Note: Descriptions are shown in the official language in which they were submitted.
~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~
11
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.
