Patent GB-327709-A (GB)
Title: Electric signalling system
IPC: H04B1/66
Filed: 1929-01-09
Abstract: 327,709. Standard Telephones &. Cables, Ltd., Sandeman, E. K., and Carter, R. O. Jan. 9, 1929. Inverting and transposing frequencies of telephone currents. - In wave transmission systems in which the width of a frequency band, such as the speech band, is reduced, with or without a general shift of the band, before transmission over a channel such as a submarine cable or a wireless channel, the reduction is effected by a scanning device which picks up varying portions of the band. In one arrangement, the frequency band is moved relatively to the scanning device (a band-pass filter) by superposing it on a periodically varying carrier wave, while in a second arrangement, the scanning device is variable and may comprise a number of band-pass filters to which the frequency band is submitted in recurring sequence. In both arrangements, the speed of scanning may vary in different parts of the frequency range, and in the second arrangement, a single transmission channel with synchronizing arrangements may be used, or if the range is divided into three bands, the side and phantom circuits of a four wire circuit may be used so as to dispense with synchronizing. First arrangement, Figs. 2, 4 and 5. A speech band B of range f1-f2 is compressed and ultimately transmitted as a band b of range f3-f4. The band B is first raised, by modulation and selection of the lower side band, to a position f1<1>-f2<1> for a purpose which will be described later. This modulation band is then supplied to a number (q) of variable frequency modulators each associated with an oscillator the frequency of which is varied between the limits f5 and f6 adjusted so that f6-f5 = f2<1>-f1<1> + f4-f3. The band f8-f7 between the upper and lower extreme positions of the lower side-band of this variable modulation has therefore a range f4-f3 = b. The frequency of the q oscillators is varied by a multiple condenser comprising groups of moving vanes angularly displaced around a spindle and groups of fixed vanes not angularly displaced, so that each oscillator operates in turn. The variable modulation products are then scanned by a filter having a range f7-f8 = b the output of which is further modulated with a steady frequency f7-f3 so as to transpose the band to the required position f4-f3. If the condenser spindle makes n revolutions per second, the band is scanned n q times per second, the arrangement being such that scanning is effective only during the increase (or decrease) of the capacity of the condensers. The Specification contains a mathematical discussion of the problem and gives formulae for determining the relative displacement of the moving vanes so that the scanning may be continuous. It can also be shown that if f1<1> is greater than ¢ (f4-f3) the upper side bands due to the variable modulation pass outside the range of the scanning filter, so that if f1 is greater than ¢ (f4-f3) the first modulation may be dispensed with. The complete transmitting and receiving) system is shown diagrammatically in Figs. 4 and 5. The speech band S is supplied to the first modulator m1 associated with oscillator 01 and the modulation products are passed through a filter F1 of range f1<1> to f2<1> from which they pass to secondary modulators m2 associated with variable frequency oscillators O<1>2-O<111>2. The variable modulation products are scanned by a pass filter F2 and the resulting band is transposed by modulator m3 and oscillator 03 to the required position in the frequency spectrum and passes through a filter F3 into the line L. At the receiving end, the process is reversed by corresponding demodulation as shown in Fig. 5 and the resulting product is passed into a delay network N. As each part of the original band is transmitted only for a fraction of any particular time considered, it is necessary to substitute for the missing parts by " repetition " at the receiving end. This is effected by taking a number of leads from the delay network N to feed amplifiers A arranged in parallel so that the complete restored band is delivered at R. The multiple variable condenser of the oscillators G<1>2-G<111>2 associated with the demodulators D2 is synchronized with the condenser at the transmitting end by any known means due allowance being made for delay between the receiving and transmitting ends. When delay is such that the required angle ot lag between the condensers is equal to the angle of displacement between adjacent groups of movable vanes, allowance for delay is not required, and consequently any delay may be compensated by adding a delay network so that the total delay requires this angle of lag. The Specification discusses the maximum permissible delay at any one frequency and deviation in delay between any two frequencies. Second arrangement, Fig. 6, and modification, Fig. 8. The speech band is fed at I, Fig. 6, to three band-pass filters BF1-BF3 in turn by means of a rotary distributor R1, the filters having ranges of 200-900, 900-1600, and 1600-2300 cycles. The three bands pass respectively through amplifiers AH1-AH3 of which AH2, AH3 operate also as frequency changers to bring all the bands to the same position, 200-900, in the frequency spectrum, the outputs being combined and passed to the line TC. Networks AN, VN are provided to equalize the attenuation and transmission velocity of the line for the range 200-900. At the receiving end, the signals are amplified at A and distributed at R2 in synchronism with the distributor R1 to amplifiers and frequency changers AH<1>1-AH<1>3 the outputs of which are fed to a delay network DN for repetition purposes as in the first arrangement. An amplifier may be arranged in front of each filter BF1-BF3 so as to present an impedance to the filter matching that of the filter itself, and as delays in the filters may vary, a phase-compensating network may be introduced between each filter and its amplifier. The time taken by R1 to pass over one contact must be short compared with the shortest transient and large compared to the time of one cycle at the lowest frequency, if distortion is to be avoided. These conditions are unattainable if the speech band is 300-2300 cycles and the number of sub-bands is three or four. To overcome this difficulty, the band is first split by filters BF into two sub-bands 300-1300 cycles and 1300-2300 cycles, as in Fig. 8, and each band is treated in a similar manner to the whole band in Fig. 6, the two input distributors R1, R2 being driven at 25 and 100 revolutions per second respectively. The two channels may, as shown, be kept separate until after the delay network stage DN so that separate and simpler networks may be used. The delay network DN may also be inserted before the received signals are split up into different bands, and in the case of the threeband system of Fig. 6, three distributors, Fig. 9 (not shown), are necessary to distribute the primary and delayed outputs of DN to the amplifiers AH<1>1-AN<1>3. In these arrangements, the distributors may be synchronized by driving them by A.C. which is sent down the line, two synchronizing frequencies being required in Fig. 8. The relative positions of the distributors, filters and frequency changers may be varied. Specifications 226,338 and 259,328 are referred to.