Note: Descriptions are shown in the official language in which they were submitted.
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Case 928
The present invention relates to an inter~ace
circuit for use with a communications signal generator and
two signal lines for the transmission of a communications
signal thereon, said communications signal having an AC
and a DC component and more particularly to interface
circuits of this type which includes a DC component
setting means for setting the relationship between the
voltage and the current of said DC component. The
interface circuit of the present invention is particulary
adap~ed for use in a telephone system, and will be
described in relation to that exemplary use, but it is to
be appreciated that the invention is not limited to this
particular use.
Conventional telephone systems consist of a number of
signal generation/reception stations and associated
speakers and microphones, in the form of handsets.
The signal created when a user speaks into a
microphone is amplified and combined with a carrier
waveform.
This modulated signal is then transmitted to another
user via two signal lines. At another station the signal
is demodulated and amplified through a speaker in that
station for reception by that user.
The modulated signal thus transmitted is requiried b~
the communications standards in each country to meet
certain specifications.
For example certain ratios of DC current to voltage
between the signal lines that are not permitted. Figure 1
shows an example of a DC current-voltage characteristic,
the shaded regions indicating unacceptable values of
current/voltage which may not be transmitted over the
signal lines. In addition, it is required that the AC
component of the modulated signal and in particular the
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AC-impedance of the signal generation/reception station,
as presented to a modulated signal transmitted on the
signal lines, meet the communications standards of these
specifications.
Conventional telephone systems are designed to meet
only one such set of specifications and are thus only
capable of use in one country, or in countries with
identical specifications. If the telephone system is to be
us~d where other specifications exist, the circuitry of
the signal generation~reception stations ~ust be analysed,
redesigned and modified.
Such redesign is time-consuming, expensive and can
only be performed by a person skilled in electronic
circuit design. Further the internal electronic circuitry
of a telephone signal generation/reception station is not
readily accessible to purchasers of telephone systems. In
order to modify such internal electronic circuitry,
special tools and expertise are required, and such
modifications can generally only be performed by skilled
tradesman. The expense and inconvenience of such
operations has thus meant that prior art telephone systems
may not easily be used in countries with differing
specifications.
An object o~ the present invention is to overcome or
alleviate the above problems in prior art communications
systems.
It is a further object of the present invention to
provide a system for permittin~ communications systems,
such as telephone systems, ~o ~e easily utilised in
countries having differing communications si~nal
speci~ications.
Another object of the present invention is to provide
a system for permitting such utilisation of telephone
systems which system may be conveniently, easily and
inexpensively effected by purchasers and users of the
telephone system.
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The present invention provides an interface circuitcharacterised in that the DC component setting means
comprises a current source connectable between said signal
lines for supplying said DC component with a constant DC
current, said current source being responsive to said DC
component voltage such that it is operable only when said
DC component voltage exceeds a predetermined value.
In one embodiment of the present invention, the
interface circuit includes DC component setting means for
setting the relationship between the voltage and current
of the DC component of the communications signal. A simple
circuit may be provided which has no significant effect on
other functions of a communications signal generation. The
voltage and impedance elements of the circuit may b~s
selected to satisfy the relationship between the DC
component voltage and current signal as required by the
communications signal specification in the area of
operation.
In another embodiment of the present invention, the
2~ interface circuit includes AC component setting means for
setting the relationship between the voltage and current
of the AC component of the communications signal. Another
simple circuit may be provided whose elements may be
selected relatively independently from their influence on
other parts of the circuit.
The following description refers in more detail to
the various features of the communication inter~ace
circuit of the present invention. To facilitate an
understanding of t~e invention reference is made in the
description to the accompanying drawings where the
communication interface circuit is illustrated in a
prefered embodiment. It is to be understood that the
communication interface circuit is not limited to the
prefered embodiment as illustrated in the drawings.
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n the drawings :
Figure 1 is a graph illustrating an example of the DC
current-voltage characteristic re~uired by communications
signal specifications.
Figure 2 is a diagrammatic view illustrating an
interface circuit according to the present invention.
F.igure 3A is a circuit diagram of a DC component
sett.ing means of the interface circuit of figure 2.
Figure 3B is a diagram representing the current~
voltage characteristic of the DC component setting means
of figure 3A.
Figure 4 is a diagram representing the circuit
diagram of another DC component setting means of the
interface circuit of figure 2.
Figure 5A is a circuit diagram of yet another DC
component setting means of the interface circuit of
figure 2.
Figure 5B is a current-voltage characteristic of the
DC component setting means of figure 5A.
Figure 6 is a circuit diagram of an AC component
setting means of the interface circuit of figure 2.
Figure 7 is a circuit diagram of the interface
circuit of figure 2.
Figure 2 shows a telephone system 1 including a
signal generation/reception station 2, a handset 3 and a
communication signal transmission means 4 comprising two
signal lines 4a and 4b respectively. Any number of similar
stations 2 may be connected to the signal lines ~a and 4b
in a like manner. The telephone system 1 further includes
an interface circuit 5 connected between the signal
generation~reception station 2 and the signal lines 4a and
4b. In this embodiment the interface circuit 5 includes an
isolation circuit 6 which provides high isolation
resistance and therefore electrical protection for the
station 2 from the rest of the telephone system.
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The interface circuit 5 comprlses DC component
setting means 7 and AC component setting means 8 connected
between the lines 4a and 4b for controlling respectively
the DC component and the ~C component of the signal.
DC component setting means 7 and AC component setting
means 8 are connected between signal lines 4a and 4b.
Although both DC component setting means 7 and AC
component settin~ means 8 are shown in Figure 2, it is to
be appreciated that both are embodiments only of the
characteristic setting means o~ the interface circuit of
the present invention. In another embodiment, not shown,
~C component setting means 7 only or AC component setting
means 8 only may be included.
The circuit of Figure 3A shows an embodiment of DC
component setting means 7.
The DC component setting means shown in this figure
comprises a current source 9 having a voltage regulator
circuit 12, a conductor 10, a switching transistor 11 and
a biasing resistor 17.
The voltage regulator circuit 12 comprises a biasing
resistor 14 and a reverse-biased Zener diode 13.
The biasing resistor 14 is connected between a
terminal 15 and the cathode of the Zener diode 13. The
collector of the switching transistor 11 is connec~ed to a
terminal of the biasing resistor 17. The other biasing
resistor 17 is connected with the anode of the Zener diode
13, to the signal line 4b.
The terminal 15 which receives the voltage U as well
as the conductor 10 are connected to a positive current
source ~rom the circuit of figure 2.
The terminal 15 and the conductor 10 are connected to
thq signal line 4a.
The current source 9 is adapted to supply a constant
current Ic across the conductor 10 when the switching
element 11 is caused to conduct.
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The voltage regulator 12 which, in the embodiment
shown, comprises a single voltage regulating element 13
and a biasing resistor 14, controls the operation the
switching element llo
As shown, voltage regulating element 13, and
switching element 11, may comprise respectively a zener
diode and switching transistor, but other circuit elements
may be used as will be appreciated by those skilled in
art. When the voltage between the two signal lines 4a and
4b in Figure 2 is such that the voltage Uref at terminal
16 is greater than the minimum voltage Umin required to
switch on switching transistor 11, constant current Ic
flows through circuit branch 10. AS the voltage across the
base-collector region o~ transistor 11 @ 0,7V, Ic is given
by :
Ic = (Uref - 0.7) / Re (where Re = the resistance
of the biasing resistor 17)
= constant
when the voltage at terminal 15 is greater than Umin.
The relationship between the current and voltage of
the circuit shown in Figure 3A is given in the current-
voltage characteristic of Figure 3s.
When the voltage between the lines 4a and 4b is less
than Umin, the voltage at terminal 16 is not sufficient to
turn switching transistor 11 on and no current flows in
the circuit in circuit branch 10. However when the voltage
U exceeds Umin, switching transistor 11 turns on and
constant DC current.Ic flows.In the circuit of Figure 3~,
the zener diode of voltage regulating element 13, and
biasing resistor 14 may ~e appropriately chosen to select
the desired voltage Uref at which switching transistor 11
conducts and the resistor 17 may be appropriately chosen
to set the value of the constant DC current Ic which flows
along circuit branch 10.
Conveniently a circuit exhibiting the voltage-current
characteristic shown in Figure 3B may be utilised to meet
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the communications signal specifications of many countries
for the D~ component of a telephone communications signal.
As can be s~en in Figure 3B, such a circuit may be used to
avoid the ~orbidden regions on a current-voltage
characteristic whilst otherwise providing a constant DC
current.
In addition, due to the short response time of
switching transistor 11, a step waveform is provided by
the circuit of Figure 3A. The AC resistance of such a step
waveform is sufficiently high so as to have no signifiant
in~luence on the AC response of the remaining circuitry in
a telephone system and may therefore be designed
independently from the circuitly affected by the AC
component of a telephone communications signal.
In other that a telephone system including a circuit
such as that shown in Figure 3A may be utilized in areas
with differing communications signal specifications at
least one of resistors 14 and 17 and the voltage
regulating element 13 may be replaced. Due to the step
response of the characteristic shown in Figure 3B, the
changing of components in this circuit has minimal effect
on the circuitry affected by the AC component of a
telephone communications signal and may easily be
effected.
In figure 4 an alternative to the simple replacement
of components is provided. DC component setting means 7 is
shown connected between ~he signal lines 4a and ~b.
The DC component setting means 7 comprise three zener
diodes 22, 23 and 24 connected in series, such that the
cathode of one diode is connected to the anode of an
adjacent diode, switching transistors 21, 25, 26, 31 and
32, a conductor 10, a biasing resistor RV and biasing
resistors 30, 33 and 34.
As in figure 3A, the collector of the switching
transistor 21 is connected to the conductor 10. The
conductor 10 is connected at its other end to the line 4a.
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The terminals of the biasing 30, 33 and 34 are
connected together to the emitter of the switching
transistor 21. The other terminal of the biasing resistor
is connected to the line 4b, whereas the other
terminals of the biasing resistors 33 and 34 axe
respectively connected to the collectors of the switching
transistors 31 and 32. The emitters of the switching
transistors 31 and 32 are each connected to the line 4a.
The bases of the switching transistors 31 and 32 are
respectively connected ~o thQ inputs 35 and 36.
The biasing resistor Rv is connected betwe~n the line
~a (terminal lS) and the cathode of the zener diode 22,
The anode of the zener diode 24 is connected to the line
4b.
The collector of the switching transistor 25 is
connected between the anode of the zener diode 22 and the
cathode of the zener diode 23, whereas the emitter of the
switching transistor 25 is connected to the collector of
the switching transistor 26 between the anode of the zener
diode 23 and the cathode of the æener diode 24~
The emitter of the transistor 26 is connected to the
line ~b. The bases of the transistors 25 and 26 are
connected respectively to the inputs 28 and 29. When the
voltage of the DC component of the con~unication signal
between terminals 18 and 19 is at a level sufficient for
the voltage at terminal 20 to exceed Uref, switching
transis~or 21 is turned on. The value of Uref, and
subse~uently Umin, can be determined by the selection of
either voltage regulating element (zener diode) 22 alone,
or in conjunction with either of the voltage regulating
elements 23 or 24, or these two elernents combined. The
circuit according to this embodiment of the invention also
includes transistors 25 and 26 in order to either include
or exclude zener diodes 23 and 24 from voltage regulator
27, and may be turned on by the application of a signal to
the base of transistors 25 and 26 via terminals 28 and 29.
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In one embodiment of the invention, this signal is
applied by way of a jumper connecting terminals 28 or 29
to the supply voltage of the telephone circuitry. In
another embodiment, terminals 28 and 29 may be connected
to a data bus and signals may be provided to transistors
25 and 26 by appropriate software. ~s will be well
appreciated by the skilled worker in the field, many other
means of applying these signals are possible.
When switching transistor 21 is t~lrned on, current
flows through resistor 30. Transistors 31 and 32 are
additionally provided however so that curren~ may also
flow through either or both of resistors 33 and 34.
Transistors 31 and 32 are turned on by the application o~
a si~nal to terminals 35 and 36.
In this way, the minimum voltage Umin between
terminals 18 and 19 at which constant current Ic flows in
circuit brauch 10 may be adjusted, and the value of the
constant current Ic itself may be adjusted, without the
need to physically replace circuit components.
Figure 5A shows another embodiment of a DC component
setting means of the interface circuit of the present
invention. This circuit includes current sources 37 and 38
of the type previously described, connected between signal
lines ~a and 4b. One can also use more than two such
current sources if necessary.
Voltage regulating elements 39 and 40, and resistors
41, 42, 43 and 44 are chosen so that switching transistor
45 of current source 37 turns on and conducts current I1
at a lower voltage level U1 than does switching transistor
46 of current source 38 at voltage level U2.
Similarly, voltage regulating element 39 and resistor
43 are chosen to provide a level of constant DC current
I1, whereas voltage regulating element ~0 and resistor 44
are chosen to provide a level of constant DC current I2
~hich is added to current I1, when transistor 46 conducts.
In this manner, and as shown by the current voltage
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characteristic of Figure 5B, a characteristic of multiple
steps may be formed which by the appropriate selection of
circuit elements can be made to match the com~unications
si~nal specifications in a variety of areas of operation.
As shown in Figure 4, current sources 37 and 38 may
include suitable adjustment means such as additional
voltage regulating elements and resistors so that one
telephone system can be used to meet the comm~nications
signal specifications in a ~-ariety of areas of operation
without the need for the physical rewiring of telephone
circuitry.
The circuit of Figure 6 shows an embodiment of the AC
setting means 8 of Figure 2. Figure 6 shows impedance
circuit branch 47 connected between signal lines 4.
Impedance circuit branch 47 includes impedance elements
48, g9 and 50 and switching transistor 51.
In the circuit 47, the resistor 48 is connected to
the collector of the switching transistor 51, and the
other terminal of the capacitor 50 and the resistor 49 are
connected together to the emitter of the switching
transistor 51.
A terminal of the circuit 47 is connected to the line
4a whilst its other terminal, for which the capacitor 50
and the resistor 49 are connected, is connected to the
line 4b.
The AC component setting means B also comprise an
input terminal 52 connected by a coupling capacitor 53 to
the base of the transistor 51.
A resistor 5A is connected to the base of the
transistor 51 and is also connected to a constant DC
current source (not shown).
A communications signal is transmitted to terminal
52, through coupling capacitor 53, to the base of
switching transistor 51 and amplifiying the communications
signal through circuit branch 47. Circuit branch 5A
supplies a porition of constant DC current Idc from the DC
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setting means 7 (as shown in Fiyure 2) to the base of
switching transistor 51 in order to ensure the switching
transistor 51 operates in the active region, so that the
current of impedance circuit branch 47 remains relatively
constant over a wide voltage range between signal lines 4.
As will be appreciated by those skilled in the art,
constant DC current Idc may be provided in a number of
ways and DC setting means 7 is not required to provide
Idc.
As shown in Figure 4, additional impedance elements
may be added or excl~lded from impedance circuit branch ~7
using additional switching transistors so that the
current-voltage characteristic required in variety of
areas of operation may more easily be met.
lS Fi.gure 7 shows a detailed circuit diagram of one
embodiment of the interface circuit of the present
invention. A signal, representing a spoken message from a
microphone for example, is received at input 55 and
transmitted to amplifier 56 where the si~nal is amplified.
Although one particular example of an amplifier circui~ is
shown, other configurations may equally be employed.
Optoelectronic coupler 57 provides high isolation
resistance and electrical prokection for the telephone
system. Having passed through optoelectronic coupler 57,
the transmitted signal is amplified by AC amplifier 58,
transistor 59 conducting when the transmitted signal is
present, ac impedance elements 60, 61 and 62 acting to
place the signal on signal lines 63 and 64 at a desired
voltage. In order that the signal may be fed back to the
user, such as through the speaker in a handset, or so that
a signal received other than that transmitted by the
interface circuit may be heard by a user, ac impedance
elements 61 and 62 are appropriately sized so that most
current flowing through transistor 59 also flows through
ac impedance element 62. This signal is than transmitted
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across optoelectronic coupler 15 and amplified by
amplifier circuit 76 be~ore being received at output 65.
In addition to ac amplifier 58, the signal placed on
lines 63 and 6~ is combined with that produced by current
sources 66 and 67. Current sources 66 and 67 produce a DC
characteristic such as that shown in Figure 5B. In
addition current sources 66 and 67 may contain a number of
additional ~oltage regulating elements and for example
impedance elements, such as shown in Figure 4, which may
be included or excluded in the interface circuit by
appropriate signals from a central processing unit
elsewhere in the telephone s~stem. Alternatively, an~
additional elements may be selected by jumpers, or other
manual control elements, as well be appreciated by those
s~illed in the elevant art.
In this particular embodiment of the present
invention, additional voltage regulating element 6~ may be
included or excluded from current source 67 by controlling
switching transistor 69. Jumper connection 70 may be used
to connect the base of switching transistor 69, through
optoelectronic coupler 71, to either ground or supply
voltage Vcc. In this manner, the characteristic of the
interface circuit of the present invention may be altered
~or various countries to match the specifications of those
countries.
The signal from AC amplifier 58 and current sources
66 and 67 is placed on lines 63 and 64, and transmitted on
signal lines 4, having passed through bridge rectifier '72.
Additional circuit elements such as line switch 73 and
ring detection circuit 74 are included in the circuit
diagram of Figure 7 or completeness but do not form part
of the present invention.
While the preferred embodiment of the present
invention has been described, it is to be understood that
the in~ention is not limited thereto, and may be otherwise
embodied within scope of the following claims.
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