CA2031180A1 - Solid state telephone line circuit - Google Patents

Abstract

A SOLID STATE TELEPHONE LINE CIRCUIT

ABSTRACT
A solid state telephone line circuit is disclosed which is connected via a subscriber loop to a subscriber station and via a PCM bus to a central office switching system. The solid state telephone line circuit of the present invention includes an interface circuit, having a voltage to current converter connected to the subscriber loop. The voltage to current converter converts feed voltage from a central office battery to loop feed current on the subscriber loop. A differential amplifier circuit connected to a sensing network, develops an output voltage that is proportional to the voltage dropped across the subscriber loop and sensed by the sensing network. A control circuit having a loop current shaping circuit is connected to the differential amplifier circuit. The loop current shaping circuit receives the differential amplifier circuit voltage and applies to the current converter a control voltage which operates the voltage to current converter to apply feed current to the subscriber loop, that is proportional to loop resistance. A PCM conversion circuit is also included that is connected to the PCM bus. The PCM
conversion circuit converts PCM encoded speech signals received on the PCM bus, into analog speech signals for transmission to the subscriber station and alternatively, converts analog speech signals received from the subscriber station to PCM encoded speech signal for transmission on the PCM bus. A receive voice path includes a receive coupling network connecting the PCM
conversion circuit to a summing amplifier on the interface circuit. The output of the summing amplifier is fed to the voltage to current converter were the receive analog voice signals are superimposed on the feed current of the subscriber loop. In the transmit voice path, analog speech signals from the subscriber station, appear as differential speech signals on the subscriber loop. The analog speech signals are picked up by the sensing network and fed to the differential amplifier circuit were the differential amplifier circuit converts the differential speech signals into single ended analog speech signals. The analog speech signals are coupled to an amplifier circuit on the control circuit. The amplified analog speech signals are then routed to the PCM conversion circuit, converted to PCM encoded speech signals and transmitted to the PCM bus.

Description

203~

A SOLID STATE TELEPHONE LINE CIRCUIT

CROSS REFERENCE TO RELATED APPLICATIONS
Cross Reference is made to the related Canadian Patent Applications entitled: "A High Voltage 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, N (Attorney Docket 89-1-035), ~A Tip-Ring Short Detector and Power Shut-Down Circuit For A Telephone Line Circuit,~ (Attorney Docket 89-1-036), nA Thermal Protection Circuit For An Integrated Circuit Subscriber Line Interface," (Attorney -~
Docket 89-1-037), "A Thermal Protection Arrangement For An Integrated Circuit Subscriber Line Interface,"
(Attorney Docket 89-1-038), "A Control Circuit For A
Solid State Telephone Line Circuit," (Attorney Docket 89-1-039), and "A Ring Trip Detector For A Solid State ~ -Telephone ~ine Circuit, N (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 solid state telephone line circuit that provides an interface between a subscriber station apparatus and a telephone switching network.
I 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 l ~ubscriber 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 ... :: . :: , '.:

. .

2~3~0 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 unction 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 jus~ described, feeds dc current to the suhscriber 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 subscxiber station is more than necessary for e~ualization. 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 aircuit. Such a device is described in the IEEE JOURNAL
OF SOLID-STATE_CIRCUITS, VOL. SC-16, NO. 4, August 1981, entitled, nA High-Voltage IC for a Transformerless Trunk and Subscriber Line Interface.n 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 s~itching s~stem.
However, presently known solid state line circuits, still suffer from deficiencies in meeting good - :

, ~ . . .
.
. . .
:. - . : :

2 0 3 ~

transmission perfor~ance specifications. Thess defici~ncies manifest themselves in poor longitudinal balance and poor longitudinal current susceptibility, which cause the circuit to fail or to become noisy.
Other problems presently encounterad 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.
Accordingly, it is an object of the present invention to provide a new and more effective solid state telephone line circuit that will effectively and efficiently couple a subscribers line station to a telephone switching system. ~ ~
':
DISCLOSURE OF THE INVENTION ~. -The above and other objects, advantages, and capabilities are realized in a solid state telephone line circuit which is connected via the tip and ring leads of the subscriber loop to a subscriber station. The solid state telephone line circuit is further connected via a Pulse Code Modulation (PCM) bus to a central office switching system. The solid state telephone line circuit of the present invention includes an interface circuit, having a voltage to current converter, connected to the tip and the ring leads of the subscriber loop. The current converter is arranged to convert feed voltage from a central office battery to loop feed current on the subscriber loop.
The interface circuit further includes a sensing network connected to the tip and the ring leads for sensing the voltage dropped across the subscriber loop, and a differential amplifier circuit connected to the sensing network. The differential amplifier circuit is disposed to develop an output voltage that is proportional to the voltage sensed by the sensing network.

". ~ ' - ' ` ' . .
: ' . . - :~ . . . .

2 ~
The solid state telephone line circuit of the present invention further includes a control circuit, having a loop current shaping circuit connected to the differential amplifier circuit. The loop current shaping circuit is arranged to receive the output voltage developed by the differential amplifier circuit and develop and apply to the current converter a control voltage. The control voltage allows the current converter to apply feed current to the subscriber loop that is proportional to loop resistance.
A PCM conversion circuit is also included that is cormected to the PCM bus. The PCM conversion circuit converts PCM encoded speech signals received on the PCM
bus from the switching system, into analog speech signals for transmission to the subscriber station.
Alternatively, the PCM conversion circuit converts analog speech signals received from the subscriber station to PCM encoded speech signal for transmission on the PCM bus to the switching system.
A receive voice path through the solid state telephone line circuit transmits speech signals from the PCM bus to the subscriber station. The receive voice path includes a receive coupling networX, connecting the PCM conversion circuit to a summing amplifier on the interface circuit. The output of the summing amplifier ' is applied to the voltage to current converter were the receive analog voice signals are superimposed on the feed current and appear as a balanced differential signal on the tip and ring leads of the subscriber loop.
A transmit voice path through the solid state telephone line circuit transmits speech signals from the subscriber station to the PCM bus. Analog speech signals from the subscriber station appear as differential speech signals on the tip and ring leads of the subscriber loop.
The analog speech ~ignals are picked up by the sensing network and fed to the differential amplifier circuit.
The differential amplifier circuit converts the . .
.
::~ . .. ` ~ ` , '' , , . : . .

: . . . .
:
, 2~3~ ~ 8~
differential speech signals into single ended analog speech signals with a dc offset. The analog speech signals output from the dif~erential amplifier circuit are connected to a transmit coupling network where the dc offset is removed and the resultant speech signals are applied to an amplifier circuit on the control circuit.
There, the signals are amplified before being routed to the PCM conversion circuit. The transmit speech signals -are then converted to PCM encoded speech signals and coupled to the PCM bus.

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 the solid state telephone line circuit, in accordance with the present invention.
Figure 2 is a detailed block diagram of the solid state telephone line circuit, in accordance with the present invention.
Figure 3 is graphical representation of the feed current charaateristics of the solid state telephone line circuit, in accordance with the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT
Directing attention ~irst to Figure 1, a broad level block diagram of the solid state telephone line circuit of the present invention 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 ' .

203~ 18~

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 works in conjunction with the interface circuit 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 ; 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 various manufacturers. Such as the TP30XX family of COMBO~
devices manufactured by the National 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 .. . . .
. ...

-, . .
. ~ , ~ ,. - ..
. . .

- 2~31~
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 circuits shown in Figure 1, in accordance with the present invention, will now be given. The interface circuit 30 shown in Figure 2, is constructed as a bipolar integrated circuit. All signals requiring high voltages and currents are interfaced by this device. With the addition of a few external discrete components a complete transformerless line circuit can be constructed.
Battery feed voltage VB and current is passed to interface circuit 30 via transistors 301-304. The transistors 301-304 are under control of the interface circuit 30 and electrically should be viewed as though they were integrated with circuit 30. Due to current and heat dissipation requirements the transistors 301-304 are ~ -located external to circuit 30. 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 ~ " , .
:: , :,. - . : . :
. ~, ' : ' ' .' ' : ' ' --~ " ~ , : -',; '.': ' ` ' . -: ~

8 ~
versus loop resistance to ensure ef f icient power usage.
This is accomplished by providing a feedback loop between the interface circuit 30 and a loop current shaping circuit 401, found on control circuit 40. 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 Differential amplifier 314. A voltage that is proportional to the voltage sensed by resistors 310-311 then appears at XMTB
of circuit 30. The voltage at XMTB is fed to a loop current shaping circuit 401, where it is properly shaped and output at the DLC node of circuit 40. This shaped voltage is fed back to the phase splitter amplifier 309 as the SUMB input voltage.
The current shaping just described generates the feed current characteristics shown on Figure 3. The smoothly decreasing current with loop resistance, allows 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.
Input impedance of the line circuit of the present invention is synthesized by the combination of an impedance network comprising, capacitor 402 and resistors ~03 and 404 for the fixed value of the feed resistance used in the design. As mentioned earlier, the feed resistance value for the line circuit is 50.0 Ohms net on each side of the subscriber loop 20. An analog represen~ation of the tip and ring difference voltage developed by the XMT differential amplifier 314 is output to node XMTA of the interface circuit 30. The voltage at node XMTA is coupled to the impedance network and fed to the AC summing node SUMA of circuit 30. The overall feedback generates an input impedance, as seen from the tip and ring leads, that is a scaled (in magnitude) ~ . .

-~'`'` 2~1 L~
version of th~ impedance network. The impedance network of the present invention yields an equivalent 900 Ohms in series with 2.16uF impedance.
In conventional 200/200 ohm inductive battery feed line circuits, the impedance to ground at either the tip or ring leads is essentially half the f~ed resistance.
In the present invention, feed resistance is synthesized by feedback techniques, so that longitudinal impedance is not related to feed resistance. A common mode voltage, (VTIP+VRING)/2, is developed between a voltage divider network comprising resistors 312 and 313 and input to interface circuit 30 at CM. This applied voltage together with the tip drive amp 307 and ring drive amp 308 keep the voltage at AT and AR on circuit 30 constant when longitudinal currents, such as 60 Hz power line induction currents, that may be induced on the tip and ring leads. Interface circuit 30 sources or sinks the longitudinal currents as required up to 9mA per lead.
Since the voltage at AR or AT appear as a virtual AC
ground to longitudinal signals, the impedance to ground is simply the series resistance of resistors 305 and 317 on the tip lead and the series resistance of resistors 318 and 306 on the ring lead, each approximately 75 Ohms per leg.
With renewed reference to Figure 2, the voice paths followed by the speech signals through the line ci~cuit of the present invention will now be described. The receive voice path, analogous to the path used to transmit speech signals to the receiver of the subscriber station 10 will be explained first. PCM representations of speech signals are applied to the PCM codec and filter 50 at DR via lead PCMR from the PCM bus of the central office switching system. The PCM signals are converted to analog signals by a digital to analog converter (not shown) internal to the PCM codec 50. The analog speech signals are then output from the PCM codec and filter 50 at VFRO and are capacitively coupled by capacitor 407 to .. . .
" '' . ' :

~' ~ : ,.
.

~ ~ 3 ~
resistor 408. Resistor 408 terminates at node SUMA of interface circuit 30. The SUMA node is a virtual ground for AC summing amplifier 315 and is also arranged to perform signal summation of the receive signal and the transmit signal output from XMTA. The summation just described is also used in the synthesis of the line circuits input impedance, as explained earlier. The speech signals are output from AC summing amplifier 315 and connected to phase splitting amp 309 where they are coupled to the subscriber line via the tip and ring drive amplifiers 307 and 308. The speech signals thus appear as a balanced differential signal on the tip and ring leads of the subscriber line 20. The ratio of resistor 40~ ov~r resistor 408 sets the initial gain for the output of AC summing amplifier 315.
The transmit voice path, analogous to the path used to transmit speech signals from the transmitter of the subscriber station will be explained now. The 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 the XMTA node of interface circuit 30. The speech signals at the XMTA node are the transmit speech signals with a dc offset. The transmit speech signals are then coupled to resistor 403 via capacitor 402. Capacitor 402 removes the dc offset and only the ac speech signals remain across resistor 403. The speech signals at resistor 403 are fed to the XMTA node of control circuit 40 were a high impedance amplifier 408 provides gain.
The amplified transmit speech signals then have a ~reflection replicaN of the receive side signal subtracted from them and the net signal is output from node XMT to the +VXI input of the PCM codec filter 50.
The speech signals are then converted into PCM digital ... .
:~ ~: , ?
- .

20311~
data and output to the central office switch via node DX
and the PCMX lead.
The line circuit of the present invention is arranged to provide either loaded or non-loaded line balance, under software control. Switched capacitor filter 405 o~ ~he control circuit 40, includes three switched capacitor filters, one for non-loaded lines and one for loaded lines and one for 9:2 loaded lines. The capacitor filters appear between the receive side speech signal at RCV of circuit 40, and AC summing amplifier 315. The switched capacitor filter 405 combines the ; filtered signal with the transmit speech signal from XMTA. The filters 405 have a response which models the gain phase of the selected (loaded, non~loaded, or 9:2 loaded) line and the interface circuit 30 plus external components. The signal output from the filters 405 is out of phase with the transmit speech signal. The net result is that the "echoN from the receive side voice path is canceled on the transmit side voice path before it is output to the transmit side listener.
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 Pollowing manner. As explained earlier for the battery faed function of the present invention, a voltage that is proportional to the voltage across the subscribers line 20 normally appears at node XMTB of control circuit 40.
This voltage is processed by loop current shaping circuit 401 and an output signal which is proportional to loop current is generated at node DLC. This signal is passed to loop sense circuit 409 and is compared to a 750mV
reference voltage. The output of the loop sense circuit .. . ..

-, :-. ~ . , :

~3~ ~3i~
409 iS connected to logic circuit 410. When, forexample, a ring trip occurs a latch internal to logic circuit 410 is set and its data read by the switching system central controller via control bus 412. Capacitor 413 together with resistance internal to loop sense circuit 409 form a symmetrical integrator. This integrator is disposed to provide "~IT" protection by blocking transient "off hookn or "on hook~ pulses of less than 12 msec from being detected by the loop sense circuit 409.
High voltage transients and power cross protection is afforded to the line circuit of the present invention by the inclusion of a diode rectifying bridge circuit 201. The bridge circuit 201 directs positive voltage transients to ground and the negative voltage transients to a special transient protection semiconductor 202.
Device 202 acts as a SCR type device ~ith an internal gate triggered by excess voltage in the range of 85 Volts that shorts the transient to ground for its duration.
Power cross protection is afforded to the line circuit via 900 mA lighting surge fuses 203 and 204 that are placed in series with each tip and ring conductor respectively.
The solid state telephone line circuit just described can be manufactured as a single compact hybrid circuit using any of the presently known thin film techniques u~ed to build microcircuits. Further, interface circuit 30 and control circuit 40 can each be manufactured as large scale integrated circuits suitable for mounting on the hybrid assembly. The hybrid assembly can thus provide the capabilities of a line circuit which in the past occupied a complete circuit card to a line circuit having a greater functional capability and occupying one sixteenth the same space. The solid state telephone line circuit of the present invention also benefits from the increased reliability inherent in solid ~' ' .
. ~, .
.
. - .
:: `
: ~ , : ' , , 203~

state construction as well the economies in labor cost and manufacture which are enjoyed by solid state 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 solid state telephone line circuit having the advantages and capabilities described herein.

.
.

::-:: . .

Claims (10)

1. A solid state line circuit connected via the tip lead and a ring lead of a subscriber loop to a subscriber station, and said solid state line circuit further connected via a PCM bus to a central office switching system, said solid state line circuit comprising:
an interface circuit including voltage to current converter means connected to said tip and said ring leads arranged to convert feed voltage from a central office battery to loop feed current on said subscriber loop, said interface circuit further including sensing means connected to said tip and said ring leads for sensing the voltage dropped across said subscriber loop, and differential amplifier means connected to said sensing means for developing an output voltage that is proportional to the voltage sensed by said sensing means:
a control circuit having loop current shaping means connected to said differential amplifier, said loop current shaping means arranged to receive said output voltage developed by said differential amplifier means and develop and apply to said current converter means a control voltage permitting said current converter means to develop and apply to said subscriber loop an amount of feed current which is proportional to loop resistance.

2. The solid state line circuit as claimed in claim 2, wherein said solid state line circuit further includes:
input impedance means connected to said differential amplifier means output voltage, and said interface circuit further including summing amplifier means connected to said current converter means, said input impedance means arranged to synthesize and output to said summing means an impedance voltage that is a scaled magnitude representation of the impedance of said input impedance means.

3. The solid state line circuit as claimed in claim 2 wherein, said solid state line circuit further includes:
a receive voice path for transmitting speech signals from said switching system to said subscriber station, said receive voice path including PCM conversion means connected to said PCM bus, said PCM conversion means arranged to convert PCM encoded speech signals to analog speech signals, and a receive coupling network connecting said PCM conversion means to said summing amplifier means, whereby, said receive voice signals appear as a balanced differential signal on said tip and ring leads of said subscriber loop and are transmitted to said subscriber station.

4. The solid state line circuit as claimed in claim 3, wherein: said solid state line circuit further includes:
a transmit voice path for transmitting speech signals to said switching system, said transmit voice path coupling differential speech signals transmitted from said subscriber station to said differential amplifier means from said sensing means, said differential amplifier means disposed to convert said differential speech signals into a single ended analog speech signals with a dc offset, said differential amplifier means connected to a transmit coupling network where said dc offset is removed, and said transmit coupling network further connected to amplifier means on said control circuit, where said analog speech signals are amplified, and said amplifier means connected to said PCM conversion means, whereby, said analog speech signals are converted to PCM encoded speech signals and transmitted on said PCM bus to said switching system.

5. The solid state line circuit as claimed in claim 4, wherein, said control circuit further includes:
logic circuit means arranged to receive and transmit control signals to a switching system central controller;
and line balancing means comprising a pair of switched capacitor filters one for non-loaded lines and one for loaded lines connected to said logic circuit means, whereby, under control of said switching system central controller said non-loaded capacitor filter or alternatively, said loaded capacitor filter is connected between said receive coupling network and said summing amplifier means.

6. The solid state line circuit as claimed in claim 5, wherein, said line balancing means is further connected to said control circuit amplifier means and to said transmit voice path, and said receive speech signals are filtered by said selected capacitor filter and combined with said transmit speech signal, whereby, the "echo" from the receive voice path is cancelled on the transmit voice path before it is output to said PCM
conversion means.

7. The solid state line circuit as claimed in claim 2, wherein, said control circuit further includes:
loop sensing means for detecting on/off hook status and dial pulsing of said subscriber station, said loop sensing means connected to said logic circuit means and to said loop current shaping means, and said loop sensing means arranged to receive from said current shaping means said control voltage, whereby, said control voltage is compared to a fixed reference voltage and a status signal transmitted to said logic circuit, and said status signal is subsequently read by said switching system central controller when a sensed event occurs.

8. In combination:
means for converting voltage to current connected to the tip and the ring leads of a subscriber loop, said means for converting arranged to convert feed voltage from a central office battery to feed current on said subscriber loop;
means for sensing connected to said tip and said ring leads of said subscriber loop, said means for sensing arranged to sense the voltage developed on said tip lead and said ring lead of said subscriber loop;
means for developing connected to said means for sensing, said means for developing arranged to develop an output voltage proportional to the voltage sensed by said sensing means; and loop current shaping means connected to said means for developing, said loop current shaping means arranged to receive said output voltage developed by said means for developing and produce and apply to said means for converting a control voltage, allowing said means for converting to apply an amount of feed current to said subscriber loop which is proportional to the resistance of said subscriber loop.

9. In combination:
a subscriber station connected to the tip lead and the ring lead of a subscriber loop;
means for converting voltage to current connected to the tip and the ring leads of said subscriber loop, said means for converting arranged to convert feed voltage from a central office battery to feed current on said subscriber loop;
means for sensing connected to said tip and said ring leads of said subscriber loop, said means for sensing arranged to sense the voltage developed on said tip and ring lead of said subscriber loop;
means for developing connected to said means for sensing, said means for developing arranged to develop an output voltage proportional to the voltage sensed by said sensing means;
loop current shaping means connected to said means for developing, said loop current shaping means arranged to receive said output voltage developed by said means for developing and produce and apply to said means for converting a control voltage, allowing said means for converting to apply an amount of feed current to said subscriber loop which is proportional to the resistance of said subscriber loop;
PCM conversion means for converting PCM encoded speech signals to analog speech signals; and a receive voice path for transmitting speech signals from said PCM conversion means to said subscriber station, said receive voice path including coupling means connecting said PCM conversion means to said means for converting, whereby, said speech signals appear as a balanced differential signal on said tip and ring leads of said subscriber loop and are transmitted to said subscriber station.

10. In combination:
a subscriber station connected to the tip lead and the ring lead of a subscriber loop;
means for converting voltage to current connected to the tip and the ring leads of said subscriber loop, said means for converting arranged to convert feed voltage from a central office battery to feed current on said subscriber loop;
means for sensing connected to said tip and said ring leads of said subscriber loop, said means for sensing arranged to sense the voltage developed on said tip and ring lead of said subscriber loop:
means for developing connected to said means for sensing, said means for developing arranged to develop an output voltage proportional to the voltage sensed by said sensing means;
loop current shaping means connected to said means for developing, said loop current shaping means arranged to receive said output voltage developed by said means for developing and produce and apply to said means for converting a control voltage, allowing said means for converting to apply an amount of feed current to said subscriber loop which is proportional to the resistance of said subscriber loop;
PCM conversion means for converting analog speech signals to PCM encoded speech signals; and a transmit voice path for transmitting speech signals from said subscriber station, said transmit voice path coupling differential analog speech signals transmitted on said tip and said ring leads of said subscriber loop, and said means for sensing coupling said differential analog speech signals to said means for developing, said means for developing converting said differential analog speech signals into single ended analog speech signals, and said analog speech signals are coupled to means for amplifying said analog speech signals, whereby, said amplified analog speech are connected to said PCM conversion means.