Noise reduction in single channel radio bearers employing privacy

Frequency inversion of the voice spectrum is a common method of providing simple privacy on single channel radio links. When combined with exponential modulation this method produces an increase in system noise of 8.00 dB. Alternative inversion methods are analyzed. The methods involve partitioning the voice spectrum into two halves, inverting one or both of these halves and, in some cases, incorporating a bandshift. The optimum method, entitled "dual inversion with down-shift," reduced the increase in system noise performance to 0.99 dB for a small increase in residual intelligibility. This was measured at 36 percent using a voice test incorporating digit identification. Frequency inversion had a residual intelligibility of 31 percent.