Optimum Pre-Emphasis for FM Transmission with Frequency-Domain Voice Scrambling

Yoshihiko AKAIWA  Eiji OKAMOTO  Yukitsuna FURUYA  

IEICE TRANSACTIONS (1976-1990)   Vol.E72   No.5   pp.507-513
Publication Date: 1989/05/25
Online ISSN: 
Print ISSN: 0000-0000
Type of Manuscript: Special Section PAPER (Special Issue on Information Theory and Its Applications)

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With popularity of radio communication, voice scrambling has received increased attention for communication privacy. It has been known that pre-emphasis at the transmit side and de-emphasis at the receive side improves the received signal to noise power ratio (SNR) both for scrambled and nonscrambled signals. In this paper, optimum pre- and de-emphasis are theoretically investigated for FM and PM transmission including frequency-domain scrambling for voice security. Using a linear system model, the optimum frequency-characteristics for pre- and de-emphasis filter are given as a function of a voice signal spectrum, demodulator output noise spectrum, and the scrambling method. Comparisons of the maximum attainable SNR are made for various case including plain voice (non scrambling) and secured voice scrambled by spectrum inversion, reverberation and FFT spectrum transformation, assuming simple voice and noise spectra.