Improvement and Evaluation of Time-Spread Echo Hiding Technology for Long-Distance Voice Evacuation Systems

Several improvements to the time-spread echo hiding method are proposed for an aerial audio data hiding in which speech signals are broadcast by the outdoor loudspeakers of a voice evacuation and mass notification system. Evaluations of the data hiding system for speech signals were conducted using computer simulations including several disturbances caused by the long-distance (from 70 to 800 m) aerial transmission of sounds. The frequency response of a distant horn-array loudspeaker system, the absorption of sound by the atmosphere, reverberation and a single long-path echo, a constant frequency shift that mimics a small Doppler shift and a mismatch of sampling frequencies between sending and receiving devices, and additive background noise are simulated as disturbances. A background noise recorded outdoors was mixed to simulate a loud outdoor space in a city at signal-to-noise ratios of −5, 0, and 5 dB. The computer simulation results showed that the suppression of high- and low-frequency regions in the logarithmic spectral domain in the decoding process was significantly effective for the decoding performance. A novel hiding method of the alternating sign of echoes was moderately effective under relatively high SNR (0 and 5 dB) conditions.