Patent GB-320091-A (GB)
Title: Secrecy systems for signalling
IPC: H04K1/04
Filed: 1928-07-04
Abstract: 320,091. Standard Telephones &. Cables, Ltd., (Western Electric Co., Inc.). July 4, 1928. Inverting and transposing frequencies of telephone currents.-Secrecy is attained in radio or line transmission of speech or like signals by (1) stepping up their frequency (by modulation and side-band selection) so that they form two consecutive bands in the frequency spectrum; (2) stepping up this double band by a frequency which wobbles some ten times a second, the extent of the wobble being equal to the band width q of the original signals; and (3) passing the wobbling double band through a band filter which transmits to line or aerial a band of width q placed centrally with regard to the wobble. Fig. 3 shows the bands in diagram form after the imposition of the wobble, frequencies being measured vertically and time horizontally. A and B together comprise the double band, and the final filter is adjusted to the frequency limits C and D, the transmitted signals comprising alternately elements of the upper band A and the lower band B. Reception is only possible with special receiving circuits, Figs. 2 and 4, with local oscillators wobbling in frequency in synchronism with those used at the transmitter. The transmitting-circuit, Fig. 1, comprises a signal source such as a microphone 1 coupled in parallel to two balanced modulators M2, M3 supplied by local oscillators 05, O6 differing in frequency by signal band width (q). The upper side bands are selected by filters BF7, BF8, and after amplification are combined to form the double band referred to in (1) and (2) above. This double band is now applied as a modulating current to an oscillator 012, the frequency of which varies, sinusoidally or otherwise and by an amount preferably equal to q, some ten times a second on either side of its mean frequency. The output filter BF20 is adjusted to pass a band C, D of width q, which is composed of alternate sections of the bands A and B. The receiving. circuit of Fig. 2 comprises two demodulators DM32, DM33 connected in parallel to the receiving aerial or line, and supplied by local oscil. lators 045, 046. The mean frequency of those oscillators differs by an amount q, and each oscillator wobbles in frequency in synchronism with the transmitting oscillator 012, so that they serve respectively to convert A and B signal elements to their original low-frequency form. Their output is amplified and re-combined, passing through a low-pass filter LPF 60 to the phones 61. The filter eliminates undesired products of the demodulation. In an alternative receiving-circuit, Fig. 4, the received currents are applied to a wobbling oscillator O33 of such frequency as to demodulate the band B to its original low-frequency form, this product being passed through an amplifier A35 and filter LPF 40 to the phones 61. The band A, still of higher frequency, passes through a filter HPF45 and is again demodulated by an oscillator O35 of frequency q. The band A is thus converted to original form and passes through an amplifier A55 to the lowpass filter and phones 61. The Specification states that the band CD may be transmitted over a telephone line as a band of speech frequencies.