Patent GB-537759-A (GB)
Title: Systems of broadcasting signals on automatic or the like telephone networks
IPC: H04H20/81
Filed: 1940-04-03
Abstract: 537,759. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. (Materiel Telephonique Soc. Anon.). April 3, 1940, No. 5993. [Group 40 (iv)] Some of the terminals of an ordinary final selector TFS give access to apparatus BDS instead of to subscribers' lines. Each apparatus BDS is individual to a particular wired broadcast programme and is associated in a characteristic way with a common multi-frequency tone-generator BFG. The apparatus BDS sends back a marking tone over the switch train and starts a switch BLF hunting for this tone in order to associate apparatus BMD with the calling line. This apparatus then controls a programme-selecting switch BLS individual to the subscriber in accordance with further tone signals received over the switch train from the apparatus BDS. In a twenty-programme system the selecting signal may comprise one of ten frequencies characteristic of the units digits of the programmes with or without another frequency to indicate the selected tens group or may comprise one of five frequencies characteristic of pairs of adjacent units digits with or without another frequency to indicate whether the selected digit is odd or even and with or without a third frequency to indicate the tens group. Circuit operations. When the apparatus BDS, shown in detail in Fig. 2, is seized, relays P, Tl therein are operated and the ring tripping relay is operated in a circuit over the B wire and a rectifier RS. This arrangement is used instead of a low-resistance loop to prevent operation of the supervisory relay and consequent operation of the meter. Apparatus BDS is marked permanently idle by the source RD. On the operation of Tl, tone currents characteristic of the particular apparatus BDS and a further tone current for marking the calling line are fed to the talking circuit and a common starting relay A is operated. Assuming the apparatus BMD, BLF are of the kind shown in detail in Fig. 4, relays D in each of the idle ones energize the driving magnet LF of the associated switch BLF which connects a tuned circuit formed by a part of inductance L1 and a condenser C1 and connected to the grid of a valve VI to the subscribers' lines in turn. When the marking tone current is encountered a relay Pt in the valve anode circuit energizes Dt which stops the switch BLF and energizes Sh. The latter energizes Et and drives the sequence switch R of the circuit BMD to its second position. Et sends back tone frequency current to operate AC, Fig. 2, and so cause the removal of the marking tone and the release of A to stop other circuits from hunting. This provision makes it possible to use a marking tone of the same frequency as one of the programmeselecting tones. In position 2 relays Dt, Sh are released and V1 is paralysed to release Pt. If the subscriber's programme-selecting switch BLS is off normal Pn now pulls up and causes interaction of relays I1, I2 with interrupters INTa, INTb to provide impulses for sending it home. Pn then falls and the sequence switch advances to position 7 in which Et is released. During this operation the circuit L1, C1 is retuned to the frequency the presence or absence of which characterizes the tens group of the wanted programme and if this frequency is present Pt energizes Dz which locks over wiper c of the control switch SS. In position 7 Pn re-starts the impulsing relays which now step the control switch SS and the programmeselecting switch BLS in synchronism. At each step, the former switch alters the tuning of a circuit connected across the line wires and to the grid of Vl and when the tuning corresponds to the frequency of one of the received tones, Pt is energized, stops the switches and .drives the sequence switch on to position 17. If Dz is energized the first ten steps are made without regard to the received tones since Dz paralyses valve VI. After ten steps it is released at wiper c. During the movement of the sequence switch to position 17 a large condenser C4 is shunted across the talking circuit to attenuate the tones heard by the subscriber, who should then hang up. At the same time, battery is applied over progressively diminishing resistance to the B-wire to shunt out the line relay at the holding bridge and release the exchange switches. In position 17 Pn re-starts the impulsing relays to send switch SS home whereupon ground is connected to the grid of V1 to operate Pt, the switch SS stops and the sequence switch is driven on to position 1. In the alternative arrangement the apparatus BMD, BLF is as shown in Fig. 5. The circuit associates itself with the calling line, the switch SS is driven home as in the other arrangement and the sequence switch advances to position 11. During the first part of this movement a test is made for the frequency the presence or absence of which indicates the tens group of the wanted programme and if this frequency is detected the relay Dz operates and locks. During the second part of the movement, L1 is tuned to a new frequency by an additional condenser C5 and if this frequency, indicative of an even units digit, is received, Cp operates and locks up. In position 11, Pn is re-operated and the control and programme-selecting switches are stepped in synchronism as in the other arrangement until the tuning of circuit L2, C4 corresponds with a received frequency, Dz if operated disabling the stopping arrangement for the first ten steps. The sequence switch then moves to position 14 and if Cp is operated a further impulse is given to switch LS via cam J and to switch SS via cam O in position 12. SS is sent home on the operation of Pn when position 14 is reached. In this position also a high-resistance relay Ir is bridged permanently and a large capacity condenser C6 intermittently across the talking circuit. Ir pulls up and the modified tone heard by the subscriber tells him to hang up. If he does so Ir falls and sends the sequence switch on to position 1. If he does not the sequence switch is sent on after an interval by a time switch TA.