A Speech Scrambler Using the Fast Fourier Transform Technique

This paper describes an analog speech scrambler using the FFT technique (FFT scrambler). The FFT scrambler provides highly secured scrambled signal by permuting a large number of FFT coefficients. Important items to be considered in the designing of the FFT scrambler are discussed, such as FFT frame length, permutation of FFT coefficients, and frame synchronization, in addition to the configuration of the experimental FFT scrambler. Possible causes for the degradation of the descrambled signal, such as transmission channel group delay and intersymbol interference, are also discussed, together with experimental results.