On Wed, Sep 21, 2011 at 10:44 PM, Hans-Christoph Steiner hans@at.or.atwrote:
I'm trying to come up with a simple voice scrambling technique that leaves voices understandable, but makes them unrecognizable. A key part of this is to make it very hard to reverse the scrambling to make the voice more recognizable.
I'm currently thinking that a ring modulator would work well for this, and it uses minimal CPU. Can anyone think of a way to reverse the ring modulation? I attached my quick sketch.
That's a nice challenge. If I were CIA, I'd try to descramble like this: demodulate (division instead of multiplication) with a sine sweep while analyzing the spectrum. At the sweep frequency where the spectrum is a harmonic recipe: bingo, a human voice. Then you could demodulate that 0.1 second of sound with the found frequency. Not something to quickly do in a Pd patch though.
If your scrambler would modulate the modulation frequency continuously, with a noise signal, speech is still intelligible, but descrambling in the above described way would no longer be possible, as you can't find a harmonic recipe from a one sample fourier transform.
Katja