Quasi-resonant transfer conferencing circuit

Abstract

A conference port for permitting simultaneous voice communication among a plurality of local stations and an external or central office line is shown. The stations may be part of a time division PBX or may be arranged as a time-sampled key telephone system having access to the same line. The conference port includes a first operational amplifier for summing the station port voltages on a capacitor during an interval that is a predetermined fraction of the resonant transfer interval. This first, or quasi-resonant, interval is sufficient to reduce the initial voltage on each of the station port sampling capacitors to a predetermined fraction of their initial level. At the conclusion of this interval the voltage on the summing capacitor in the conference port is multiplied by an appropriate factor in a second operational amplifier and is applied back simultaneously to all of the conference ports together with the signal incoming over the external or central office line. The voltage impulse, so applied, causes the signal contributed by each station port to appear at each other station port attenuated by a suitable factor, illustratively 6dB, while the incoming signal appears full strength at each station port. Accordingly, good return loss is provided at each station port and contrast between signals from stations and the incoming line is reduced via the controllable station-to-station loss.