[CM] audio leveler

[Bill Schottstaedt]

> So, can I use SND for leveling?

Yes, but you might need to hack a bit. 

There's a first pass at an automatic gain control in dsp.scm,
but it's commented out.  I was going to experiment with 
moving-rms to detect portions with just background noise
(to be squelched, rather than amplified); it's on my
"TODO" list...

Anyway, to give it a try, load dsp.scm, paste the agc function
into the listener, load the sound you want to try it on, ideally
set optimization to 6: (set! (optimization) 6), and (agc):

(load "dsp.scm")
(set! (optimization) 6)

(define* (agc :optional (ramp-speed .001) (window-size 512))
  (let ((maxer (make-moving-max window-size))
        (mult 1.0))
    (map-channel
     (lambda (y)
       (let* ((curmax (moving-max maxer y))
              (diff (- 0.5 (* mult curmax)))
              (this-incr (* diff ramp-speed)))
         (set! mult (+ mult this-incr))
         (* y mult))))))

(open-sound "learn-Snd-while-you-drive.snd")

(agc)