BACKGROUND OF THE INVENTION

The present invention relates generally to sub¬ scriber interfaces of a telephone network, and more par¬ ticularly to a method and an arrangement for generating a ringing signal to a subscriber set. This method and arrangement is employed to generate a ringing signal by means of which the alarm device of a subscriber set (e.g. the AC bell of a telephone set) is sounded.

The function of a ringing signal generator is to generate the voltage needed for ringing the alarm device of a telephone. This has been specified as a 70 V,,,. (100 V peak-to-peak), 25 Hz voltage in international standards. Such a sinusoidal voltage is depicted in Fig¬ ure 1. In some telephone exchanges, a ringing voltage frequency of 50 Hz may also be used. The alarm device to be activated by the ringing voltage is typically the AC bell of a conventional telephone set or an electronic alarm device, the latter being employed in modern tele¬ phones instead of a conventional (mechanical) bell.

In prior art approaches, the ringing signal generator is incorporated in a local exchange or a subscriber multiplexer (a subscriber multiplexer is a multiplexer that has been brought as close to the sub¬ scribers as possible and connects several subscribers to a local exchange) in accordance with the principle shown in Figure 2, so that it is common to the telephone sets SD1...SDN of several subscribers. The subscriber line interface circuit SC1...SCN of each subscriber in¬ corporates a relay RL by means of which the ringing signal is switched via a subscriber line SUBL to the subscriber set when necessary. In Figure 2, a ringing signal has been switched to subscriber set SD2 from a

common generator RG, while the other subscriber sets are connected to the speech path VP.

The ringing signal generator RG may be for ex¬ ample a device of the kind shown in Figure 3, comprising a power supply 31, an oscillator 32 and an amplifier 33. The power supply provides the supply voltages needed by the oscillator and the amplifier, which in this exem¬ plary case are +12V and -12V and +55V and -55V. The power supply may be for example a flyback switched-mode power supply. The oscillator 32 generates a 25 Hz AC voltage VI, which is amplified in amplifier 33 into a ringing voltage V2 in accordance with Figure 1.

However, ringing signal generators are attended particularly by two drawbacks. First, the ringing signal generator is comparatively expensive, thus adding to the cost of the exchange or subscriber multiplexer. More¬ over, the ringing signal generator has considerable bulk and will take up much room at its installation site. The latter drawback is accentuated particularly in sub- scriber multiplexers which possibly have to be installed in small spaces.

SUMMARY OF THE INVENTION

It is an object of the present invention to eliminate the above drawbacks. This object is achieved with the method in accordance with the present inven¬ tion, which is characterized in that the ringing signal is generated by means of a known subscriber line inter- face circuit provided with a polarity reversal function, by reversing the polarity supplied by such a circuit to the subscriber line at a rate determined by the ringing signal. The arrangement in accordance with the present invention is characterized in that it comprises control means for continuous reversal of the polarity supplied

by the SLIC circuit to the subscriber line at a rate determined by the ringing signal for generating to the subscriber set a ringing signal by means of said control means. The idea of the invention is to utilize the known subscriber line interface circuit (SLIC circuit) and particularly its capacity for polarity reversal, by reversing the polarity of the subscriber line at a rate determined by the ringing signal by means of such a cir- cuit. By this means, the ringing voltage specified in the standards can be approximated with sufficient accuracy, yet without any ringing signal generator as shown in Figure 3 being needed.

Hence, on account of the solution in accordance with the invention, the inconvenient ringing signal gen¬ erator can be omitted entirely, and yet not extra com¬ ponents are needed to generate the ringing signal.

The solution in accordance with the invention is particularly suitable for subscriber multiplexers located close to the subscriber, since no strict ringing voltage distortion requirements have been set for such multiplexers.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention and its pre¬ ferred embodiments will be described in greater detail with reference to Figures 4 to 7 in the accompanying drawings, in which Figure 1 depicts the ringing voltage generated by a prior art ringing signal generator,

Figure 2 illustrates a known method of gener¬ ating a ringing signal,

Figure 3 shows a prior art ringing signal generator,

Figure 4 shows a subscriber multiplexer in which a ringing signal is generated in accordance with the invention,

Figure 5 illustrates the generation of a ring¬ ing signal in the apparatus shown in Figure 4,

Figure 6 shows a ringing signal generated in accordance with the invention, and

Figure 7 illustrates a preferred embodiment of the apparatus shown in Figure 5.

DETAILED DESCRIPTION OF THE INVENTION

Figure 4 illustrates a subscriber multiplexer 40 in which a ringing signal is generated in accordance with the invention. (More closely, the figure shows the subscriber end of a subscriber multiplexer in which the solution in accordance with the invention is employed. ) The subscriber multiplexer includes a number of ident¬ ical subscriber line interface circuits SUB, each typic- ally being common to several subscribers. Each sub¬ scriber line interface circuit is coupled to a multi¬ plexer/demultiplexer part 41, which performs multiplex¬ ing of the data transmitted towards the exchange and de¬ multiplexing of the data received from the exchange. The data stream towards the exchange and the data stream from the exchange may form for example a known basic multiplexing system (2048 kbit/s) comprising 30 speech channels in accordance with the ITU-T (formerly CCITT) recommendations (G.704). At the exchange end, the 2048 kbit/s connection shown in Figure 4 may be connected either directly to the digital subscriber interface of the exchange or via a separate adaptation unit to the analog subscriber interfaces of the exchange. In the case of an analog ex- change, the requisite adaptation unit comprises, simil-

arly as the subscriber end, a multiplexer/demultiplexer part and exchange interface units, which on the one hand simulate the functions of the exchange to the subscriber and on the other hand transmit the signalling of the analog subscriber loop to the exchange.

In each subscriber line interface circuit SUB shown in Figure 4, the interface of an individual sub¬ scriber has been implemented in accordance with Figure 5. The actual subscriber interface is implemented with an interface circuit known per se, an SLIC circuit 50 (Subscriber line interface Circuit). The SLIC circuit in practice attends to current supply and signalling of the subscriber loop. Hence, the SLIC circuit recognizes the selections made by the subscriber, lifting off hook and other similar steps, and attends to the signalling related to them. The SLIC circuit also in practice per¬ forms the 2-wire/4-wire transformation, and is coupled to the analog 4-wire interface of codec 51. The codec 51 converts the digital signal re¬ ceived from the exchange into analog form and the analog signal transmitted towards the exchange into digital form, respectively. The SLIC circuit applies the analog signal obtained from the codec to the 2-wire subscriber line SUBL together with a DC component (the above-men¬ tioned current supply). The SLIC circuit separates the speech signal of the subscriber from the signal it receives from the exchange and supplies it to the codec for the above A/D conversion. It is essential to the present invention that the SLIC circuit has a polarity reversal function, i.e., the polarity of wires a and b in the subscriber line SUBL may be reversed from normal (in the normal situ¬ ation the polarity of wire a is positive and that of wire b negative).

An SLIC circuit of the kind described above may be for example a circuit of the Am795XX or Am79M5XX series, manufacturer Advanced Micro Devices, U.S.A.

Similar circuits are available from other manufacturers as well.

In accordance with the present invention, the capability of an SLIC circuit to reverse the polarity of the subscriber wires is utilized in the generation of a ringing signal. When a ringing signal is to be gen- erated to a subscriber, the SLIC circuit is controlled to reverse the polarity of the subscriber wires at a rate determined by the ringing signal. This means that the polarity varies for example at intervals of about 20 ms (which corresponds to a 25 Hz ringing voltage fre- quency). In Figure 5, the control that performs con¬ tinuous polarity reversal at a rate determined by the ringing signal is denoted by reference 52. Such a con¬ trol of an SLIC circuit can be easily implemented with a microprocessor, for example. (For instance in the above circuits by AMD, the control is performed by means of inputs C1-C3 of an input decoder. )

Figure 6 shows a ringing signal generated in accordance with the invention. The voltage rise and fall rate (i.e., polarity reversal rate) may be set at a desired constant value by means of components (resistors and capacitors) incorporated in the SLIC circuit, so that the voltage waveform is made to resemble a sinu¬ soidal form as closely as possible. (For example in the case of the above circuit family, the setting is made with resistors R _DC1 and R _DC2 and capacitor C _DC , as is apparent from the data sheets for the above circuit family. ) In SLIC circuits, it is not normally desired to make too abrupt polarity reversals, since an abrupt change will cause interference to other telephones. This is of advantage also for the present invention, since

the voltage waveform will thus be inherently closer to sinusoidal. The voltage peak value is dependent on the SLIC circuit employed, being typically about ±30V, which in practice is a fully sufficient ringing voltage. (It is also possible to use additional amplifiers to provide voltage gain. )

It is also possible to incorporate in connec¬ tion with the SLIC circuit a separate circuit wherewith the rise rate of the ringing voltage (i.e. polarity re- versal rate) is made selectable (a few rate values may be selected). Hence, such a circuit has several alterna¬ tives for the above components determining the polarity reversal rate, one of the components being coupled to the SLIC circuit at a time. This principle is illus- trated in Figure 7, in which circuits having different component values are denoted by references A, B and C. The SLIC circuit employs one of these at a time, depend¬ ing on which of them has been selected for use by the selector means 53. (For clarity, Figure 7 shows these components determining the polarity reversal rate as components external to the SLIC circuit, whereas in Figure 5 it has been presumed that one such component combination is included in the SLIC circuit 50. )

As is apparent from the foregoing, very small changes to prior art solutions are necessary to imple¬ ment the present invention. The only change required relates to the control of the SLIC circuit, that is, polarity reversal is made continuous at a rate deter¬ mined by the ringing signal. Information on the fact that a ringing signal is to be generated is obtained in a known manner, as is information on the fact that the generation of a ringing signal is to be discontinued.

Even though the invention has been explained in the foregoing with reference to the examples in accordance with the accompanying drawings, it is obvious

that the invention is not restricted thereto, but it can be modified within the scope of the inventive idea set forth above and in the attached claims. For example, the ringing voltage and its frequency may be modified depending on where and for what kind of subscriber sets the ringing signal is to be generated.