Flemming Richter Mikkelsen wrote:
There are no simple voice transforms at all that will get through the codec,
and actually encrypt.
Voice changing is possible, but encryption is not.
You _cannot_ - for example - exepect frequency inversion - to get through
the codec chain.
What about correlating (multiplying) the input signal with a
different signal (encoding) and in the other end extract your
signal (decoding) by removing the added signal?
If it does not sound like voice when it leaves the phone, it will be
massively and unpredictably distorted by the codecs.
In a call between two GSM phones, there are at least two encode/decode
with occasionally different codecs between the microphone on one side,
and the speaker on the other.
You cannot (usually) pick the codec.
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