| 摘要 |
A system and method are disclosed for preventing intelligible interception of information signals transmitted over a two-direction line. A masking signal is applied through a hybrid circuit at the receiving end of the line, and this masking signal, which appears on the line together with the information signal, prevents intelligible decoding. The masking signal includes a series of frequencies which are sequentially applied. Only at the receiving end of the line, where the hybrid circuit attenuates the masking signal which it receives at its receive port, can intelligible decoding take place. The amplitude of the information signals is sensed at the receiving end and the amplitude of the masking signals on the line is adjusted to be great enough to provide security, by confusing an eavesdropping detector, yet small enough at the receiving end so that the receiving detector is not confused.
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