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

Communication 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 becomes attractive due to its higher degree of security compared to the 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 complicates the implementation and makes the transmission very sensitive to channel conditions. In this paper, a new frequency domain scrambling algorithm is presented, which is an extension of the DFT scrambler previously proposed. The use of short-time Fourier analyis and the filter bank concept leads to the special feature that frame synchronization is completely unnecessary. This simplifies the implementation and improves the reliability and feasibility. The theoretical developments, simulation results, hardware implementation, and test results are all discussed in detail.