[CM] duplicating each N samples of an audio file

[Tito Latini]

On Sun, Aug 25, 2019 at 06:03:17PM -0400, David O'Toole wrote:
> Hello there! I'm trying to duplicate each N samples of an audio file so
> that ABCD becomes AABBCCDD and so on. I have modified one of the examples
> to produce the following code below, but it doesn't work. It makes a
> properly-sized blank audio file, but only the first block is written.
> Indeed, putting in a print statement shows that the loop is only run once.
> But NUM-BLOCKS turns out to be 51 which is what I expected. Forgive me if
> this is a simple Scheme error, as I am more familiar with Common Lisp and
> Elisp than I am with Scheme. Or I may be misunderstanding something about
> Snd itself. I would greatly appreciate any help you can offer. Thank you.
> ---_David
> 
> (when (= 0 (length (sounds)))
>   (open-sound "/home/dto/Desktop/beatloop.wav"))
> 
> (define echo-mosaic
>   (lambda* (block-len snd chn)
>     (let* ((len (framples snd chn))
>   (num-blocks (floor (/ len (srate snd) block-len)))
>   (new (new-sound #f :size (* 2 len))))
>       (if (> num-blocks 1)
>  (let ((actual-block-len (ceiling (/ len num-blocks))))
>    (do ((n 0 (+ n 1)))
> ((= n num-blocks))
>      (let ((beg (* n actual-block-len)))
> (let ((reg (make-region beg (+ beg actual-block-len) chn)))
>  (mix-region reg (* 2 beg) new chn)
>  (mix-region reg (+ (* 2 beg) actual-block-len) new chn)
>  (forget-region reg))))
>    new)))))
> 
> (echo-mosaic 0.25 0 0)

There is a trivial error; it is correct:

  (let ((reg (make-region beg (+ beg actual-block-len -1) snd chn))) ...)

The result of your echo-mosaic is a new empty sound file with pending mixes.
Perhaps you want save-sound after loop:

  (do ((n 0 (+ n 1)))
-     ((= n num-blocks))
+     ((= n num-blocks) (save-sound new))

possibly with a filename for new-sound.