| 摘要 |
1465923 Secrecy transmission systems SIEMENS AG 18 March 1974 [3 April 1973] 11881/74 Heading H4R In a speech secrecy transmission system in which sub-bands of the speech band are intercharged in a time-varying pattern, the speech content of one sub-band is replaced by a corresponding band of interference signals. The speech at E1 is modulated with fixed frequencies f1, f2, f4, f5, filtered F11, F12 and F14, F15 into sub-bands of 550 Hz or of varying width and are modulated again with frequencies f1 to f5 in a varying pattern controlled by unit U1. The central band 1350-1900 Hz which contains little intelligence is replaced by a noise signal G which is preferably derived from frequency division of a master oscillator providing the carriers so as to produce a harmonic series which varies at speech rhythm, has a level or duration dependent on speech level or duration and which may provide the pilot signal of a compander. At the receiver (Fig. 2, not shown) the signals are modulated in four modulators according to the transmission pattern, filtered and modulated again, no separate path for the central sub-band being provided till the interference band is suppressed in the receiver. |
