CA2032651C - Telephone subscriber line voltage change detection - Google Patents
Telephone subscriber line voltage change detectionInfo
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- CA2032651C CA2032651C CA 2032651 CA2032651A CA2032651C CA 2032651 C CA2032651 C CA 2032651C CA 2032651 CA2032651 CA 2032651 CA 2032651 A CA2032651 A CA 2032651A CA 2032651 C CA2032651 C CA 2032651C
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Abstract
TELEPHONE SUBSCRIBER LINE VOLTAGE CHANGE DETECTION
Abstract of the Disclosure Changes in a telephone subscriber line voltage are detected in a telephone set by supplying a voltage dependent upon the subscriber line voltage to a potential divider having first, second, and third tapping points, the second tapping point being between the first and third tapping points, and comparing the voltages at the first and third tapping points with a smoothed version of the voltage at the second tapping point to produce pulses at respective terminals when the line voltage is falling or rising. An algorithm is described for responding to the pulses to distinguish changes in hook state of another telephone connected to the same subscriber line from interfering signals. The arrangement facilitates remote release from hold and privacy indication functions for the telephone set.
Abstract of the Disclosure Changes in a telephone subscriber line voltage are detected in a telephone set by supplying a voltage dependent upon the subscriber line voltage to a potential divider having first, second, and third tapping points, the second tapping point being between the first and third tapping points, and comparing the voltages at the first and third tapping points with a smoothed version of the voltage at the second tapping point to produce pulses at respective terminals when the line voltage is falling or rising. An algorithm is described for responding to the pulses to distinguish changes in hook state of another telephone connected to the same subscriber line from interfering signals. The arrangement facilitates remote release from hold and privacy indication functions for the telephone set.
Description
~3~X~
TELEPHONE SUBSCRIBER LlNE VOLTAGE CHANGE DETECTION
This invention relates to the detection of voltage changes on a telephone subscriber line, and is particularly concerned with the detection of such voltage changes, at a first telephone set especi~lly when it is off-hook, due to changes in the on-hook or off-hook 5 state of another telephone set conn~cted to the sarne subscriber line.
Back~round of the ]nvention It is wcll known to connect two or more te1ephone sets, for example in differentrooms of a residence, to the same two-wire telephone subscriber 1ine, thus enabling two or more people in the lesidence to participate simultaneously in a telephone conversation 10 via the telephone subscriber line. A disadvantage of such an arrangement is that the privacy of a person using a first telephone set may be lost due to another person taking a second telephone set, connected to the same telephone subscriber line, off-hook (lifting the handset). While in some circumsta~ces a click may be audible to the person using the first telephone set when the second telephone is taken of ~-hook, this does not provide a 15 convenient or reliable indication that priYacy has been interrupted. Accordingly, it is desirable to provide a more reliable indication at one telephone set of the hook state of another telephone set connected to the same line.
Such an indication is also useful even when the first telephone set is on-hook, as it can then serve to indicate to a prospective user of this telephone set whether or not the 20 telephone subscriber line is already in use by another person using one of the other telephone sets connected to this line.
Furthermore, it is desirable for a user of the first telephone set to be able to place this in a hold state during a telephone call, to go to a second telephone set connected to the same line to continue the call, and to have the first telephone set automatically release itself 25 from the hold state when the second telephone set is taken off-hook. This is referred to as remote release from hold.
For each of these situations, a change in the voltage between the two wires of the telephone subscriber line can conceivably be detected and used to take an appropriate action, for e~mple to prwide an indication at the first telephone set or to effect a release 30 of this telephone set from the hold state. However, it has been found to be very difficult to monitor the subscriber line voltage in an effective manner to accomplish this, because of the variability of this voltage and the presence of interfering signals on the line.
More particularly, the actual subscriber line voltage is dependent upon a number of variab1e factors, such as the resistance of the telephone subscriber line and the number and 35 nature of the telepholle sets connected to the line. Interfering signals on the line can occur due to hook switch flashing or pulse dialling activities at other telephone sets connected to the line, and due to short-duration open switch intervals at the central o~fice to which the subscriber line is connected. Open switch intervals comprise temporary interruptions in ~(~3~
voltage supplied from the central of fice ~o thc subscriber line during switchin~ activity at the central of fice, and known methods of remote release from hold are susceptible to incorrectly releasing from hold in response to open switch intervals.
An object of this invention, therefore, is to provide an improved method of and S apparatus for detecting voltage changes on a telephone subscriber line.
Summarv of the Invention According to one aspect of this invention there is provided apparatus for detecting changes in a subscriber line voltage between two wires of a telephone subscriber line, comprising: a potential divider having first, serond, and third tapping points, the second 10 tapping point b ing between the first and third tapping points; means for supplying to the potential divider a voltage dependent upon the subscriber line voltage; a low pass filter circuit having an input connected to the second tapping point; a first voltage comparator, having inputs connected to the first tapping point and an output of the low pass filter circuit, for producing an output pulse in response to the subscriber line voltage falling;
15 and a second vo1tage comparator, having inputs connected to the third tapping point and the output of the low pass filter circuit, for producing an output pulse in response to the subscriber line voltage rising.
The means for supplying a voltage dependent upon the subscriber line voltage preferably comprises means for attenuating audlo frequency signals, so that audio 20 frequency signals vvhich occur Oll the telephone subscriber line in use do not interfere with the voltage comparisons.
It is convenient for the potential divider to comprise a first resistance between the first and second tapping points and a second resistance between the second and Ihird tapping points, the first resistance being less than the second resistance so that a greater 25 sensitivity is provided for falling than for rising subscriber 1ine voltages.According to another aspect this invention provides apparatus for detecting changes in a subscriber line voltage between two wires of a telephone su~scriber line, comprising: means for a~tenuating audio frequency signals; a buf~er coupled via the means for attenuating audio frequency signals to the subscriber line, the buffer having an 30 output at which it prcduces, relative to a common rail, a voltage dependent upon the subscriber 1ine voltage; a potential divider having first, second, and third tapping points, the potential divider comprising a first resistor between the output of the buffer and the first tapping point, a second resistor between the first and second ~apping points, a third resistor between the second and third tapping points, and a four~h resistor between the 35 third tapping point and the common rail; a smoothing circuit having an input connected to the second tapping point; a first voltage comparator, having inputs connected to the first tapping point and an output of the smoothing circuit, îor producing an output pulse in response to the subscriber line voltage falling; and a second voltage comparator, having ~ ~ 3 ~ ~J ~ ~
inputs connected to the third tapping point and the output of the smoo~ing circuit, for producing an output pulse in response to the subscriber line voltage rising.
Conveniently each of the m~aDs for attenuating audio frequency signals and the smoothing circuit comprises a resistance and a capacitance whose product defines a 5 respective time constant, and the time constant of the smoothing circuit is of the order of ten times the time constant of the means for attenuating audio frequency signals.
In accordance with a further aspect of this invention there is provided a method of detecting, at a first te1ephone set connected to a two-wire telephone subscriber line, a change from an on-hook state to an off-hook state of a second telephone set connected to 10 the same subscriber line, comprising the steps of: monitoring a subscriber line voltage between the two wires of the subscriber line; producing a pulse at a first circuit tenninal in response to the subscriber line voltage falling; producing a pulse at a second circuit terminal in response to the subscriber line voltage rising; in response to a pulse at the fisst termina1 having at least a predetermined duration, starting a window timer period;
15 detecting any pulse at the second terminal during the window timer period and, in the absence of such a pulse and in the absence of an action at the first telephone set causing the subscriber line voltage to fall, determining that the second telephone set is in an off-hook state.
According to another aspect this invention provides a method of detecting, at a 20 first telephone set connected to a two-wire te1ephone subscriber line~ hook state changes of a second telephone set cormected to the same subscriber line, comprising the steps of:
(i) monitoring a subscriber line voltage between the two wires of the subscriber line;
(ii~ producing a pulse at a first terminal in response to t~e subscriber line voltage falling;
25 (iii) producing a pulse at a second termina1 in response to the subscriber line voltage rising;
(iv) detecting a first pulse at either of the fi~st and the second terminals and noting at which tenninal such a first pulse is produced;
(v3 determining whether the pulse has at least a predeterrnined dumtion, if so 30 proceeding to step (ix) and i3 not p~oceeding to step (vi);
(vi) detecting any pulse at either tenninal starting within a predetermined cancellation period, in the absence of any pulse returning to step (iv) and in response to such a pulse proceeding to step (vii);
(vii) detennining whether the most recent pulse has at least a predetelmined duration, if 35 so proceeding to step (viii) and if not returning to step (vi);
(viii) determining whether the mos~ recent pulse is at the same terminal as that noted in step (iv) at which the first pulse was produced, if so proceeding to step (ix) and if not returning to step (iv);
~03~5:1 (ix) detecting any pulse, at the other terminal than that noted in step (iv) at which the fist pulse was produced, starting within a predeter nined window period, in the absence of any pulse proceeding to step (x) and in the presence of such a pulse returning to step (iv);
S (x) in the absence of any action at the first telephone set to cause the first pulse detected in step (iv), determining that the second tdephone set is off-hook if the first pulse was produced at the fi~t termina1, and deter nining that the second telephone set is on-hook if the first pu1se was at the second ter ninal; and (xi) returning to step (iv).
Although none of the times is partic!arly critical, the predetermined duration in each of steps (v) and (vii) is preferably at least about 10 ms, and desirably is at least about 60 ms. In the embodiments of the invention described below this period is 106 ms, but it can conceivablybe anywhere in a range from about 10 ms to about 400 ms. The predetermined cancellation period in step (vi) i8 conveniently of the order of 1.2 seconds, 15 and the predetermined window period in step (ix) is conveniently of the order of 600 ms, but again these periods are not critical and may be varied to suit particular circumstances.
Preferably steps (i) to (iii) of the al~ove method comprise the steps of: deriving from the subscriber line voltage first, second, and third comparison voltages, the second comparison voltage being produced by smoothing a voltage between the first and third 20 comparison voltages; and comparing the first and third comparison voltages with tlle second comparison voltage to produce the pulses at the first and second terminals respectively.
The second comparison voltage is conveniently produced by integrating a voltage between the first and third comparison voltages with an integration time constant of the 25 order of 200 ms. It should be appreciated that such integration is the same as a low pass filtering or smoothing as recited above.
Brief Description of the I)rawin~s The invention will be further understood from the following descliption with reference to the accornpanying drawings, in which similar references are used in different 30 figures to denote similar components and in which:
Figs. I and 2 are circuit diagrams illustrating alternative forms of apparatus, in accordance with embodiments of the invention, for detecting changes in voltage between the two wires of a telephone subscriber line;
Fig. 3 is a flow chart illustrating the processing, in accordance with an 35 embodiment of the invention, of signals produced by ~e apparatus of Fig. 1 or 2; and Figs. 4 to 8 are signal diagrams with reference to which the operation of the apparatus and the processing of the signa1s are explained.
Descliption of the Preferred Embodiments Referring to Fig. 1, there is illustrated a f~rst form of a subscriber line voltage change detector in accordance with a preferred embodiment of the invention. The detector comprises a potential divider 10, a low pass fllter circuit constituted by a series resistor 12 S and a shunt capacitor 14, two voltage comparators 16 and 18, and circuitry 20 for supplying to the potential divider 10 a voltage dependent upon the voltage between the tip and ring wires of a two-wire telephone subscriber line (not shown).
The circuitry 20 includes a diode bridge 22 having an a.c. input connected to input terrninals referenced Tip and ~ing, which are coupled to the tws) wires of the telephone subscriber line as described further below, and a d.c. output. A positive termina1 of the d.c. output of the diode bridge is connected to a line 24, which constitutes a common rail in the apparatus of Fig. 1, and a negative terrninal is connected to a grounded line 26. The circuitly 20 alsv includes a potential divider comprising resistors 28 and 30 connected between the lines 24 and 26, with a capacitor 32 connected in parallel with the resistor 30 between the common rail or line 24 and the junction between the resistors 28 and 30. In addition, the circuitry 20 includes a buffer 34 having an input connected to the junction between the resistors 28 and 30. The potential divider 10 is connected between the output of the buffer 34 and the common rail 24.
The apparatus of Fig. 1 forrns part of a telephone set, t~e remainder of which is not shown and can be of generally known form, which is connected to the telephone subscriber line. The diode bridge 20 may be a diode bridge which is already provided for telephone polarity guard purposes. The l`ip and Ring terrninals shown in Fig. 1 are connected to the tip and ring wires, respectively, of the telephone subscriber line. Tbey can be connected on either side of the book switch of the telephone set, i.e. either on the central office side of the hook switch so that they are always connected to the subscriber line regardless of whether or not the telephone is off-hook, or on the voice circuit (terminal) side of the hook switcll so that they are comlected to the line via the hook switch. In the latter case, the appamtus of Fig. 1 is only responsive to subscriber line voltage changes when the telephone is off-hook (the hoolc switch is closed), but in this case the resistors 28 and 30 can have moderate resistances because tbey do not continuously load the subscriber line. In the former case, which is preferred and is assumed throughout the following description, the apparatus is responsive to subscriber 1ine voltage changes regardless of whether or not the telephone is off-hook. In this case the resistors 28 and 30 continuously load the subscriber line and therefore have very high resistances, and the buffer 34 also has a ve~y high input impedance.
By way of example, the resistors 28 and 3Q can have resistances of 47 MQ and 16 MQ respectively. The capacitor 32, which can bave a capacitance of 2.2 nF, serves to attenuate audio frequency signals at the input of the buffer 34 without unduly attenuating longer duration voltage changes which are to be dete~ted. In tl~is example the capacitor 32 forms with the resistors 28 and 30 (which the capacitor sees in parallel combination as a resistance of a~out 12 MQ) a smoo&ing (or low pass filter or integrating) circuit with a time constant of the order of 26 ms. The buffer 34 serves to isolate the potential divider 5 10 from the subscriber 1ine.
The potential divider 10 ~omprises four resistors 36, 38, 40, and 42 connected in series, thus providing three tapping points or nodes which are referenced N I, N2, and N3. Ths node N I is between the resistor 36, connected to the output of the buffer 34, and the resistor 38; the node N2 is between the resistors 38 and 40; and the node N3 is between the resistors 40 and 42, the resistor 42 being connected to the common rail 24.
The low pass filter circuit comprising the components 12 and 14 has an input connected to the node N2 and an output constituting a node N4; thus the series resistor 12 is connected between the nodes N2 and N4, and the shunt capacitor 14 is connected behveen the nod~
N4 and the common ail 24. The low pass filter circuit serves as described ~urther below 15 to smooth the voltage at the node N4 relative to that at the node N2.
By way of example, the potential divider resistors 36, 38, 40, and 42 can have resistances of respectively 10 kQ, 500 Q, 750 Q, and 10 kQ; the resistor 12 can have a resistance of 100 kQ and the capacitor 14 can haYe a capacitance of 2.2 ~lF. The low pass filter circuit (which can alternatively be considered as a smoothing or integrating circuit) 20 has a time constant determined by the product of the resistance of ~e resistor 12 with the capacitance of the capacitor 14, and in this case is 220 ms. This is of the order of 200 ms, or of the order of ten times ~he time constant of the audio firequency attenuating components 28, 30, and 32.
The comparator 16 has a non-inverting (+) input connected to the node N 1, an 25 inverting (-) input connected to the node N4, and an output connected to a circuit terrninal F at which as described further below it provides a pulse in response to the subscriber line voltage falling. Conversely, the comparator 18 has a non-~verting (~) input connected to the node N4, an inverting (-) input connected to the node N3, and an output connected to a circuit terminal R at which as described fur~er below it provides a pulse in response to 30 the subscriber line voltage dsing. In each case the duration of each output p~se is dependent upon the magnitude of the subscriber line voltage chang~ and its timing relative to, and the timing and magnitude of, previous subscriber line voltage changes. Although not shown in Fig. I, additional circuitry (e.g. positive feedback or culrent soulces switched by the comparator outputs) may be provided for each comparator to implement 35 hysteresis to prevent oscillation or spurious outputs ~rom the comparators at the switching thresholds.
The apparatus of Fig. 2 is similar to that of Fig. 1 and opera~es in a corresponding manner, except that the common rdil to which the potential divider 10 is connected is 3.,'?_ constituted by the grounded line 26 ~ather ~an ~he line 24, and cons~quently t~econnections of the inYerti~ and non-inverting inputs of each comparator are interchanged (e.g. for the comparator 16, the inve~ting (-) input is connected to the node Nl and the non-inverting (~) input is connected to the node N4). As it is more usual to think of a S common rail as being grounded as in Fig. 2, tlle following description relates to this arrangement rather than that of Fig. 1. However, it should be appreciated that the two arrangements are interchangeable and equivalent.
In addition to the apparatus of Fig. 1 or 2, the telephone set includes a control microprocessor (not shown) which, in addition to performing conventional functions such 10 as controlling the storage, retrieval, and display of dialled numbers, serves to monitor the pulses which are produced at the terminals F and R and to take appropriate consequential actions, such as controlling an indicator to indicate that another telephone set connected to the same subscriber 1ine is off-hook, and controlling remote release of a hold state of this telephone set. To this end, the microprocessor carries out, in a time sharing manner with 15 its othf~r functions, an algorithrn whîch is repsesented in the flow chart of Fig. 3.
Before describing t~e flow c~art of Fig. 3 in detail, reference is also made to Fig. 4, which illustrates by way of example signals whtch may occur in operation of the apparatus of Fig. 2 when the subscriber line voltage, between the two wires of the telephone subscriber line, falls, for example as a result of another telephone set ~onnected 20 to the same subscri~er line being taken off-hook. The upper part of Fig. 4 represents t~e voltages of the nodes N 1 to N4 as a function of time; ~he lower parts of Fig. 4 represent the resulting signals at the terminals F and R, which signals are accordingly denoted by the sarne references F and R respectively.
Initially, assuming a static situation, as shown at the left-hand side of Fig. 4 the 25 voltage of the node N4 is the same as that of the node N2 and lies between the voltages of the nodes Nl and N3, and the outputs of the compalators 16 and 18 are both low.
Because the resistor 38 has a smaller resistance than the resistor 40, the voltage of the node N2 is closer ~o that of the node N 1 than that of the node N3, as can be seen in Fig. 4. This dif~erence makes the arrangement more sensitive to falling subscAber line 30 voltage, as occurs in the event of loss of privacy and remote release from hold, ~an rising subscriber line voltage, but if desired equa1 sensltivity ca n be provided by making the resistances of the resistors 38 and 40 equal.
At a time tl m Fig. 4, it is assumed that the subscriber 1ine voltage falls due to another telephone connected to the line being t~ken off-~ook. The voltages of the nodes 35 N 1, N2, and N3 all fall in a corresponding manner, relatively quickly and slowed only by the effect of the capacitor 32, to lower 1evels as represented at the right-hand side of Fig. 4. The voltage of the node N4, due to the smoothing effect and time constant of the low pass filter circuit constituted by the components 12 and 14, falls exponentially and more slowly from the original voltage of the node N2 to the final voltage of the node N2 Consequently, from the time tl until a time t2, the node N4 is more positive than the node N 1, with the result that during this time period Ihe comparator 16 produces a pulse of the si8nal F, i.e. a pulse indicating that the subscriber line voltage has fallen. The output of 5 the compamtor 18, as represented by the signal R, remains low because ~he node N4 stays more positive than the node N3.
As represented by a box 50 in Fig. 3, the microprocessor waits for a pulse to occur at either of the terminals F and R, and on the occurrence of a pulse records whether it is at the terminal F or the terminal R. It then determines, as represented by a decision 10 box 52, whether or not the pulse has a valid duration. In this embodiment a pulse with a duration greater than about 0.1 second (actually 106 ms, selected in view of the time sharing of the microprocessor), is considered to be valid, and a pulse of lesser duration is considered to be invalid.
Assuming that the pulse has a valid duration (i.e. t2-t l> 106 ms), then at the end of the pulse (i.e. at the time t2) the micropsocessor starts a ~00 ms window timer at a box 54 and checks in a decision box 56 whether a pulse starts at the other terminal during this window period. This is discussed further below with reference to Fig. 8. Assuming that no other pulse oceurs during this window period, then a decision box 58 is reached in which it is determined whether or not the pulse which has been detected is a result of actions which have been taken at this particular telephone set. If the answer to this decision is yes (for example the pu1se at the terminal F is due to this set being taken of ~-hook), then no action is necessary from this algorithm and a return is made to the box 50 via a line 60 in Fig. 3. If, however, the answer to this decision in the box 58 is no, then it is concluded that the pulse is due to a change in the hook state of another telephone set oonnected to the same telephone subscriber line. In o~er words, it is determined that this other telephone set is now in an off~hook state if the pulse was at the terminal F, and that this other telephone set is now in an on-hook state if the pulse was at t~e tenninal R.
In this case appropriate action is taken at a box 62, and again a return is made to the initial box 50.
The action represented by the box 62 may be any of a number of possible actions,depending on the state of this telephone set, t~e te~minal at which the pulse occurs, and the previous conditions recognized by the microprocessor. For example, if the pulse is at the terminal F as represented in Fig. 4, then the microproc~ssor conc1udes that ~nother telephone set connected to the sarne subscriber line has been taken off-hook. If this telephone set (i.e. the telephone set containing ~e microprocessor) is on-hook, the action may be to provide an indication that the telephone subscriber line is in use. If this telephone set is on hold, then the action may be to release this set from the hold state, on the understanding that a telephone call is being continued from another set (remote release ~0~ ~3~3~_ from hold). If this set is off-hook but not on hold, the action may be to prwide an indication that another telephone set has been taken off-hook and that privacy has been lost. Conversely, if the pulse were at the terminal R instead of the terminal F, the action may be to terminate a previous indication that the subscriber line is in use or ~hat privacy S has been lost.
The other boxes and parts of the flow chart of Fig. 3 serve for distinguishing against various pulses which may occur at one or both of the terminals F and R due to various interfering signals such as those which have already been mentioned. These are discussed further below with additional reference to the diagrarns in Figs. 5 to 8. These diagrams are provided for explanation and by way of example only, to illustrate the types of signals which may occur in operation. In each of these diagrams, the signals F and R
are represented in a similar manner to the representation of these signals in Fig. 4, and for sirnplicity the corresponding node voltages are not illustrated. In these diagrams, also for sirnplicity, pulses are illustrated all with either a fi~ed short duration (narrow pulses) or a fixed longer duration (wide pulses). It should be appreciated that the narrow pulses represent pulses with arbitrary and various invalid durations less than 106 ms, and the wide pulses represent pulses with arbitrary and various valid durations greater than 106 ms. In addition, the intervals between pulses shown in the diagrams of Figs. S to 8 may vary widely.
As has already been indicated, the duration of 106 ms for a valid pulse is not critical, and is selected in ~is embodiment of ~e invention as being about 100 ms in vi~w of component values of the appalatus of Figs. I and 2, and specifically 106 ms to match tirning characteristics of the microprocessor and other functions which it performs. A
shorter or 10nger validation duration may be used, possibly as little as 10 ms or as much as 400 ms, but it is preferably at 1east 60 ms in view of pulse dialling timing considerations .
In Fig. 5, the signal F has two sonsecutive pulses, the first having an invalid duration and the second a valid duration. Such a pulse sequence may occur when atelephone set is taken off-hook, because the hook switch is usually a mechanical switch which is sllbject to intennittent contact on being closed. In the flow chart of Fig. 3, in this case the first pulse is detected at the box 50 and it is decided at the box 52 that its duration is invalid. Consequently, as representi d by a box 64 a cancellation timer, having a time period of 1.2 seconds, is started, and it is determined at a decision box ~6 whether or no~
ano2her pulse, at either t~rminal F or R, starts within this cancellation time period. If not, then it is assumed that the invalid dulation pulse is an erroneous pulse, and its detection is effectively cancelled by a return being made to the box 50 via a line 68.
If, as is assumed in t~e case of Fig. 5, another pulse starts within the 1.2 second cancellation timerperiod, then a decision box 70 is reached in which it is detennined 5~
whether or not this nf~w pulse has a valid duration, the criterion for this being the same as at the box 52. If the duration of this pulse is valid, then a decision box 72 is reached in which it is determined whether or not this pulse is at the same terminal (F or R~ as the pulse which was initially detected at the box 50. In the case of Fig. 5 the answer to this 5 decision is yes, and consequently a branch is taken via a line 74 to the box 54, and the same sequence as described above is followed in this and subsequent boxes.
In Fig. 6, the signal F has two consecutive pulses, the first having an invalid duration and the second a valid duration, and between these pulses a pulse having an invalid duration also occurs at the other terminal R. Such a pulse sequence may occur 10 when a telephone set is taken off-hook and there is a mechanical switch bounce. In the flow chart of Fig. 3, in this case the sarne sequence as described above for Fig. S is followed until the decision box 70 is reached. Now in the box 70 it is determined that the new pulse at the terminal R has an invalid duration, and a return is made via a line 76 to the box 64, the cancellation timer being restarted at the end of 1dle pulse at the terminal R.
In the subsequent decision boxes 66, 70, and 72 lhe valid duration pulse at the terminal F
is respectively detected, validated, and detennined to be at the same terminal (F) as the original pulse detected at the box 50, so that a branch is made to the box 54 via the line 74 as described above.
In Fig. 7, both of the signals F and R have an arbitrary number of pulses each of 20 invalid duration, each following the immediately preceding pulse within the cancellation timer period, with a final pulse of valid duration at ~he opposite terminal to ~at at which the first pulse occurred. In Fig. 7 the first pulse occurs at the terminal R and the final, valid duration, pulse occurs at the tenninal F. This is typical of a sequence of pulses generated when a pulse dialling telep~one, which has previously been taken off-hook (this 25 having been detected as described above) is dialled. In t~is case, as in the case of Fig. 6 each invalid duration pulse results in a return via the line 76 to restart ~he cancellation timer and to wait for a valid duration pulse starting within the cancellation time period. The f1nal pulse at the tem~inal F is a valid duration pulse, and results in the decision box ?2 being reac~ed. In this case the valid duration pulse (at the term1nal F) is not at the same 30 terminal as the initial pulse (at the tenn~al R) detected at the box 50, and accordingly a return is made via the line 68 to the box 50 and no action is taken.
In Fig. 8, a valid duration i~ulse occurs at the terminal F, and after a delay a pulse (which is shown as a valid duration pulse bu~ need not be) occurs at the terminal R. This is ~ypical of an open switch interval, in which power to the subscriber line is momentarily 35 removed dwring a switching operation, causing the su~scriber line voltage to fall and subsequently to rise. In this case the initia1 valid duration pulse at the terminal F results in the decision box 56 being reached as described above in relation to Fig. 4. As the window timer period of 600 ms is selected to be greater than the delay between the end of fi~
the first pulse and the start of the second pulse, in this case ~he answer at the decision box 56 is yes, and as a result a return is made to the box 50 via the line 60 without any other action being taken. Thus an erroneous remote release from hold due to t~e spen switch interval is avoided.
From the above description, it can be seen that the algorithm which the microprocessor follows, corresponding to the flow chart of Fig. 3, enables significant subscriber line voltage changes to be detected and responded to, without being adversely affected by interfering signals due to hook switch bounce or flashing, pulse dialling, and open switch intervals. Thus with reference to the pulse sequences in ~igs. 4 to 6, in each case the valid duration pulse of the signal F is correctly determined to represent a fall in the subscriber line voltage, regardless of the presence of preceding invalid duration pulses of the signals F and R. With reference to Fig. 7, the pulse sequence shown therein, corresponding to pulse dialling at another telephone set, does not result in any erroneous deterrnination of a change in the hook state of t~at te1ephone set, because the valid duration final pulse is at the other terrninal than that at which the first pulse is detected. In addition, an open switch interval, producing pu1ses such as shown in Fig. 8, also does not result in any erroneous determination of a change in the hook state of another telephone set.
It should also be appreciated from the circuit diagrams in Figs. 1 and 2 and t~Pdiagram in Fig. 4 that the voltage at the node N4 is al tomatica11y positioned, when the subscriber line voltage is not char~ing, at a desired position between the voltages at the nodes N 1 and N3. This self-adjusting (self-centeAng if the resistors 38 and 40 have the same resistance) propeny of the apparatus enables the apparatus to operate effectivelywith relatively arbitrary abso1ute va1ues of subscriber 1ine voltage, and hence arbit~ary subscriber line resistances and numbers and types of telephone sets connected to the subscAber line.
It should be recognized, however, that the arrangements described above do not necessarily respond to every situation in which ~ere is a change in the hook state of another te1ephone set cormected to the same subscriber line. In particular, if the other telephone set's hook state changes at su~stantially the s~ne time as a Cllange in ~e hook state of the telephone set containing the microprocesss)r and the apparatus of Fig. 1 or 2, this will not be recognized by th~ ar~angement described above. Thus the above arrangement provides a useful, rather ~an an absolutely reliable, indica~ion of the hook state of the other telephone set. It should be appreciated that the use~ulness of this indication can be enhanced by modifying the above ar~gement ~o monitor differentduratios of valid duration pulses and to respond accordin~y; such modification may facilitate the recognition of sirnultaneous hook state changes at more than one telephone set.
Furthermore, the above descliption relates only to the existence of one other telephone set connected to the subscriber line, whereas in practice several other telephone sets may be so connected and the hooX state of each of these may be independently changed. Obviously, the microprocessor may be arranged to track simultaneously the S hook state of a plurality of other telephone sets connected to the same subscriber line, and again in this case pulse durations may be monitored to assist in this tracking.
Numerous other modifications, variations, and adaptations may be made to the described embodiments without departing from the scope of the invention as defined in the claims.
TELEPHONE SUBSCRIBER LlNE VOLTAGE CHANGE DETECTION
This invention relates to the detection of voltage changes on a telephone subscriber line, and is particularly concerned with the detection of such voltage changes, at a first telephone set especi~lly when it is off-hook, due to changes in the on-hook or off-hook 5 state of another telephone set conn~cted to the sarne subscriber line.
Back~round of the ]nvention It is wcll known to connect two or more te1ephone sets, for example in differentrooms of a residence, to the same two-wire telephone subscriber 1ine, thus enabling two or more people in the lesidence to participate simultaneously in a telephone conversation 10 via the telephone subscriber line. A disadvantage of such an arrangement is that the privacy of a person using a first telephone set may be lost due to another person taking a second telephone set, connected to the same telephone subscriber line, off-hook (lifting the handset). While in some circumsta~ces a click may be audible to the person using the first telephone set when the second telephone is taken of ~-hook, this does not provide a 15 convenient or reliable indication that priYacy has been interrupted. Accordingly, it is desirable to provide a more reliable indication at one telephone set of the hook state of another telephone set connected to the same line.
Such an indication is also useful even when the first telephone set is on-hook, as it can then serve to indicate to a prospective user of this telephone set whether or not the 20 telephone subscriber line is already in use by another person using one of the other telephone sets connected to this line.
Furthermore, it is desirable for a user of the first telephone set to be able to place this in a hold state during a telephone call, to go to a second telephone set connected to the same line to continue the call, and to have the first telephone set automatically release itself 25 from the hold state when the second telephone set is taken off-hook. This is referred to as remote release from hold.
For each of these situations, a change in the voltage between the two wires of the telephone subscriber line can conceivably be detected and used to take an appropriate action, for e~mple to prwide an indication at the first telephone set or to effect a release 30 of this telephone set from the hold state. However, it has been found to be very difficult to monitor the subscriber line voltage in an effective manner to accomplish this, because of the variability of this voltage and the presence of interfering signals on the line.
More particularly, the actual subscriber line voltage is dependent upon a number of variab1e factors, such as the resistance of the telephone subscriber line and the number and 35 nature of the telepholle sets connected to the line. Interfering signals on the line can occur due to hook switch flashing or pulse dialling activities at other telephone sets connected to the line, and due to short-duration open switch intervals at the central o~fice to which the subscriber line is connected. Open switch intervals comprise temporary interruptions in ~(~3~
voltage supplied from the central of fice ~o thc subscriber line during switchin~ activity at the central of fice, and known methods of remote release from hold are susceptible to incorrectly releasing from hold in response to open switch intervals.
An object of this invention, therefore, is to provide an improved method of and S apparatus for detecting voltage changes on a telephone subscriber line.
Summarv of the Invention According to one aspect of this invention there is provided apparatus for detecting changes in a subscriber line voltage between two wires of a telephone subscriber line, comprising: a potential divider having first, serond, and third tapping points, the second 10 tapping point b ing between the first and third tapping points; means for supplying to the potential divider a voltage dependent upon the subscriber line voltage; a low pass filter circuit having an input connected to the second tapping point; a first voltage comparator, having inputs connected to the first tapping point and an output of the low pass filter circuit, for producing an output pulse in response to the subscriber line voltage falling;
15 and a second vo1tage comparator, having inputs connected to the third tapping point and the output of the low pass filter circuit, for producing an output pulse in response to the subscriber line voltage rising.
The means for supplying a voltage dependent upon the subscriber line voltage preferably comprises means for attenuating audlo frequency signals, so that audio 20 frequency signals vvhich occur Oll the telephone subscriber line in use do not interfere with the voltage comparisons.
It is convenient for the potential divider to comprise a first resistance between the first and second tapping points and a second resistance between the second and Ihird tapping points, the first resistance being less than the second resistance so that a greater 25 sensitivity is provided for falling than for rising subscriber 1ine voltages.According to another aspect this invention provides apparatus for detecting changes in a subscriber line voltage between two wires of a telephone su~scriber line, comprising: means for a~tenuating audio frequency signals; a buf~er coupled via the means for attenuating audio frequency signals to the subscriber line, the buffer having an 30 output at which it prcduces, relative to a common rail, a voltage dependent upon the subscriber 1ine voltage; a potential divider having first, second, and third tapping points, the potential divider comprising a first resistor between the output of the buffer and the first tapping point, a second resistor between the first and second ~apping points, a third resistor between the second and third tapping points, and a four~h resistor between the 35 third tapping point and the common rail; a smoothing circuit having an input connected to the second tapping point; a first voltage comparator, having inputs connected to the first tapping point and an output of the smoothing circuit, îor producing an output pulse in response to the subscriber line voltage falling; and a second voltage comparator, having ~ ~ 3 ~ ~J ~ ~
inputs connected to the third tapping point and the output of the smoo~ing circuit, for producing an output pulse in response to the subscriber line voltage rising.
Conveniently each of the m~aDs for attenuating audio frequency signals and the smoothing circuit comprises a resistance and a capacitance whose product defines a 5 respective time constant, and the time constant of the smoothing circuit is of the order of ten times the time constant of the means for attenuating audio frequency signals.
In accordance with a further aspect of this invention there is provided a method of detecting, at a first te1ephone set connected to a two-wire telephone subscriber line, a change from an on-hook state to an off-hook state of a second telephone set connected to 10 the same subscriber line, comprising the steps of: monitoring a subscriber line voltage between the two wires of the subscriber line; producing a pulse at a first circuit tenninal in response to the subscriber line voltage falling; producing a pulse at a second circuit terminal in response to the subscriber line voltage rising; in response to a pulse at the fisst termina1 having at least a predetermined duration, starting a window timer period;
15 detecting any pulse at the second terminal during the window timer period and, in the absence of such a pulse and in the absence of an action at the first telephone set causing the subscriber line voltage to fall, determining that the second telephone set is in an off-hook state.
According to another aspect this invention provides a method of detecting, at a 20 first telephone set connected to a two-wire te1ephone subscriber line~ hook state changes of a second telephone set cormected to the same subscriber line, comprising the steps of:
(i) monitoring a subscriber line voltage between the two wires of the subscriber line;
(ii~ producing a pulse at a first terminal in response to t~e subscriber line voltage falling;
25 (iii) producing a pulse at a second termina1 in response to the subscriber line voltage rising;
(iv) detecting a first pulse at either of the fi~st and the second terminals and noting at which tenninal such a first pulse is produced;
(v3 determining whether the pulse has at least a predeterrnined dumtion, if so 30 proceeding to step (ix) and i3 not p~oceeding to step (vi);
(vi) detecting any pulse at either tenninal starting within a predetermined cancellation period, in the absence of any pulse returning to step (iv) and in response to such a pulse proceeding to step (vii);
(vii) detennining whether the most recent pulse has at least a predetelmined duration, if 35 so proceeding to step (viii) and if not returning to step (vi);
(viii) determining whether the mos~ recent pulse is at the same terminal as that noted in step (iv) at which the first pulse was produced, if so proceeding to step (ix) and if not returning to step (iv);
~03~5:1 (ix) detecting any pulse, at the other terminal than that noted in step (iv) at which the fist pulse was produced, starting within a predeter nined window period, in the absence of any pulse proceeding to step (x) and in the presence of such a pulse returning to step (iv);
S (x) in the absence of any action at the first telephone set to cause the first pulse detected in step (iv), determining that the second tdephone set is off-hook if the first pulse was produced at the fi~t termina1, and deter nining that the second telephone set is on-hook if the first pu1se was at the second ter ninal; and (xi) returning to step (iv).
Although none of the times is partic!arly critical, the predetermined duration in each of steps (v) and (vii) is preferably at least about 10 ms, and desirably is at least about 60 ms. In the embodiments of the invention described below this period is 106 ms, but it can conceivablybe anywhere in a range from about 10 ms to about 400 ms. The predetermined cancellation period in step (vi) i8 conveniently of the order of 1.2 seconds, 15 and the predetermined window period in step (ix) is conveniently of the order of 600 ms, but again these periods are not critical and may be varied to suit particular circumstances.
Preferably steps (i) to (iii) of the al~ove method comprise the steps of: deriving from the subscriber line voltage first, second, and third comparison voltages, the second comparison voltage being produced by smoothing a voltage between the first and third 20 comparison voltages; and comparing the first and third comparison voltages with tlle second comparison voltage to produce the pulses at the first and second terminals respectively.
The second comparison voltage is conveniently produced by integrating a voltage between the first and third comparison voltages with an integration time constant of the 25 order of 200 ms. It should be appreciated that such integration is the same as a low pass filtering or smoothing as recited above.
Brief Description of the I)rawin~s The invention will be further understood from the following descliption with reference to the accornpanying drawings, in which similar references are used in different 30 figures to denote similar components and in which:
Figs. I and 2 are circuit diagrams illustrating alternative forms of apparatus, in accordance with embodiments of the invention, for detecting changes in voltage between the two wires of a telephone subscriber line;
Fig. 3 is a flow chart illustrating the processing, in accordance with an 35 embodiment of the invention, of signals produced by ~e apparatus of Fig. 1 or 2; and Figs. 4 to 8 are signal diagrams with reference to which the operation of the apparatus and the processing of the signa1s are explained.
Descliption of the Preferred Embodiments Referring to Fig. 1, there is illustrated a f~rst form of a subscriber line voltage change detector in accordance with a preferred embodiment of the invention. The detector comprises a potential divider 10, a low pass fllter circuit constituted by a series resistor 12 S and a shunt capacitor 14, two voltage comparators 16 and 18, and circuitry 20 for supplying to the potential divider 10 a voltage dependent upon the voltage between the tip and ring wires of a two-wire telephone subscriber line (not shown).
The circuitry 20 includes a diode bridge 22 having an a.c. input connected to input terrninals referenced Tip and ~ing, which are coupled to the tws) wires of the telephone subscriber line as described further below, and a d.c. output. A positive termina1 of the d.c. output of the diode bridge is connected to a line 24, which constitutes a common rail in the apparatus of Fig. 1, and a negative terrninal is connected to a grounded line 26. The circuitly 20 alsv includes a potential divider comprising resistors 28 and 30 connected between the lines 24 and 26, with a capacitor 32 connected in parallel with the resistor 30 between the common rail or line 24 and the junction between the resistors 28 and 30. In addition, the circuitry 20 includes a buffer 34 having an input connected to the junction between the resistors 28 and 30. The potential divider 10 is connected between the output of the buffer 34 and the common rail 24.
The apparatus of Fig. 1 forrns part of a telephone set, t~e remainder of which is not shown and can be of generally known form, which is connected to the telephone subscriber line. The diode bridge 20 may be a diode bridge which is already provided for telephone polarity guard purposes. The l`ip and Ring terrninals shown in Fig. 1 are connected to the tip and ring wires, respectively, of the telephone subscriber line. Tbey can be connected on either side of the book switch of the telephone set, i.e. either on the central office side of the hook switch so that they are always connected to the subscriber line regardless of whether or not the telephone is off-hook, or on the voice circuit (terminal) side of the hook switcll so that they are comlected to the line via the hook switch. In the latter case, the appamtus of Fig. 1 is only responsive to subscriber line voltage changes when the telephone is off-hook (the hoolc switch is closed), but in this case the resistors 28 and 30 can have moderate resistances because tbey do not continuously load the subscriber line. In the former case, which is preferred and is assumed throughout the following description, the apparatus is responsive to subscriber 1ine voltage changes regardless of whether or not the telephone is off-hook. In this case the resistors 28 and 30 continuously load the subscriber line and therefore have very high resistances, and the buffer 34 also has a ve~y high input impedance.
By way of example, the resistors 28 and 3Q can have resistances of 47 MQ and 16 MQ respectively. The capacitor 32, which can bave a capacitance of 2.2 nF, serves to attenuate audio frequency signals at the input of the buffer 34 without unduly attenuating longer duration voltage changes which are to be dete~ted. In tl~is example the capacitor 32 forms with the resistors 28 and 30 (which the capacitor sees in parallel combination as a resistance of a~out 12 MQ) a smoo&ing (or low pass filter or integrating) circuit with a time constant of the order of 26 ms. The buffer 34 serves to isolate the potential divider 5 10 from the subscriber 1ine.
The potential divider 10 ~omprises four resistors 36, 38, 40, and 42 connected in series, thus providing three tapping points or nodes which are referenced N I, N2, and N3. Ths node N I is between the resistor 36, connected to the output of the buffer 34, and the resistor 38; the node N2 is between the resistors 38 and 40; and the node N3 is between the resistors 40 and 42, the resistor 42 being connected to the common rail 24.
The low pass filter circuit comprising the components 12 and 14 has an input connected to the node N2 and an output constituting a node N4; thus the series resistor 12 is connected between the nodes N2 and N4, and the shunt capacitor 14 is connected behveen the nod~
N4 and the common ail 24. The low pass filter circuit serves as described ~urther below 15 to smooth the voltage at the node N4 relative to that at the node N2.
By way of example, the potential divider resistors 36, 38, 40, and 42 can have resistances of respectively 10 kQ, 500 Q, 750 Q, and 10 kQ; the resistor 12 can have a resistance of 100 kQ and the capacitor 14 can haYe a capacitance of 2.2 ~lF. The low pass filter circuit (which can alternatively be considered as a smoothing or integrating circuit) 20 has a time constant determined by the product of the resistance of ~e resistor 12 with the capacitance of the capacitor 14, and in this case is 220 ms. This is of the order of 200 ms, or of the order of ten times ~he time constant of the audio firequency attenuating components 28, 30, and 32.
The comparator 16 has a non-inverting (+) input connected to the node N 1, an 25 inverting (-) input connected to the node N4, and an output connected to a circuit terrninal F at which as described further below it provides a pulse in response to the subscriber line voltage falling. Conversely, the comparator 18 has a non-~verting (~) input connected to the node N4, an inverting (-) input connected to the node N3, and an output connected to a circuit terminal R at which as described fur~er below it provides a pulse in response to 30 the subscriber line voltage dsing. In each case the duration of each output p~se is dependent upon the magnitude of the subscriber line voltage chang~ and its timing relative to, and the timing and magnitude of, previous subscriber line voltage changes. Although not shown in Fig. I, additional circuitry (e.g. positive feedback or culrent soulces switched by the comparator outputs) may be provided for each comparator to implement 35 hysteresis to prevent oscillation or spurious outputs ~rom the comparators at the switching thresholds.
The apparatus of Fig. 2 is similar to that of Fig. 1 and opera~es in a corresponding manner, except that the common rdil to which the potential divider 10 is connected is 3.,'?_ constituted by the grounded line 26 ~ather ~an ~he line 24, and cons~quently t~econnections of the inYerti~ and non-inverting inputs of each comparator are interchanged (e.g. for the comparator 16, the inve~ting (-) input is connected to the node Nl and the non-inverting (~) input is connected to the node N4). As it is more usual to think of a S common rail as being grounded as in Fig. 2, tlle following description relates to this arrangement rather than that of Fig. 1. However, it should be appreciated that the two arrangements are interchangeable and equivalent.
In addition to the apparatus of Fig. 1 or 2, the telephone set includes a control microprocessor (not shown) which, in addition to performing conventional functions such 10 as controlling the storage, retrieval, and display of dialled numbers, serves to monitor the pulses which are produced at the terminals F and R and to take appropriate consequential actions, such as controlling an indicator to indicate that another telephone set connected to the same subscriber 1ine is off-hook, and controlling remote release of a hold state of this telephone set. To this end, the microprocessor carries out, in a time sharing manner with 15 its othf~r functions, an algorithrn whîch is repsesented in the flow chart of Fig. 3.
Before describing t~e flow c~art of Fig. 3 in detail, reference is also made to Fig. 4, which illustrates by way of example signals whtch may occur in operation of the apparatus of Fig. 2 when the subscriber line voltage, between the two wires of the telephone subscriber line, falls, for example as a result of another telephone set ~onnected 20 to the same subscri~er line being taken off-hook. The upper part of Fig. 4 represents t~e voltages of the nodes N 1 to N4 as a function of time; ~he lower parts of Fig. 4 represent the resulting signals at the terminals F and R, which signals are accordingly denoted by the sarne references F and R respectively.
Initially, assuming a static situation, as shown at the left-hand side of Fig. 4 the 25 voltage of the node N4 is the same as that of the node N2 and lies between the voltages of the nodes Nl and N3, and the outputs of the compalators 16 and 18 are both low.
Because the resistor 38 has a smaller resistance than the resistor 40, the voltage of the node N2 is closer ~o that of the node N 1 than that of the node N3, as can be seen in Fig. 4. This dif~erence makes the arrangement more sensitive to falling subscAber line 30 voltage, as occurs in the event of loss of privacy and remote release from hold, ~an rising subscriber line voltage, but if desired equa1 sensltivity ca n be provided by making the resistances of the resistors 38 and 40 equal.
At a time tl m Fig. 4, it is assumed that the subscriber 1ine voltage falls due to another telephone connected to the line being t~ken off-~ook. The voltages of the nodes 35 N 1, N2, and N3 all fall in a corresponding manner, relatively quickly and slowed only by the effect of the capacitor 32, to lower 1evels as represented at the right-hand side of Fig. 4. The voltage of the node N4, due to the smoothing effect and time constant of the low pass filter circuit constituted by the components 12 and 14, falls exponentially and more slowly from the original voltage of the node N2 to the final voltage of the node N2 Consequently, from the time tl until a time t2, the node N4 is more positive than the node N 1, with the result that during this time period Ihe comparator 16 produces a pulse of the si8nal F, i.e. a pulse indicating that the subscriber line voltage has fallen. The output of 5 the compamtor 18, as represented by the signal R, remains low because ~he node N4 stays more positive than the node N3.
As represented by a box 50 in Fig. 3, the microprocessor waits for a pulse to occur at either of the terminals F and R, and on the occurrence of a pulse records whether it is at the terminal F or the terminal R. It then determines, as represented by a decision 10 box 52, whether or not the pulse has a valid duration. In this embodiment a pulse with a duration greater than about 0.1 second (actually 106 ms, selected in view of the time sharing of the microprocessor), is considered to be valid, and a pulse of lesser duration is considered to be invalid.
Assuming that the pulse has a valid duration (i.e. t2-t l> 106 ms), then at the end of the pulse (i.e. at the time t2) the micropsocessor starts a ~00 ms window timer at a box 54 and checks in a decision box 56 whether a pulse starts at the other terminal during this window period. This is discussed further below with reference to Fig. 8. Assuming that no other pulse oceurs during this window period, then a decision box 58 is reached in which it is determined whether or not the pulse which has been detected is a result of actions which have been taken at this particular telephone set. If the answer to this decision is yes (for example the pu1se at the terminal F is due to this set being taken of ~-hook), then no action is necessary from this algorithm and a return is made to the box 50 via a line 60 in Fig. 3. If, however, the answer to this decision in the box 58 is no, then it is concluded that the pulse is due to a change in the hook state of another telephone set oonnected to the same telephone subscriber line. In o~er words, it is determined that this other telephone set is now in an off~hook state if the pulse was at the terminal F, and that this other telephone set is now in an on-hook state if the pulse was at t~e tenninal R.
In this case appropriate action is taken at a box 62, and again a return is made to the initial box 50.
The action represented by the box 62 may be any of a number of possible actions,depending on the state of this telephone set, t~e te~minal at which the pulse occurs, and the previous conditions recognized by the microprocessor. For example, if the pulse is at the terminal F as represented in Fig. 4, then the microproc~ssor conc1udes that ~nother telephone set connected to the sarne subscriber line has been taken off-hook. If this telephone set (i.e. the telephone set containing ~e microprocessor) is on-hook, the action may be to provide an indication that the telephone subscriber line is in use. If this telephone set is on hold, then the action may be to release this set from the hold state, on the understanding that a telephone call is being continued from another set (remote release ~0~ ~3~3~_ from hold). If this set is off-hook but not on hold, the action may be to prwide an indication that another telephone set has been taken off-hook and that privacy has been lost. Conversely, if the pulse were at the terminal R instead of the terminal F, the action may be to terminate a previous indication that the subscriber line is in use or ~hat privacy S has been lost.
The other boxes and parts of the flow chart of Fig. 3 serve for distinguishing against various pulses which may occur at one or both of the terminals F and R due to various interfering signals such as those which have already been mentioned. These are discussed further below with additional reference to the diagrarns in Figs. 5 to 8. These diagrams are provided for explanation and by way of example only, to illustrate the types of signals which may occur in operation. In each of these diagrams, the signals F and R
are represented in a similar manner to the representation of these signals in Fig. 4, and for sirnplicity the corresponding node voltages are not illustrated. In these diagrams, also for sirnplicity, pulses are illustrated all with either a fi~ed short duration (narrow pulses) or a fixed longer duration (wide pulses). It should be appreciated that the narrow pulses represent pulses with arbitrary and various invalid durations less than 106 ms, and the wide pulses represent pulses with arbitrary and various valid durations greater than 106 ms. In addition, the intervals between pulses shown in the diagrams of Figs. S to 8 may vary widely.
As has already been indicated, the duration of 106 ms for a valid pulse is not critical, and is selected in ~is embodiment of ~e invention as being about 100 ms in vi~w of component values of the appalatus of Figs. I and 2, and specifically 106 ms to match tirning characteristics of the microprocessor and other functions which it performs. A
shorter or 10nger validation duration may be used, possibly as little as 10 ms or as much as 400 ms, but it is preferably at 1east 60 ms in view of pulse dialling timing considerations .
In Fig. 5, the signal F has two sonsecutive pulses, the first having an invalid duration and the second a valid duration. Such a pulse sequence may occur when atelephone set is taken off-hook, because the hook switch is usually a mechanical switch which is sllbject to intennittent contact on being closed. In the flow chart of Fig. 3, in this case the first pulse is detected at the box 50 and it is decided at the box 52 that its duration is invalid. Consequently, as representi d by a box 64 a cancellation timer, having a time period of 1.2 seconds, is started, and it is determined at a decision box ~6 whether or no~
ano2her pulse, at either t~rminal F or R, starts within this cancellation time period. If not, then it is assumed that the invalid dulation pulse is an erroneous pulse, and its detection is effectively cancelled by a return being made to the box 50 via a line 68.
If, as is assumed in t~e case of Fig. 5, another pulse starts within the 1.2 second cancellation timerperiod, then a decision box 70 is reached in which it is detennined 5~
whether or not this nf~w pulse has a valid duration, the criterion for this being the same as at the box 52. If the duration of this pulse is valid, then a decision box 72 is reached in which it is determined whether or not this pulse is at the same terminal (F or R~ as the pulse which was initially detected at the box 50. In the case of Fig. 5 the answer to this 5 decision is yes, and consequently a branch is taken via a line 74 to the box 54, and the same sequence as described above is followed in this and subsequent boxes.
In Fig. 6, the signal F has two consecutive pulses, the first having an invalid duration and the second a valid duration, and between these pulses a pulse having an invalid duration also occurs at the other terminal R. Such a pulse sequence may occur 10 when a telephone set is taken off-hook and there is a mechanical switch bounce. In the flow chart of Fig. 3, in this case the sarne sequence as described above for Fig. S is followed until the decision box 70 is reached. Now in the box 70 it is determined that the new pulse at the terminal R has an invalid duration, and a return is made via a line 76 to the box 64, the cancellation timer being restarted at the end of 1dle pulse at the terminal R.
In the subsequent decision boxes 66, 70, and 72 lhe valid duration pulse at the terminal F
is respectively detected, validated, and detennined to be at the same terminal (F) as the original pulse detected at the box 50, so that a branch is made to the box 54 via the line 74 as described above.
In Fig. 7, both of the signals F and R have an arbitrary number of pulses each of 20 invalid duration, each following the immediately preceding pulse within the cancellation timer period, with a final pulse of valid duration at ~he opposite terminal to ~at at which the first pulse occurred. In Fig. 7 the first pulse occurs at the terminal R and the final, valid duration, pulse occurs at the tenninal F. This is typical of a sequence of pulses generated when a pulse dialling telep~one, which has previously been taken off-hook (this 25 having been detected as described above) is dialled. In t~is case, as in the case of Fig. 6 each invalid duration pulse results in a return via the line 76 to restart ~he cancellation timer and to wait for a valid duration pulse starting within the cancellation time period. The f1nal pulse at the tem~inal F is a valid duration pulse, and results in the decision box ?2 being reac~ed. In this case the valid duration pulse (at the term1nal F) is not at the same 30 terminal as the initial pulse (at the tenn~al R) detected at the box 50, and accordingly a return is made via the line 68 to the box 50 and no action is taken.
In Fig. 8, a valid duration i~ulse occurs at the terminal F, and after a delay a pulse (which is shown as a valid duration pulse bu~ need not be) occurs at the terminal R. This is ~ypical of an open switch interval, in which power to the subscriber line is momentarily 35 removed dwring a switching operation, causing the su~scriber line voltage to fall and subsequently to rise. In this case the initia1 valid duration pulse at the terminal F results in the decision box 56 being reached as described above in relation to Fig. 4. As the window timer period of 600 ms is selected to be greater than the delay between the end of fi~
the first pulse and the start of the second pulse, in this case ~he answer at the decision box 56 is yes, and as a result a return is made to the box 50 via the line 60 without any other action being taken. Thus an erroneous remote release from hold due to t~e spen switch interval is avoided.
From the above description, it can be seen that the algorithm which the microprocessor follows, corresponding to the flow chart of Fig. 3, enables significant subscriber line voltage changes to be detected and responded to, without being adversely affected by interfering signals due to hook switch bounce or flashing, pulse dialling, and open switch intervals. Thus with reference to the pulse sequences in ~igs. 4 to 6, in each case the valid duration pulse of the signal F is correctly determined to represent a fall in the subscriber line voltage, regardless of the presence of preceding invalid duration pulses of the signals F and R. With reference to Fig. 7, the pulse sequence shown therein, corresponding to pulse dialling at another telephone set, does not result in any erroneous deterrnination of a change in the hook state of t~at te1ephone set, because the valid duration final pulse is at the other terrninal than that at which the first pulse is detected. In addition, an open switch interval, producing pu1ses such as shown in Fig. 8, also does not result in any erroneous determination of a change in the hook state of another telephone set.
It should also be appreciated from the circuit diagrams in Figs. 1 and 2 and t~Pdiagram in Fig. 4 that the voltage at the node N4 is al tomatica11y positioned, when the subscriber line voltage is not char~ing, at a desired position between the voltages at the nodes N 1 and N3. This self-adjusting (self-centeAng if the resistors 38 and 40 have the same resistance) propeny of the apparatus enables the apparatus to operate effectivelywith relatively arbitrary abso1ute va1ues of subscriber 1ine voltage, and hence arbit~ary subscriber line resistances and numbers and types of telephone sets connected to the subscAber line.
It should be recognized, however, that the arrangements described above do not necessarily respond to every situation in which ~ere is a change in the hook state of another te1ephone set cormected to the same subscriber line. In particular, if the other telephone set's hook state changes at su~stantially the s~ne time as a Cllange in ~e hook state of the telephone set containing the microprocesss)r and the apparatus of Fig. 1 or 2, this will not be recognized by th~ ar~angement described above. Thus the above arrangement provides a useful, rather ~an an absolutely reliable, indica~ion of the hook state of the other telephone set. It should be appreciated that the use~ulness of this indication can be enhanced by modifying the above ar~gement ~o monitor differentduratios of valid duration pulses and to respond accordin~y; such modification may facilitate the recognition of sirnultaneous hook state changes at more than one telephone set.
Furthermore, the above descliption relates only to the existence of one other telephone set connected to the subscriber line, whereas in practice several other telephone sets may be so connected and the hooX state of each of these may be independently changed. Obviously, the microprocessor may be arranged to track simultaneously the S hook state of a plurality of other telephone sets connected to the same subscriber line, and again in this case pulse durations may be monitored to assist in this tracking.
Numerous other modifications, variations, and adaptations may be made to the described embodiments without departing from the scope of the invention as defined in the claims.
Claims (25)
1. Apparatus for detecting changes in a subscriber line voltage between two wires of a telephone subscriber line, comprising:
a potential divider having first, second, and third tapping points, the second tapping point being between the first and third tapping points;
means for supplying to the potential divider a voltage dependent upon the subscriber line voltage;
a low pass filter circuit having an input connected to the second tapping point;a first voltage comparator, having inputs connected to the first tapping point and an output of the low pass filter circuit, for producing an output pulse in response to the subscriber line voltage falling; and a second voltage comparator, having inputs connected to the third tapping point and the output of the low pass filter circuit, for producing an output pulse in response to the subscriber line voltage rising.
a potential divider having first, second, and third tapping points, the second tapping point being between the first and third tapping points;
means for supplying to the potential divider a voltage dependent upon the subscriber line voltage;
a low pass filter circuit having an input connected to the second tapping point;a first voltage comparator, having inputs connected to the first tapping point and an output of the low pass filter circuit, for producing an output pulse in response to the subscriber line voltage falling; and a second voltage comparator, having inputs connected to the third tapping point and the output of the low pass filter circuit, for producing an output pulse in response to the subscriber line voltage rising.
2 . Apparatus as claimed in claim 1 wherein the means for supplying a voltage dependent upon the subscriber line voltage comprises a buffer having an input coupled to the telephone subscriber line and an output connected to the potential divider.
3 . Apparatus as claimed in claim 2 wherein the means for supplying a voltage dependent upon the subscriber line voltage further comprises two resistances forming a second potential divider coupled to the telephone subscriber line, the input of the buffer being connected to a junction between the two resistances.
4. Apparatus as claimed in claim 3 wherein the means for supplying a voltage dependent upon the subscriber line voltage further comprises a capacitor connected in parallel with one of the two resistances for attenuating audio frequency signals.
5 . Apparatus as claimed in claim 3 wherein the means for supplying a voltage dependent upon the subscriber line voltage further comprises a diode bridge via which the two resistances forming the second potential divider are coupled to the telephone subscriber line.
6. Apparatus as claimed in any of claims 1 to 5 wherein the means for supplying a voltage dependent upon the subscriber line voltage comprises means for attenuating audio frequency signals.
7 . Apparatus as claimed in any of claims 1 to 5 wherein the low pass filter circuit comprises a series resistor connected between the input and the output of the filter circuit, and a shunt capacitor connected between the output of the filter circuit and a common rail.
8. Apparatus as claimed in any of claims 1 to 5 wherein the low pass filter circuit comprises a series resistor connected between the input and the output of the filter circuit, and a shunt capacitor connected between the output of the filter circuit and a common rail, and wherein the product of the resistance of the series resistor and the capacitance of the shunt capacitor of the filter circuit correspond to a time constant of the order of 200 ms.
9. Apparatus as claimed in any of claims 1 to 5 wherein the potential divider comprises a first resistance between the first and second tapping points and a second resistance between the second and third tapping points, the first resistance being less than the second resistance.
10. Apparatus for detecting changes in a subscriber line voltage between two wires of a telephone subscriber line, comprising:
means for attenuating audio frequency signals, a buffer coupled via the means for attenuating audio frequency signals to the subscriber line, the buffer having an output at which it produces, relative to a common rail, a voltage dependent upon the subscriber line voltage;
a potential divider having first, second, and third tapping points, the potential divider comprising a first resistor between the output of the buffer and the first tapping point, a second resistor between the first and second tapping points, a third resistor between the second and third tapping points, and a fourth resistor between the third tapping point and the common rail;
a smoothing circuit having an input connected to the second tapping point;
a first voltage comparator, having inputs connected to the first tapping point and an output of the smoothing circuit, for producing an output pulse in response to the subscriber line voltage falling; and a second voltage comparator, having inputs connected to the third tapping point and the output of the smoothing circuit, for producing an output pulse in response to the subscriber line voltage rising.
means for attenuating audio frequency signals, a buffer coupled via the means for attenuating audio frequency signals to the subscriber line, the buffer having an output at which it produces, relative to a common rail, a voltage dependent upon the subscriber line voltage;
a potential divider having first, second, and third tapping points, the potential divider comprising a first resistor between the output of the buffer and the first tapping point, a second resistor between the first and second tapping points, a third resistor between the second and third tapping points, and a fourth resistor between the third tapping point and the common rail;
a smoothing circuit having an input connected to the second tapping point;
a first voltage comparator, having inputs connected to the first tapping point and an output of the smoothing circuit, for producing an output pulse in response to the subscriber line voltage falling; and a second voltage comparator, having inputs connected to the third tapping point and the output of the smoothing circuit, for producing an output pulse in response to the subscriber line voltage rising.
11. Apparatus as claimed in claim 10 wherein each of the means for attenuating audio frequency signals and the smoothing circuit comprises a resistance and a capacitance whose product defines a respective time constant, and wherein the time constant of the smoothing circuit is of the order of ten times the time constant of the means for attenuating audio frequency signals.
12. A method of detecting, at a first telephone set connected to a two-wire telephone subscriber line, a change from an on-hook state to an off-hook state of a second telephone set connected to the same subscriber line, comprising the steps of:
(i) monitoring a subscriber line voltage between the two wires of the subscriber line;
(ii) producing a pulse at a first circuit terminal in response to the subscriber line voltage falling;
(iii) producing a pulse at a second circuit terminal in response to the subscriber line voltage rising;
(iv) in response to a pulse at the first terminal having at least a predetermined duration, starting a window timer period;
(v) detecting any pulse at the second terminal during the window timer period and, in the absence of such a pulse and in the absence of an action at the first telephone set causing the subscriber line voltage to fall, determining that the second telephone set is in an off-hook state.
(i) monitoring a subscriber line voltage between the two wires of the subscriber line;
(ii) producing a pulse at a first circuit terminal in response to the subscriber line voltage falling;
(iii) producing a pulse at a second circuit terminal in response to the subscriber line voltage rising;
(iv) in response to a pulse at the first terminal having at least a predetermined duration, starting a window timer period;
(v) detecting any pulse at the second terminal during the window timer period and, in the absence of such a pulse and in the absence of an action at the first telephone set causing the subscriber line voltage to fall, determining that the second telephone set is in an off-hook state.
13. A method as claimed in claim 12 wherein steps (i) to (iii) comprise the steps of:
deriving from the subscriber line voltage first, second, and third comparison voltages, the second comparison voltage being produced by smoothing a voltage between the first and third comparison voltages; and comparing the first and third comparison voltages with the second comparison voltage to produce the pulses at the first and second terminals respectively.
deriving from the subscriber line voltage first, second, and third comparison voltages, the second comparison voltage being produced by smoothing a voltage between the first and third comparison voltages; and comparing the first and third comparison voltages with the second comparison voltage to produce the pulses at the first and second terminals respectively.
14. A method as claimed in claim 13 wherein the second comparison voltage is produced by integrating a voltage between the first and third comparison voltages with an integration time constant of the order of 200 ms.
15. A method as claimed in claim 13 wherein the step of deriving the first, second, and third comparison voltages from the subscriber line voltage comprises the step of attenuating audio frequency signals.
16. A method as claimed in any of claims 12 to 15 wherein the window timer period is of the order of 600 ms.
17. A method as claimed in any of claims 12 to 15 wherein the predetermined duration is at least about 60 ms.
18. A method of detecting, at a first telephone set connected to a two-wire telephone subscriber line, hook state changes of a second telephone set connected to the same subscriber line, comprising the steps of:
(i) monitoring a subscriber line voltage between the two wires of the subscriber line;
(ii) producing a pulse at a first terminal in response to the subscriber line voltage falling;
(iii) producing a pulse at a second terminal in response to the subscriber line voltage rising;
(iv) detecting a first pulse at either of the first and the second terminals and noting at which terminal such a first pulse is produced;
(v) determining whether the pulse has at least a predetermined duration, if so proceeding to step (ix) and if not proceeding to step (vi);
(vi) detecting any pulse at either terminal starting within a predetermined cancellation period, in the absence of any pulse returning to step (iv) and in response to such a pulse proceeding to step (vii);
(vii) determining whether the most recent pulse has at least a predetermined duration, if so proceeding to step (viii) and if not returning to step (vi);
(viii) determining whether the most recent pulse is at the same terminal as that noted in step (iv) at which the first pulse was produced, if so proceeding to step (ix) and if not returning to step (iv);
(ix) detecting any pulse, at the other terminal than that noted in step (iv) at which the first pulse was produced, starting within a predetermined window period, in the absence of any pulse proceeding to step (x) and in the presence of such a pulse returning to step (iv);
(x) in the absence of any action at the first telephone set to cause the first pulse detected in step (iv), determining that the second telephone set is of off-hook if the first pulse was produced at the first terminal, and determining that the second telephone set is on-hook if the first pulse was at the second terminal; and (xi) returning to step (iv).
(i) monitoring a subscriber line voltage between the two wires of the subscriber line;
(ii) producing a pulse at a first terminal in response to the subscriber line voltage falling;
(iii) producing a pulse at a second terminal in response to the subscriber line voltage rising;
(iv) detecting a first pulse at either of the first and the second terminals and noting at which terminal such a first pulse is produced;
(v) determining whether the pulse has at least a predetermined duration, if so proceeding to step (ix) and if not proceeding to step (vi);
(vi) detecting any pulse at either terminal starting within a predetermined cancellation period, in the absence of any pulse returning to step (iv) and in response to such a pulse proceeding to step (vii);
(vii) determining whether the most recent pulse has at least a predetermined duration, if so proceeding to step (viii) and if not returning to step (vi);
(viii) determining whether the most recent pulse is at the same terminal as that noted in step (iv) at which the first pulse was produced, if so proceeding to step (ix) and if not returning to step (iv);
(ix) detecting any pulse, at the other terminal than that noted in step (iv) at which the first pulse was produced, starting within a predetermined window period, in the absence of any pulse proceeding to step (x) and in the presence of such a pulse returning to step (iv);
(x) in the absence of any action at the first telephone set to cause the first pulse detected in step (iv), determining that the second telephone set is of off-hook if the first pulse was produced at the first terminal, and determining that the second telephone set is on-hook if the first pulse was at the second terminal; and (xi) returning to step (iv).
19. A method as claimed in claim 18 wherein steps (i) to (iii) comprise the steps of:
deriving from the subscriber line voltage first, second, and third comparison voltages, the second comparison voltage being produced by smoothing a voltage between the first and third comparison voltages; and comparing the fist and third comparison voltages with the second comparison voltage to produce the pulses at the first and second terminals respectively.
deriving from the subscriber line voltage first, second, and third comparison voltages, the second comparison voltage being produced by smoothing a voltage between the first and third comparison voltages; and comparing the fist and third comparison voltages with the second comparison voltage to produce the pulses at the first and second terminals respectively.
20. A method as claimed in claim 19 wherein the second comparison voltage is produced by integrating a voltage between the first and third comparison voltages with an integration time constant of the order of 200 ms.
21. A method as claimed in claim 19 wherein the step of deriving the first, second, and third comparison voltages from the subscriber line voltage comprises the step of attenuating audio frequency signals.
22. A method as claimed in any of claims 18 to 21 wherein the predetermined duration in each of steps (v) and (vii) is at least about 10 ms.
23. A method as claimed in any of claims 18 to 21 wherein the predetermined duration in each of steps (v) and (vii) is at least about 60 ms.
24. A method as claimed in any of claims 18 to 21 wherein the predetermined cancellation period in step (vi) is of the order of 1.2 seconds.
25. A method as claimed in any of claims 18 to 21 wherein the predetermined window period in step (ix) is of the order of 600 ms.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2032651 CA2032651C (en) | 1990-12-19 | 1990-12-19 | Telephone subscriber line voltage change detection |
| JP35234691A JPH0541732A (en) | 1990-12-19 | 1991-12-13 | Device and method of detecting voltage change of telephone subscriber line |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2032651 CA2032651C (en) | 1990-12-19 | 1990-12-19 | Telephone subscriber line voltage change detection |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2032651A1 CA2032651A1 (en) | 1992-06-20 |
| CA2032651C true CA2032651C (en) | 1995-02-14 |
Family
ID=4146692
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2032651 Expired - Fee Related CA2032651C (en) | 1990-12-19 | 1990-12-19 | Telephone subscriber line voltage change detection |
Country Status (2)
| Country | Link |
|---|---|
| JP (1) | JPH0541732A (en) |
| CA (1) | CA2032651C (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP6323021B2 (en) * | 2014-01-20 | 2018-05-16 | オムロン株式会社 | Object detection sensor and game machine |
-
1990
- 1990-12-19 CA CA 2032651 patent/CA2032651C/en not_active Expired - Fee Related
-
1991
- 1991-12-13 JP JP35234691A patent/JPH0541732A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| CA2032651A1 (en) | 1992-06-20 |
| JPH0541732A (en) | 1993-02-19 |
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| EEER | Examination request | ||
| MKLA | Lapsed |