TELEPHONE LINE INTERFACE CIRCUIT

Abstract: Telephone line interface circuit for use in switching a call connection through between subscriber telephone sets via a switching matrix. This is provided with two terminals (c, d) for connection, optionally via a cable, to a telephone set and with two terminals for connection to one side of the switching matrix. This connection can be switched on and off for speech signals. The interface circuit comprises, between the telephone set and switching matrix terminals, a first (1) and a second (2) bidirectional amplifier which are connected to each other via a common port, said cascade circuit behaving as an accurate one-to-N transformer for speech signals, and a third (3) bidirectional amplifier, one port of which is connected in parallel to the common port of the first and second (1, 2) bidirectional amplifiers and the other port of which is terminated with an impedance such that a direct-voltage decoupling is obtained between the telephone set terminals and the switching matrix terminals.

Description: The invention relates to a telephone line interface circuit for use in switching a call connection through between subscriber telephone sets via a switching matrix, which interface circuit is provided with two terminals for connection, optionally via a cable, to a telephone set and with two terminals for connection to one side of the switching matrix, a connection which can be switched on and off being achieved for speech signals. Such a telephone line interface circuit is known from practical engineering. For the purpose of the DC decoupling between the subscriber telephone set and the switching matrix, large capacitors are used in the line connection in such circuits in order to make a difference possible in voltage level between the circuits on both sides. As a result, such a circuit cannot be constructed in integrated form. The object of the invention is to eliminate this problem and to provide an interface circuit which can be integrated and which provides a transparent connection for speech signals which can be switched on and off in a simple manner. In a telephone line interface circuit of the type described in the introduction, this is achieved in an embodiment according to the invention in that said circuit comprises a cascade circuit, incorporated between the telephone set- and switching matrix terminals, of a first and a second bidirectional amplifier which are connected to each other via a common port, said cascade circuit behaving as an accurate one-to-N transformer for speech signals, and a third bidirectional amplifier, one port of which is connected in parallel to the common port of the first and second bidirectional amplifiers and the other port of which is terminated with an impedance such that a direct-voltage decoupling is obtained between the telephone set terminals and the switching matrix terminals. In another embodiment according to the invention, this is achieved in that said circuit comprises a first bidirectional amplifier incorporated between the telephone set- and switching matrix terminals, said first bidirectional amplifier forming a cascade circuit with a second bidirectional amplifier at the other side of the switching matrix, which cascade circuit behaves as an accurate one-to-one transformer for speech signals, and a third bidirectional amplifier, one port of which is connected in parallel to the switching matrix terminals of the first bidirectional amplifier and the other port is terminated with an impedance such that a directvoltage decoupling is obtained between the telephone set terminals and the switching matrix terminals. Advantageously, a part of said circuit can be used for supplying the activation voltage or for detecting loop currents as a result of which the guiescent and speech states of the telephone set can be determined. The invention will be explained in more detail on the basis of some embodiments, reference being made to the drawings in which: Figure 1 shows the circuit diagram of a bidirectional amplifier; Figure 2 shows a block diagram of a practical embodiment of a bidirectional amplifier; Figure 3 shows the setting up of a call connection between a system line and an interface circuit with the insertion of a switching matrix; Figure 4 shows a circuit diagram of an embodiment of the interface circuit according to the invention; Figure 5 shows the circuit diagram of a further embodiment of the interface circuit according to the invention; Figure 6 shows the block diagram of a system for setting up a connection between two subscriber telephone sets with the insertion of a switching matrix and two interface circuits; and Figure 7 shows a further embodiment of the system used in Figure 6 with a variant of the interface circuit according to the invention. The following applies to the circuit diagram, shown in Figure 1, of a bidirectional amplifier: U1 = R1.I2 U2 = R2.I1 Zi = U1/I1 = -(R1.R2)/ZO U2/U1 = -ZO/R1 I2/I1 = R2/ZO In the block diagram shown in Figure 2 of a practical embodiment of a bidirectional amplifier or gyrator, the blocks may be constructed, in a known manner, as balanced amplifiers which can easily be switched on and off. Figure 3 shows diagrammatically the block diagram of a small PABX system 11. The system line 12 is shown on the left and a number, in this case four, of subscriber telephone sets 13, each connected to a separate telephone line interface circuit 14, are shown on the right. A switching matrix 16, which is controlled by a microprocessor 15, is furthermore shown in the PABX. Figure 4 shows a first embodiment of the interface circuit according to the invention. The cascade circuit of a bidirectional amplifier 1 and a bidirectional amplifier 2 guarantees a completely transparent connection for speech signals in the band from 300 Hz to 3400 Hz. The impedance Z2 which can be seen on the lefthand side of the amplifier 2 is in fact equal to 30k.150/Zsm (sm = switching matrix). The impedance Z1 which can be seen on the lefthand side of the amplifier 1 is therefore equal to 30k.150/Z2 = Zsm. In the same manner, the impedance ZO which is seen on the righthand side of the amplifier 2 is equal to the impedance Zsub (subscriber = subscriber telephone set). It is clear that the cascade circuit of the bidirectional amplifiers 1 and 2 behaves as a one-to-N transformer for speech signals, it being possible for N to be greater or less than, or equal to, one. Two of these cascade circuits (optionally separated by a connection line of the switching matrix) in series behave as a one-to-one transformer for speech signals. The bidirectional amplifier 3 is added to achieve, under these circumstances, the galvanic separation with respect to the switching matrix which is obtained in the known circuit with the aid of large capacitors. A parallel circuit of a capacitor of 33 µ and a resistor of 600 k is incorporated at the output port. For low frequencies, said impedance at the output terminals of said amplifier 3 is equal to 600 k. The impedance Z3 which can be seen at the other side of the amplifier 3 is then equal to 15 k.600/600 k = 15 ohm. Between the terminals a and b there is therefore a short circuit for DC, and this implies that the DC impedance Zi and the DC impedance ZO at the input and output respectively is very high (300 k). Normally, the parallel circuit of 33 µ and 600 k at the output of the amplifier 3 is converted into a series circuit of 15 ohm and 297 H. It is obvious that these resistance and capacitance values are only examples and that other values are possible. An inductive nature is furthermore added to the impedance Zi, which can be seen between the subscriber telephone set lines, with the aid of the bidirectional amplifier 4. A parallel circuit of a 3.3 µ capacitor and a 15 k resistor is incorporated at the output terminals thereof. Said parallel circuit is converted in the other direction into a series circuit of a 300 ohm resistor and a 14.85 H selfinductance. In this case, the low-frequency impedance Z4 at the input of the amplifier 4 is equal to 30 k.150/15 k = 300 ohm. The output of the amplifier 4 is therefore short-circuited for high frequencies and the impedance Z4 is high. Various modes of operation are possible with this interface circuit. In the speech mode, a transparent speech connection for alternating current signals is achieved via the amplifiers 1 and 2. Here the amplifier 3 is used for the required direct-voltage decoupling and the amplifier 1, 4 or a further amplifier 5 (see Fig.5) can be used for impressing the line voltage or activation voltage (ringing) through a suitable summing point. In the activation mode (ringing), the amplifier 1 is switched off and the amplifier 4 provides the inductive nature, the state of the amplifiers 2 and 3 being neutral. In the standby mode, the amplifier 2 is switched off and the amplifiers 1, 3 and 4 are switched on. In this case, a correct impedance can be presented to the subscriber telephone set in a simple manner, for example, with the aid of a further bidirectional amplifier 5, as shown in Figure 5. In this interface circuit according to the invention, the magnitude of the voltage across, and/or the current through, the telephone set for detection purposes can, moreover, be measured in a simple manner by measuring the current and/or voltage at the port of the amplifier 1, 4 or 5 not connected to the telephone set. Thus, the "hook-on/hook-off" state can be detected by determining the loop current in the telephone set with the aid of the measurement of a low voltage at the other side of the amplifier 3. A separate measuring resistor such as that in the normal and known circuits is no longer necessary. Any voltage on the line can also be detected by measuring the current at one port of a bidirectional amplifier, a port of which is connected to the subscriber telephone set line, for example such as amplifier 5 as is shown in Figure 5. This gives an indication of the voltage on the line. Maximum line voltages and maximum line currents can also be monitored with the aid of said bidirectional amplifier 5 since these are now measured at a more measurable point. In Figure 6, the block diagram is given as an example of the setting up of a call connection between two subscribers via the said interface circuits according to the invention and a switching matrix. The two interface circuits 21, 22 are connected respectively to subscriber telephone set 1 and to subscriber telephone set 2. As also stated earlier, these two interface circuits 21, 22 form a one-to-one transformer for speech signals. Furthermore, each interface circuit 21, 22 can be connected via one of the four switching matrix (or PABX) lines to one of the four (arbitrarily chosen number) connection lines c1 to c4. The other lines can therefore still be used to connect other interface circuits to each other. The switches shown in Figure 6 are realized by making the amplifier 2 switchable in each interface circuit. In the embodiment shown in figure 7 of the system in Figure 6, a variant is used for the interface circuit. In the two interface circuits 23, 24 shown, the bidirectional amplifiers 2 are omitted and the amplifiers 1a, 1b and 3a, 3b are rendered switchable after they have been connected with a particular connection line. Here, too, the two interface circuits 23, 24 again form a one-to-one transformer for speech signals. Figure 7 also shows a further bidirectional amplifier 4a, 4b, one port of which is connected in parallel to the telephone set terminals, as in Figures 4 and 5. In Figure 7, one port of yet another bidirectional amplifier 5a, 5b (not shown) may be connected in parallel, as in Figure 5, to the telephone set terminals. The various modes of operation as described in relation to Figures 4 and 5 are also possible with the circuit of Figure 7. If the connection is obtained in this last "simple" embodiment via one of the four connection lines c1 to c4, two bidirectional amplifiers are incorporated in the speech connection which again form a transparent connection. In this simple embodiment, a prerequisite is, however, that the corresponding resistors in the various interface circuits formed on a chip must be as equal to each other as possible. This is simpler in the more "extended" embodiment of Figure 6 since the transfer is determined by the resistance ratios of resistors on the same chip.

Claims: Telephone line interface circuit for use in switching a call connection through between subscriber telephone sets via a switching matrix, which interface circuit is provided with two terminals (c, d) for connection, optionally via a cable, to a telephone set and with two terminals for connection to one side of the switching matrix, a connection which can be switched on and off being achieved for speech signals, characterized in that the interface circuit comprises a cascade circuit, incorporated between the telephone set- and switching matrix terminals, of first (1) and second (2) bidirectional amplifiers which are connected to each other via a common port (a, b), said cascade circuit behaving as an accurate one-to-N transformer for speech signals, and a third bidirectional amplifier (3), one port of which is connected in parallel to the common port (a, b) of the first and second bidirectional amplifiers and the other port of which is terminated with an impedance such that a direct-voltage decoupling is obtained between the telephone set terminals and the switching matrix terminals. Interface circuit for use in switching a call connection through between subscriber telephone sets via a switching matrix, which interface circuit is provided with two terminals for connection, optionally via a cable, to a telephone set and with two terminals for connection to one side of the switching matrix, a connection which can be switched on and off being achieved for speech signals, characterized in that the interface circuit comprises a first bidirectional amplifier (1a) incorporated between the telephone set- and switching matrix terminals, said first bidirectional amplifier forming a cascade circuit with a second bidirectional amplifier (1b) at the other side of the switching matrix, which cascade circuit behaves as an accurate one-to-one transformer for speech signals, and a third bidirectional amplifier (3a), one port of which is connected in parallel to the switching matrix terminals of said first bidirectional amplifier and the other port is terminated with an impedance such that a direct-voltage decoupling is obtained between the telephone set terminals and the switching matrix terminals. Interface circuit according to Claim 1 or 2, characterized in that a fourth bidirectional amplifier (4, 4a) is provided, the one port of which is connected in parallel to the telephone set terminals (c, d) and the other port of which is terminated with an impedance such that a defined impedance is connected in parallel to the telephone set terminals, and in that, furthermore, a direct voltage or direct current which provides the telephone set with the required line voltage can be supplied at a suitable summing point in said first or fourth bidirectional amplifier. Interface circuit according to Claim 2 or 3, characterized in that an alternating voltage or alternating current which provides the telephone set with the required activation voltage can be supplied at a suitable summing point in said first or fourth bidirectional amplifier. Interface circuit according to Claim 4, characterized in that a fifth bidirectional amplifier (5) is provided, the one port of which is connected in parallel to the telephone set terminals and the other port of which is terminated with a resistor such that said fifth bidirectional amplifier can be used for supplying the required activation voltage. Interface circuit according to one of the preceding Claims 2 to 5, characterized in that the magnitude of the voltage across and/or the current through the telephone set can be measured for detection purposes by measuring the current and/or the voltage at the port of said first (1, 1a) or fourth (4, 4a) or fifth (5) bidirectional amplifier not connected to the telephone set. System provided with at least two interface circuits according to one of the preceding claims, the one of which is provided at the one side of the switching matrix and the other is provided at the other side of the switching matrix.

Inventors: NORDHOLT, ERNST HUGO, VAN DER CAMMEN, PETRUS ADRIANUS MARIE
Applicants: ALCATEL TELECOM SYSTEMS B.V.
Application year: 1989
Main classification: H04M19
IPC codes: H04M19, H04M3