Hitachi, Ltd.

Nippon Telegraph and Telephone Public Corporation

Yuichi Morikawa

Eiichi Amada

Hirotoshi Shirasu

Toshiro Suzuki

Hiroshi Kuwahara

Kazuo Saito

Hirohiko Sato

In an interface circuit which couples a two-wire line of bidirectional transmission and unidirectional four-wire receiving and transmitting lines; in order to permit the interface circuit to operate in adaptation to the fluctuation of an impedance with the two-wire line side viewed from the four-wire receiving line, the four-wire receiving line is provided with a plurality of filters and a group of switches for selecting the filters, an output of the four-wire transmitting line is compared with outputs obtained by scanning the filters, and the filter providing the minimum output difference in the comparisons is selected and connected.

Interface circuit interconnecting a bidirectional two-wire line with

For the purpose of realizing a line circuit, especially a two-wire four-wire conversion circuit of a telephone exchange in the form of LSI and providing the conversion circuit with an impedance which is suitable to a side tone characteristic of the telephone set, in a line circuit composed of a hybrid circuit wherein the four-wire input line is connected to the two-wire subscriber line via a buffer amplifier and a terminating impedance and in addition the four-wire output line is connected to the output of a subtractor which subtracts an input signal of the four-wire input line passed through a filter circuit from a signal fed from the two-wire subscriber line, the terminating impedance is formed by a resistor component and in addition a feedback circuit having such a transfer characteristic that the impedance of the line circuit seen from the two-wire subscriber line side will become a specified complex impedance is installed between a line which couples the two-wire subscriber line to the subtractor and an input of the buffer amplifier.

Subscriber line interface circuit with complex impedance

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Zushi, Japan

Yuichi Morikawa