General outline of HF digital radiotelephone systems

considering a) that voice communications in the HF band use 3 kHz channels; b) that security is essential for some communications; c) that scrambling is the only means of obtaining a sufficient level of security; d) that the required level of security can easily be achieved using digitized speech technology; e) that there is therefore a need for speech signal coders (vocoders) associated with HF modems; f) that, for good quality HF channels, the maximum permissible bit rate is 4 800 bit/s; g) that interference and propagation effects such as fading cause an increase in the bit error ratio for digital communications, thereby calling for correction processes (error-correcting codes, interleaving),