshbank j)))))
            (vct-set! output i (* (env amp-env) wvsum ))))
        (vct->channel output start end ))))

;;;  (load "arpeggio.scm")  
;;;
;;;  (arpeggio 0 10 400 0.5)
;;;  (arpeggio 0 10 400 0.5 :offset 1.25)
;;;