Li : Dev Actionscript Development Blog

Pitch shifting and time stretching in AS3

Audio time stretching is the process of slowing down or speeding up a piece of audio without altering its pitch. Its complement is pitch shifting, which is the process of altering the pitch of a piece of audio without altering its tempo.

This transformations are not as straight forward as they seem to be. This is primarily because any transformations applied to an audio signal in the time domain usually affect its frequency domain, and vice versa. I've been researching this a lot lately and I must admit I bumped into a pretty big and mean realm. If an attempt to preserve audio quality is made, very complex algorithms (made by some very smart people) start showing up. Different DSP (digital signal processing) techniques exist for this purpose such as the Phase Vocoder, TDHS, WSOLA, PSOLA, etc... you can read more about the topic on wikipedia here.

So, what I did was grabbed Ryan Berdeen's port of the C Sountouch library to AS3 (soundtouchAS3) and modified it to act as a StretchFilter in Joe Berkovitz's StandingWave2 (standingwave) dynamic audio library for flash 10. After adding this filter to standingwave, it was just a question of combining it with the lib's ResamplingFilter, and voila, pitch shifting.

DEMO: here.

****WARNING****: This is quite cpu intensive (the implementation is far from optimized), so be prepared to turn the volume down! Also, please be patient with the sound loading, and sorry for the crappy interface! This is just an experiment. Avoid touching the controls before a sound is loaded (:P). When the sound loads, try altering the pitch or the tempo, one at a time.

Clearly, my adaptation of the time stretching algorithm needs to be optimized and perhaps even ported back to C! (for use with Alchemy) but the main purpose of this experiment is just to show that time stretching and pitch shifting in AS3 is possible... which is pretty good news for us Flash geeks, right?

This should have a few uses. I can think of a couple: Dynamically altering the tempo or pitch of media the way Quicktime player does with the A/V controls (would be pretty good for tutorials, specially music tutorials), being able to warp audio sources in some sort of online audio editor, etc.