[BC] Disguising voices

[RichardBJohnson at comcast.net]

I accidentally deleted the Email asking about disguising voices. Somebody said to use something like a "flanger" or other pitch-change device designed for music. Although it might seem to work fine, the problem is that there is almost always some constant-frequency noise in the background when a caller is calling. It might be a computer fan, a clock, some 120Hz hum, etc. This means that some security specialist could always recover the caller's original voice from a recording by realigning the constant-frequency bins to a constant time and amplitude. Therefore, any attempt to disguise a caller's voice may lead to a false sense of security. Once one convinces a caller that his/her comments are secure, the caller may state something that should never have been divulged on a public forum. This could lead to lawsuits should the caller subsequently be identified.

It it really trivial to take a digital recording, produce a FFT, realign the background noise bins to a constant time and amplitude scale, and then take its inverse. The result will be exactly what existed before the time/amplitude distortion. If somebody has an easily recognized voice, they can then be recognized. Note that analyzing the frequency components of speech has become a commonplace method of secretly determining if somebody may be lying. 'Taint rocket-science no more. 

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/