CA2029140A1 - Control circuit for a solid state telephone line circuit - Google Patents
Control circuit for a solid state telephone line circuitInfo
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- CA2029140A1 CA2029140A1 CA 2029140 CA2029140A CA2029140A1 CA 2029140 A1 CA2029140 A1 CA 2029140A1 CA 2029140 CA2029140 CA 2029140 CA 2029140 A CA2029140 A CA 2029140A CA 2029140 A1 CA2029140 A1 CA 2029140A1
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- loop
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- interface
- subscriber
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Abstract
A CONTROL CIRCUIT FOR A SOLID STATE TELEPHONE LINE
CIRCUIT
ABSTRACT
A control circuit for a solid state line circuit is disclosed that includes, in combination, a logic interface circuit connected to a central controller. The logic interface circuit receives and stores digital data representing commands from the central controller and transmits status signals from the control circuit to the central controller. A DC loop control circuit provides loop control voltage to the interface circuit which adjusts the loop current fed to a subscriber loop. A
loop sense comparator receives a representation of the subscriber loop voltage and compares the loop voltage to a threshold. When the threshold is exceeded, the loop sense comparator transmits to the logic interface a status signal. A ring trip circuit monitors the subscriber loop and transmits to the logic interface a status signal representing an interruption in ringing current applied to the subscriber loop. An AC summing amplifier, amplifies voice signals transmitted on the subscriber loop and subtracts from the transmit voice signals, a receive voice component. A switched capacitor filter, under control of the logic interface, provides either loaded or non-loaded network line balance operation. The switched capacitor filter also receives a receive voice signal which it is uses to generate the subtraction signal used by the AC summing amplifier.
CIRCUIT
ABSTRACT
A control circuit for a solid state line circuit is disclosed that includes, in combination, a logic interface circuit connected to a central controller. The logic interface circuit receives and stores digital data representing commands from the central controller and transmits status signals from the control circuit to the central controller. A DC loop control circuit provides loop control voltage to the interface circuit which adjusts the loop current fed to a subscriber loop. A
loop sense comparator receives a representation of the subscriber loop voltage and compares the loop voltage to a threshold. When the threshold is exceeded, the loop sense comparator transmits to the logic interface a status signal. A ring trip circuit monitors the subscriber loop and transmits to the logic interface a status signal representing an interruption in ringing current applied to the subscriber loop. An AC summing amplifier, amplifies voice signals transmitted on the subscriber loop and subtracts from the transmit voice signals, a receive voice component. A switched capacitor filter, under control of the logic interface, provides either loaded or non-loaded network line balance operation. The switched capacitor filter also receives a receive voice signal which it is uses to generate the subtraction signal used by the AC summing amplifier.
Description
2~291~
A CONTROL CIRCUIT FOR A SOLID STATE TELEPHONE LINE
CIRCUIT
CROSS REFERENCE TO RELATED APPLICATIONS
Cross Reference is made to the related Canadian Patent Applications entitled: nA Solid State Telephone Line Circuit,~ (Attorney Docket 89-1-032), ~A High ~oltage Subscriber Line Interface Circuit,~ (Attorney Docket 89-1-034), ~A Circuit For Synthesizing An Impedance Across The Tip And Ring Leads Of A Telephone Line Circuit,~ (Attorney Docket 89-1-035), ~A Tip-Ring Short Detector and Power Shut-Down Circuit For A
Telephone Line Circuit, n (Attorney Docket 89-1-036), ~A
Thermal Protection Circuit For An Integrated Circuit Subscriber Line Interface, n (Attorney Docket 89-1-037), ~A Thermal Protection Arrangement For An Integrated Circuit Subscriber Line Interface, n (Attorney Docket 89-1-038), and ~A Ring Trip Detector For A Solid State Telephone Line Circuit,~ (Attorney Docket 89-1-040) filed on the same date, and by the same assignee as this Application.
BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates to the field of telecommunications and, more particularly, to a control circuit that provides the low voltage control functions for an integrated circuit subscriber line interface.
2. Description of the Prior Art ;
~ Telephone line circuits are customarily found in the telephone switching system or central office of a telecommunications network. The telephone line circuit ;~
interfaces the central office, to a telephone or subscriber station found at a location remote from the central office. The telephone line circuit functions to supply power or battery feed to the subscriber station o9 1 ~ 0 via a two wire transmission line or subscriber loop and to couple the intelligence or voice signal to and from the telephone switching system.
In many presently known telephone line circuits the battery feed function has been performed by using a passive, highly balanced, split winding transformer and or inductors which carry up to 12Oma dc. This passive circuit has a wide dynamic range, passing noise-free differential signals while not overloading with the 60Hz longitudinal induced currents. The line circuit just described, feeds dc current to the subscriber loop and also provides the voice path for coupling the voice signal between the subscriber station and the central office. The electromagnetic components of passive line circuits are normally bulky and heavy and consume large amounts of power for short subscriber loop lengths where the current fed to the subscriber station is more than necessary for equalization. Active line-feed circuits can be less bulky and require lower total power, but meeting dynamic range and precision balance requirements dictates an overly complex circuit design.
Recently, solid state replacements for the electromagnetic components of the aforementioned line circuits have been developed. Devices such as high voltage bipolar transistors and other specialized integrated circuits are being designed to replace the heavy and bulky components of the electromagnetic line circuit. Such a device is described in the IEEE JOURNAL
OF SOLID-STATE CIRCUITS, VOL. SC-16, NO. 4, August 1981, entitled, ~A High-Voltage IC for a Transformerless Trunk and Subscriber Line Interface.~ These smaller and lighter components allow the manufacture of telephone switching systems having more line circuits per circuit card as well as decreasing the physical size of the switching system. ~ ;
However, presently known solid state line circuits, still suffer from deficiencies in meeting good 2 02~
transmission performance specifications. These deficiencies manifest themselves in poor longitudinal balance and poor longitudinal current susceptibility, which cause the circuit to fail or to become noisy.
Other problems presently encountered are excessive power dissipation at short loops that consume prodigious amounts of central office power and 2 wire input impedance circuits that are complex and that exhibit poor return loss.
lo Accordingly, it is an object of the present invention to provide a new and more effective control circuit that will effectively and efficiently provide control functions to a subscriber line interface circuit.
DISCLOSURE OF THE INVENTION
The above and other objects, advantages, and capabilities are realized in a control circuit used in solid state line circuit. The solid state line circuit includes an interface circuit connected to a subscriber loop and a PCM codec filter. The control circuit includes, in combination, a logic interface circuit connected via an address/data bus and a control bus to a central controller. The logic interface circuit is arranged to receive and store digital data representing commands from the central controller and to transmit ~
status signals from the control circuit to the central ~ -controller.
A ~C loop control circuit, provides loop control voltage to the interface circuit. The DC loop control circuit, receives from the interface circuit, a representation of loop voltage sensed across the subscriber loop. It then develops and feeds back to the interface circuit, a control voltage, that adjusts the loop current fed to the subscriber loop.
A loop sense comparator is connected to the DC loop control circuit and to the logic interface. The loop sense comparator receives a representation of the ~29~
subscriber loop voltage and compares the loop voltage to a threshold derived by a predetermined reference voltage.
When the threshold is exceeded, the loop sense comparator transmits to the logic interface a status signal.
A ring trip circuit is connected to the subscriber loop and the logic interface. The ring trip circuit is arranged to monitor the subscriber loop and transmit to the logic interface a status signal representing an interruption in the ringing current applied to the subscriber loop.
An AC summing amplifier amplifies voice signals transmitted on the subscriber loop. The AC summing amplifier also receives a subtraction signal which it uses to subtract from the transmit voice signals a receive voice component before sending the voice signals to the PCM codec filter.
A switched capacitor filter, provides under control of the logic interface, loaded or non-loaded network line balance operation. The switched capacitor filter is further arranged to receive a receive voice signal from the PCM codec filter which is used to generate the subtraction signal used by the AC summing amplifier.
Finally, a voltage reference generator is included that provides a first reference voltage used by the loop sense comparator as the threshold voltage. The voltage reference generator also develops a second reference voltage used hy the ring trip circuit for detecting the interruption in ringing current applied to the subscriber loop.
BRIEF DESCRIPTION OF THE DRAWINGS
A better understanding of the invention may be had from the consideration of the following detailed ^~
description taken in conjunction with the accompanying drawings in which:
Figure 1 is a broad level block diagram of a solid state telephone line circuit, using the control circuit in accordance with the present invention.
Figure 2 is a detailed block diagram of the control circuit, in accordance with the present invention.
DESCRIPTION OF A PREFERRED EMBODIMENT
Directing attention first to Figure 1, a broad level block diagram of a solid state telephone line circuit is shown. The line circuit is shown driving a subscriber station or telephone 10, via a subscriber loop 20. The subscriber loop 20 is comprised of a twisted two wire (2W) loop pair having a tip and a ring lead. The 2W loop is connected from the subscriber station 10 to an interface circuit 30. The interface circuit 30 feeds a -48 V dc voltage to the subscriber loop across the tip and ring leads from a central office battery (not shown). `
The interface circuit 30 further functions to superimpose a voice signal on the dc feed voltage and also feed ringing current to subscriber loop 20 for signalling.
The Interface circuit 30 further functions to provide the ~;
2W to 4W (four wire) hybrid function of splitting the balanced signal on the tip and ring leads into separate -transmit and receive paths that are ground referenced.
The control circuit 40, in accordance with the ~ ~-present invention, works in conjunction with the interface cir~uit 30 to provide the DC loop current shaping and the line balance impedance portion of the 2W
to 4W hybrid function. The control circuit 40 further controls various detection functions, such as, ring trip detection and loop sense detection, as well as, providing a logic interface to the central controller of the central office switching system.
Most modern digital telephone switching systems use Pulse Coded Modulation (PCM) digital data to convey voice traffic through the central office switching system.
Therefore, some method of signal translation is required ~02~
-to convert the analog voice signals received by the interface circuit 30 to PCM digital data. This is typically accomplished by a PCM codec and filter circuit such as shown at 50. These devices are commercially available as a so called CODEC/FILTER COMBO~ from varisus manufacturers. Such as the TP30XX family of COMBO~
devices manufactured by the Naticnal Semiconductor Company. Analog voice data from subscriber station 10 is processed by the PCM codec 50 and applied to the PCM bus of the central office switching system for transmission to its destination. Similarly, the codec 50 receives PCM
data from the switching system and converts the PCM data into analog signals which are superimposed on the dc feed voltage of the subscriber loop 20.
The three solid state circuits 30, 40 and 50 just described, in combination embody a complete line circuit adapted to connect a single subscriber station to a central office switch.
Referring now to Figure 2, a more detailed explanation of the functions of the control circuit 40 shown in Figure 1, will now be given. The control circuit 40 shown in Figure 2, is constructed as a bipolar integrated circuit. All signals requiring low voltages and currents are interfaced by circuit 40 while those signals requiring high voltages, typically central office battery (-48v), are interfaced by device 30.
The control circuit 40 of the present invention includes in combination, a logic interface 410, a DC loop control circuit 401, a loop sense comparator circuit 409, a switched capacitor filter 405, an AC summing amplifier 408, a ring trip circuit 403 and a Vref circuit 411.
The logic interface 410 functions to provide a digital interface between the control circuit 40 and a central controller of a central office switching system.
The control circuit operating states are defined by a bi-directional 4-bit address and one bit data bus NDATA 413 transmitted to control circuit 40. The reading back to -6- ;~
2~2~4~
the central controller is also accomplished using the NDATA 413 bus. Write (WR), read (RD), and chip select signals (CEN) are transmitted to logic interface 410 via lead 412.
The DC loop control circuit 401 is used to generate a control voltage to drive phase splitter amplifier 309 on interface circuit 30. The feedback control voltage generated by loop control circuit 401 controls loop current allowing for predetermined and smoothly decreasing current curve for increasing loop resistance up to 2000 ohms.
The loop sense comparator 409, is used to sense the condition of the subscriber loop 20, such as loop closure, due to an ~off-hook~ condition, or rotary dial pulses. ~ -~
The ring trip circuit 403, is used to detect if the subscriber has gone ~off-hook~ during a ringing period.
AC summing amplifier 408, functions to amplify the ac voice signals transmitted from the subscriber and to ~ ;
cancel out the receive voice signals that are fed back to the line interface. The transmit voice signals are output to the PCM codec 50 via the XMT lead for transmission to the cen~ral office switch.
The switch capacitor filter 405, functions to reshape the ac voice signals received from the PCM codec 50 on lead RCV providing echo cancellation and line balance usually required in 2-4 wire conversions. The capacitors are selectable under control of the logic interface, via leads L and NL, to provide loaded (L) or non-loaded (NL) network lines.
Vref 411 generates a 750mV voltage used by the loop sense comparator 409 and a -36mV reference voltage used by the ring trip circuit 403.
Since control circuit 40 works in association with interface circuit 30, some of the functions of interface circuit 30 will be explained in combination with those of control circuit 40 in order to aid in the understanding 6~
of the present invention. A more complete explanation of the functionality of interface circuit 30 may be had by reference to applicant's copen~ing Application titled, nA
High Voltage Subscriber Line Interface Circuitn, (Attorney Docket No. 89-1-034), filed concurrently with the present Application.
Current on either the tip or ring lead of the subscriber loop 20 is sensed by a network of parallel resistors 305 and 306 whose value is approximately 50.0 ohms net on each side of the loop 20. The voltage drop across each resistor 305 and 306 is fed back to a tip drive and a ring drive amplifier 307 and 308, respectively. A phase splitter amplifier 309, couples input voltage SUMB to the tip drive amplifier 307 and the ring drive amplifier 308 and together with the feedback voltage from resistors 305 and 306 create a voltage to current converter. Input voltage SUMB controls the loop feed current, such that, a given voltage at SUMB results ;
in a given current in the subscriber loop 20 flowing from the tip lead to the ring lead. The feed circuit of the present invention is designed to provide a typical gain of 20mA/Volt at SUMB.
It should be noted that the current feed in subscriber loop 20 is not a constant current, as the aforementioned discussion may imply. Rather, the current is shaped to provide sufficient variation of current versus loop resistance to ensure efficient power usage.
This i8 accomplished by providing a feedback loop between the interface circuit 30 and a DC loop control circuit 401. As loop resistance becomes less than 2K Ohm, a resistor network comprising resistors 310-311 apply the voltage drop sensed across subscriber loop 20 to a XMT
Dif~erential amplifier 314. A voltage that is approximately half, but proportional to the voltage sensed by resistors 310-311 then appears at XMTB. The voltage at XMTB is fed to DC loop control circuit 401, where it is properly shaped and output to circuit 30.
~Q2~
This shaped voltage is fed back to the phase splitter amplifier 309 as the SUMiB input voltage.
The current shaping just described, generates feed current characteristics that exhibit smoothly decreasing current with loop resistance, allowing for power savings at short loops, while still providing sufficient variation of current versus loop resistance to ensure proper transmit levels from the subscriber station. As the total external resistance becomes greater than 2K
Ohms, the battery feed reverts to a constant voltage feeding scheme.
The control circuit of the present invention is arranged to provide either loaded or non-loaded line -; -balance, under control of logic interface 410. Switched capacitor filter 405, includes two switched capacitor filters, one for non-loaded lines and one for loaded lines. The individual filters are selected by signal lines NL and L from logic interface 410. The capacitor filters appear between the receive side speech signal at RCV of circuit 40, and phase splitter amplifier 309 on interface circuit 30. The switched capacitor filter 405 combines the filtered signal with the transmit speech signals passed on XMTA. The filters have a response which models the gain phase of the selected (loaded or non-loaded) line and the interface circuit 30 plus external components. The signal output from the filters 405 is 180 degrees out of phase with the transmit speech signals. The net result is that the ~echo~ from the receive side voice path, is sent to the ac summing amplifier 408 via lead -RCV. The receive volce signals are then canceled on the transmit side voice path before the transmit voicè signals are output via XMT for PCM
conversion by the PCM codec 50.
The AC sum amplifier 408 is used to amplify the voice signals received from the subscriber station and to cancel out the receive voice signals fed back from the -~:` 20~40 subscribers loop, before they are converted to PCM
representations by PCM codec filter 50.
Differential tip/ring speech signals transmitted from the subscriber station 10 over the subscribers line 20, are initially coupled to the interface 30 via resistors 310 and 311. The received differential speech signals are applied to XMT differential amplifier 314 which converts the speech signals to a single ended signal that appears at XMTA of interface circuit 30. The speech signals at XMTA are coupled to control circuit 40 were a high impedance amplifier in circuit 408 provides a gain of 2.536. The amplified transmit speech signals -then have a ~reflection replica~ of the receive side signal subtracted from them, as explained above in the description of the switched capacitor filter circuit 405, the net signal is output from XMT to PCM codec filter 50.
Loop sensing, is the detection of on/off hook status of the subscriber and the replication of dial pulsing for dial pulsing equipped subscriber stations. All loop sensing detection in the present invention occurs within -the control circuit 40. This detection is accomplished by sensing an analog representation of the loop voltage and comparing the analog representation to a fixed threshold with a comparator. This is accomplished in the following manner. As explained earlier, for the battery ~--feed function of the present invention, a voltage that is proportional to the voltage across the subscribers line 20 normally appears at lead XMTB. This voltage is processed by the DC loop control circuit 401 and an output signal SUMB is generated which is proportional to loop current. The generated SUMB signal is also passed to the loop sense comparator circuit 409 where it is compared to a 750mV reference voltage generated by circuit VREF 411. Upon detection of a sensed loop condition, such as for example, a ring trip, loop sense comparator 409 sends an output signal to input LS of logic interface 410. A latch internal to logic circuit 2~29~ 4~ --~10 then is set and its data read by the switching system central controller via bus 413.
The control circuit 40, is further disposed to provide a means of applying ringing voltage to the subscriber loop 20 for signaling the subscriber instrument 10. A ringing relay RR associated with the subscriber loop 20 is operated or released to provide the desired on/off ringing period, under control of the central office central controller. T~ ring the ~
subscribers instrument, the central controller writes to ~-the RR control point of the logic interface 410. This sets an internal RR latch in interface 410. The now set control point, through the RR output lead, pulls the RR
relay via its NPN transistor driver. When the relay is activated RR contacts 318 and 315 are closed allowing ac ringing voltage and current from a ringing generator (not ;
shown), to be applied in series with the dc battery potential via leads RBB and RBA. ~ -To detect a ring trip during ringing, such as when ;
~0 the substation instrument is placed ~off-hook~, a resistor 317 develops a voltage drop which is proportional to the applied ringing current. The differential voltage developed by resistor 317 is divided down and converted to a ground referenced signal by resistor network 319 and an operational amplifier internal to ring trip circuit 403. The ring trip circuit 403 filters out the ac ringing component of the signal and keeps any dc component. Normally, when a substation is ~on-hook~ during a ringing cycle, ringing current has only an ac component. However, when the subscriber goes ~off-hook~, DC loop current from the battery biased ringing generator is drawn into the loop. The dc component is then compared against a -36mV reference voltage developed by Vref circuit 411. If the dc signal -sensed by circuit 403 is lower than the -36mV reference, a signal is generated and sent to the RT sense point of logic interface 410. This resets the RR latch, ;.. ,, ., . . .".. : :.. : : : ,: ,: , :.:: :.,.. :.:.. ~:: :.. : :. : :-.. - ; . :
~291~0 deactivating relay RR and removing the ringing generator from the loop. The RT sense point is also monitored by the central controller to detect if a ring trip has occurred.
The control circuit 40 just described, can be manufactured as a large scale integrated circuit suitable for mounting on a hybrid assembly. The integrated circuit can thus provide the capabilities of a line circuit control device, which in the past, occupied a complete circuit card, to a control circuit having greater functional capability and occupying one sixteenth the same space. The control circuit of the present invention also benefits from the increased reliability inherent in integrated circuit construction as well the economies in labor cost and manufacture which are enjoyed by such devices. -~
It will be obvious to those skilled in the art that numerous modifications to the present invention can be made without departing from the scope of the invention as defined by the appended claims. In this context, it ~ ~
should be recognized that the essence of the invention `~
resides in a control circuit for a solid state telephone line circuit having the advantages and capabilities ~-described herein.
A CONTROL CIRCUIT FOR A SOLID STATE TELEPHONE LINE
CIRCUIT
CROSS REFERENCE TO RELATED APPLICATIONS
Cross Reference is made to the related Canadian Patent Applications entitled: nA Solid State Telephone Line Circuit,~ (Attorney Docket 89-1-032), ~A High ~oltage Subscriber Line Interface Circuit,~ (Attorney Docket 89-1-034), ~A Circuit For Synthesizing An Impedance Across The Tip And Ring Leads Of A Telephone Line Circuit,~ (Attorney Docket 89-1-035), ~A Tip-Ring Short Detector and Power Shut-Down Circuit For A
Telephone Line Circuit, n (Attorney Docket 89-1-036), ~A
Thermal Protection Circuit For An Integrated Circuit Subscriber Line Interface, n (Attorney Docket 89-1-037), ~A Thermal Protection Arrangement For An Integrated Circuit Subscriber Line Interface, n (Attorney Docket 89-1-038), and ~A Ring Trip Detector For A Solid State Telephone Line Circuit,~ (Attorney Docket 89-1-040) filed on the same date, and by the same assignee as this Application.
BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates to the field of telecommunications and, more particularly, to a control circuit that provides the low voltage control functions for an integrated circuit subscriber line interface.
2. Description of the Prior Art ;
~ Telephone line circuits are customarily found in the telephone switching system or central office of a telecommunications network. The telephone line circuit ;~
interfaces the central office, to a telephone or subscriber station found at a location remote from the central office. The telephone line circuit functions to supply power or battery feed to the subscriber station o9 1 ~ 0 via a two wire transmission line or subscriber loop and to couple the intelligence or voice signal to and from the telephone switching system.
In many presently known telephone line circuits the battery feed function has been performed by using a passive, highly balanced, split winding transformer and or inductors which carry up to 12Oma dc. This passive circuit has a wide dynamic range, passing noise-free differential signals while not overloading with the 60Hz longitudinal induced currents. The line circuit just described, feeds dc current to the subscriber loop and also provides the voice path for coupling the voice signal between the subscriber station and the central office. The electromagnetic components of passive line circuits are normally bulky and heavy and consume large amounts of power for short subscriber loop lengths where the current fed to the subscriber station is more than necessary for equalization. Active line-feed circuits can be less bulky and require lower total power, but meeting dynamic range and precision balance requirements dictates an overly complex circuit design.
Recently, solid state replacements for the electromagnetic components of the aforementioned line circuits have been developed. Devices such as high voltage bipolar transistors and other specialized integrated circuits are being designed to replace the heavy and bulky components of the electromagnetic line circuit. Such a device is described in the IEEE JOURNAL
OF SOLID-STATE CIRCUITS, VOL. SC-16, NO. 4, August 1981, entitled, ~A High-Voltage IC for a Transformerless Trunk and Subscriber Line Interface.~ These smaller and lighter components allow the manufacture of telephone switching systems having more line circuits per circuit card as well as decreasing the physical size of the switching system. ~ ;
However, presently known solid state line circuits, still suffer from deficiencies in meeting good 2 02~
transmission performance specifications. These deficiencies manifest themselves in poor longitudinal balance and poor longitudinal current susceptibility, which cause the circuit to fail or to become noisy.
Other problems presently encountered are excessive power dissipation at short loops that consume prodigious amounts of central office power and 2 wire input impedance circuits that are complex and that exhibit poor return loss.
lo Accordingly, it is an object of the present invention to provide a new and more effective control circuit that will effectively and efficiently provide control functions to a subscriber line interface circuit.
DISCLOSURE OF THE INVENTION
The above and other objects, advantages, and capabilities are realized in a control circuit used in solid state line circuit. The solid state line circuit includes an interface circuit connected to a subscriber loop and a PCM codec filter. The control circuit includes, in combination, a logic interface circuit connected via an address/data bus and a control bus to a central controller. The logic interface circuit is arranged to receive and store digital data representing commands from the central controller and to transmit ~
status signals from the control circuit to the central ~ -controller.
A ~C loop control circuit, provides loop control voltage to the interface circuit. The DC loop control circuit, receives from the interface circuit, a representation of loop voltage sensed across the subscriber loop. It then develops and feeds back to the interface circuit, a control voltage, that adjusts the loop current fed to the subscriber loop.
A loop sense comparator is connected to the DC loop control circuit and to the logic interface. The loop sense comparator receives a representation of the ~29~
subscriber loop voltage and compares the loop voltage to a threshold derived by a predetermined reference voltage.
When the threshold is exceeded, the loop sense comparator transmits to the logic interface a status signal.
A ring trip circuit is connected to the subscriber loop and the logic interface. The ring trip circuit is arranged to monitor the subscriber loop and transmit to the logic interface a status signal representing an interruption in the ringing current applied to the subscriber loop.
An AC summing amplifier amplifies voice signals transmitted on the subscriber loop. The AC summing amplifier also receives a subtraction signal which it uses to subtract from the transmit voice signals a receive voice component before sending the voice signals to the PCM codec filter.
A switched capacitor filter, provides under control of the logic interface, loaded or non-loaded network line balance operation. The switched capacitor filter is further arranged to receive a receive voice signal from the PCM codec filter which is used to generate the subtraction signal used by the AC summing amplifier.
Finally, a voltage reference generator is included that provides a first reference voltage used by the loop sense comparator as the threshold voltage. The voltage reference generator also develops a second reference voltage used hy the ring trip circuit for detecting the interruption in ringing current applied to the subscriber loop.
BRIEF DESCRIPTION OF THE DRAWINGS
A better understanding of the invention may be had from the consideration of the following detailed ^~
description taken in conjunction with the accompanying drawings in which:
Figure 1 is a broad level block diagram of a solid state telephone line circuit, using the control circuit in accordance with the present invention.
Figure 2 is a detailed block diagram of the control circuit, in accordance with the present invention.
DESCRIPTION OF A PREFERRED EMBODIMENT
Directing attention first to Figure 1, a broad level block diagram of a solid state telephone line circuit is shown. The line circuit is shown driving a subscriber station or telephone 10, via a subscriber loop 20. The subscriber loop 20 is comprised of a twisted two wire (2W) loop pair having a tip and a ring lead. The 2W loop is connected from the subscriber station 10 to an interface circuit 30. The interface circuit 30 feeds a -48 V dc voltage to the subscriber loop across the tip and ring leads from a central office battery (not shown). `
The interface circuit 30 further functions to superimpose a voice signal on the dc feed voltage and also feed ringing current to subscriber loop 20 for signalling.
The Interface circuit 30 further functions to provide the ~;
2W to 4W (four wire) hybrid function of splitting the balanced signal on the tip and ring leads into separate -transmit and receive paths that are ground referenced.
The control circuit 40, in accordance with the ~ ~-present invention, works in conjunction with the interface cir~uit 30 to provide the DC loop current shaping and the line balance impedance portion of the 2W
to 4W hybrid function. The control circuit 40 further controls various detection functions, such as, ring trip detection and loop sense detection, as well as, providing a logic interface to the central controller of the central office switching system.
Most modern digital telephone switching systems use Pulse Coded Modulation (PCM) digital data to convey voice traffic through the central office switching system.
Therefore, some method of signal translation is required ~02~
-to convert the analog voice signals received by the interface circuit 30 to PCM digital data. This is typically accomplished by a PCM codec and filter circuit such as shown at 50. These devices are commercially available as a so called CODEC/FILTER COMBO~ from varisus manufacturers. Such as the TP30XX family of COMBO~
devices manufactured by the Naticnal Semiconductor Company. Analog voice data from subscriber station 10 is processed by the PCM codec 50 and applied to the PCM bus of the central office switching system for transmission to its destination. Similarly, the codec 50 receives PCM
data from the switching system and converts the PCM data into analog signals which are superimposed on the dc feed voltage of the subscriber loop 20.
The three solid state circuits 30, 40 and 50 just described, in combination embody a complete line circuit adapted to connect a single subscriber station to a central office switch.
Referring now to Figure 2, a more detailed explanation of the functions of the control circuit 40 shown in Figure 1, will now be given. The control circuit 40 shown in Figure 2, is constructed as a bipolar integrated circuit. All signals requiring low voltages and currents are interfaced by circuit 40 while those signals requiring high voltages, typically central office battery (-48v), are interfaced by device 30.
The control circuit 40 of the present invention includes in combination, a logic interface 410, a DC loop control circuit 401, a loop sense comparator circuit 409, a switched capacitor filter 405, an AC summing amplifier 408, a ring trip circuit 403 and a Vref circuit 411.
The logic interface 410 functions to provide a digital interface between the control circuit 40 and a central controller of a central office switching system.
The control circuit operating states are defined by a bi-directional 4-bit address and one bit data bus NDATA 413 transmitted to control circuit 40. The reading back to -6- ;~
2~2~4~
the central controller is also accomplished using the NDATA 413 bus. Write (WR), read (RD), and chip select signals (CEN) are transmitted to logic interface 410 via lead 412.
The DC loop control circuit 401 is used to generate a control voltage to drive phase splitter amplifier 309 on interface circuit 30. The feedback control voltage generated by loop control circuit 401 controls loop current allowing for predetermined and smoothly decreasing current curve for increasing loop resistance up to 2000 ohms.
The loop sense comparator 409, is used to sense the condition of the subscriber loop 20, such as loop closure, due to an ~off-hook~ condition, or rotary dial pulses. ~ -~
The ring trip circuit 403, is used to detect if the subscriber has gone ~off-hook~ during a ringing period.
AC summing amplifier 408, functions to amplify the ac voice signals transmitted from the subscriber and to ~ ;
cancel out the receive voice signals that are fed back to the line interface. The transmit voice signals are output to the PCM codec 50 via the XMT lead for transmission to the cen~ral office switch.
The switch capacitor filter 405, functions to reshape the ac voice signals received from the PCM codec 50 on lead RCV providing echo cancellation and line balance usually required in 2-4 wire conversions. The capacitors are selectable under control of the logic interface, via leads L and NL, to provide loaded (L) or non-loaded (NL) network lines.
Vref 411 generates a 750mV voltage used by the loop sense comparator 409 and a -36mV reference voltage used by the ring trip circuit 403.
Since control circuit 40 works in association with interface circuit 30, some of the functions of interface circuit 30 will be explained in combination with those of control circuit 40 in order to aid in the understanding 6~
of the present invention. A more complete explanation of the functionality of interface circuit 30 may be had by reference to applicant's copen~ing Application titled, nA
High Voltage Subscriber Line Interface Circuitn, (Attorney Docket No. 89-1-034), filed concurrently with the present Application.
Current on either the tip or ring lead of the subscriber loop 20 is sensed by a network of parallel resistors 305 and 306 whose value is approximately 50.0 ohms net on each side of the loop 20. The voltage drop across each resistor 305 and 306 is fed back to a tip drive and a ring drive amplifier 307 and 308, respectively. A phase splitter amplifier 309, couples input voltage SUMB to the tip drive amplifier 307 and the ring drive amplifier 308 and together with the feedback voltage from resistors 305 and 306 create a voltage to current converter. Input voltage SUMB controls the loop feed current, such that, a given voltage at SUMB results ;
in a given current in the subscriber loop 20 flowing from the tip lead to the ring lead. The feed circuit of the present invention is designed to provide a typical gain of 20mA/Volt at SUMB.
It should be noted that the current feed in subscriber loop 20 is not a constant current, as the aforementioned discussion may imply. Rather, the current is shaped to provide sufficient variation of current versus loop resistance to ensure efficient power usage.
This i8 accomplished by providing a feedback loop between the interface circuit 30 and a DC loop control circuit 401. As loop resistance becomes less than 2K Ohm, a resistor network comprising resistors 310-311 apply the voltage drop sensed across subscriber loop 20 to a XMT
Dif~erential amplifier 314. A voltage that is approximately half, but proportional to the voltage sensed by resistors 310-311 then appears at XMTB. The voltage at XMTB is fed to DC loop control circuit 401, where it is properly shaped and output to circuit 30.
~Q2~
This shaped voltage is fed back to the phase splitter amplifier 309 as the SUMiB input voltage.
The current shaping just described, generates feed current characteristics that exhibit smoothly decreasing current with loop resistance, allowing for power savings at short loops, while still providing sufficient variation of current versus loop resistance to ensure proper transmit levels from the subscriber station. As the total external resistance becomes greater than 2K
Ohms, the battery feed reverts to a constant voltage feeding scheme.
The control circuit of the present invention is arranged to provide either loaded or non-loaded line -; -balance, under control of logic interface 410. Switched capacitor filter 405, includes two switched capacitor filters, one for non-loaded lines and one for loaded lines. The individual filters are selected by signal lines NL and L from logic interface 410. The capacitor filters appear between the receive side speech signal at RCV of circuit 40, and phase splitter amplifier 309 on interface circuit 30. The switched capacitor filter 405 combines the filtered signal with the transmit speech signals passed on XMTA. The filters have a response which models the gain phase of the selected (loaded or non-loaded) line and the interface circuit 30 plus external components. The signal output from the filters 405 is 180 degrees out of phase with the transmit speech signals. The net result is that the ~echo~ from the receive side voice path, is sent to the ac summing amplifier 408 via lead -RCV. The receive volce signals are then canceled on the transmit side voice path before the transmit voicè signals are output via XMT for PCM
conversion by the PCM codec 50.
The AC sum amplifier 408 is used to amplify the voice signals received from the subscriber station and to cancel out the receive voice signals fed back from the -~:` 20~40 subscribers loop, before they are converted to PCM
representations by PCM codec filter 50.
Differential tip/ring speech signals transmitted from the subscriber station 10 over the subscribers line 20, are initially coupled to the interface 30 via resistors 310 and 311. The received differential speech signals are applied to XMT differential amplifier 314 which converts the speech signals to a single ended signal that appears at XMTA of interface circuit 30. The speech signals at XMTA are coupled to control circuit 40 were a high impedance amplifier in circuit 408 provides a gain of 2.536. The amplified transmit speech signals -then have a ~reflection replica~ of the receive side signal subtracted from them, as explained above in the description of the switched capacitor filter circuit 405, the net signal is output from XMT to PCM codec filter 50.
Loop sensing, is the detection of on/off hook status of the subscriber and the replication of dial pulsing for dial pulsing equipped subscriber stations. All loop sensing detection in the present invention occurs within -the control circuit 40. This detection is accomplished by sensing an analog representation of the loop voltage and comparing the analog representation to a fixed threshold with a comparator. This is accomplished in the following manner. As explained earlier, for the battery ~--feed function of the present invention, a voltage that is proportional to the voltage across the subscribers line 20 normally appears at lead XMTB. This voltage is processed by the DC loop control circuit 401 and an output signal SUMB is generated which is proportional to loop current. The generated SUMB signal is also passed to the loop sense comparator circuit 409 where it is compared to a 750mV reference voltage generated by circuit VREF 411. Upon detection of a sensed loop condition, such as for example, a ring trip, loop sense comparator 409 sends an output signal to input LS of logic interface 410. A latch internal to logic circuit 2~29~ 4~ --~10 then is set and its data read by the switching system central controller via bus 413.
The control circuit 40, is further disposed to provide a means of applying ringing voltage to the subscriber loop 20 for signaling the subscriber instrument 10. A ringing relay RR associated with the subscriber loop 20 is operated or released to provide the desired on/off ringing period, under control of the central office central controller. T~ ring the ~
subscribers instrument, the central controller writes to ~-the RR control point of the logic interface 410. This sets an internal RR latch in interface 410. The now set control point, through the RR output lead, pulls the RR
relay via its NPN transistor driver. When the relay is activated RR contacts 318 and 315 are closed allowing ac ringing voltage and current from a ringing generator (not ;
shown), to be applied in series with the dc battery potential via leads RBB and RBA. ~ -To detect a ring trip during ringing, such as when ;
~0 the substation instrument is placed ~off-hook~, a resistor 317 develops a voltage drop which is proportional to the applied ringing current. The differential voltage developed by resistor 317 is divided down and converted to a ground referenced signal by resistor network 319 and an operational amplifier internal to ring trip circuit 403. The ring trip circuit 403 filters out the ac ringing component of the signal and keeps any dc component. Normally, when a substation is ~on-hook~ during a ringing cycle, ringing current has only an ac component. However, when the subscriber goes ~off-hook~, DC loop current from the battery biased ringing generator is drawn into the loop. The dc component is then compared against a -36mV reference voltage developed by Vref circuit 411. If the dc signal -sensed by circuit 403 is lower than the -36mV reference, a signal is generated and sent to the RT sense point of logic interface 410. This resets the RR latch, ;.. ,, ., . . .".. : :.. : : : ,: ,: , :.:: :.,.. :.:.. ~:: :.. : :. : :-.. - ; . :
~291~0 deactivating relay RR and removing the ringing generator from the loop. The RT sense point is also monitored by the central controller to detect if a ring trip has occurred.
The control circuit 40 just described, can be manufactured as a large scale integrated circuit suitable for mounting on a hybrid assembly. The integrated circuit can thus provide the capabilities of a line circuit control device, which in the past, occupied a complete circuit card, to a control circuit having greater functional capability and occupying one sixteenth the same space. The control circuit of the present invention also benefits from the increased reliability inherent in integrated circuit construction as well the economies in labor cost and manufacture which are enjoyed by such devices. -~
It will be obvious to those skilled in the art that numerous modifications to the present invention can be made without departing from the scope of the invention as defined by the appended claims. In this context, it ~ ~
should be recognized that the essence of the invention `~
resides in a control circuit for a solid state telephone line circuit having the advantages and capabilities ~-described herein.
Claims (5)
1. A control circuit used in solid state line circuit, said solid state line circuit including an interface circuit connected to a subscriber loop and a PCM codec filter, said control circuit comprising:
a logic interface circuit connected via a address/data bus and a control bus to a central controller, said logic interface arranged to receive and store digital data representing commands from said central controller, and to transmit status signals from said control circuit to said central controller;
a DC loop control circuit providing loop control voltage to said interface circuit, said DC loop control circuit arranged to receive from said interface circuit a representation of loop voltage sensed across said subscriber loop and develop and feedback to said interface circuit a control voltage adjusting the loop current fed to said subscriber loop;
a loop sense comparator connected to said DC loop control circuit and to said logic interface, said loop sense comparator receiving said representation of said loop voltage and comparing said loop voltage to a predetermined reference voltage representing a specific subscriber loop sensed condition, and responsive to the detection of said sensed condition, said loop sense comparator transmits to said logic interface a status signal;
a ring trip circuit connected to said subscriber loop and said logic interface, said ring trip circuit arranged to monitor said subscriber loop and transmit to said logic interface a status signal representing an interruption in ringing current applied to said subscriber loop;
an AC summing amplifier, arranged to amplify voice signals transmitted on said subscriber loop and subtract from said transmit voice signals a receive voice component before sending said voice signals to said PCM
codec filter; and a switched capacitor filter, arranged to provide under control of said logic interface loaded or non-loaded network line balance operation, said switched capacitor filter further arranged to receive a receive voice signal from said PCM codec filter and to generate a subtraction signal used by said AC summing amplifier for subtracting the receive voice component from said transmit voice signal.
a logic interface circuit connected via a address/data bus and a control bus to a central controller, said logic interface arranged to receive and store digital data representing commands from said central controller, and to transmit status signals from said control circuit to said central controller;
a DC loop control circuit providing loop control voltage to said interface circuit, said DC loop control circuit arranged to receive from said interface circuit a representation of loop voltage sensed across said subscriber loop and develop and feedback to said interface circuit a control voltage adjusting the loop current fed to said subscriber loop;
a loop sense comparator connected to said DC loop control circuit and to said logic interface, said loop sense comparator receiving said representation of said loop voltage and comparing said loop voltage to a predetermined reference voltage representing a specific subscriber loop sensed condition, and responsive to the detection of said sensed condition, said loop sense comparator transmits to said logic interface a status signal;
a ring trip circuit connected to said subscriber loop and said logic interface, said ring trip circuit arranged to monitor said subscriber loop and transmit to said logic interface a status signal representing an interruption in ringing current applied to said subscriber loop;
an AC summing amplifier, arranged to amplify voice signals transmitted on said subscriber loop and subtract from said transmit voice signals a receive voice component before sending said voice signals to said PCM
codec filter; and a switched capacitor filter, arranged to provide under control of said logic interface loaded or non-loaded network line balance operation, said switched capacitor filter further arranged to receive a receive voice signal from said PCM codec filter and to generate a subtraction signal used by said AC summing amplifier for subtracting the receive voice component from said transmit voice signal.
2. The control circuit as claimed in claim 1 wherein, said control circuit further includes:
a ringing relay connected to a ringing generator and to said logic interface, and said logic interface is arranged to receive from said central controller control signals rendering said ringing relay operated and alternatively non-operated, thereby, connecting and alternatively disconnecting said ringing generator from said subscriber line.
a ringing relay connected to a ringing generator and to said logic interface, and said logic interface is arranged to receive from said central controller control signals rendering said ringing relay operated and alternatively non-operated, thereby, connecting and alternatively disconnecting said ringing generator from said subscriber line.
3. The control circuit as claimed in claim 1 wherein, said control circuit further includes:
a voltage reference generator connected to said ring trip circuit and said loop sense comparator, said voltage reference circuit arranged to develop a first reference voltage used by said loop sense comparator for detecting said specific loop sense condition, and a second reference voltage used by said ring trip circuit for detecting the interruption in ringing current applied to said subscriber loop;
a voltage reference generator connected to said ring trip circuit and said loop sense comparator, said voltage reference circuit arranged to develop a first reference voltage used by said loop sense comparator for detecting said specific loop sense condition, and a second reference voltage used by said ring trip circuit for detecting the interruption in ringing current applied to said subscriber loop;
4. In combination:
interface means connected via an address/data bus and a control bus to a central controller, said interface means accepting and storing data representing commands from said central controller and transmitting status signals to said central controller;
loop control means arranged to receive a representation of the voltage of a subscriber loop and develop a control voltage which controls the loop current fed to said subscriber loop;
loop sensing means connected to said loop control means and to said interface means, said loop sensing means arranged to receive said representation of said subscriber loop voltage and to transmit to said interface means status signals representing a predetermined loop condition;
ring detection means connected to said subscriber loop and to said interface means said ring detection means arranged to transmit to said interface means status signals representing detection of interruption of a ringing signal on said subscriber loop;
amplifier means arranged to receive and amplify voice signals transmitted on said subscriber loop and subtract from said transmit voice signals a receive voice component before transmitting said voice signals to a PCM
codec filter; and filter means arranged to provide under control of said interface means loaded, or alternatively, non-loaded network line balance, said filter means further arranged to develop a subtraction signal from a receive voice signal transmitted by said PCM codec filter, and to transmit said subtraction signal to said amplifier means.
interface means connected via an address/data bus and a control bus to a central controller, said interface means accepting and storing data representing commands from said central controller and transmitting status signals to said central controller;
loop control means arranged to receive a representation of the voltage of a subscriber loop and develop a control voltage which controls the loop current fed to said subscriber loop;
loop sensing means connected to said loop control means and to said interface means, said loop sensing means arranged to receive said representation of said subscriber loop voltage and to transmit to said interface means status signals representing a predetermined loop condition;
ring detection means connected to said subscriber loop and to said interface means said ring detection means arranged to transmit to said interface means status signals representing detection of interruption of a ringing signal on said subscriber loop;
amplifier means arranged to receive and amplify voice signals transmitted on said subscriber loop and subtract from said transmit voice signals a receive voice component before transmitting said voice signals to a PCM
codec filter; and filter means arranged to provide under control of said interface means loaded, or alternatively, non-loaded network line balance, said filter means further arranged to develop a subtraction signal from a receive voice signal transmitted by said PCM codec filter, and to transmit said subtraction signal to said amplifier means.
5. In combination:
means connected via an address/data bus and a control bus to a central controller for accepting and storing data representing commands from said central controller and transmitting status signals to said central controller;
means receiving voltage representations of a subscriber loop, arranged to develop signals that control loop current;
means arranged to receive said voltage representations of said subscriber loop and develop status signals representing a predetermined loop condition;
means arranged to transmit status signals representing detection of interruption of a ringing signal on said subscriber loop;
means arranged to receive and amplify voice signals transmitted on said subscriber loop, and further arranged to receive a subtraction signal for subtracting from said transmit voice signals a receive voice component; and means arranged to provide loaded, or alternatively, non-loaded network line balance, and further arranged to develop said subtraction signal from a receive voice signal.
means connected via an address/data bus and a control bus to a central controller for accepting and storing data representing commands from said central controller and transmitting status signals to said central controller;
means receiving voltage representations of a subscriber loop, arranged to develop signals that control loop current;
means arranged to receive said voltage representations of said subscriber loop and develop status signals representing a predetermined loop condition;
means arranged to transmit status signals representing detection of interruption of a ringing signal on said subscriber loop;
means arranged to receive and amplify voice signals transmitted on said subscriber loop, and further arranged to receive a subtraction signal for subtracting from said transmit voice signals a receive voice component; and means arranged to provide loaded, or alternatively, non-loaded network line balance, and further arranged to develop said subtraction signal from a receive voice signal.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US44582689A | 1989-12-04 | 1989-12-04 | |
| US445,826 | 1989-12-04 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2029140A1 true CA2029140A1 (en) | 1991-06-05 |
Family
ID=23770348
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2029140 Abandoned CA2029140A1 (en) | 1989-12-04 | 1990-11-01 | Control circuit for a solid state telephone line circuit |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2029140A1 (en) |
-
1990
- 1990-11-01 CA CA 2029140 patent/CA2029140A1/en not_active Abandoned
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| Date | Code | Title | Description |
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| FZDE | Dead |