Patent GB-560925-A (GB)
Title: Electric signalling systems
IPC: H03H7/42
Filed: 1940-12-24
Abstract: 560,925. Secret transmission systems. ECKERSLEY, P. P., and CARPENTER, R. E. H. Dec. 24, 1940, Nos. 18126/40 and 16253/41. [Class 40 (iv)] [Also in Group XL] In a system using polyphase transmission for a band of signal frequencies such as speech or television, the signal band is subjected to polyphase modulation with a suitable frequency (carrier) as explained below and the modulation products restricted by filters to a band width substantially less than the original, the arrangements being such that pairs of original frequencies are represented by single frequencies distinguished by, opposite directions of phase rotation in the transmission system. If, for example, the speech band fs, 0-3000 (Fig. 2), modulates a mid-band carrier frequency fr, 1500, and the upper sideband is removed, the remaining side band comprises a portion 1500-0 representing the original frequencies 0-1500 and a further portion 0-1500 representing frequencies 1500- 3000, the spectrum being therefore folded back as shown at 12. In the two-phase system shown in Fig. 1, utilizing rotary transformers as described in Specifications 521,191 and 560,923, speech from microphone 1 is fed to the rotor 2 of a two-phase rotary transformer, the rotor being rotated at the carrier frequency 1500 and the two stator windings connected over filters 5, 6 (0-1500) to the stator windings of a similar receiving transformer the rotor 2a of which is rotated at carrier frequency and is connected to telephone receivers 15. Any particular pair of speech frequencies, e.g. 500, 2500 is represented by common frequency components 1000, but phase rotation of the two components takes place in opposite directions. At the receiver, if corresponding connections and direction of rotation of the rotor are made, the output for the frequency components in question comprises 1500 + 1000 = 2500 and 1500 - 1000 = 500, so that the original frequencies are restored. The transmission may be over radio channels. If an attempt were made to tap a transmission line or channel with a telephone or radio frequency receiver, the result would be an unintelligible folded spectrum in which the original frequencies of the lower half of the band are inverted. If, however, one channel was tapped with apparatus for demodulating at a frequency 1500, the result would comprise a correct reproduction of the lower half of the band with a superimposed inverted upper half which might be intelligible owing to the relatively low level of the upper speech frequencies. To overcome this, a microphone with a level energy-frequency characteristic, or a pre-modulation equalizer may be used, with compensating arrangements at the receiver. The transmission may be further confused by displacement of one phase at the transmitter and a corresponding displacement of the other phase at the receiver. Suitable networks are described. The twophase transmitter and receiver may also be arranged out of quadrature ; the demodulated components would be sufficiently intelligible if the phase angles at the transmitter and receiver were complementary but some inverted components would be present. The relative phases and frequency characteristics at the transmitter and receiver may be varied according to a predetermined plan. Similar arrangements may be used for multi-phase transmission. Multiplex transmission. Further channels above 0-1500 may be provided by feeding the line through filters from other arrangements of rotary transformers. For example, if two transformers at a transmitter are rotated in opposite directions at speeds corresponding to carriers 3000 and 4500 and the lower side bands are taken, subjected to filters 1500-3000 and combined, the result is two bands 16, 17, Fig. 2, each representing one half of the speech band with opposite direction of phase rotation but not relatively inverted. The original band is reproduced by corresponding arrangements at the receiver. Modifications using static multiphase modulators. In the arrangement shown in Fig. 3, separate thermionic or dry-rectifier ring modulators 18, 19 are supplied with carriers in quadrature from generator 22 over phase changing networks 23, 23a. A two-phase circuit, Fig. 10; with means for varying the phase angle, comprises a generator 22 connected to a tuned circuit L, C in which L and C are shunted by equal resistances R and respectively provide input for valves 72, 73 which feed the two modulators at 25, 25a. The phase angle between the two outputs is dependent on the value of R. Impedance networks. The equalizers may be two-element inverse arm networks at the transmitter and receiver, Figs. 5 and 6 (not shown) ; the phase shifter may be a low pass filter, Fig. 7 (not shown) in which phase shift is dependent on frequency, or on a constant resistance lattice network, Fig. 8 (not shown). The network characteristics may be changed according to plan by varying the saturation of the core of inductance 67, Fig. 9. For this purpose an auxiliary winding 69 is variably energized by battery 70 and resistance 71. Interchangeable code wheels, cams &c. could be provided to vary the values of components.