Interface circuits for connection to non-dedicated telephone lin

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179 2DP, H04M 1100

Patent

active

043784704

ABSTRACT:
Receive and transmit paths, for example for frequency shift keyed signals, are coupled to an interface circuit including a line transformer which is permanently connected to the telephone line via a high impedance. The effect of this high impedance on received signals is counteracted by an amplifier and/or a tuned circuit comprising a winding of the transformer. Switches in the interface circuit, responsive to received signals, serve to by-pass the high impedance, and reduce the gain of the amplifier and/or eliminate the effect of the tuned circuit, to permit normal coupling of signals between the telephone line and the receive and transmit paths.

REFERENCES:
patent: 3376389 (1968-04-01), Fair
patent: 3448215 (1969-06-01), Engel
patent: 3551597 (1970-12-01), Russell
patent: 3842206 (1974-10-01), Barsellotti et al.
patent: 4009342 (1977-02-01), Fahrenschon et al.
patent: 4104486 (1978-08-01), Martin et al.
patent: 4126762 (1978-11-01), Martin et al.
patent: 4321429 (1982-03-01), Takatsuki et al.
IBM Technical Disclosure Bulletin, vol. 17, No. 2, Jul. 1974, pp. 477-478, Chrisfield, R. P., Wrage, R. H. "Time Sharing Circuit for Voice and FSK Data".

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