Automatic speed-loudness control

Special Interest Groups Audio Processing
1

Hi. I’m looking for an automatic effect that will make an audio clip a bit slower and faster depending on how loud a particular section of audio is.

Like, I’m looking for an effect that will slightly slow down the audio clip when the audio file plays a loud tone and speeds up as it gets quieter.
(Eg. If the audio is: at -15db it will be fullspeed, at -10db is will be a little slower, at -6db it will be even slower, going back to -10 it will start speeding up again)

Something that will have an adjustable Attack Time, Release Time, Threshold, and Ratio, just like a compressor.

I hope I make sense to you guys, its kinda hard to explain.

Through this, I’m trying to simulate: how an audio playback circuit (mostly used in digital toys) will slow down and speed up on a low battery depending on how much power/loudness is output to its speaker. With what I’m trying to do I’ll likely use 0.1 second attack, 1.0 second release, and threshold of approx -14 db

2

Have you tried creating the effect using a “Time track”? http://manual.audacityteam.org/o/man/time_tracks.html

3

In fact, I HAVE tried that already. But, I find that it’s for one thing not bang time-accurate.

And secondly, I work around that by pre-slowing down the track by 500%, doing the timetrack, then putting the speed back to normal after saving. From there, I take like 10 minutes just to attempt to do this on 10 seconds worth of audio (approx. 50 points made), but I still don’t feel I’ve achieved the effect I wanted due to not doing an accurate job (its hard).

That’s why I feel I need something to do this job for me automatically.

4

At first glance frequency-modulating the sound by its envelope will do it …
Waveform and its envelope.png
but the whole thing increasingly goes out of sync :cry: …

(defun fm (s-in)
   (let* ((s-in (lowpass8 s-in (* 0.2 *sound-srate*)))
         (s-in (lowpass8 s-in (* 0.2 *sound-srate*)))
         (map (integrate (db-to-linear (scale 5 (aref s-in 0))))))
      (snd-resamplev (aref s-in 1) *sound-srate* map)))

(fm s)

The above code frequency-modulates the right track by the left one
paste it in “Nyquist_Prompt” … http://manual.audacityteam.org/man/Nyquist_Prompt
Change “scale 5” to a lower number for less pitch wobble.

5

and it would be difficult to automatically calculate the necessary timing corrections.

I think the way to do it would be to loop through short sections so that the timing drift (per section) is not significant.
Also, I think that the perceived quality could be improved by creating a smoother envelope that rides the peaks (see follow and snd-avg).

Do you have a lot of these to do?
How accurate does it need to be?
I had a quick go and it seemed to be pretty effective. I worked from left to right making the adjustments. I also set the tempo range so that the “Upper speed limit” was 100%.
Can you think of any other applications where this effect could be useful?