Patent GB-722174-A (GB)
Title: Improvements in or relating to communication circuit arrangements
Inventor: FLOWERS THOMAS HAROLD
IPC: H04Q3/52
Filed: 1949-10-05
Abstract: 722,174. Automatic exchange systems. POSTMASTER GENERAL. Aug. 21, 1951 [Oct. 5, 1949], No. 25653/49. Class 40 (4). A subscriber's exchange equipment includes receiving apparatus which responds to an audiofrequency signal which is transmitted over the switch train from common apparatus and initiates ringing of the subscriber, means responsive to the current which flows when the line is looped preventing operation of the receiving apparatus during a call by speech currents of the same frequency as the ringing- control signal. Metering is controlled by another audio-frequency signal reverted to the calling subscriber's exchange equipment. The invention is especially applicable to systems in which electronic switches unable to carry the normal strong ringing or metering currents are employed. As shown in Fig. 2, the subscribers' line 1 is connected by way of a hybrid transformer H1 to the unidirectional go and return paths 2, 3 with the former of which is associated a private wire or signalling path 4. To control ringing and metering two audio-frequencies fR and fM are employed, e.g. with a bandwidth of 5 kc./s.fR and fM could be 2.5 and 4.2 kc/s. respectively. These are injected into the return path by a common unit (not shown) and reach the subscriber's equipment over the exchange switches which may be electronic. Metering. Transformer T1 with capacitor C4 forms a band-pass filter passing fM to thyratron V1, and inductor L1 and capacitor C5 a band-stop filter for the same frequency. The latter together with a pad R8, R9, R10, R11 and a similar band-stop filter on the other side of the common unit prevents simulation of the meter-control signal by speech currents. The grid of V1 is biased by battery B2 to a voltage less than the grid striking voltage so that in the absence of a meter-control signal V1 remains non-conducting. The anode of V1 is supplied with an alternating voltage by G1 and a direct-current voltage by B1, these voltages being such that the sum of the generator peak voltage and the battery voltage is less than the anode striking voltage and their difference is less than the anode sustaining voltage of V1. The reception of the meter-control signal on the grid of V1 causes the tube to fire when its anode-to-cathode voltage exceeds the sustaining voltage so that the anode current charges C1 and flows through the meter M. When the anode voltage drops below the sustaining value V1 ceases to conduct and C1 discharges through M, but when it again exceeds the sustaining value the tube re-fires if the meter-control signal still persists and the meter remains operated. On cessation of the meter-control signal V1 does not re-fire and after C1 has discharged the meter restores. Ringing. The ringing-control signal at frequency fR is selected by the winding of T2 tuned by C6 and applied to the grid of thyratron V2 which operates in the same manner as V1 so that a pulsating unidirectional current at the frequency of the ringing current generator G2 flows in winding ab of hybrid transformer H2 as long as the ringing-control signal persists. Hybrid H2 with C2 and C3 forms a bridge network of the kind referred to in Specification 722,173 which is balanced when the line 1 is disconnected therefrom but unbalanced when it is connected. Accordingly if the line 1 and its bell-set (not shown) are connected to hybrid H1 the ringing current will appear between points d and g of H2 and the subscriber will be rung. The voltage at point g of H2 is also applied via resistor R3 to the private wire 4 and on the reply of the called party the current flow in the loop causes this voltage to rise. Thus rectifier W1, normally biased to cut-off by B6, is rendered conducting and connects the transferred capacitance of C7 across the tuned winding of T2. This effectively short-circuits the grid circuit of V2, preventing the connection of ringing current to the subscriber's line by the action of speech currents at frequency fR and reducing the attenuation of speech currents by transformer T2.