Patent GB-408320-A (GB)
Title: Improvements in or relating to carrier signalling systems
IPC: H04J1/04
Filed: 1932-10-07
Abstract: 408,320. Wired wireless systems; impedance networks. STANDARD TELEPHONES & CABLES, Ltd., HODGSON, K. G., JACOBSEN, B. B., and RALPH, F., Connaught House, Aldwych, London. Oct. 7, 1932, No. 27939. [Classes 40 (iii) and 40 (v).] In a line carrier - wave signalling-system a programme channel of wide band-width is transmitted as a single sideband spaced above all other channels in the frequency range, a pilot frequency for the programme channel being transmitted in the space between the highest of the other channels and the programme channel. A crystal filter circuit comprising a single crystal associated in series or parallel with a pure resistance circuit is used in addition to electrical filter devices for removing the unwanted side-band of the programme channel and the carrier wave is in addition normally suppressed. Switching-devices may be provided at a terminal station for utilizing the crystal filter either for transmission or reception, and to connect a receiving amplifier direct to a transmitting-amplitier, whereby the station operates as a repeating station. As an example, the frequency range from zero to 3 kc. may be occupied by a telephone channel, and from 3 kc. to 30 kc. by a number of carrier telephone or telegraph channels. The pilot frequency for the programme channel is 34 kc. while the transmitted lower side band of the programme channel occupies the range 34.5 kc. to 42.4 kc., the carrier frequency of 42.5 kc. and the upper side band being suppressed. At the transmitter, Fig. 2, the low-frequency programme from the line VFL passes through a tone compensator VFE to a balanced, grid-current, carriersuppressing modulator MOD, the side-bands entering a low-pass filter and crystal filter MOD.F and CRY.F respectively to eliminate the upper side-band. The modulator MOD is supplied with the carrier frequency from an oscillator C.OSC through a filter CF which prevents the pilot frequency reaching the modulator. The pilot - frequency oscillator P.OSC controls the carrier-wave oscillator C.OSC and also sends pilot-frequency current to the line, at a level of about 35 db. below the maximum level of the programme side-band, both of which traverse amplifier T.AMP, line equalizers LE and line filter LF reaching the line CL, the other lower carrier channels being transmitted to line through filters LF1, LF2 &c. in known manner. At the receiver, Fig. 3, the programme channel and pilot frequency pass through filters DE.MOD.F and CRY.F, identical with MOD.F and CRY.F of the transmitter, and the pilot frequency is selected by filter PF amplified at PA and is used to control the frequency of the carrier-wave oscillator RC.OSC. The latter supplies carrier frequency to the demodulator DEMOD, the demodulated side-band passing to the voice frequency line V1F1L through another filter DE.MOD.F and tone compensator VFE. A pilot indicator PI may be provided to give warning of any change in line attenuation. Crystal filter, Fig. 10. The crystal CRY has electrodes deposited on it by cathodic deposition and rests on its edge on cotton wool, contact being made with the electrodes by light spiral springs resting at the nodal line and serving to support the crystal. It is included between resistances R4, R5 as the coupling member between the valves V1, V2 of a resistance coupled amplifier. Valve V1 has an impedance of about 60,000 ohms, while that of V2 is about 6,000 ohms. The anode resistance R3 is about 60,000 ohms and resistances R4, R5 in series about 150,000 ohms. Resistance R8 is a high resistance leak, while R6 is about 25,000 ohms, the condenser C3 having a capacity of .01 mf. A mathematical analysis of the circuit is given in the Specification. Pilot and carrier oscillators, Fig. 11. The pilot oscillator comprises a back-coupled valvo V3, the output of which is amplified in valve V4 and fed into the grid circuit of the carrier oscillator V5. Synchronism is effected by the coincidence of the 5th harmonic of the pilot frequency, which latter is 34 kc., and the 4th harmonic of the carrier frequency, which latter is 42.5 kc. The vernier condenser C4 is provided to adjust the pilot frequency, and hence the carrier frequency, with reference to the resonant frequency of the crystal filter. The output of the carrier oscillator is amplified by valve V6. Terminal switching-arrangements, Fig. 12. By means of eight double-pole two-way switches 1 .... 8, three carrier lines CL1, CL2, CL3, may be associated with a voicefrequency line VFL through a common crystal filter CRY.F, a modulator MOD, a demodulator DEMOD, a common receiving amplifier RA, and individual transmitting amplifiers TA1, TA2, TA3. The equipment may act as (1) a receiving terminal from any line ; (2) a transmitting terminal to any line ; (3) a transmitting terminal with the output branching to several lines ; (4) a through repeater ; (5) a through repeater with branching; (6) a through repeater with branching, the programme also being tapped and demodulated locally.