| 摘要 |
PURPOSE:To increase number of ciphering keys without increasing an error of a recovered digital signal and to intensity very much the security of privacy call by changing a memory output irregularly in the unit of not only bits but also frames. CONSTITUTION:A digital signal S and an irregular signal Y are added at an exclusive OR circuit 10 at the transmission side so as to obtain a ciphered signal Z. The signal Z is inputted to a register 12 and stored in the unit of bits. Any of parallel outputs r1-rm is selected by a random selection circuit 11 while the selection is changed in the units of frames to obtain selection signals x1-xn. A memory 22 uses the signal as address information to output the signal Y. The signal Z formed in this way is synthesized with a frame synchronizing signal FS and a start signal START and the result is sent to a transmitter 20. A transmission switch 21 is turned on, the transmitter 20 is arisen and a counter 15 of the circuit 11 is set to an initial value. Since each counter is operated from an initial set value at the transmission and reception side in this way, when each memory content is coincident, the same selection is attained.
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