Speech scrambler

DETAILED DESCRIPTION In view of the above shortcomings of the prior art, an object of the present invention is to provide a speech scrambler which makes it difficult to distinguish silence and fricative sounds and produces a scrambled voice signal with no trace of intonation of the original voice, ensuring the security of communication.
To attain the above object, the present invention is characterized in that a low power band portion of a voice spectrum obtained by an orthogonal transform of the voice signal, such as a fast Fourier or Hadamard transformation, is adaptively removed and is substituted by a dummy spectrum; the resulting spectrum is rearranged, and the voice signal is transformed by an inverse transform into a time domain signal for transmission.
The power of the spectrum removed should be low enough to have no influence on the descrambled speech quality.
At the receiving side, the received signal is subjected to an orthogonal transform, the resulting spectrum is inversely rearranged to its original order, the dummy spectrum is eliminated therefrom, and then the signal is transformed into a time domain by an inverse orthogonal transform, thereby obtaining a descrambled voice signal.
The dummy spectrum has such an energy distribution that it does not appear in the actual voice spectrum, and is distinguishable only when descrambled by a correct descrambling key.
The spectrum to be removed has little effect on the original voice signal, and hence will not affect the descrambled speech quality.
On the other hand, the dummy spectrum inserted has certain energy, so that after rearranged, it acts as interference components and suppresses the intonation of original speech, producing an effect of masking voice information when the volume of the original speech is small.