TELEPHONE CIRCUIT FOR SUPPLYING RINGING SIGNALS TO A SUBSCRIBER TELEPHONE LINE AND FOR DETECTING UNHOOKING

Abstract: A telephone circuit, which may be monolithically integrated, for supplying ringing signals to a subscriber telephone line and for detecting unhooking during ringing comprises a logic control circuit (C) connected to exchange components which control the supply of the ringing signals and determine their rhythm. The logic circuit (C) is connected to a circuit means (RS) for detecting direct current on the line and a current comparator (COMP) for generating a signal when the value of the line current exceeds a predetermined current value. This signal is also supplied to the exchange components via a transfer circuit (HS) to inform them that unhooking has taken place. The logic control circuit is connected to a timing signal generator (FC) which synchronizes its functions with instants in which the ringing signal has a zero amplitude, to a signal amplifier circuit (SA) which supplies the ringing signals to the line and to the transfer circuit (HS). When the direct current detecting circuit means (RS) signals a probable unhooking, the logic control circuit (C) disables the amplifier circuit (SA) from supplying ringing signals to the line and inhibits the transfer of any signals to the exchange components, checking after a predetermined time interval that unhooking has actually taken place.

Description: The present invention relates to a telephone circuit, which is capable of being monolithically integrated, for forming an interface between a subscriber's telephone line and exchange control components for supplying ringing signals to the telephone line and detecting unhooking during ringing. A subscriber's telephone set is generally connected to a telephone exchange by a two-wire line, to whose terminals both the speech circuit and the ringer of the telephone set are connected in parallel. The speech circuit is connected in series with a switch which is closed only when unhooking takes place, whereas a capacitor is connected in series with the ringer and is designed to block the direct current component from the line. A subscriber's telephone line is supplied by a direct voltage generator in series with which there is connected an alternating voltage generator which constitutes the source of ringing signals when the exchange components command the supply of these signals to the subscriber. The exchange control components also determine the ringing rhythm at which the ringing signals are sent to the subscriber's set. The ringing signals, which have a sinusoidal wave shape and a predetermined duration are supplied to the line in succession spaced by predetermined pauses with no signal. When unhooking takes place during the supply of a ringing signal, the line receives a direct current component superimposed on the alternating ringing current, which alternating current is absent both during the pauses between the ringing signals and during the time intervals between one call and the next by the subscriber. In modern telephone exchanges, unhooking is detected by circuit means of electronic type designed to detect line current variations and to detect the presence of direct current on the line due to actual unhooking, even when the alternating current of a possible ringing signal is superimposed thereon. The sinusoidal ringing signals generally have a frequency of between 16 and 66 Hz and working voltage values which are much higher (60-80 V) than those of normal telephone speech signals and are thus, if not immediately discontinued at the time of unhooking, converted by the speech circuit of the subscriber's telephone set into high intensity acoustic signals which may damage the subscriber's hearing and the set. A telephone circuit for the detection of unhooking during ringing must therefore have extremely short response times to make it possible immediately to discontinue the supply of ringing signals when unhooking takes place during ringing. In certain circumstances, however, variations in the line current due to other factors (for example interference generated by other subscribers) could be interpreted as line current variations following the supply of a direct current component due to unhooking. In order to ensure in all cases that unhooking has been reliably detected a second check may be carried out, immediately after the discontinuation of the supply of ringing signals, to ascertain whether direct current is present on the line and to confirm that unhooking by the subscriber has taken place. This solution is disclosed in the Italian Patent Application 30050 A/76 in the name of CSELT for telephone systems of conventional type, comprising ringing apparatus with circuits which cannot be monolithically integrate. However, the detection of unhooking is not in practice absolutely reliable with this method since any discontinuation of the ringing signals during the transmission of one of these signals may cause an instantaneous voltage and line current variation with the generation of harmonics likely to prevent a reliable check immediately thereafter. According to the invention, there is provided a telephone circuit as defined in the appended Claim 1. Preferred embodiments of the invention are defined in the other appended claims. It is thus possible to provide a telephone circuit, which may be monolithically integrated, for the supply of ringing signals to a subscriber's telephone line and for the detection of unhooking during ringing, designed to form an interface between the subscriber's telephone line and exchange control components, which does not generate interference on the line and enables unhooking to be detected more rapidly and reliably. The invention will be further described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a block diagram of a telephone circuit for supplying ringing signals and for detecting unhooking during ringing constituting a preferred embodiment of the invention; Figure 2 is a block diagram of a transfer circuit with a memory of the telephone circuit of Figure 1; and Figure 3 is a flow chart illustrating in symbolic form the possible evolution over time of the logic states of a logic control circuit suitable for use in the telephone circuit of Figure 1. The same reference letters are used in the drawings for corresponding components. The telephone circuit shown in Figure 1 comprises a signal amplifier circuit SA having at least one input terminal RT for connection to exchange components designed to generate an alternating signal of sinusoidal type with a frequency identical to that of the ringing signals to be sent to the line, but having a smaller amplitude. The signal amplifier circuit SA also has an enabling terminal and first and second output terminals for connection to a two-wire telephone line (not shown). The exchange components referred to herein are not shown in Figure 1. The telephone circuit further comprises a timing signal generator circuit FC, having at least one input terminal connected to the input terminal RT and at least one output terminal. The timing signal generator circuit FC generates a timing signal at each instant at which the sinusoidal signal supplied to the terminal RT passes through zero in the same direction, i.e. either only when the sinusoidal signal is increasing or only when the sinusoidal signal is decreasing. A logic control circuit C has a first terminal CS for connection to exchange components which generate control signals for the supply of ringing signals to the line. These control signals determine the beginning and end of the subscriber call signal and the ringing rhythm. A circuit means RS detects direct current on the line, even when alternating current is simultaneously present on the line, and generates a signal when direct current on the line is detected. A current comparator COMP generates a signal when the value of the line current exceeds a predetermined value. Both the circuit means RS and the current comparator COMP have at least one input terminal IL for connection to the line. The circuit means RS has an output terminal connected to a second terminal PS of the logic circuit C and the comparator COMP has an output connected both to a third terminal SG of the logic circuit C and to an input terminal of a transfer circuit HS also incorporated in the telephone circuit. The logic control circuit C has a fourth input terminal CK connected to the timing signal generator circuit FC and can change its logic state only in the presence of the timing signals. The logic circuit C has a first output terminal IS connected to the enabling terminal of the signal amplifier circuit SA and a second output terminal INH connected to an inhibiting terminal of the transfer circuit HS. The transfer HS has at least one output terminal for connection to exchange control components (not shown) designed to receive and process signals generated and stored by the transfer circuit and indicating that unhooking has actually taken place. The operation of a telephone circuit and possible practical embodiments of the various circuit blocks shown in Figure 1 will now be described. The signal amplifier circuit SA may be a normal amplifier of a type known to persons skilled in the art which is switched on and off by a control or "enabling" terminal. The switching on and off of the amplifier gives rise, from the sinusoidal signal coming from the exchange components, to the ringing signals which are spaced in time and also have a wave shape of sinusoidal type. The amplifier circuit SA is in general not coupled to the telephone line in a direct manner, but via further circuit amplification and supply means of the line. The timing signal generator circuit FC may be constructed as a circuit known in the technical literature as a "zero crossing detector". When a signal of alternating type is supplied to the input of this type of circuit, it is possible to obtain a pulse signal each time that the input signal passes through zero or reaches a predetermined threshold, or each time that it passes through zero in a particular direction. In the latter case, if a signal of sinusoidal type is supplied to the input terminal, the circuit generates output pulse signals with a frequency equal to the frequency of the sinusoidal input signal. In the telephone circuit, the pulse signals act as timing signals to synchronize all the functions of the circuit exactly with instants at which the ringing signals to be supplied to the line pass through zero. The circuit means RS for detecting the presence of direct current on the line may take the form, using known solutions, of a line current transducer and integrator circuit which integrates the current supplied by the transducer over one or more whole periods of the line voltage. A solution of this type is disclosed, for example, in the Italian Patent Application 23832 A/83 in the name of the present applicants. When the integrator detects the presence on the line of a direct current having a value greater than a predetermined value, an information signal showing that there is direct current on the line and therefore that unhooking may have taken place is generated. This information signal supplied by the circuit means RS is processed, however, by the logic control circuit C in such a way that the exchange components cannot be erroneously advised of unhooking which has not, in fact, taken place. The logic circuit C is connected for this purpose to the current comparator COMP, of a type known to those skilled in the art, which generates an information signal when the instantaneous value of the line current exceeds a predetermined value. This signal, which is taken by the logic circuit C as indicating that unhooking has taken place, is also transmitted to the exchange components via the transfer circuit HS which is controlled, via its inhibiting terminal, by the logic circuit C such that the signal generated by the current comparator COMP is transmitted only when the information that unhooking has taken place is certain. The transfer circuit HS may simply be an electronic switch which prevents the transfer of the signal generated by the comparator when a signal generated by the logic circuit C is supplied to the inhibiting terminal. For particular design reasons, it may, however, take the form of the more complex circuit comprising a memory component shown in Figure 2. In this case, the transfer circuit HS is designed, when no signal is being supplied to the inhibiting terminal, to transfer as output the signals supplied to the input terminal and is, in contrast, designed to store, at the instant at which a signal is supplied to the inhibiting terminal, the presence or absence of an input signal, generating, when a signal is supplied in this instant to the input terminal, an output signal until the signal supplied to the inhibiting terminal is discontinued. The main component of the telephone circuit is the logic control circuit C, whose operating stages are fully synchronized with the instants at which the ringing signal crosses zero by the timing signals supplied to the input terminal CK. The logic control circuit C comprises circuit means for generating at the output terminal IS a signal enabling the signal amplifier circuit SA to supply ringing signals to the line when a control signal from the exchange is supplied to the control terminal CS. The control circuit starts to generate this signal, however, only after a predetermined number of timing signals from the instant at which the control signal is supplied and continues to generate the signal, if no unhooking (whether actual or assumed) is detected, up to the instant at which the first timing signal following the cessation of the control signal is supplied to the terminal CK. This avoids uncertainties as regards the control signal from the exchange and avoids, as mentioned above, the generation of interference on the line. The circuit means included in the logic control circuit C also generate, via the output terminal INH, from the same instant as that at which the enabling signal begins to be generated via the output terminal IS, a signal inhibiting the transfer circuit HS from transmitting any signals to the exchange control components. This inhibiting signal continues to be generated, when unhooking has not been confirmed, up to the instant at which a predetermined timing signal, following the cessation of the control signal supplied to the terminal CS, is supplied to the terminal CK. This predetermined timing signal is not in general the first timing signal after the cessation of the control signal, but a subsequent signal selected such that, before transmission of an information signal advising that unhooking has taken place to the exchange components, there is time for the confirmation of unhooking by the logic control circuit C and such that this transmission takes place when the telephone network is stabilized after the supply of ringing signals to the line has been discontinued. The logic control circuit C also comprises first and second timed control circuits which make it possible to interrupt or prevent the generation of signals via the output terminals IS and INH. The first timed control circuit is designed to detect any signal of assumed unhooking supplied to the input terminal PS only at those instants at which a control signal is being supplied to the input terminal CS and a timing signal is simultaneously being supplied to the input terminal CK, with the exception of a predetermined number of such instants after each occasion at which the first timed control circuit detects the supply of a signal to the input terminal PS. When it detects the supply of a signal to the terminal PS, the first timed control circuit inhibits the generation, via the output terminal IS, of the enabling signal until a predetermined timing signal is subsequently supplied to the input terminal CK. In this way, during the call stage, as soon as the logic control circuit C receives advice from the circuit means RS of possible unhooking, it discontinues for a predetermined period, needed for the subsequent confirmation of actual unhooking, the supply of ringing signals to the line. If unhooking has not in fact taken place, the supply of ringing signals may be restarted if, obviously, a control signal is still being supplied to the input terminal CS. The operation to check whether unhooking has actually taken place is essentially carried out by the second timed control circuit, which detects via the terminal SG a possible signal generated by the current comparator COMP only when the input terminal CK is being supplied with that predetermined timing signal which also enables the generation of the enabling signal to be continued via the output terminal IS, after every interruption thereof, and when, at the same time, a control signal is being supplied to the input terminal CS. In this way the possible signal from the comparator COMP may be detected at an instant at which the network is stabilized and there is still no alternating current on the line, starting to supply ringing signals to the line only at that instant, since this possible signal is undoubtedly a confirmation of direct current on the line due to actual unhooking. When the second timed control circuit detects the supply of a confirmation signal to the terminal SG, it disables the first timed control circuit from detecting the supply of signals to the input terminal PS and inhibits the generation of signals via the two output terminals IS and INH until the first timing signal following the cessation of the supply of the control signal from the exchange components to the input terminal CS is supplied to the input terminal CK. As soon as the generation of the inhibiting signal via the terminal INH is discontinued the transfer circuit HS enables the transmission of the signal generated by the current comparator COMP to the exchange components as advice that unhooking has taken place and this interrupts the control signal being supplied to the first input terminal CS of the logic control circuit C which, as described below, returns to an initial state in which it waits for further control signals from the exchange. There is no difference between a discontinuation of the control signal for determining the ringing rhythm and a discontinuation due to the cessation of the call phase. A relatively simple circuit embodiment of the logic control circuit C is immediately suggested to persons skilled in the art on the basis of the characteristics described above in respect of the circuit means which generate the output signals and the first and second timed control circuits. These may be constructed directly in a known manner, or, obviously to the person skilled in the art, using logic gates of AND type, D type bistable multivibrators and timing pulse signal counters. However, the logic control circuit C may be constructed as a logic state machine designed to carry out a number of functions, not just those listed, comprising an assembly of constituent logic components from which it would not be possible to pick out, unequivocally in time, circuit means generating signals and timed control circuits, even though it would be possible to pick out at each instant the various logic components designed to make them up in practice. Figure 3 thus shows a flow chart illustrating the evolution over time of the logic states of a logic state machine for forming the logic control circuit C. A person skilled in the art could construct the circuit of the logic state machine directly from this flow chart using the circuit components which he has available. The illustration of Figure 3 is of a conventionally known type. Each rectangle, which represents a logic state, shows the state of the outputs of the logic control circuit C, indicating the symbol of the output terminal(s) via which a signal is being generated. Each diamond, which represents a decision function of the logic circuit, shows the symbol of the input terminal to which a signal is being supplied causing the subsequent passage, shown by an arrow, to another logic state. The letter Y indicates the passage caused by the supply of a signal to this terminal, while the letter N indicates the passage which takes place if no signal is being supplied. Passage from one logic state to the next takes place at each timing signal supplied to the input terminal CK. The rectangles shown in broken lines represent the possibility of one or more repetitions of the prior state shown by a rectangle shown in unbroken lines. The telephone circuit not only makes it possible to prevent interference on the line and to obtain at the same time reliable detection of unhooking actually taking place, but also allows its integration in more complex interface circuits whose completely separate functions may be carried out by the same logic circuit. Although a single embodiment of the invention has been described and illustrated, it is evident that many variants are possible without departing from the scope of the invention.

Claims: A telephone circuit for supplying ringing signals to a subscriber line and for detecting unhooking, comprising a timing signal generator (FC) for generating timing signals having the same frequency as the ringing signals and coinciding with zero-crossing of the ringing signals, characterised by further comprising: first means (RS) for detecting direct current in the subscriber line; second means (COMP) for detecting when subscriber line current exceeds a predetermined value; and a control circuit (C) responsive to the first means (RS), the second means (COMP), the timing signals (CK), and a ringing control signal from a telephone exchange to control the supply of ringing signals to the subscriber line and to control the supply of an unhooking detection signal to the telephone exchange. A circuit as claimed in Claim 1, characterised in that the control circuit (C) is arranged, upon detection of direct current by the first means (RS), to prevent the supply of ringing signals synchronously with the timing signals and subsequently, upon detection by the second means (COMP) of a current exceeding the predetermined value, to supply the unhooking detection signal to the telephone exchange. A circuit as claimed in Claim 1 or 2, characterised by a signal amplifier circuit (SA) having at least one input terminal (RT) for receiving from the telephone exchange an alternating signal of sinusoidal type for forming the ringing signal, an enabling terminal, and first and second output terminals for connection to the subscriber line, the timing signal generator (FC) having at least one input terminal (RT) for receiving from the telephone exchange the alternating signal of sinusoidal type and being arranged to generate the timing signals at each instant at which the alternating signal passes through zero in a predetermined direction, the control circuit (C) having a first input terminal (CS) for receiving the ringing control signal, a second input terminal (PS) connected to the first means (RS), a third input terminal (SG) connected to the second means (COMP), and a fourth input terminal (CK) connected to the timing input signal generator (FC), the logic control circuit (C) further having a first output terminal (IS) connected to the enabling terminal of the amplifier circuit (SA) and a second output terminal (INH) connected to an inhibiting terminal of a transfer circuit (HS), which transfer circuit has an input terminal connected to the output terminal of the second means (COMP) and an output terminal for connection to the telephone exchange components for supplying the unhooking detection signal. A telephone circuit as claimed in Claim 3, characterised in that the transfer circuit (HS) comprises memory arranged to transfer, when no signal is being supplied to the inhibiting terminal, to the output terminal thereof a signal supplied to the input terminal thereof and to store and hold at the output thereof, at an instant at which a signal is supplied to the inhibiting terminal thereof, the presence or absence of signals at the input terminal thereof. A telephone circuit as claimed in Claim 3 or 4, characterised in that the control circuit (C) comprises: a circuit means for generating a signal via the first output terminal (IS) during the whole time interval between the supply to the fourth input terminal (CK) of an initial predetermined timing signal following the supply of a signal to the first input terminal (CS), and the supply to the fourth input terminal (CK) of the first timing signal after the cessation of the signal supplied to the first input terminal (CS), and for generating a signal via the second output terminal (INH) during the whole time interval between the supply to the fourth input terminal (CK) of the initial predetermined timing signal following the supply of a signal to the first input terminal (CS), and the supply to the fourth input terminal (CK) of a predetermined timing signal following the cessation of the signal supplied to the first input terminal (CS); a first timed control circuit for detecting the supply of a signal to the second input terminal (PS) only at those instants at which a signal is being supplied to the first input terminal (CS) and a timing signal is simultaneously being supplied to the fourth input terminal (CK), with the exception of a predetermined number of these instants following each instant at which the supply of a signal to the second input terminal (PS) is detected, and for inhibiting, when the supply of a signal to the second input terminal (PS) is detected, the generation of a signal via the first output terminal (IS) until a subsequent predetermined final timing signal is supplied to the fourth input terminal (CK); and a second timed control circuit for detecting the supply of a signal to the third input terminal (SG) only when the fourth input terminal (CK) is being supplied with the predetermined final timing signal following each instant at which the first timed control circuit detects the supply of a signal to the second input terminal (PS) and a signal is simultaneously being supplied to the first input terminal (CS), for disabling, when the supply of a signal to the third input terminal (SG) is detected, the first timed control circuit from detecting the supply of a signal to the second input terminal (PS), and for inhibiting the generation of signals via the first (IS) and second (INH) output terminals until the fourth input terminal (CK) is supplied with the first timing signal following the cessation of the signal supplied to the first input terminal (CS). A monolithically integrated circuit containing a telephone circuit as claimed in any one of the preceding claims.

Inventors: LAVOISARD, JEAN-LOUIS, SAVIOTTI, VANNI, SILIGONI, MARCO
Applicants: SAT SOCIETE ANONYME DE TELECOMMUNICATIONS, SGS MICROELETTRONICA SPA
Application year: 1987
Main classification: H04M19
IPC codes: H04M3, H04M19