Code from class, 2012-11-14

tiny-envelope.rkt

(require (planet clements/rsound:4))
(require "event-channel.rkt")
(require 2htdp/image)
(require 2htdp/universe)

(define ec (make-event-channel))

(send-event ec 400)

;; an event is either
;; - (make-note-start number)
;; - (make-note-stop)
(define-struct note-start (pitch))
(define-struct note-stop ())

;; latch : remember the last non-false value
(define (latch input old)
  (cond [(false? input) old]
        [else input]))

(check-expect (latch 34 49) 34)
(check-expect (latch false 49) 49)

(define (env-fun last-evt old)
  (cond [(note-start? last-evt) 0.5]
        [else 
         (max 0.0 (- old (/ 1 44100)))]))

(signal-play 
 (network ()
          [last-evt (latch (try-get-event ec) 
                           (prev last-evt 
                                 (make-note-stop)))]
          [env-amp (env-fun last-evt (prev env-amp 0.0))]
          [tone (sine-wave 440)]
          [out (* tone env-amp)]))


;; a world is false

;; keh : world key -> world
(define (keh w k)
  (cond [(key=? "1" k) 
         (begin (send-event ec (make-note-start 440.0))
                w)]
        [(key=? "2" k)
         (begin (send-event ec (make-note-start 920.0))
                w)]
        [else w]))

;; kreh : world key -> world
(define (kreh w k)
  (begin
    (send-event ec (make-note-stop))
    w))

(define (fake-draw w) (empty-scene 100 100))

(big-bang #f
          [on-draw fake-draw]
          [on-key keh]
          [on-release kreh])