Patent GB-560794-A (GB)
Title: Improvements in telephone or like systems
IPC: H04Q3/00
Filed: 1942-10-15
Abstract: 560,794. Automatic and semi-automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd., GILLINGS, C., BEALE, C. E., and TERRONI, T. B. D. Oct. 15, 1942, No. 14466. [Class 40 (iv)] The system described in Specification 559,016, in which numerical trains of impulses are converted at the outgoing end of a junction into voice-frequency code signals, is modified in the following respects-( 1 ) the talking circuit remains complete at the outgoing end during the setting up of a connection, (2) the timing of the supervisory signals is relied upon to distinguish them from disturbing currents of the same frequency, (3) the receipt at the incoming end of the preparatory signal preceding a code signal serves to block the talking circuit to prevent the latter signal reaching a later stage in the connection. On a call involving two VF junctions in tandem, a simplex signal sent back from the outgoing relay set at the tandem exchange causes the incoming relay set to switch through and cut out the decoding apparatus. Similarly, on a call involving two VF junctions with an intermediate DC junction, a simplex signal over the latter causes switch-through at the first tandem exchange. If there are two intermediate DC junctions, however, the transmission of the simplex signal back to the first tandem exchange is impracticable and a re-conversion to VF working is necessary at the third tandem exchange: this explains the importance of feature (3) above, which forms the subject of Specification 560,807. Outgoing end, Figs. 1 ... 4. When the operator takes the junction into use, a timing switch TS is started and relay CO switches through to the repeating coils RPC1 and removes the termination TER, which when two-way working is employed forms part of the incoming relay set. A seizing signal of frequency X is sent for 100 milliseconds by relays FXY, CS, the latter being operative whenever a signal requires to be sent, after which FC pulls up. During dialling, impulse relay A is connected to the tip and ring of the jack and the numerical digits are registered and converted into code form as in the prior Specification. The registers DSA ... DSD are used in cyclic order and an arrangement may be provided to prevent the register DSA taking precedence if the fifth dialled digit is received before the first four digits have been sent. On the return of the dialling key, relay KR energizes over the sleeve and releases A, whereupon the digit distributer DD advances to its mid-position and the supervisory lamp relights. The operator may now listen to tones and, if necessary, speak without receiving a reply signal : if she restores her speaking key, increased current over the sleeve operates RR to maintain a suitable termination on the junction. The reply signal, which consists of intermittent Y frequency, 140 ms. on and 360 ms. off, is checked by a pair of slow-release relays MS, MT which operate and fall back in turn under the control of relay BS. The latter releases sluggishly during each " on " period, and if the timing is correct the second " on " period will find MS back and MT still up, whereupon relay SA energizes and connects up AA to step on the distributer DD to normal and send an acknowledgment signal of Y frequency. The supervisory lamp is dimmed, and RR, if up. is released. The release signal, which is the same as the reply signal, is checked in the same way and brings about the operation of SY which relights the supervisory lamp. The clearing signal when the operator takes down consisting of frequency X for 2 seconds or 6 seconds if no reply has been received followed by frequency Y for 300 ms. is sent as in the prior Specification. If the operator clears accidentally while the called party is talking, the clearing signal may fail to get through. It is arranged, therefore, that the release signal when the called party hangs up in these circumstances starts a further rotation of the switch TS during which a fresh clearing signal is sent. When the junction is seized over the local selector train, starting relay SR energizes over the test wire and after the seizing signal has been sent the operation of FC makes a loop circuit for impulse relay A. The connections of the latter relay are reversed when SA responds to the reply signal. When the junction is seized over a selector from an incoming VF relay set, relay AC operates in a simplex circuit with the result that, after the seizing signal has been sent, the switch TS continues to step to position 20, FC does not operate and the incoming VF signals pass directly to the junction. The supervisory signals pass back without affecting the outgoing relay set. Incoming end, Figs. 5, 6. The seizing signal and numerical code signals are received as described in the prior Specification, and relay CA which responds to each preparatory signal converts the line termination consisting of resistances YB, YC, YD, YE, YJ and condenser QA into an infinite loss pad. The reply of the called party energizes relay EE in place of KK, whereupon IR operates to cut out the loss pad, MA energizes and FY is controlled over pulse wire 14 to send the reply signal. The acknowledgment signal, frequency X for 440 ms., releases relay GX and operates GY momentarily to end the reply signal and release MA. When the called party hangs up, relay KK reoperates instead of EE, the relapse of IR replaces the loss pad, and the release signal is sent in the same way as the reply signal. In response to the clearing signal, frequency X releases relay GX and brings up CC whereupon the following frequency Y connects up CR to initiate release. The loss pad is such that, with CA back, tones can pass without serious attenuation. If the incoming selector switches through to a short-circuited trunk, neither KK nor EE is operative and a circuit is made for relay SC which opens the forward loop to allow the clearing signal to be satisfactorily received. If another VF junction is seized, both KK and EE energize in the simplex circuit of a relay like AC, Fig. 1, and IR operates to cut out the loss pad and disconnect CB, but MA is inoperative since PY is held by KK. Subsequent VF signals, therefore, pass directly to the outgoing junction without affecting the incoming relay set, which responds only to the operator's clearing signal. If a DC junction is seized, the normal outgoing repeater is cut out and, therefore, a simplex signal can be received from the next tandem exchange if a VF junction is taken into use thereat in order to provide a straight-through path for subsequent VF signals. A modification is described, Fig. 9 (not shown), for preventing the cutting out of the loss pad by busy flash if this is encountered as well as busy tone.