Optimum pre-emphasis for FM transmission with frequency-domain voice scrambling

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 filters are given as functions of the voice signal spectrum, demodulator output noise spectrum, and the scrambling method. Comparisons of the maximum attainable SNR (signal/noise ratio) are made for various cases including plain voice (nonscrambling) and secured voice scrambled by spectrum inversion, reverberation, and DFT (discrete Fourier transform) spectrum transformation, assuming simple voice and noise spectra.<
>