Patent GB-814141-A (GB)
Title: Improvements in or relating to telephone ringing equipment
Inventor: LEMAN HUGH SPENCE
IPC: H04Q3/52
Filed: 1956-06-22
Abstract: 814,141. Automatic exchange systems. STANDARD TELEPHONES & CABLES Ltd. June 21, 1957 [June 22, 1956]. No. 19415/56. Class 40 (4). Ringing current for a subscriber consists of an audio-frequency modulated at a lower frequency the subscriber's subset comprising means to demodulate the signal to derive a current at the modulation frequency for ringing a bell. The method of ringing is particularly suited to electronic exchanges. The audio-frequency may be of comparatively low-frequency (e.g. 300 c.p.s.) at fairly high level, or at a higher frequency at speech level, accepted at the substation by a frequency selective circuit. At the exchange the signal may be transmitted from a conventional ring modulator via a Stone bridge. At the receiver the signal may be accepted by a frequency-sensitive circuit, e.g. a resistance-capacity circuit or a resonant circuit and rectified to produce the modulation signal. This is preferably amplified by a transistor to produce currentat the modulation frequency for operating a polarized bell. The transistor may be operated as a detector in which case a separate rectifier is not required. Fig. 9 illustrates the basic circuit. The induction coil L1 provides impedance matching to the line and resistance R3 and capacity C5 provides decoupling, reduces negative feed-back on the transistor and makes for a steady (as opposed to a square-wave) drain on the exchange battery. Two transistors in cascade may be used in which case a resistance capacity coupling is preferred, Fig. 11 (not shown). In further embodiments a condenser is provided across the collector-emitter circuit arranged to be resonant with the bell impedance at the modulation frequency, Figs. 12, 14 (not shown). In the arrangements so far described the bell current always has a unidirectional component which necessitates fitting a bias spring on the bell or alternatively the provision of a permanently energized polarizing winding which may simply be connected in series in the supply circuit from the line to the collector, Fig. 23 (not shown). In an alternative embodiment the bell is provided with two windings alternately energized. Thus, as shown in Fig. 17, two transistors are provided arranged to conduct 180 degrees out of phase, bell windings PR1, PR2 being provided in the respective collector circuits. In an alternative arrangement, Fig. 18 (not shown), three transistors are provided, the first acting as a phase splitter for feeding two further transistors operating in push-pull, each transistor having a bell winding in its collector circuit. Instead of a resistance such as R 3  Fig. 9, in the decoupling circuit an inductance (which may be part of the usual induction coil in the subset) or parallel tuned circuit may be used. Fig. 27 illustrates various alternatives for the decoupling circuit and also for the input to the transistor base. Fig. 24 illustrates a complete subset utilizing the transistor TR1 as an amplifier-rectifier for demodulating the incoming ringing current, operation of the gravity switch GS inserting the transmitter TX into the transistor emitter circuit for amplification of the outgoing speech signals. In party line systems different audio frequencies may be used to call different substations.