[linux-audio-user] extreme time stretching

[Eric Dantan Rzewnicki]

I've done some pretty extreme stretching/compressing of time with 
ecasound's -ei effect (labeled as a pitch shifter). The default builtin 
implementation (linear interpolation, I think?) will do arbitrary 
percentages up or down. I don't have a great critical ear for sound 
quality (things tend to just sound different in different ways rather 
than better or worse) but it seems fine to me and works. If you compile 
ecasound with SRC support the shift is limited to 12x in either 
direction. But, as Steve pointed out to me on #lad last week(?) you can 
do multiple iterations to get greater shifts/stretches. Using SRC the 
resulting sound is somehow smoother sounding than with the internal 
ecasound version.

-Eric Rz.

derek holzer wrote:
> Doing some quick experiments with Soundtouch, I hear that it probably 
> isn't going to work for "extreme" timestretching. Even doubling the 
> length of a 4 minute soundfile results in *very* noticeable artifacts... 
> a kind of stuttering effect as the samples get pulled apart. You won't 
> be making a 24 hour Beethoven stretch with this one...
> Seems like an FFT-based approach, or a very good synchronous 
> granulation, might be better. Pure Data has a nice phase vocoder demo 
> patch in its documentation. You'll have to fiddle with it to get stereo, 
> of course. But hey, it's only a help patch! ;-)
> Other option is the Synchgrain external for PD, as I might have 
> mentioned earlier. I haven't tested the limits of it yet, but I know 
> that it sounded smooth at pretty slow playbacks.
> d.