Nippon Telegraph and Telephone Public Corporation

Hitachi, Ltd.

Kazuo Hamazato

Toshio Hayashi

Junjiro Kitano

Michio Tokunaga

Tetsuo Takeshita

Akio Sagawa

Toshiyuki Tahara

Hironobu Uehara

Tatsuro Takahashi

Tetsuaki Egawa

Kanzi Tawara

A hybrid circuit composed of semiconductor current amplifiers without an inductance and large capacitance has been found. The DC speech current supplied by the DC power source (33) and the speech signal from the modem (28) are adjusted by the current amplifiers (17a, 17b) so that the subscriber line (2) is terminated with the predetermined impedance (600 ohms), and the speech signal from the subscriber line (2) is transferred to the modem (28) through the AC monitor circuits (24a, 24b). The common mode noise induced on the subscriber line (2) is cancelled by adjusting the input current of said current amplifiers (17a, 17b) by the common mode noise detector (22) and the common mode noise monitor circuits (23a, 23b). The speech signal from the modem (28) to the subscriber line (2) does not return to the modem (28) because of the cancellation operation by the return signal detector circuit (32). The present hybrid circuit is suitable to compose a semiconductor integrated circuit (IC) because no inductance is used and no large capacitor is used.

Hybrid circuit in a telephone subscriber interface circuit

A current supply circuit comprises a DC power feed circuit exhibiting a constant current characteristic for a power source and a constant resistance characteristic for a load, a first detecting circuit for detecting a load current or a load voltage, a DC-DC converter circuit inserted between the power source and the DC power feed circuit, a second detecting circuit for detecting an output voltage from the DC-DC converter circuit; and an operation circuit coupled at the input with the output of the first detecting circuit and the output of the second detecting circuit for controlling the DC-DC converter circuit by the output thereof. The DC-DC converter circuit is so controlled as to produce a voltage representative of the sum of a voltage drop across the load and a fixed voltage necessary for the operation of the DC power feed circuit.

Current supply circuit

A circuit for detecting a closed loop formed by actuation of a telephone set, the loop being indicative of a connection between the telephone set and an exchange through a ring line and a tip line. The detecting circuit comprises generators for generating first and second detection signals having amplitudes proportional to a ring line current and a tip line current respectively, a summing circuit for summing the detection signals and a comparator for comparing the outputs of the summing circuit and a reference value and delivering an output depending upon the result of comparison.

Loop detecting circuit

The telephone switching system comprises a plurality of speech path units, a juncture switch unit for interconnecting the speech path units, a central processing unit for controlling the speech path units and the junctor switch unit. Each speech path unit comprises a time switch which shifts a signal which is time-division-multiplexed from time slots of a plurality of incoming highways to other time slots on a plurality of outgoing highways, a digital trunk circuit for processing a register signal received by the time slot and other control signals, and a speech path control equipment which receives and distributes a control signal between the time division switch, the digital trunk circuit and the central processing unit. The junctor switch unit is constructed to change the connections between the speech path units by using a plurality of time slots as one group, whereby the change of connection between respective speech path units by the junctor switch and effects changes in terms of a group units of a plurality of time slots when the number of the speech path units is varied.

Time division telephone switching systems

Growing community of inventors

Tokyo, Japan

Kazuo Hamazato