On Fri, 18 Dec 2009, [email protected] wrote:
>
> Hi,
>
> long time ago I had an audio file, which I think it was
> generate via csound, which plays a tone pitch, which /seems/
> to increase endlessly. It was the audible counterpart of an
> optical illusion.
>
> I like to know, how this "trick" was constructed and how I
> can rebuild the effect ?
>
> Thank you very much in advance for your help!
>
Hi,
The instrument below implements a shephard tone in snd-rt.
(taken from rt-example.scm). I don't really
know very well what I'm doing when it comes
to signal processing, but by tweaking enough I got it to sound
quite nice. Hopefully someone will shout out if there's
something fundamentally wrong with it. :-)
Hopefully the code is somewhat understandable.
(definstrument (risset startpitch endpitch num-oscs loop-duration)
(define starttime (+ 1 (rte-time))) ;; Ensure all starts simultaniously by
scheduling one second into the future.
(for-each (lambda (n)
(let ((osc (make-oscil #:frequency 0))
(e-p (make-env `(0 ,startpitch 1 ,endpitch ) #:duration
loop-duration #:base (expt 2 num-oscs)))
(e-a (make-env `(0 0 1 0.25 2 0) #:duration
loop-duration #:base 100)) ;; Linear amplitude-change didn't sound very nice.
(start-location (* (/ (* (mus-srate) loop-duration)
num-oscs)
n)))
(set! (mus-location e-p) start-location)
(set! (mus-location e-a) start-location)
(<rt-play-abs> starttime
(lambda ()
(if (>= (mus-location e-p) (mus-length e-p))
(begin
(set! (mus-location e-p) 0)
(set! (mus-location e-a) 0)))
(out (* (env e-a)
(oscil osc (hz->radians (env
e-p)))))))))
(iota num-oscs)))
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