Patent GB-455279-A (GB)
Title: Improvements in or relating to systems for the exchange of intelligence at carrier frequencies over a common medium
IPC: H04J1/00
Filed: 1935-04-12
Abstract: 455,279. Exchange and transmission systems. STANDARD TELEPHONES & CABLES, Ltd., 63, Aldwych, London.- (Assignees of Espenschied, L., Tidd, W. H., and Green, E. I.) April 12, 1935, No. 11446. Convention date, May 26, 1934. [Class 40 (iv)] [See also Group XL] A large number of subscribers are given telephone interconnection facilities resembling those provided by an ordinary exchange or direct - call intercommunication system, individual subscriber's lines, however, being replaced by individual carrier frequencies which are guided by a transmission circuit common to all the subscribers. Preferably, ultra highfrequencies are used. Even in the case in which subscribers select each other directly, a central station may be provided for supplying exact frequencies or for amplifying or both. The amplifying may involve a temporary or permanent frequency shift, Figs. 6, 8 (not shown). Two or more high-frequency areas may be linked by junctions. Automatic volume control may be employed to ensure constancy of level for all subscribers. Specification 455,348 is referred to. The Specification as accepted is confined to wired systems but as open to inspection under Sect. 91 makes it clear that all the exchange and direct call systems described below are equally suitable for wireless transmission. Exchange system substation, Fig. 3. On the removal of the receiver from the switch-hook, circuits are made for energizing the oscillator OC, modulator MO and microphone ST1 from the power means AC. The subscriber then controls automatic switches to extend the connection over frequency changing links to a similar substation at which RL1 connects up a bell to notify the wanted subscriber. When the latter removes his receiver, RL2 cuts off the bell and the local transmitter apparatus is energized, after which two-way conversation can proceed. The circuit may be modified as indicated by the dotted line YY to avoid switch contacts in the speech path. According to the Specification as open to inspection under Sect. 91, but not as accepted, the extension of the call at the exchange may be done by a manual operator. Automatic exchange, Fig. 20. Incoming currents selected by the subscriber's individual apparatus OF1 are rectified at OD1 and operate the line reply LR. Line switch SU1 then hunts in known manner and, when an idle connector is found, COR cuts off the line relay and switches through the test, impulsing and speech circuits. Line relay FR pulls up over the impulsing circuits and energizes release relay RR which connects dialling tone to the speech circuit. The first digit is repeated through dialling relay SR to the stepping magnet MM2. On the relapse of SR at the end of the digit the impulse circuit is connected to magnet MM3 and parallel dialling relay AR. If the line is busy, BR operates over the test wire before the relapse of AR after the second digit, locks up and sends back busy tone. If it is idle, the relapse of AR energizes the switching relay WR in two steps in series with the wanted party's cut-off relay. The latter switches on the power to the called party's oscillator equipment at the exchange so that a ringing signal is given at the substation as described above. Alternatively, ordinary ringing current could be sent as a modulation of the carrier current. When the calling party hangs up, FR, RR, WR fall back and the switches go home. Automatic exchange, Fig. 21 (not shown). In this system there is no line-switch. The control of the switch mechanism is the same as in the system shown in Fig. 20, but although the switches are shown with wipers, these, except the test and homing wipers, are without function and the setting of the switch serves to tune the radio equipment to the wavelengths of the wanted party by means of variometers, Fig. 15 (not shown). Automatic exchange, Fig. 22. The tuning of the circuits at the incoming side of the connecting circuit shown in Fig. 22 are being constantly adjusted by a motor DMO, it being arranged that the difference between the characteristic transmitting and receiving frequencies is the same for all subscribers. When a call arrives, the operation of FR, RR stops the motor by energizing YR. When the tuning arrangement is in positions corresponding with a busy subscriber, XR is operated to disconnect FR. Dialled impulses control the tuning mechanisms SU2, SU3 as described above. Automatic exchange, Fig. 23 (not shown), in which there is no translation of incoming currents to voice-frequencies. The selective arrangements are as described in connection with Fig. 20. Dialling tone is given by transmitting a second high-frequency to beat with the receiving frequency of the calling party. Instead of transmitting the conversation through the exchange circuits at voice frequency, means are provided for converting the frequency received from the calling party to that necessary to call the wanted party and vice versa. Direct call systems. In one arrangement, Fig. 5 (not shown), in which each subscriber is given a characteristic pair of wavelengths for incoming calls, a calling subscriber operates a knob which simultaneously tunes transmitting and receiving circuits to the wavelengths individual to the wanted party. He then removes his receiver to switch on the power supply and to operate a relay for switching the oscillator and detector from the fixed tuned circuits characteristic of himself to the variably tuned circuits controlled by the knob referred to above. If the wanted party is busy, carrier current at his transmitting frequency will be at once received and will cause the operation of a busy signal. If the wanted party is idle, a relay, operated (say) when the oscillator has warmed up, prevents the operation of the busy signal. At the receiving station, the arrival of a carrier at the characteristic receiving frequency closes a local bell circuit which is interrupted when the called party lifts his receiver. This operation also connects the power supply to the oscillator and two-way conversation is now possible. In a modification, Fig. 9, a single carrier frequency is used for both directions. The subscriber's characteristic frequency is selected from those radiated by a central station by a fixed circuit FF1, whereas those characteristic of others are selected by the circuit VF1 which is tunable by means of the apparatus HC. The selected frequency is led to a modulator MO2 which is supplied by a local oscillator VO. The difference frequency is selected by a sharply tuned filter FF3, amplified at AM and fed to a modulator MO3 which is also fed by VO. The output of MO3 thus includes a component of exactly the same frequency as the currents selected by FF1 or VF1 and after selection by filters FF2 or VF2 this component passes to the combined transmitting and receiving modulator MO1. When the receiver is removed to initiate a call, RL3 locks up and switches over from the fixed tuning elements to the adjustable ones which have previously been set in accordance with the frequency of the wanted subscriber. If the wanted party is busy, his frequency will be modulated at 60 cycles, and 60 cycle currents will pass filter LP and, after rectification in RF2, will operate RL4 to give a busy signal. If the wanted party is idle, RL3 connects up 1000-cycle calling frequency current to the hybrid. This current, after selection by filter FF4 and rectification at RF1, energizes RL1 which is followed by RL2. At the same time the 1000-cycle current modulates the transmitted carrier current and is received at the distant station where, after selection by the filter FF4, it energizes RL1 which rings the bell until the subscriber answers. RL2 then locks up, disconnects RL3, connects 60 cycle current to the modulator to cut off the calling current (by operating RL4 at the calling party's station) and to mark the channel busy, and completes receiver and transmitter circuits. Wired wireless transmission systems. A network as shown in Fig. 10 may be employed and use may be made of coaxial conductors, e.g. as described in Specifications 352,011 and 353,020, or air-spaced shielded pairs of dielectric wave-guides. For example, if very short waves are used, the central conductor of a coaxial cable may be discontinued a short distance from the terminal stations. Preferably, the more remote subscribers have longer wavelengths. Fig. 13 shows a repeating arrangement for systems using networks of coaxial conductors. Each line is terminated with a resistance equal to the high-frequency characteristic impedance, and the subscriber's sets are connected through high impedances to the line. The amplifier CR amplifies a broad band and is coupled to the line in known manner for two-way amplification. Fig. 14 shows a modification in which the central conductors are all connected together at a central point through resistances JR. The junction point is connected through an amplifier AM and the common return. Carrier frequencies may be provided by the apparatus CS. The method of calculating the values of the resistances JR to avoid reflection is given in the Specification. As shown in Fig. 25, the sets are given an impedance equal to the characteristic impedance of the line and are not connected up as in Fig. 13 over high impedances. Various schemes, Figs. 16, 19 (not shown) are shown for the arrangement of coaxial or other conductors to provide a network covering an area. Such networks may be designed so that the impedance in either direction measured at any point and any frequency is always the same. The Specification as open to inspection under Sect. 91 refers to the application of the invention to television and also includes the matters abridged in the subsequent paragraphs. This subject-matter does not appear in the Specification as accepted. Manual exchange system, Figs. 5 and 6 (Cancelled), working with the substation shown in Fig. 3. At the exchange, a number of receivers OF1, OF3, &c.,