Secure Speech Communication Over a CCITT-Speech Channel
Today’s main emphasis in secure speech communication is on vocoders. However there are still a lot of CCITT-speech communication channels in use for which secure speech communication is desired. It has generally been found that to do so a compromise between cost, security and speech quality has to be chosen.
In cases where vocoders cannot be used because digital transmission of the required bit rate (normaly 2.4–4.8 kbits/sec) cannot be safely achieved, the method of speech scrambling has to be looked at.
Starting with an introduction to the “speech iceberg” which shows the elements of speech relevant to communication, the problem of scrambler security is dealt within the context of a communication system. It is shown how scramblers relate to vocoders on the one hand and waveform coding schemes suitable for analog transmission on the other hand. A definition of the security of scramblers is presented and its application is indicated and compared to the criterions used to evaluate digital ciphering systems.
It is also shown how scramblers have developped and that secure systems are possible with the modern digital technology.
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