Telephone system signalling circuit - uses AC with synchronisation circuit timing control pulses to coincide with zero voltage crossover

摘要

The system uses alternating current on the conversion line of a telephone switching system. A signalling voltage generator (GEN) has its output connected to the conversation line. A shaping circuit (REU) modifies the alternating voltage provided by the generator, and produces a rectangular voltage with pulses in synchronism with the alternating current. A logic current synchronises the control signals which are produced, with the passing through zero of the reshaped alternating voltage. The signalling voltage generator comprises a Wien-Robinson bridge oscillator. The generator (GEN) may be coupled to the conversation lines (A,B) via a switchable pre-amplifier (VOV), a voltage doubler, (SPV) an isolating transformer (T1) and a transmission amplifier (SEV).