Patent GB-660867-A (GB)
Title: Improvements in or relating to privacy arrangements for radio communication systems
Inventor: MACMULLAN JOHN SUMNER MUNRO
IPC: H04K1/04
Filed: 1949-06-08
Abstract: 660,867. Secret transmission. STANDARD TELEPHONES & CABLES, Ltd. June 8, 1949, No. 15258/49. Class 40 (iv). [Also in Group XL (c)] A privacy arrangement for a two-channel independent side-band radio system comprises switching means at the transmitter for producing the channel side-bands inverted and erect in odd and even numbered periods of time by the use, respectively, in such periods of carrier waves of two different frequencies, and means for transmitting each carrier wave with the channel side-bands during the appropriate period. At the receiver, switching means synchronized by the received carrier waves supply appropriate carrier waves for demodulating the channel side-bands. As shown in Fig. 1, the transmitter includes a relay 10 operated by a commutating device 11 to throw three two-way switches into their respective positions during odd-numbered periods T1 and even-numbered periods T2. Three carrier sources 8, 7, 9 are of frequencies f1, f, f2, such that f1 - f is equal to f - f2 and both differences approximate to the width of the signal bands to be transmitted. During the periods T1 the carrier f is switched to the modulator 5 for the channel A, and the carrier f to modulator 6 for channel B, whilst during the periods T2 carrier f is switched to modulator 5 and carrier f2 to modulator 6. A filter 12 selects the lower side-band of f1  and the upper side-band of f to constitute the channel A, and a filter 13 the lower side-band of f and the upper side-band of f2 to constitute the channel B, the two channels being combined at 14. The switches are also arranged to send out weak unmodulated carrier f1 through the attenuator 16 during period T1, and carrier f2 during periods T2; the carrier f being sent out continuously through an attenuator 15. From the combining circuit 14 the mixture passes to a modulator 17 where it combines with waves from an intermediate frequency oscillator 18 followed by a filter 19 for selecting the upper side-band. This is mixed in a balanced modulator 20 with the output of the final oscillator 21, the resulting sidebands being amplified at 22 and radiated. At the receiving station, Fig. 2, the stages 24 ... 30 reproduce at X the mixed frequencies present at 14, Fig. 1, these frequencies being fed to a series of filters of which 31 selects the band of frequencies f1 to f constituting channel A, 35 the band f to f2 constituting channel B, whilst 32, 33, 34 select respectively the carrier frequencies f1, f, and f2. The frequencies f1, f2 are fed through amplifiers and limiters to rectifiers 50, 51 which operate a trigger circuit 52 during periods T1, T2, respectively, so that the relay 53 operates synchronously with the relay 10 at the transmitter. The relay contacts are arranged to supply during periods T1 the frequency f1 to demodulator 41 for channel A, and the frequency f to demodulator 42 for channel B. During the periods T2, f is supplied to channel A and f2 to channel B. The amplified output from the filter 33 may be used to operate an automatic frequency control device 54 for the local oscillator 30. The commutator 11 may comprise a free-running or synchronized multivibrator (Fig. 4, not shown) with the relay 10 in one of its anode circuits, while a similar circuit with its grids connected respectively to the rectifiers 50, 51 may be used as the trigger 52 at the receiver. In a modified receiver circuit (Fig. 3, not shown), the three carriers f, f1, f2 are supplied by crystal-controlled oscillators. Further shuffling of the side-bands may be effected by providing an additional multi-vibrator at the transmitter synchronized on a sub-multiple of the frequency of the commutator 11 and arranged to interchange the connections of the carrier sources 8, 9 periodically.