Patent GB-560930-A (GB)
Title: Improvements relating to electrical signalling systems
IPC: H04K1/00
Filed: 1942-07-21
Abstract: 560,930. Secret transmission systems. ECKERSLEY, P. P., and CARPENTER, R. E. H. July 21, 1942, No. 10215. [Class 40 (iv)] [Also in Group XL] In the two-phase transmission system described in Specification 560,925 arrangements are provided whereby a single channel is used between the transmitter and receiver. The use of different frequency bands for the several phases is discussed and regarded impracticable. In the system shown in Fig. 1, the arrangements to the left and right of lines AB and CD respectively comprise the two-phase transmitter and receiver of the prior Specification. Speech current from microphone M is given a level amplitude-frequency characteristic at VM and is modulated in quadrature at M1, M2 at the mid speech-frequency the lower side band being passed by LP1, LP2, each phase thus comprising the speech spectrum folded back on itself, each frequency of the folded band representing two original frequencies, which are, however, differentiated by phase rotation in opposite directions. These two phases are again submitted to modulation in quadrature at M3, M4 at a frequency higher than the highest present in the folded spectrum (the mid speech-frequency may be used). In the absence of a variable phase displacing network N1, the combined output of M3, M4 would, as in. the prior Specification, give intelligible speech. In the present case, the output comprises a " crossed " spectrum with erect and inverted speech superimposed. The individual outputs of M3, M4 are similar but differently phased. The combined output is transmitted over a two-wire line or by a single carrier frequency to quadrature modulators M5, M6 similar to and synchronized with M3, M4 after making allowance for phase displacement in the line. The outputs of M5, M6 after removing the upper side bands at LP3, LP4 are equivalent to the inputs of M3, M4. The phase relationships are restored by a network N2 similar to N1 and the two phases are demodulated at M7, M8 as in the Provisional Specification and combined to give correct speech after equalization at VE. In the system shown more fully in Fig. 6, the modulators and demodulators are all of motordriven commutator type. For example, modulator CM1 combines the functions of the quadrature modulators M1, M2, Fig. 1, the commutator segments represented as fixed contacts being engaged by brushes 11, 12 which are angularly disposed so as to provide the quadrature relationship of their outputs. The commutators may, however, comprise vibrating switches. Such a modulator normally produces no output of modulating, i.e. commutating, frequency, but if it is supplied with D.C. superimposed on the frequency band to be modulated, an output of commutating frequency is also obtained and this is used for synchronizing as will appear later. In this system, the main components have the same references as those in Fig. 1 except that the pairs of quadrature modulators are denoted CM1 ... 4. Modulator CM2 which passes the crossed spectra into the single channel over low pass filter LP having a cut-off at the highest modulating component, is fed with D.C.. from battery 24 over an A.C. isolating pad 23 so that it transmits the modulating frequency. The D.C. is isolated by condenser 19. At the receiver, modulator CM3 in addition to supplying the crossed spectra to the two phase lines, also rectifies the incoming modulating frequency, the resultant D.C. separated out by network 31 being dependent in intensity on the lack of synchronism and used by a winding 32 to control the driving motor of commutator modulator CM3. It is essential for the correct reproduction of speech that the channel should be " balanced by arranging that phase and attenuation variations over the transmitted band are symmetrical about the mid-band frequency. Modifications. The phase-shifting networks N1, N2 may be replaced by magnetic storage devices providing for substantial time delay in one of the phases at the transmitter to enhance the secrecy. The same object is achieved by making the frequency of the single channel modulators M3 .. M6 different from that of the two-phase system modulators M1 .. M4. The effect of this is to depress the erect frequency components and raise those of the inverted components if the frequency of M3 .. M6 is decreased, and vice versa. As intelligibility of speech is decreased more by a reduction of frequency than by an increase, the preferred arrangement reduces the erect components, say, by 200 c.p.s. The frequency of modulators M1, M2, M7, MS may be wobbled, e.g. by simultaneous variation of series resistance in the driving motors of commutators CM1, CM4, Figs. 6 and 7. The balanced characteristics of the channel may be upset by a network at the outgoing end and restored by a network at the incoming end. Rotary transformers may be used as modulators and the Provisional Specification describes their use in some detail, but as an alternative to rotary commutators, equivalent static modulators, including thermionic valves, are preferred. The Provisional Specification also refers to Specification 521,191.