Note: Descriptions are shown in the official language in which they were submitted.
10~39131
2he present invention relates to telecommunication
exchanges and more particularly to the problems encountered
in providing compatibility between new equipment inserted
into an existing telecommunications network.
In particular, in the existing telephone system the
present subscribers instruments represent a considerable
i~vestment and in designing a new exchange it is difficult
not to impose restrictions on such design by the necessity
of compatible working with the subscribers instruments.
A number of alterations have already been proposed to over-ome ~-
this problem including the conversion of bells into tone
activated "callers" when every subscriber has an M.~. Key phone. `
One problem which remains however is the compatibility between
the telephone transducers and the exchan~e equipment. ~he
problem is that it is too expe~sive to change all the subscribexs
instruments when a new exchange is installed so the tendency has
been to design the exchange to be compatible with the old ~ ;
subscribers instruments. ~ater, when the time comes to change
the subscribers instruments, possibly on an individual ~ -
replacement basis, they have to be compatible with the exchange
the design of which was made compatible with the old -~
subscribers instruments etc. -
,:
~` One possible solution to this problem is to impose a~
amplification or attenuation factor on signals received from r
sent to a subscribers instrument. In the case of a large
exchange however~ it may be impractical to provide all the
information relating to a line-circuit and its losses ~rom a ~ ~`
theoretical calculation of the subscribers position with respect
to the excha~ge, particularly since the line loss for a -~
particular subscriber may vary depending on the routing of
cables and the line impedance may be increased by for example
the insertion of a new higher resistance cable in the
cable network.
~V~39131
It is therefore desirable to provide a relatively simple
method for compensating, at least approximately, for variations
in individual line-circuits. The present invention provides a
method and apparatus for this purpose.
In accordance with the invention there is provided in a
time-division multiplex telephone system in which subscribers
line circuits have different loss characteristics and in which a
gain/attenuation regulator is provided in the exchange, a method
of compensating for differences in line impedances, including
the steps of feeding a subscriber's line circuit with a constant
current of a value outside the operating range of any regulator
in the subscriber's instrument, detecting the voltage across the
constant current source at the exchange, applying said voltage
to an analogue/digital converter to provide a digital represent-
ation of the line impedance, and applying said digital represent-
ation to a digital gain/attenuation regulator for the selection
of a digital gain/attenuation characteristic corresponding to ~-
said line impedance. ~ ~ -
According to the present invention there is provided a
method of compensating for variations in individual telephone ;
subscribers line circuits, including the steps of feeding a
subscriber's line circuit with a constant current of a value
outside the operating range of any regulator in the subscriber's
instrument, detecting the voltage across the constant current
feed at the exchange, and controlling the gain/attenuation of a
regularor in the exchange in dependence upon the magnitude of `~
said voltage to tend to maintain a standard relation between ; `~
sound levels at the subscriber's instrument and corresponding
signal levels on the exchange side of said regulator.
The method preferably includes the step of controlling
said gain/attenuation in dependence upon the known class of
subscriber's instrument, in addition.
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~08913~
The telephone system may be a time-division-multiplex
system in which said regulator comprises a signal level trans- :
lator having a plurality of input/output signal-level gain/ -~
attenuation values, said voltage being quantised and applied to
the regulator to select a gain/attenuation value.
According to another aspect of the invention, apparatus i:
for compensating for variations in individual telephone subscrib-
ers line circuits, includes constant current generating means
arranged for connection to a subscriber's line circuit and cap- -~
able of supplying a constant current of a value outside the
operating range of any regulator in the subscriber's instrument, ~ ~
voltage detecting means for detecting the voltage developed .~ ~ -
across said constant current generating means, and a regulator -
having ~
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108913il
controllable gain/attenuation and being controlled i~ dependence
upon the ma~nitude of said voltage, the arrangement being such
that a standard rel~tion between sound levels at the subscriber 18
instrument and corresponding signal levels o~ the e~change side
of said regulator is achieved or approached.
Preferably storage means is provided containing
information as to the class of the subscribers~ instruments
together with means for controlling said regulator in dependen-e
upon the class of the particular subscriber' 8 instrument bein~
0 set up for a call.
~ he regulator may comprise a signal-level translator
having a plurality of input/output signal-level gai~/attenuation
values, and there may be included quantising means whereby said
voltage is quantised to one of a plurality of values for the
selection of a corresponding gain/attenua~ion value.
~he apparatus may i~clude means for modifying the `
qua~tised voltage in dependence upon a voltage corresponding to
said class of instrument, the regulator being controlled by the
modified voltage. ~he regulator is preferably time shared with other
~20 subscribers' line circuits, the gain/attenuation value being
selected in accordance with each li~e-circuit.
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An embodiment of the present invention will now be
described by way of example with reference to the accompanying
drawings in which;-
Figure 1 shows symbolically a line-circuit including
a subscribers instrument, connected to a current generator and
voltage detection device according to the present i~vention; and ~ -
Figure 2 shows an exchange circuit connected to a
line circuit and regulating means according to the present invention.
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lV891~
Referring to ~igure 1, a constant current source C in
the exchange ~upplie~ a constant current I to the line ~ of a
line circuit LC, the latter including the line L and the
telephone instrument ~. ~he source C may be a constant current
source of the exchange used for normal line energisation, in which
case it must provide a constant current of a value which i8 outside
the operating range of an~ regulator iD tbe telephone instrument
and in particular, insufficient to operate such a regulator (which
may have been provided for a constant voltage supply system to
cater for low values of line length and consequent high line
curre~ts). Alternatively the source C may be a special source
provided purely for the purposes of the inve~tion (where for ~ -
example the normal source is a constant voltage source). If the
regulator is a non-linear shunt resistor, for example, it will
remain in its high resistance coDdition and this will be the
standard conditioD of all line circuits when u~der test. The
required current level will, of course, ~ary with the type and
manufacture of the equipme~t i~ question but a suitable value in
one know~ case is 25 mA. It must in ~ny event be outside the
range of operation of the instrument regulator and thus should
commonly be less tha~ about 50 mA. Instrument regulators do,
of course, vary according to the system for which they were
originally designed and the constant current level for the
purposes of the i~veution should be chose~ accordingly. A
current level above the useful range of the instrument regulator
may be possible in some cases but is clearly not preferred,
for reasons of power loss if no other.
~ he line resistance is represe~ted in ~igure 1 by the
lumped components ~ , the resista~ce of the telephone instrument
(i~ the con~ta~t current condition) being R~.
Clearly then, the voltage V across the curreDt source C
varies with this line circuit resistance and in fact is equal to
I(2RL ~ R~). The voltage may be measured on a meter M but
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~089131
primaril~ is detected and applied to a ~uanti~ing circuit Q the
output of which is a digital approximation of the detected
t voltage and thu~ of the combination of line circuit resistance
a~d telephone resistance.
Tbe process performed by the block Q may be performed `~
initially in respect of each subscriber during the period of
iDstallation of the subscribers equipment and the value of the
line loss may be permane~tly stored in the exchange. The value
would then onl~ be updated if a change in line loss was
10 suspected. However, this would have the disadvantage that if
as a result of an oversight a change in line conditions i5 not
reported the previously calculated line loss would be incorrect. ~ `
It is therefore preferred that the process performed by
the block Q for the value of line loss is performed each time
that a call is set up to or from a subscriber.
, . . .
It is conceivable that during the duration of a call
the length of line and hence the li~e loss may be altered.
. This can occur wheD on a PABX a call is transferred from an -
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;~ extenRio~ on a short line to an extension on a long li~e. Also
Y~` 20 it i8 possible in future excha~ges that automatic transfer of
calls from the subscribers normal number to a preselected or
~- predialled alternative number may alter the line length during
the period of the set up of the call. ~hus it may be advantageous -
to mo~itor the voltage drop across the ~ource C during a call
~ so that any necessary correction can be made to the determination
; of li~e losses.
The basic approach of this invention is that, ideally,
~ i .
- definea sound levels at the telephone transducers shall be
produced by, or shall produce, defined signal levels, a~d iD ~ :
;.` .
particular, PCM codes, in the exchange, irrespective of the line
length and characteristics and the class of telephone instrument ~r,
(i.e. the type of tra~sducers).
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~L0~913~
Xe~ulators are therefore employed in the excbange
which can adaust the signal level, received or transmittedt to
at least tend to achieve this result. Such re~llators may
operate in many ways on ~CM signals. Por example, shifting
the signal bit pattern by one bit produces either a 6 dB
attenuation on a 6 dB amplifi¢ation dependin~ on the direction
of shift-care must be taken ~ot to disturb the sign bit.
Constants can for example be added to or subtracted ~rom
the segment value.
The signal can be decompanded, operated o~
rithmetically and recompa~ded.
The incoming signal can be applied as an address to
a read only memory (ROM) producing a completely defined output,
the gain/attenuation being ~ixed by the stored output values
inserted against the input "addresses". In one example the
ROM contai~s 256 words of 8 bits, each word corresponding to a
PCM code. The ROM i8 addressed by the incoming PCM word.
~he address location in the ROM provides PCM code which has a~
appropriately attenuated or amplified value.
To achieve a raDge of traDslatio~ values from ~ ;~
atte~uation to amplification~, the ROM is divided iDto a rumber
of complete field~ each field giving a complete set of
translations of voltage values at a particular value of
attenuation or amplificatior, the attenuatio~/gain factor
varyi~g from field to field and having, say, values of -~
-lOdB, -5dB, O, 1 5dB a~d ~lOdB.
A corresponding range of fi~e control voltage values
in digital form is thus required, these control words beiDg
made up of a line length compone~t deriYed from the quantiser
Q of ~igure 1 a~d a telephone class component derived from
stored informatio~. Tbe latter adjusts tbe gain/attenuation
in dependenee UpOD the type of transducer and associated
circuitry of the instrument.
- 1089131
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This particular method of gain/attenuation adjustment
!, appears to be the one most generally applicable and most
j amenable to Large Scale Integration (~SI) techniques. It can
be used on a companded or a decompanded signal although the
number of addresses in the latter case would be much higher. .
A telephone system incorporating two such ~DM
regulators 20~ 21, one in the transmit and one in the re¢eive
path is shown in ~igure 2. ~he constant current source C of
Figure 1 is included in a constant current line feed and loop
detection circuit 22, and the quantising circuit Q includes
analogue switch 23 and an analogue to digital converter 24.
F ~he analogue to digital converter 24 provides the output
indication of the line length which corresponds to the ~ -
`~ anticipated line 109s in dbls.
There is provided an analogue switch 23 for each
line circuit, the switches 23 being selected by an address
supplied from central control as the particular call is set up. ~ :
At the same time, a store 32, containing a digital weighting ~
, ~
for each subscriber in respect of the class of telephone
instrument, is addressed to provide the particular weighting
as an output to a digital adder 33.
The analogue switch output is applied to a common
i. . . .
~:;; analogue/digital converter 24 which makes an approximate -~
(2-bit) digital conversion to cover the two levels of
attenuatio~ and two levels of gain. Zero gain is effected by
R select bit supplied from central control, which has the
:~ effect of by-passing the regulators 20, 21. :~:
~ .
~he 2-bit output of the adder 33 and the one 'select : ;~
~ bit' are entered in a register 35 the contents of which are then
~ 30 loaded into a 3 x 32 bit oyclic shi~t register 34 so that each
channel has a 3-bit regulator-select word associated with it. ::~
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1089i3~
The register 34 is synchronised with tho channel c~cle ~o that
the appropriate select word appears at its output in synchronism
with the occurrence of the associated channel. ~oading of the
register 34 at the appropriate channel period is effected by
channel counti~g and matching circuitry in known manner.
~he 3-bit output of the register 34 is applied t- the
regulators 20 and 21 to determine whether they are to be used -
for the particular channel (1 bit) and if so, what level
of gain or attenuation is to be applied. Accordingly, one
of four memory blocks in the regulator is selected by the
select word (2 bits), and in this block the coded signal
envelope is applied as an address to extract a corresponding
attenuated or amplified signal level. ~he original signal
level is thus modified in accordance with the line
characteristics and the type of telephone instrument. -~
It will be apparent that as far as the type of ;
telephone instrument is defined by its resistance, the type will ;~
be a factor in the voltage detecbed across the constant curre~t
source. However, the resistance of a telephone instrument is
, ~ .
only one factor in determining its sensitivity and it will be~
~ recalled that this invention is concerned with matching exchange
`~ signal levels with sound levels at the telephone instrument.
herefore it will be clear that the store 32 contains information ;~
as to instrument sensltivity quite apart from its resistance.
The modlfication provided by the store 32 content may be a simple
addition effected in the digital adder 33 or may consist o~ any
other kind of modification multiplication etc. so that the
modification may be greatly dependent on the significance of
the stored sensitivity factor.
3~ ~he remaining parts of the circuitry of ~igure 2 are
conventional, the analogue components including the li~e feed
and loop detector 22, hybrid 26 to separate the go and return
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10~39~31
paths, and filters 27 and 28. The digit~L components include
the coder 29 and decoder 30 associated with the in~i~idual
subscriber, a~d multiplexing circuitry 31 common to all of
the channels. ~he output of the digital multiplex circuit 31
is fed via the ~DM regulator 20 to the exchange and the input
of circuit 31 is connected to the exchange via the ~DM
regulator 21. .~
If ~he constant current generator, describea above for .
use in the test and impedance-adjusting process, is also employed -
: 10 as the normal line energising source, several advantages may
accrue~ ~he low level of (constant) current employed, say
25m~, involves a generally low power rating for the exchange
in comparison with constant voltage systems where much higher
line currents are commonly involved.
Another factor of some significance is that in a low- ~ .
value constant~current exchange-source system the source
resistance does not have to have a large heat-dissipation capacity
to cater f~r earth faults on the line. The constant curre~t
system of the invention therefore has valuable incidental
advantages.
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