GB852892A - Improvements relating to telephone systems - Google Patents

Abstract

852,892. Exchange systems. SIEMENS EDISON SWAN Ltd. March 11, 1957 [March 13, 1956], No. 7830/56. Class 40 (4). In a system using pulses of a single frequency for signalling over a junction, a voice-frequency receiver is linked to the incoming line circuit of the junction by a hybrid TR1, Fig. 1A, so that it does not respond to voice-frequency from the exchange side of the circuit. The hybrid is unbalanced when a call over the junction is answered or is intercepted by an operator. The receiver then responds to voice-frequency signals from either direction by splitting the line at ax 1, ax2 between the receiver and the exchange. An answer signal is repeatedly transmitted over the junction from the line circuit until it is acknowledged by a single pulse. On release, clear forward pulses are received over the junction and the signal is acknowledged by a single release pulse. Windings I and II of the hybrid TR1 are each divided, as shown, in the proportion of 1: 25. The voice-frequency receiver comprises two pentodes V3, V4, a signal circuit SA7, a guard circuit SA6 and a normally-operated relay XX arranged as described in Specification 827,174. Contacts xxl, xx2 are shown back. Non-linear resistances quench sparks at relay contacts. Line circuit operation.-In response to a pickup pulse of 2280 c/s., relay XX falls back. Dialling relay CC pulls up with guard relay BH and relays AA, DA follow. Relay AA loops the line 28, 29 over contacts cc1, dr6 to call the exchange. At the end of the pick-up pulse, relay BR pulls up in series with BH and, as CC falls back, relay AX pulls up and removes the split condition so that dialling tone can be transmitted over the junction to the calling- exchange. Pulsing relay AA is now held over front contact of xx2 and dialled pulses of signal current are repeated as break impulses on the line 28, 29, relay AX falling back to split the line again whilst dialling relay CC is up. On completion of dialling, AX pulls up again but is short-circuited so as to split the line when relay DP responds to battery reversal on reply. The circuit of DA is opened at dp3 and during the release period, relay SX pulls up and connects a signal source (2280 c/s.) to leads 26; 27 to signal reply over the junction. Relays DA, DR (both slow to release) interact and relay SX transmits further pulses over the junction until a single pulse acknowledgement signal is received. Relay XX falls back in response to the signal and relays CC, AK pull up and the hybrid is unbalanced. At the end of the signal, CC falls back quickly, DR being up, and relay AA, AX are held operated to provide a speech path through the circuit. Relays DA, DR, BR, BH remain up. The receiver, at this stage, splits the line in response to signals from either direction. Release.-When the called party clears, line current is reversed and DP falls back. Relay DA is de-energized and SX transmits a clear back signal over the junction during the release period of DA. Relay AX falls back to split the line for the duration of the clear-back signal. When DA falls back, AK changes the input circuit of the receiver to the normal hybrid form. Relay DR falls back slowly and DA pulls up again, bringing the circuit back to the condition it was in before the call was answered. When the caller clears, one or more two-second pulses of clear forward signal are received over the junction. Relay XX is back long enough to release guard relay BH. Relay CC pulls up and falls back again at the end of the pulse. Relay SX transmits a release signal over the junction during the release period of DA. Four-wire junction, Fig. 2.-The hybrid TRlA is connected in the incoming pair with the smaller sections of the windings I and II permanently in series with the line wires. The acknowledgement relay AKA (winding not shown) when up, short-circuits the balance resistance R47. Line blocking relay BL (winding not shown) in effect splits the two lines at bl1, bl2. Another hybrid H links the junction with a two-wire exchange line 49, 50. Contacts sx correspond to contacts sx of Fig. 1A.