I didn't try to compile and run your code, but you're on the right track.
APH has a newer box for speech compression; I don't know the selling price, but I would expect it to sound better than the old unit.
Do a web search for "time-scale modification" and start reading. One approach to cleaning up the output involves doing autocorrelation calculations on the audio data, looking for places where one piece of waveform (approx 20 milliseconds long) can smoothly join another piece within a useful skip interval. Other approaches do fast Fourier transforms or sinusoidal speech coding/decoding in an attempt to match similar segments. Most systems, one way or another, do a windowing function to overlap the ends of the signals; one window is fading down while another is fading up, so that any mismatch is partly masked. You might find something useful on this by looking for "synchronous overlap add" in a search. The rotating tape heads did this in an analog way; the playback head had more than one gap in it, and you controlled the amount of overlap by the wrap angle.
There are a lot of time scale modification algorithms out there; some are better for speech; some are computationally less complex, and some are built to achieve the maximum amount of time compression. Look for "Malcolm Slaney" and see if he has a demonstration page of his really fast speech somewhere.
At 08:49 AM 10/6/2004 -0500, you wrote:
I tried an experiment this last weekend to see how hard it is to write code that compresses audio similarly to what the old APH pitch restoring speech compressers used to do. In my case, mine still does, but it is almost 30 years old and I know it will one day bite the dust. ...
... Creating implements of mass instruction.
Lloyd Rasmussen, Senior Staff Engineer
National Library Service f/t Blind and Physically Handicapped
Library of Congress (202) 707-0535 <http://www.loc.gov/nls/z3986>
HOME: <http://lras.home.sprynet.com>
The opinions expressed here are my own and do not necessarily represent those of NLS.
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