A New Time Domain Speech Scrambling System Which Does Not Require Frame Synchronization

Abstract

Communications security is becoming more and more Important today. There is thus an increasing interest in analog scramblers due to the desire for secure speech communications over existing telephone channels with standard telephone bandwidth at acceptable speech quality and reasonable cost. The concept of scrambling the sample values of the speech waveforms is attractive due to its higher degree of security compared to traditional scramblers. But all these sample value scramblers require frame synchronization, i.e., the signal segments used in scrambling and descrambling processes have to be exactly the same for signal recovery. This not only complicates the implementation but makes the transmission very sensitive to channel conditions. A new frequency domain scrambling system was proposed recently, which eliminates the requirement for frame synchronization. However, FFT algorithms are used in this system to transform speech samples back and forth between time and frequency domains. This not only requires higher complexity and cost, but introduces significant roundoff errors and enhances the channel noise and distortion. In this paper, a new time domain scrambling system was proposed which eliminates all the FFT's in the previous system but still does not require frame synchronization. This simplifies the implementation and improves the quality and reliability. The theoretical derivation, synchronization analysis, software simulation, hardware implementation, and residual intelligibility tests of this new system are discussed in great detail in this paper.