A voice scrambler for mobile communication

A new voice scrambling method for mobile communication is proposed. The principle is to invert the voice sample polarity selectively according to the scrambling code, smooth it with a low-pass filter, and feed it to the radio transmitter. To utilize the principle, the necessary techniques such as filtering, synchronizing and scrambling code design, are described. The whitened spectrum of the scrambled voice and a large number of key codes provide a high security communication. The scrambler presented here is designed for an adaptor to press-to-talk mobile radio. Voice spectrum variation by scrambling is demonstrated and articulation is measured under nonfading and fading conditions with signal to noise (S/N) variation. Those results show that the scrambling method is suitable for mobile communication with rapid synchronization for press-to-talk radio, noise, and fading immunity.