A conference system for establishing simultaneous interchange of information among a plurality of participants (E.sub.o -E.sub.k) comprises a conference network (CN). Each participant is connected to a terminal (n.sub.o -n.sub.k) which is connected to a hybrid network (CS.sub.o -CS.sub.k). The output (SW.sub.o -SW.sub.k) from the hybrid networks (CS.sub.o -CS.sub.k) are fed to summing amplifiers (SA.sub.o -SA.sub.k) in the conference network CN. The outputs of the summing amplifiers are fed to the participants via the hybrid networks (CS.sub.o -CS.sub.k). The summing amplifiers (SA.sub.o -SA.sub.k) are arranged so that the signals contributed by the participant to which the particular summing amplifier output is connected is multiplied by a factor .beta. while the signals contributed by the other participants are multiplied by a factor .alpha.. By returning a portion of the signal generated by a participant to that participant it is possible to construct a system having a lower attenuation while maintaining stability.

A key telephone system comprised of a key service unit connected to a plurality of local sets via four-wire conductors for providing full duplex signal communication between the key service unit and sets. Signals are carried by a first pair of each of the conductors for unidirectional transmission of signals from each set to the key service unit, and the remaining pair of conductors carries signals transmitted by the key service unit for reception by one or more of the sets. Four-to-two wire signal conversion and line impedance balancing is effected by one or more trunk circuits of the key service unit in the event one or more of the local sets are connected via the key service unit for communication between the sets and the outside telephone lines. As a result of separate undirectional communication paths being provided between individual ones of the subscriber sets and the key service unit, via the four-wire conductors, the impedance balancing function of the trunk circuits is independent of the length and quality of the four-wire conductors. Each of the trunk circuits preferably incorporates an electronic telephone circuit for performing the four-to-two wire signal conversion as well as AC and DC termination and loop length gain equalization for signals carried by the outside telephone lines. The key service unit and subscriber sets of the telephone system are inexpensive, compact and highly efficient as a result of minimizing the number of four-to-two wire conversions therein.

For use in multiport digital conference arrangements wherein speech samples of selected speakers are summed for distribution to the conferees, the disclosed method and apparatus controls the level of speech represented by information samples to be included in an output sample for distribution to the ports and equalizes the speech level between speakers to reduce speech level contrast heard by the conferees. In addition, a speech detector for each port and microprocessor-controlled switching hardware also adjust the signal level represented by samples received on the ports to effect speaker selection. Furthermore, gain coefficients for a port may be incrementally adjusted during a predetermined period of time to avoid noticeble signal level changes when implementing speaker selection.