Patent GB-1434706-A (GB)
Title: Mehtod and installation for masked speech transmission over a telephone channel
IPC: H04K1/04
Filed: 1974-07-02
Abstract: 1434706 Speech secrecy system GRETAG AG 2 July 1974 [2 July 1973] 29360/74 Heading H4R In a speech secrecy system an incoming signal is split into at least two complementary frequency bands, one of which is delayed relative the other before they are combined to form an aggregate signal. The aggregate signal is divided into two complementary frequency bands, the ratio of widths of which are varied by a control signal generated at the transmitter, and the frequency bands are interchanged to provide the signal for transmission. At the receiver the transmission signal is subjected to substantially the same process, in reverse order, to restore the original speech. Fig. 14 shows a preferred embodiment of a transmitter/receiver for the secrecy system with the transmit-receive switch 90a-90f in the receive position. As described an input speech signal from microphone 1 is passed through a.g.c. amplifier 91, pre-emphasis circuit 92, and band limiting filter 93 to the unit 6 which splits the signal into at least two non-overlapping frequency bands one of which is delayed by delay 36 before the bands are combined by adder 35. The filters 33 and 34 can be just high pass and low pass filters, but preferably have interdigitated comb type response and formed by filters of transversal type using a common delay device, either analogue, of bucket brigade type, or digital. Similarly the delay is preferably of bucket brigade or digital type which allows that both the filter responses, and the delay time can be varied by a cipher key signal. The aggregate signal from adder 35 is applied to a modulation system 7 (8) where, in a first modulator 14 and filter 15 the aggregate signal in the band 300 to 3,400 Hz is shifted to a band 3,400 to 6,500 Hz and added to the unchanged aggregate signal in adder 13. The resulting signal is shifted by a variable frequency oscillator and modulator 17 to a higher frequency band and a portion of the signal which contains complementary parts of the aggregate signal which has been shifted only by modulator 17 and the aggregate signal which has been shifted by both modulators 14 and 17, is selected by band pass filter 19 which has a pass band of 3,100 Hz. The output of filter 19 is shifted down to the band 300 to 3,400 Hz for transmission to the receiver. At the receiver the received signal is fed through an identical modulator system 7 (8) to that used at the transmitter, fed with the same carrier frequencies to restore the frequency shift and band inversion applied at the transmitter. The resulting signal, a replica of the aggregate signal at the transmitter, is applied to the filter delay unit 6 (9) where, due to the changeover of switches 90d and 90e from their position during transmission, delay is applied to that part, or parts, of the frequency spectrum of the signal which was not delayed at the transmitter, thus restoring the original speech in intelligible form. The variable frequency carrier f 2  for modulator 17 is derived by modulation of a signal which is a sub multiple of a 4 MHz signal produced by a divider 104 the division ratio of which is varied by the cipher key S derived from the cipher generator 106. The cipher generators at transmitter and receiver are synchronized by causing the division ratio of divider 104 to switch alternately between two conditions which cause, due to unbalance of modulator 17 by operation of switch 119, the transmission of a synchronizing signal comprising alternate bursts of 1190 and 1744 Hz. This is detected at transmission by the transmitter and on reception by the receiver in unit 124 to cause the cipher generator at both transmitter and receiver to be reset to a predetermined state so that the cipher generators will run in synchronism. The synchronizing system can be arranged to be transmitted each time a unit is switched to the transmit condition. An alternative embodiment of the modulation system 7 (8), is described with respect to Fig. 4 (not shown) two adjacent signal spectrums are produced, for selection of the two complementary part spectrums, by a double modulation using two carriers, spaced by 3,100 Hz, which are varied simultaneously to provide the variable ratio of sub-band width.