Patent GB-1465393-A (GB)
Title: Method and apparatus for the scrambled transmission of speech via a telephony channel
IPC: H04K1/04
Filed: 1974-02-12
Abstract: 1465393 Scrambled transmission GRETAG AG 12 Feb 1974 [13 Feb 1973] 06411/74 Heading H4R In a secret transmission system, the speech band is divided into interchanged complementary bands by a multistage modulation process, the ratio of the widths of the bands varied by a control signal, a single or multiple pre- or postecho added, the scrambled signal transmitted, the echo removed, and the original speech restored by subjecting the complementary bands to the same modulation process as employed at the transmitter. The control signal is not transmitted but is generated at the receiver by means of a basic stored code in conjunction with a supplementary code transmitted at each change of transmission direction.  In the transceiver of Fig. 20 (switches 84a- 84e shown in the receive position) microphone signals are fed 85, 86 to a modulation. Stage 6 where the speech band is added at 13 to the upper sideband of its modulation product with carrier f 1 , the two bands modulated with variable carrier f 2  a portion of the composite upper sideband removed by a filter 19 having 0À3-3 kHz speech band width and brought down to the audio band by modulation with carrier f 3 . The product on line 87 consists of two complementary inverted bands (Fig. 3) which are then routed to a circuit 7 in which a positive feedback loop 50-52 adds multiple echos at 138 for transmission 89. At an identical receiver (switches 84a-84e as shown) the scrambled signal is routed first to the circuit 7 which operates in inverse manner to regenerate the echo and subtract it from the input leaving only the complementary inverted bands to be fed over line 88 to an identical modulation circuit 6 which restores the speech bands to their original positions for reproduction over loud-speaker 5. The frequency f 2  is derived by mixing a 175 kHz signal from filter 94 with a variable frequency obtained by division at 97 of a 4 MHz frequency from an 8 MHz crystal oscillator 101. A code generator 102 provides digital signals s on five lines such as 116 to vary the division ratio in discrete steps according to a predetermined pattern imposed on a base code keyed-in to store 103. Depression of "splat" key 107 causes command unit 108 to operate a relay setting the switches 84a-84e appropriately and transmitting a 21 bit code three times to the receiver to synchronize a code generator in the receiver which thereby sends a reply pulse to the transmitter to clear any residual signals in the delay 52 (e.g. shift register) of the echo circuit 7. The sync. pulses and supplementary 21 bit code are transmitted at each change of transmission direction and a "close down" pulse is also envisaged. As well as f 2 , the shift frequency f 5  through the delay 52 may be varied to improve secrecy and the complementary speech bands may be transmitted alternately in the normal and inverse positions. Variations on the modulation process and echo formation are described; an alternative modulator (Figs. 4-5, not shown) includes a double modulator replacing modulator 17 and enabling identical constructions of modulators 14, 20 without the need for adder 13. Alternative echo circuits (Figs. 6-9) include positive or negative feed forward or feedback circuits to add single or multiple pre- or post-echos. The delay elements may be analogue or digital shift registers whose output tops are connected to an adder through amplifiers whose gains are in geometric progression apart from that associated with the last top so that the engines associated with positiva and negative pulses balance.