Speech watermarking for air traffic control

In air traffic control the voice communication between a controller and all pilots in a delimited airspace are handled on a single VHF frequency. To identify a speaking aircraft, pilots have to start all verbal communications with the aircraft call sign. For automatic identification, it is desirable to transmit additional hidden aircraft identification data in time with this voice message over the VHF channel. That means the additional digital data has to be embedded into the speech signal. Such watermarking systems are used e.g. for CD-audio copyright protection, where copyright data is embedded into the music without any audible distortion. A system for speech watermarking in an air traffic control environment is developed. The system uses spread spectrum technology with linear prediction for spectral shaping of the watermark to achieve perceptual hiding. Error control coding is done using the BCH-code, which can both detect and correct errors. Results show that, for 12 bits/s, the watermark can be transmitted at a level which is hardly audible with very low error rate (≤ 0.1%). For the transmission of 24 and 36 bits/s the watermark level has to be increased to an audible but not annoying level so as to stay in a low error rate region.