Technical Field

5 This invention relates to a method for trans¬ mitting telecommunication signals over a two-wire line to an analog interface of a telephone exchange, where¬ in a direct-current signal, i.e. loop current, is fed by means of a current generator to the telephone ex¬ it) change over said two-wire line. The invention also relates to an interface circuit for transmitting tele¬ communication signals over a two-wire line to an analog interface of a telephone exchange, the inter¬ face circuit comprising a current generator for 15 producing a direct-current signal, i.e. loop current, and feeding it to the telephone exchange over the two- wire line.

The invention concerns particularly connecting a telephone exchange having a conventional analog inter- 20 face to a digital transmission link. This requires an interface circuit simulating the functions of a tele¬ phone set adapted to be connected to the analog inter¬ face of the telephone exchange.

As used in this connection, the OFF-hook state 25 refers to a state that is created e.g. when the sub¬ scriber picks up the handset of the telephone set. Correspondingly, the ON-hook state refers e.g. to a state prevailing when the handset has not been picked up. Among other things, the interface circuit has to 30 transmit and simultaneously receive electric voice- frequency signals; connect a DC loop current in the OFF-hook state, be currentless or connect a very low current in the ON-hook state; form dialling pulses on one hand by chopping and on the other hand by short- 35 circuiting the loop current; have a high impedance in

the ON-hook state; separate components connected to the telephone line and floating relative to the local earth and components connected to the earth plane of the interface circuit at a high impedance while resisting the high voltage of the line.

Background Art

The above-mentioned functions have been realized in a known solution by means of circuits separate from each other, which comprise a great number of electro¬ mechanical components, such as relays and trans¬ formers. This solution comprises a dedicated coupling for almost each function of the interface circuit, and the coupling or circuit to be used at a given time can be selected by means of the relays.

Another known solution utilizes a FET transistor establishing with an RC loop a connection called a gyrator. In this known solution, the function of the gyrator is to simulate a series connection of an inductance and a resistance, whereby a DC loop is obtained by using active components. Further in this known solution, the loop is switched on and off by means of a separate switch connected in series. The series resistance of the gyrator, however, is not controlled dynamically in this solution, wherefore a separate switch is needed to form the dialling pulses at a low ohm level.

The biggest drawback of the above-mentioned known solutions is that the components they comprise are complicated, which makes the required coupling large and expensive, particularly in cases where electromechanical components are used.

Disclosure of the Invention

The object of the present invention is to eliminate the above-mentioned problems and offer a solution which is simpler than the known solutions and in which the required components are cheaper than in the known solutions. These objects are achieved by means of a method according to the invention, which is characterized in that the loop current to be fed to the telephone exchange is controlled by adjusting the current generator in response to the telecommunication signals so as to simulate the signals of a telephone set adapted to be connected to the analog interface of the telephone exchange. The invention also relates to an interface circuit, by means of which the method according to the invention can be applied. The interface circuit according to the invention is characterized in that the interface circuit comprises adjusting means for adjusting the current generator in response to the telecommunication signals so as to simulate the signals of a telephone set adapted to be connected to the analog interface of the telephone exchange.

The invention is based on the idea that the interface circuit is significantly smaller in outer dimensions than known interface circuits and simpler and cheaper in configuration when it is configured in such a way that the telecommunication signals to be forwarded to the telephone exchange are applied in an isolated manner by means of the adjustable current generator to the telephone exchange having an analog interface over a two-wire line, i.e. by connecting the isolated signal as a modulation of the loop current. In this way extra components required for several parallel couplings are avoided, which also reduces the

space requirement of the components, as all required signals can be forwarded by means of a single common coupling. The most important advantages of the method and interface circuit according to the invention are thus that all of its functions can be realized by a single common coupling, whereby the interface circuit according to the invention is simpler and smaller than known interface circuits, in addition to which the price of the interface circuit according to the invention is lower than that of known interface circuits.

In a preferred embodiment of the interface cir¬ cuit according to the invention, the current generator is adjusted by means of a dual opto-coupler in response to telecommunication signals. This embodiment of the invention is particularly advantageous as it allows the required isolation to be achieved in a simple way between a signal applied by the pre¬ amplifier and the current generator. Preferred embodiments of the method and the interface circuit according to the invention appear from the attached dependent claims 2 to 3 and 5 to 9.

Description of the Drawings

The invention will be described below more fully by means of its first preferred embodiment with refer¬ ence to the attached figure, which is a block diagram illustrating a preferred embodiment of the interface circuit according to the invention.

The interface circuit 1 shown in the figure transmits signals between a telephone exchange 2 hav¬ ing an analog interface and a PMC connection. A so- called PCM multiplexer, of which the interface circuit according to the invention forms a part, comprises

several (e.g. 30) corresponding analog two-wire line connections, of which one will be described by way of example here.

In the case shown in the figure, a control unit 3 acts as an interface unit for the PCM connection having a data transmission rate of 2 Mbit/s. The control unit 3 comprises e.g. a PCM codec having AD and DA converters. Signals are transmitted between the telephone exchange 2 and the interface circuit 1 over a two-wire line a-b. A diode bridge (not shown in the figure) is preferably provided at the junction point between the lines so as to allow the change of polar¬ ity, and possibly a ringing detector.

When e.g. normal speech is transmitted via the interface circuit 1 shown in the figure, the control unit 3 applies the speech signals forwarded over the PCM connection towards the exchange to a preamplifier 4 through an output 5. A voice-frequency signal obtained from the output of the preamplifier 4 is applied to a dual opto-coupler 6. The dual opto- coupler 6 allows the signal produced by the pre¬ amplifier 4 to be transmitted in an isolated manner, i.e. by means of light, to a current-controlled current generator 7. The signal obtained from the output 8 of the dual opto-coupler 6 is used in the control of the current generator 7, i.e. for modulat¬ ing a voice-frequency signal transmitted over the PCM connection to the loop formed by the two-wire line a- b. Variation in a loop current, i.e. a current supplied from the exchange to the two-wire line a-b, causes the desired signal to appear in the reception circuit of the telephone exchange 2 at the end of the two-wire line a-b.

The voice-frequency signals applied by the tele- phone exchange to the two-wire line a-b are amplified

by a differential amplifier 9. The differential amplifier 9 known per se in type converts the voice- frequency signal from the differential voltage between the a and b lines to a voltage against the reference earth of the connection. As the amplifier is DC-con¬ nected, the DC value of the voltage between the a and b lines is also obtained from the amplifier output 10. The signal obtained from the output 10 of the differential amplifier 9 is applied to the control unit 3, preferably through a bandstop filter 11 filter¬ ing an advice-of-charge frequency, and to a loop filter, preferably implemented as an active RC integrator 12. The time constant of the integrator 12 is so long that the voice-frequency signal components will be filtered off the signal obtained from its output 13 almost completely. The signal obtained from the output 13 of the integrator 12 is fed as a refer¬ ence signal to the preamplifier 4 in order to maintain the voltage between the lines a and b at a desired level, that is, constant over a long time period. The magnitude of this constant voltage is selected so that the entire connection has a high resistance to useful signals and noise signals even at low loop current values. The OFF-hook state (as well as the ON-hook state) is controlled through the output 15 of the control unit 3. In the OFF-hook state, signals fed from the output 15 are forwarded to the RC integrator 12 and the preamplifier 4. A detector 16 detects transition to the OFF-hook state as an increase in the loop current. After a predetermined limit value has been achieved, an impedance Z is thus connected to the line by means of a transistor switch 14 so that the impedance appearing to the telephone exchange 2 will be low, e.g. 600 ohms. Upon transition to the ON-hook

state, the control unit 3 switches off the loop cur¬ rent, whereby the transistor switch 14 disconnects the impedance Z from the line when the detector 16 detects a decrease in the loop current. As a consequence, the impedance appearing to the telephone exchange in the ON-hook state will be high.

In the ON-hook state a bias voltage is generated for the dual opto-coupler 6 of the interface circuit and the current generator 7 from the same point by means of a connection vb. This bias voltage is gen¬ erated from the loop current.

Dialling pulses are applied to the telephone exchange 2 by chopping and short-circuiting the loop current by means of the current generator 7. This is done by applying the dialling pulses transmitted over the PCM connection via the output 15 of the control unit to the preamplifier 4 and the RC integrator 12. In practice, the dialling pulses are applied in such a way that the output 15 of the control unit 3 altern- ates between the OFF-hook state, where current is fed through the output 15 to the preamplifier and the RC integrator, and the ON-hook state, where no current is fed through the output 15 to the preamplifier and the RC integrator. During the ON-hook state created on forwarding dialling pulses, the control signal obtained from the output 15 affects the current generator 7 through the preamplifier 4 and the dual opto-coupler 6, thus switching off the loop current. In addition, the control signal is connected to the RC integrator 12, which is forced into a state corresponding to a high loop current. Upon transition from the ON-hook state to the OFF-hook state when forwarding dialling pulses, a very strong impulse is transmitted through the preamplifier 4 and the dual opto-coupler 6 to the

current generator 7, which causes a marked instan¬ taneous increase in the loop current. The time constant of the loop is such that a normal loop cur¬ rent is achieved slightly slower than the time re- quired for a dialling impulse.

It is to be understood that the above de¬ scription and the drawing relating to it are only intended to illustrate a preferred embodiment of the interface circuit according to the invention without limiting the invention to it. Accordingly, the method and interface circuit according to the invention can, of course, be utilized in other connections than with telecommunication signals transmitted over a PCM connection. The preferred embodiments of the method and interface circuit according to the invention may thus vary within the scope of the attached claims.