CA1089131A - Method and apparatus for compensating for variations in subscribers line lengths and apparatus - Google Patents
Method and apparatus for compensating for variations in subscribers line lengths and apparatusInfo
- Publication number
- CA1089131A CA1089131A CA286,926A CA286926A CA1089131A CA 1089131 A CA1089131 A CA 1089131A CA 286926 A CA286926 A CA 286926A CA 1089131 A CA1089131 A CA 1089131A
- Authority
- CA
- Canada
- Prior art keywords
- digital
- line
- gain
- regulator
- attenuation
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04M—TELEPHONIC COMMUNICATION
- H04M19/00—Current supply arrangements for telephone systems
- H04M19/001—Current supply source at the exchanger providing current to substations
- H04M19/005—Feeding arrangements without the use of line transformers
Landscapes
- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Signal Processing (AREA)
- Interface Circuits In Exchanges (AREA)
Abstract
ABSTRACT
A method and apparatus for compensating for variations in subscribers apparatus and individual line lengths by measuring during the set up period of a call, the voltage variation across the feed at constant current.
This voltage variation is then used, by application to a simple A/D converter, to provide a coarse digital evaluation of the line length.
A method and apparatus for compensating for variations in subscribers apparatus and individual line lengths by measuring during the set up period of a call, the voltage variation across the feed at constant current.
This voltage variation is then used, by application to a simple A/D converter, to provide a coarse digital evaluation of the line length.
Description
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.
c ~ - 3 ~
, . ",: , -~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 ~
, ;`,f,'': . ' ,.',' ,,~,~
.~'~' ~'.
''~ :' '~' . ' '.
~:~
~ ~' :.
-3a-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.
.:--, ~ .
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.
~ ' - .- - .~
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 ~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 -~........................................................................... .
;~ 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).
, ''~, ;
~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.
,, , 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. ::~
, .. . . ....
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 _9_ ~
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.
`''- ':
}~
~ ~' ',' ' ' ~' i, ,.
~ , .
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.
c ~ - 3 ~
, . ",: , -~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 ~
, ;`,f,'': . ' ,.',' ,,~,~
.~'~' ~'.
''~ :' '~' . ' '.
~:~
~ ~' :.
-3a-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.
.:--, ~ .
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.
~ ' - .- - .~
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 ~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 -~........................................................................... .
;~ 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).
, ''~, ;
~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.
,, , 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. ::~
, .. . . ....
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 _9_ ~
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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Claims (6)
1. 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 and 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 representation of the line r impedance, and applying said digital representation to a digital gain/attenuation regulator for the selection of a digital gain/attenuation characteristic corresponding to said line impedance.
2. A method according to Claim 1 including the steps of deriving from a memory a digital representation of subscriber instrument sensitivity, combining said digital representations of line impedance and instrument sensitivity, and selecting said digital gain/attenuation characteristic is accordance with the combined representation.
3. A method according to Claim 1 or Claim 2 wherein said digital gain/attenuation regulator comprises signal level translator means having a plurality of memory fields, each of which fields stores a range of output signal values related by a predetermined gain/attenuation value, different for each field, to a corresponding range of input signal values, and wherein said digital representation selects a corresponding one of said fields.
4. In a time-division-multiplex telephone system in which subscriber line circuits have different loss characteristics, apparatus for compensating for differences in line impedances includes constant current generating means for connection to a subscriber's line and supplying a constant current of a value outside the operating range of any regulator in the subscriber's instrument, voltage detection means for detecting the voltage developed across said constant current generating means, said voltage detection means including an analogue/digital converter.
providing a digital representation of the line impedance, the apparatus further including a digital gain/attenuation regulator providing gain/attenuation at a level dependent upon said digital representation, for the purpose of achieving a standard relation between sound levels at the subscriber's instrument and corresponding signal levels on the exchange side of said regulator.
providing a digital representation of the line impedance, the apparatus further including a digital gain/attenuation regulator providing gain/attenuation at a level dependent upon said digital representation, for the purpose of achieving a standard relation between sound levels at the subscriber's instrument and corresponding signal levels on the exchange side of said regulator.
5. Apparatus according to Claim 4, including digital storage means containing a digital representation of the class of the subscribers instruments, means for combining the digital representations of the line impedance and of the class of subscribers instrument, and means for controlling said digital gain/attenuator regulator in response to said combined digital representations.
6. Apparatus according to Claim 4, wherein said digital gain/attenuation regulator comprises signal level translator means having a plurality Or memory fields, each of which fields stores a range of output signal values related by a predetermined gain/attenuation value, different for each field, to a corresponding range of input signal values, and wherein said digital representation constitutes an address for selecting a particular one of the memory fields.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB38555/76A GB1584778A (en) | 1976-09-17 | 1976-09-17 | Telecommunication exchanges |
| GB38555/76 | 1976-09-17 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1089131A true CA1089131A (en) | 1980-11-04 |
Family
ID=10404250
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA286,926A Expired CA1089131A (en) | 1976-09-17 | 1977-09-16 | Method and apparatus for compensating for variations in subscribers line lengths and apparatus |
Country Status (8)
| Country | Link |
|---|---|
| BE (1) | BE858816A (en) |
| CA (1) | CA1089131A (en) |
| DE (1) | DE2741667A1 (en) |
| FR (1) | FR2365252A1 (en) |
| GB (1) | GB1584778A (en) |
| NL (1) | NL7710118A (en) |
| SE (1) | SE425537B (en) |
| ZA (1) | ZA775490B (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4254305A (en) * | 1979-02-07 | 1981-03-03 | International Telephone And Telegraph Corporation | Current limited subscriber line feed circuit |
| US4272648A (en) * | 1979-11-28 | 1981-06-09 | International Telephone And Telegraph Corporation | Gain control apparatus for digital telephone line circuits |
| DE3902439A1 (en) * | 1989-01-27 | 1990-08-02 | Basf Ag | PLANT PROTECTIVE AGENTS BASED ON 1-ARYL OR 1-HETARYLIMIDAZOLE CARBONIC ACID ESTERS |
| US5274703A (en) * | 1991-05-23 | 1993-12-28 | At&T Bell Laboratories | Automatic loss control circuit |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1299735B (en) * | 1965-08-04 | 1969-07-24 | Siemens Ag | Circuit arrangement for telephone systems, in particular private branch exchanges with telephone stations, which have an automatic attenuation compensation |
| US3781481A (en) * | 1971-09-30 | 1973-12-25 | Stromberg Carlson Corp | Amplifier select and control circuit |
| DE2250651B2 (en) * | 1972-10-16 | 1975-06-19 | Telefonbau Und Normalzeit Gmbh, 6000 Frankfurt | Circuit arrangement for attenuation compensation in a private branch exchange with four-wire connection paths |
| DE2347696C3 (en) * | 1973-09-21 | 1978-12-21 | Siemens Ag, 1000 Berlin Und 8000 Muenchen | Circuit arrangement for feeding subscriber lines in telephone switching systems |
| US3903378A (en) * | 1974-03-27 | 1975-09-02 | Gte Automatic Electric Lab Inc | Arrangement for controlling the gain of two-way amplifiers in accordance with loop lengths |
-
1976
- 1976-09-17 GB GB38555/76A patent/GB1584778A/en not_active Expired
-
1977
- 1977-09-13 ZA ZA00775490A patent/ZA775490B/en unknown
- 1977-09-15 NL NL7710118A patent/NL7710118A/en not_active Application Discontinuation
- 1977-09-15 SE SE7710346A patent/SE425537B/en not_active IP Right Cessation
- 1977-09-16 CA CA286,926A patent/CA1089131A/en not_active Expired
- 1977-09-16 FR FR7728078A patent/FR2365252A1/en active Granted
- 1977-09-16 DE DE19772741667 patent/DE2741667A1/en active Granted
- 1977-09-16 BE BE181006A patent/BE858816A/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| FR2365252B1 (en) | 1982-03-26 |
| GB1584778A (en) | 1981-02-18 |
| SE425537B (en) | 1982-10-04 |
| FR2365252A1 (en) | 1978-04-14 |
| NL7710118A (en) | 1978-03-21 |
| SE7710346L (en) | 1978-03-18 |
| BE858816A (en) | 1978-01-16 |
| DE2741667C2 (en) | 1987-09-24 |
| ZA775490B (en) | 1978-07-26 |
| DE2741667A1 (en) | 1978-03-23 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |