A full‐duplex analog speech scrambler using the FFT technique and its performance
Akira Matsunaga, Michihisa Ohkawa, Keiichiro Koga, Keiichi Sakurai · Electronics and Communications in Japan (Part III Fundamental Electronic Science) · 1990
Abstract The FFT scrambling method based on the manipulations of the Fast Fourier Transform (FFT) coefficients of speech is one of the analog speech scramblers which realize high‐level security and excellent descrambled speech quality. It is not clear, however, how the descrambled speech quality and syn‐chrnoization performance are affected by transmission impairments. This paper analyzes the effects of transmission impairments on the descrambled speech quality by computer simulations and provides a guide for the system design. Then practical methods to cope with the effect of transmission impairments are proposed. Based on the results of the simulations, experimental scramblers are developed using general‐purpose DSP's (Digital Signal Processors). The system configuration and the result of the performance evaluation tests are shown. The experimental scramblers are equipped with an automatic equalizer for group delay and amplitude distortions, as well as the automatic frequency control (AFC). An echo canceller is also provided so that the full‐duplex operation is achieved using two‐wire lines. As a result of the test under the simulated environments, it is proved that the descrambled speech quality is satisfactory and the proposed method is effective in practice.