Interface circuits for connection to non-dedicated telephone lines
First Claim
1. 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.
5 Assignments
0 Petitions
Accused Products
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.
229 Citations
14 Claims
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1. An interface circuit for connection to a non-dedicated telephone line, said circuit comprising:
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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. - View Dependent Claims (2, 3, 4, 5, 6, 7, 14)
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8. An interface circuit for coupling a receive path and a transmit path to a non-dedicated telephone line, said circuit comprising:
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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.
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9. An interface circuit for coupling a receive path and a transmit path to a non-dedicated telephone line, said circuit comprising:
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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.
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10. An interface circuit for coupling a receive path and a transmit path to a non-dedicated telephone line, said circuit comprising:
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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. - View Dependent Claims (11, 12, 13)
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Specification