Full-duplex speech for HF radio systems

For HF radio systems the maximum rate that can be reliably carried within the assigned band is not sufficient for TDD voice transmission given the present state of speech coding and data transmission technology. To overcome these problems the system described in this paper uses a technique called time-scale modification (TSM) which can compress a T second segment of analog voice by a factor of two. An advantage of this approach is that the compressed voice signal occupies the same bandwidth and dynamic range and consequently no radio modifications are required. Additionally since the frequency occupation remains unchanged, all existing radio regulations remain satisfied and the type approval process for the radio need not be repeated. As the timescale modification process is reversible the time compressed speech segments can be restored to their original form with minimal distortion. By using analog time compression and expansion full-duplex audio can be achieved when the available bandwidth of the system does not allow for digital TDD operation. The real-time implementation and test results of an analog time division duplex system (ATDD) are presented in this paper.