Inverse the audio selection from the Nyquist console

Programming and Development Nyquist
1

Hello,

I’m new in the forum, and I try to learn nyquist language.

I use Audacity version 3.7.1 and I’m on Windows.

I would just like to reverse the selected audio from the Nyquist console.

Could someone give me the code I need to use?

Thanks in advance for your help.

2

For short audio selections (the limit is a bit more than 1 million samples), you can read the audio into an array, reverse the data, then convert back to a sound and return the sound to Audacity.

(defun reverse-sound (sig ln)
  (let* ((srate (snd-srate sig))
         (ar (snd-fetch-array sig ln ln))
         (ra (make-array ln)))

    (do ((i 0 (1+ i))
         (j (1- ln) (1- j)))
        ((= i ln))
      (setf (aref ra j) (aref ar i)))

    (snd-from-array 0 srate ra)))

; Limit to max 1 million samples
(setf snd-length (min (truncate len) 1000000))
(multichan-expand #'reverse-sound *track* snd-length)

For longer selections you would have to use Audacity’s “Reverse” effect. This can be called from Nyquist using AUD-DO or AUD-DO-COMMAND, but only in a Nyquist Macro:

(aud-do-command "Reverse")
""
1 Like
3

Hi Steve,

Thanks a lot, your function works like a charm.

Kind regards.

4

Hi Steve,

Could I have the equivalent of your previous LISP code in SAL if possible?

Thanks in advance.

5

I don’t do SAL.

1 Like
6

Hi Steve,

Finally, I was able to make the same operation on a small selection, using SAL syntax.

Here is the code below, I hope it will be useful to other developers who wanted to do the same operation.

Thanks again Steve, because your LISP code helped me a lot.

define function reverse-sound(sig, ln)
  begin
    set srate = snd-srate(sig);
    set ar = snd-fetch-array(sig, ln, ln);
    set ra = make-array(ln);
    set j = ln;
    loop
      for i from 0 below ln by 1
        set j = j - 1;
        set ra[j] = ar[i];
    end;
    return snd-from-array(0, srate, ra);
  end;


; Limit to max 1 million samples
set snd_length = min(truncate(len), 1000000);
return reverse-sound(*track*, snd_length);

Kind regards.

7

If you want to support stereo tracks, you will need to adjust your SAL code to process each channel separately. This is accomplished in the LISP syntax version using multichan-expand.

1 Like
8

Hi Steve,

Thanks a lot, you’re perfectly right.

I forgot to use Multichan-Expand for stereo channels.

Here is the corrected code, always in sal:

define function reverse-sound(sig, ln)
  begin
    set srate = snd-srate(sig);
    set ar = snd-fetch-array(sig, ln, ln);
    set ra = make-array(ln);
    set j = ln;
    loop
      for i from 0 below ln by 1
        set j = j - 1;
        set ra[j] = ar[i];
    end;
    return snd-from-array(0, srate, ra);
  end;


; Limit to max 1 million samples
set snd_length = min(truncate(len), 1000000);
return multichan-expand(quote(reverse-sound), *track*, snd_length);

Kind regards.