Abstract: A conference system is disclosed which is capable of freely setting the sound volume level by making the sound volume levels of the input and output of the conference circuit variable for each participant, by pluralities of input and output ROM converters so that the levels of voices of participants of large and small line losses are made as close to each other as possible and that the participant of a large line loss is supplied with a voice of low attenuation.

Abstract: A conference system, in which a buffer memory is connected to the input-output of each highway for storing all its channel data for each frame. The stored data is time-divided so that a time-length obtained by dividing one frame length by an appropriate number (for example, eight) may be the operation time length of each trunk, thereby implementing real-time processing a number of trunks. Conference circuits capable of inhibiting an output for each channel are connected in parallel to the highways. During an ordinary two-party communication operation an output on the corresponding channel of the conference circuit is inhibited, and during a conference operation an output on the corresponding channel of the highway switching circuit is inhibited, by which channels allocated to telephone sets or CO lines are used for the conference without the necessity of providing channels specifically therefor.

Abstract: A synchronization system for a key telephone system, in which information signals are transmitted in a time-division manner between a key service unit and each key telephone set. The information signal is formed by a control signal, a first signal transmitted from the key service unit to the key telephone set and a second signal from the key telephone set to the key service unit. The control signal is formed by a start signal of one-pulse and a variable synchronizing code, which is placed in a time slot following the start signal and which is formed by a plurality of pulses assuming one of predetermined consecutive values in a predetermined order for each transmission frame period. The control signal is transmitted from the key service unit to each key telephone set for each frame period in synchronism with a first counter provided at the key service unit for developing standard of transmission timings.

Abstract: A power source circuit having a reference voltage generating circuit for generating a DC reference voltage a current output circuit provides across output lines a current proportional to an input voltage, and an error voltage integration circuit is provided for integrating a voltage proportional to a difference between the DC reference voltage and the voltage across the output lines at a required integration time constant to apply the integrated output as the input voltage to a current output circuit. The integration time constant is set so that an output terminal connected to the output lines may have a low output impedance in terms of direct current to maintain a predetermined constant DC output voltage and have a high output impedance with respect to an AC signal applied to the output lines.