GB2204761A - Telephone system and terminal - Google Patents

Abstract

Some recent telephone systems, especially PBX's, use four-wire lines between the exchange and the terminals with specialised signalling. Conventionally this uses four-wire to two-wire converters at the subscriber's end, to which an ordinary telephone set is connected. In the present case no such converter is used at the terminal, with the receive pair (RX PAIR) directly connected to the earpiece (2) and signal detection means. in addition the telephone transmitter (10) is connected to the transmit pair (TX PAIR). All the circuitry, except for an external power supply, is in the same case as the telephone. A tone caller 7 is operated by control processor 6 on detection of a received 2280 Hz signal at 3, of a level above noise detected at 4. Dialed digits are sent as 2280 Hz bursts at 10 pulses per minute. <IMAGE>

Description

TELEPHONE SYSTEM AND TERMINAL This invention relates to a telephone system such as a PBX in which some at least of the telephones served are connected to the PBX via a relatively long line. The invention is, of course, also usable in the subscriber's lines served by a public telephone exchange.

It is sometimes necessary for a telephone to have to work over a long distance from an exchange such as a PBX, and this distance is limited to a few kilometres because of difficulties in supplying the necessary voltage to the telephone, and in the deterioration of transmission performance.

One solution to the above problem is to use what is known as an AC 15C private circuit. This uses a four-wire 2280 Hz single frequency signalling private network, and achieves an unlimited long line extension.

Two-wire to AC 15C and AC 15C to two-wire converters are commercially available, but the use of two such converters is an expensive solution to the problem.

An object of the present invention is to provide a solution to this problem which is cheaper than currently-used solutions.

According to the invention, there is provided an automatic telephone exchange system which includes a telephone exchange to which a number of subscribers' sets are connected, some at least of said sets being connected to the exchange over four-wire lines each of which includes a transmit pair and a receive pair, in which each said four-wire line is connected to the exchange by a four-wire tie line, and in which at the subscriber's terminal for a said four-wire line the transmit pair and the receive pair are connected to the terminal's transducers and other circuit elements without the use of four-wire to two-wire conversion means.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which Fig. 1 shows a PBX long-line extension using AC 15C to line and Fig. 2 shows how the arrangement of Fig. 1 is physically implemented.

Fig. 1 is a simplified block diagram of a telephone terminal embodying the invention, and which is connected to the exchange via a four-wire line having a receive pair RX PAIR and a transmit pair TX PAIR.

The receive pair is connected to a line matching circuit 1 to match the receive circuitry to the line's parameters, and its output is connected via an amplifier to the telephone receiver 2. The circuit 1 is also connected to a signal detector 3 and a noise detector 4 and there are connected to a comparator 5 whose output goes to a control and signalling processor 6. The incoming signalling is thus detected, since it consists of a tone at a defined frequency, 2280Hz while the noise detector also feeds the comparator. Thus the signalling is protected from noise since it has to exceed in value the noise.

The processor responds to a sequence of incoming tones, which in the simple case enables the tone caller 7 to drive a ringing signal to the transducer 8.

Where necessary, during the idle state, the detector 4 is disabled by the connection shown, under control of the processor 6 and is enabled as soon as the line is seized.

When the local subscriber initiates a call he does so by off-hooking the handset, which includes transmitter 10 and receiver 2. When he hears dial tone sent back to the exchange in response to a tone off, he emits the wanted number using a keypad 11. As a result the processor 6 outputs the necessary digital signals to control the trnsmission circuit via the SEND TONE input, which transmits signals of frequency 2280Hz from oscillator 12. The required digits are transmitted by switching the 2280Hz tone on and off at a rate of 10pups.

Signalling is sent using a HIGH TONE white LOW TONE represents the idle state.

The tones to be sent pass via an amplifier 14 and a line matching circuit 15 to the TX PAIR.

When the caller speaks his speech passes as can be seen to the TX PAIR. As usual a sidetone connection 16 is provided to ensure that sidetone is not a nuisance.

The system is driven by an external power supply via a DC-DC converter.

All of the elements shown in Fig. 1, except for the power supply are in the same case, i.e. it does not need separate circuit units from the telephone proper.

The combination of the units, i.e. conversion between line and telephone and the telephone itself being in one unit has several advantages: (a) there is no need for four-wire to two-wire conversion at the subscriber's end.

(b) a simple low-cost tone caller can be used.

(c) a low-cost standard external power supply, British Standards approval can be used.

The general arrangement can be seen in Fig. 2, where we see an AC 15C private circuit outgoing from a PBX of the SDX40 type. This is described in our Patent No. 2128446B (G.A. Sarson et al 5-4-1). The four-wire private circuit is connected to a terminating box at the subscriber's premises from which the two pairs go to the telephone instrument. This is shown with a separate power supply for connection to the mains.

Claims (4)

1. An automatic telephone exchange system which includes a telephone exchange to which a number of subscribers' sets are connected, some at least of said sets being connected to the exchange over four-wire lines each of which includes a transmit pair and a receive pair, in which each said four-wire line is connected to the exchange by a four-wire tie line, and in which at the subscriber's terminal for a said four-wire line the transmit pair and the receive pair are connected to the terminal's transducers and other circuit elements without the use of four-wire to two-wire conversion means.

2. A telephone subscriber's terminal, for connection to a four-wire line to a telephone exchange, said four-wire line including a transmit pair and a receive pair, which terminal includes (a) an input circuit connected to the receive pair of the four-wire line via which incoming signalling information and speech signals are received, the speech signals being applied to an electro-acoustic receiving transducer; (b) a signal detector connected to said input circuit which detects incoming signalling information and applies it to a control processor for the terminal; (c) an output circuit connected to the transmit pair of the four-wire line via which outgoing signalling information and speech signals are sent to the remote exchange, the speech signals being acquired from an acousto-electric transmitting transducer;; (d) signal sending circuitry from which outgoing signalling information including wanted number identifying information is transmitted to the exchange via the output circuit and under the control of the control processor; and (e) a local power supply for the terminal, wherein the input and output circuits which perform conversion between the four-wire line and the circuit elements of the terminal are installed in the same casing as is the telephone circuitry, so that the terminal consists of a single assembly, and wherein said conversion is effected without using a four-wire to two-wire converter.

3. A terminal as claimed in claim 2, and which includes a processor which controls the operations of a tone caller in response to incoming calling signals and the transmission of wanted number identifying signals from the terminal.

4. A telephone subscriber's terminal, substantially as described with reference to the accompanying drawings.