Speech Watermarking and Air Traffic Control

PhD Student 
Konrad Hofbauer
Research Area


 Air traffic control (ATC) voice radio communication between aircraft pilots and controllers is subject to technical and functional constraints owing to the legacy radio system currently in use worldwide. This thesis investigates the embedding of digital side information, so called watermarks, into speech signals. Applied to the ATC voice radio, a watermarking system could overcome existing limitations, and ultimately increase safety, security and efficiency in ATC. In contrast to conventional watermarking methods, this field of application allows embedding of the data in perceptually irrelevant signal components. We show that the resulting theoretical watermark capacity far exceeds the capacity of conventional watermarking channels. Based on this finding, we present a general purpose blind speech watermarking algorithm that embeds watermark data in the phase of non-voiced speech segments by replacing the excitation signal of an autoregressive signal representation. Our implementation embeds the watermark in a subband of narrowband speech at a watermark bit rate of the order of magnitude of 500 bit/s. The system is evaluated experimentally using an ATC speech corpus and radio channel measurement results, both of which were produced specifically for this purpose and constitute contributions on their own. The adaptive equalization based watermark detector is able to recover the watermark data in the presence of channels with non-linear phase, time-variant bandpass filtering, amplitude gain modulation, desynchronization and additive noise. In the aeronautical application the scheme is highly robust and outperforms current state-of-the-art speech watermarking methods. The theoretical results show that the performance could be increased even further by incorporating perceptual masking.  


This thesis is supervised by Gernot Kubin.