Interface circuits for connection to non-dedicated telephone lines

Claim 2

An interface circuit for connection to a non-dedicated telephone line, said circuit comprising: switching means having a first state and a second state; means for presenting a high impedance to the telephone line in the first state of the switching means, whereby normal use of the telephone line is undisturbed; means for coupling signals from a transmit path to the telephone line and from the telephone line to a receive path in the second state of the switching means; and means for counteracting the effect of said high impedance on signals received via the telephone line in the first state of the switching means, whereby said signals are coupled to the receive path.

Claim 3

An interface circuit as claimed in claim 1 wherein the means for coupling signals comprises a transformer and the means for presenting a high impedance comprises a high impedance via which a winding of the transformer is coupled to the telephone line in the first state of the switching means, the high impedance being by-passed in the second state of the switching means.

Claim 4

An interface circuit as claimed in claim 1 wherein the means for counteracting the effect of said high impedance comprises a tuned circuit for counteracting the effect of said high impedance on signals received via the telephone line in the first state of the switching means and having frequencies at about the resonance frequency of the tuned circuit.

Claim 5

An interface circuit as claimed in claim 2 wherein the means for counteracting the effect of said high impedance comprises a tuned circuit, comprising a capacitor and a winding of the transformer, for counteracting the effect of said high impedance on signals received via the telephone line in the first state of the switching means and having frequencies at about the resonance frequency of the tuned circuit.

Claim 6

An interface circuit as claimed in claim 1, 3, or 4 wherein the means for coupling signals from the telephone line to the receive path comprises an amplifier, and the means for counteracting the effect of said high impedance comprises means for increasing the gain of the amplifier in the first state of the switching means.

Claim 7

An interface circuit as claimed in claim 2 wherein the means for coupling signals from the telephone line to the receive path comprises an amplifier and the means for counteracting the effect of said high impedance comprises means for increasing the gain of the amplifier in the first state of the switching means.

Claim 8

Apparatus for receiving and transmitting signals via a non-dedicated telephone line, said apparatus comprising an interface circuit as claimed in claim 1, 4, or 6 and means responsive to signals supplied via the telephone line to the receive path for controlling the switching means.

Claim 9

An interface circuit for coupling a receive path and a transmit path to a non-dedicated telephone line, said circuit comprising: a transformer having first and second windings; first impedance means for coupling the first winding of the transformer to the telephone line with a sufficiently high impedance that normal use of the telephone line is undisturbed; second impedance means coupled between the transmit path and the second winding of the transformer; amplifier means coupled between the second winding of the transformer and the receive path; and switching means selectively operable to decrease the impedances of the first and second impedance means and to decrease the gain of the amplifier means.

Claim 10

An interface circuit for coupling a receive path and a transmit path to a non-dedicated telephone line, said circuit comprising: a transformer having first and second windings; first impedance means for coupling the first winding of the transformer to the telephone line with a sufficiently high impedance that normal use of the telephone line is undisturbed; second impedance means coupled between the transmit path and the second winding of the transformer; amplifier means coupled between the second winding of the transformer and the receive path; and switching means selectively operable to decrease the impedances of the first and second impedance means and to decrease the gain of the amplifier means.

Claim 11

An interface circuit for coupling a receive path and a transmit path to a non-dedicated telephone line, said circuit comprising: a transformer having first and second windings; first impedance means for coupling the first winding of the transformer to the telephone line with a sufficiently high impedance that normal use of the telephone line is undisturbed; second impedance means coupled between the transmit path and the second winding of the transformer; differential amplifier means having differential inputs coupled to the second winding of the transformer and to the transmit path and having an output coupled to the receive path; and switching means selectively operable to decrease the impedances of the first and second impedance means and to decrease the gain of the differential amplifier means.

Claim 12

An interface circuit for coupling a receive path and a transmit path to a non-dedicated telephone line, said circuit comprising: a transformer having first and second windings; first impedance means for coupling the first winding of the transformer to the telephone line with a sufficiently high impedance that normal use of the telephone line is undisturbed; second impedance means coupled between the transmit path and the second winding of the transformer; differential amplifier means having differential inputs coupled to the second winding of the transformer and to the transmit path and having an output coupled to the receive path; and switching means selectively operable to decrease the impedances of the first and second impedance means and to decrease the gain of the differential amplifier means.

Claim 13

An interface circuit for coupling a receive path and a transmit path to a non-dedicated telephone line, said circuit comprising: a transformer having first and second windings; first impedance means for coupling the first winding of the transformer to the telephone line with a sufficiently high impedance that normal use of the telephone line is undisturbed; second impedance means coupled between the transmit path and the second winding of the transformer; coupling means coupling the second winding of the transformer to the receive path; a capacitor connected to the second winding of the transformer and forming a tuned circuit therewith, whereby signals having frequencies at about the resonance frequency of the tuned circuit and coupled to the first winding of the transformer via the first impedance means are coupled via the coupling means to the receive path; and switching means selectively operable to decrease the impedances of the first and second impedance means.

Claim 14

An interface circuit for coupling a receive path and a transmit path to a non-dedicated telephone line, said circuit comprising: a transformer having first and second windings; first impedance means for coupling the first winding of the transformer to the telephone line with a sufficiently high impedance that normal use of the telephone line is undisturbed; second impedance means coupled between the transmit path and the second winding of the transformer; coupling means coupling the second winding of the transformer to the receive path; a capacitor connected to the second winding of the transformer and forming a tuned circuit therewith, whereby signals having frequencies at about the resonance frequency of the tuned circuit and coupled to the first winding of the transformer via the first impedance means are coupled via the coupling means to the receive path; and switching means selectively operable to decrease the impedances of the first and second impedance means.

Claim 15

An interface circuit as claimed in claim 10 wherein the coupling means comprises a differential amplifier having a non-inverting input coupled to the second winding of the transformer, an inverting input coupled to the transmit path, and an output coupled to the receive path.

Claim 16

An interface circuit as claimed in claim 8, 9, or 10 wherein said switching means is selectively operable to decrease the impedance of the first impedance means to substantially zero and to decrease the impedance of the second impedance means to substantially match the characteristic impedance of the telephone line.

Claim 17

Apparatus for receiving and transmitting signals via a non-dedicated telephone line, said apparatus comprising an interface circuit as claimed in claim 8, 9, or 10 and means responsive to signals supplied via the telephone line to the receive path for controlling the switching means.

Claim 18

Apparatus for connection to a non-dedicated telephone line, said apparatus comprising an interface circuit as claimed in claim 1, 3, or 4 and signalling means connected to the receive and transmit paths of the interface circuit, the signalling means being responsive to a predetermined combination of signals supplied to the receive path via the telephone line and the interface circuit with the switching means of the interface circuit in said first state to cause the switching means to adopt said second state.