This invention relates generally to telephone switches for interconnecting telephone subscribers, and more particularly the invention relates to a new and improved line interface unit.

Telephone subscribers are interconnected by two wire loop circuitry to a central switching exchange center from where incoming and outgoing signals are carried on four wire trunks. Long haul four wire trunks are eventually returned to a two wire subscriber loop. The two wire to four wire conversion is accomplished by a subscriber line interface at the switch.

The line interface performs what is referred to as BORSHT functions: battery power, over voltage protection, ringing, signalling, hybrid (2-4 wire conversion)-, and testing. Heretofore, the subscriber line interface for performing all of these functions have been expensive and bulky. A hybrid transformer has been necessary for the 2-4 wire conversion and for overvoltage protection. Additionally, mechanical relays have been used to connect and disconnect the high voltage ringing signal, which adds to the size and cost of the interface. Further, the mechanical nature of the relays causes reliability problems.

Accordingly, an object of the present invention is an improved telephone subscriber line interface circuit.

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Another object of the invention is a subscriber line interface which is smaller, more economical, and more reliable.

Briefly, in accordance with the invention the size of the transformer in the subscriber line interface is reduced by employing pulse width modulation in the interface. Further, the interface circuitry lends itself to VLSI technology thus reducing size and cost while improving reliability. Addi- ^• tionally, the line interface circuitry improves overall reliability by eliminating ringing relays and providing the ringing voltage directly from the electronic circuit through the small isolating transformer to the telephone receiver. Dynamic balancing is employed to improve transmission quality by reducing echoes encountered in long distance calls.

The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawing, in which:

Figure 1 is a functional block diagram of a central switch exchange.

Figure 2 is a generalized functional block diagram of a line interface unit in the exchange of Figure 1.

Figure 3 is an electrical schematic of a line interface unit in accordance with one embodiment of the present invention.

Figure 4 is a detailed schematic for outgoing signal circuitry in the interface unit of Figure 3.

Figure 5 is a schematic of a divider used in the circuitry of Figure 4.

Figure 6 is an electrical schematic of additional circuitry for generating signals used in the circuitry of Figure 4.

Figure 7 is an electrical schematic of incoming signal circuitry in the line interface unit of Figure 3.

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Referring now to the drawings. Figure 1 is a functional block diagram of a central switch exchange in which the present invention is used. A plurality of telephone users

10 are interconnected through two wire loops 12 to line interface units 14 in the switch. Each interface unit is interconnected by four wires 16 (e.g. two wires for signal transmission and two wires for signal reception) to PCM

Codec units 18. The units 18 convert audio analog signals

^* from the interface unit 14 to a pulse code digital format, and the digital signals from the PCM Codec units are then multiplexed on a PI bus. Conversely, digital signals from the PI bus 20 are received by the PCM Codec units 18 and converted to analog audio signals for application to the line interface unit 14. The Pi bus is an eight bit wide time division multiplex PCM databus. The PI bus and the PCM Codec units are controlled by a microprocessor 22 at the switch.

The line interface units 14 perform the BORSHT functions described above. Figure 2 is a functional block diagram of a line interface unit in accordance with the invention in which the analog audio signals from the subscriber's telephone 10 are modulated and demodulated within the line interface unit 12. Analog signals from the telephone are modulated by a modem 26 and a high frequency signal is then transferred through coupling transformer 28 to a second modem 30 which demodulates the signal and applies the audio analog signal to PCM Codec unit 18. Conversely, analog audio signals from the PCM Codec unit 18 are modulated by modem 30, transferred through coupling transformer 28, and demodulated by modem 26 and applied as audio analog signals to the two wire loop 12 and the telephone 10. Further, the circuitry lends itself to VLSI circuit technology, thus reducing size and cost and improving reliability. The control circuitry 32 communicates with the microprocessor at the switch for the exchange of information such as line status and ringing command.

By modulating the analog signals the size of transformer 28 can be reduced and the BORSHT functions can be more reliably implemented. _/ ^UR£ '

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Figure 3 is a schematic of one embodiment of a line interface illustrated in Figure 2. The voice signals from the PCM Codec unit 18 and destined for the subscriber set 10 are applied to pulse width modulator 36 which modulates a high frequency carrier with the voice signals. The modulated signal is divided in divider 38, and the two divided signals SI and S2 are applied to switch transistors 40 and 42 connected to the primary winding of transformer 28. The center tap of the primary winding is connected to a voltage source, +V. The alternating conduction of transistors 40 and 42 generates a high frequency pulse train with no dc value and in which the duty cycle varies with the voice frequency. The pulse train is transmitted by transformer 28 and generates voltage across sense resistor 44.

Advantageously, the absence of a dc value prevents the transformer core from saturating before full use of the transformer maximum flux density is made by the transmitted signal. Since the size of the core tends to decrease as the frequency of the signals increases, the high frequency modulated signal requires a very small ferrite core while a conventional transformer for a voice frequency signal needs a substantially larger core. Further, the high frequency pulse train provides line power to the receiver 10 through the full wave rectifier comprising diodes 48 which are connected to the secondary winding of transformer 28. The photo SCRs 50 which connect the bridge through lowpass filter 51 to the loop 12 are used for ringing and testing, as will be described further hereinbelow. Since the sub¬ scriber loop 12 is floating and has no common ground with the electronic circuits of the switch, full isolation of the switch from the outside loop is provided, thus protecting the switch from high voltages caused by lightning and power line crossing.

A signal coming from the subscriber 10 modulates the resistance of the telephone set's microphone, and the resulting audio signal amplitude modulates the voltage across sense resistor 44 connected between transistors 40, 42 and circuit ground.

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The voltage across sense resistor 44 is applied through amplifier 52 to differential amplifier 54. Any echo signal from the pulse width modulator 36 is applied through a balancing network 54 and subtracted from the detected signal in amplifier 54. The output of the amplifier 54 is then filtered and applied as an input to the PCM unit 18. Impor¬ tantly, the balancing network 56 simulates the subscriber loop impedance transformation through the pulse width modulator in order to avoid echos of the signal to the telephone 10 being heard by the other transmitting party after a round trip delay of the signal. As will be described further hereinbelow with reference to Figure 1 , the balancing network comprises a combination of one capacitor and two dynamically adjustable resistors to simulate the impedance of the subscriber loop.

When the switch processor demands ringing of the receiver 10, circuitry is provided to control switches 40 and 42 and increase the. duty cycle to nearly 100%. The circuitry also controls the photodiodes 54 to provide a 25 Hertz, or other ringing frequency, pulse for ringing. As will be described further hereinbelow, line status detection is achieved by comparing the voltage across the sense resistor 44 with a reference voltage.

Consider now Figures 4, 5, 6 and 7 which are detailed schema- tics of the circuitry illustrated in Figure 3. The circuitry for an outgoing signal from the switch to the subscriber is illustrated in Figures 4, 5, and 6 and the circuitry for the incoming signal from the subscriber to the switch is illus¬ trated in Figure 7. Referring now to Figure 4, the outgoing voice on line 16 is applied to the pulse width modulator 36 which includes two 100K hertz timers 60 and 62. The output . of modulator 36 is a 100 Khz signal designated PWM which is pulse width modulated by the audio signal on line 16. The disable terminal of timer 62 is connected to a RING bit line from the switch processor whereby the modulator is controlled during the ring command.

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The PWM signal is applied through an OR gate 64 to the divid¬ er 38 and thence through buffers 66 to the gate terminals of transistors 40 and 42. Divider 38 is conventional circuitry as illustrated in Figure 5 wherein the 100 Hz input signal drives flip-flops 67 and 68 which in turn are interconnected with NAND gate 69 and 70 which generate 5OK hertz signals Q and Q for driving transistors 40 and 42, respectively.

Ring circuitry 72 responds to a RING bit signal from the switch processor and provides a ring signal through OR gate 0 64 to control switches 40 and 42 at nearly 100% duty cycle. The circuitry includes a 50 Hertz timer 74 which drives cascaded flip-flops 75, 76 and 77. The 50 hertz signal is also applied to a divider 80 (similar to the divider in Figure 5) , and the 25 Hertz signals from divider 80 are 5 applied to NAND gate 82 and NOR gate 84 along with the RING bit and - RING bit, respectively, which in turn drive light emitting diodes 86 and 88. The light emitting diodes 86 and 88 in turn control the photo SCR devices 50 whereby a 25 Hertz signal is applied to ring the telephone receiver 10.

Q Constant current control and status detection is accomplished by the circuit illustrated in Figure 6. The voltage across the sense resistor 48, designated V , is applied through a low pass filter 90, and the filtered signal (V _f ) is applied to one input of comparator 92. The other input to comparator 5 92 is the pulse width modulation (PWM) signal from modulator 36. The output of comparator 92, designated V_, is applied through resistor 94 to one input of differential amplifier 96. A reference voltage, ^v _ref , is also applied through resistor 98 to the input of amplifier 96. When is 0 greater than V - then an ON HOOK status is indicated. When

Vr is less than Vre _r then an OFF HOOK is indicated,

Figure 7 is hybrid circuitry for balancing an incoming signal from the subscriber T.Q to the central switch. As above indicated, the subscriber line simulation is- accomplished by 5 a three component network consisting of a variable resistor 102, a shunt capacitor 104. and a series variable resi. _r „ ___

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106. The signal V _f , obtained in the circuitry of Figure 6 by low pass filtering the voltage V_ across the sense resistor 44, is applied to control an analog to digital converter 108. The output of the converter 108 controls the conductance of transistors bridging the resistive elements in variable resistors 102 and 106 in accordance with the required echo suppression. The modulated echo signal PWM and complement, PWM, control the voltage applied by transistors 112 and 114 through inductor 116 and capacitor 118 to junction 120 and the voltage applied by transistor 113 to junction 120.

Resistor Rss at junction 120 simulates the sense resistor 44.

The three component network provides the requisite line impedance simulation for balance of the PWM signal for echo cancellation. The signal at junction 120, across R__, is then the adjusted echo signal which is applied through amplifier 122 to the junction 124.

Operation of transistors 112, 113, and 114 in establishing voltage across resistor Rss simulate the effect of the PWM pulses across the sense resistor 114. When the pulse is pres- ent, transistor 112 applies- a voltage to terminal 120. When the pulse is absent, transistor 113 removes voltage at termina 120 and simulates the diode short seen by the telephone user. The impedance network including resistors 107 and 106 and capacitor 104 simulate the driving impedance of the modulator-demodulator 26.

The voltage V_ , which includes both the subscriber and echo signals, is applied through a passive low pass filter 127 to amplifier 128 and thence to the terminal 124. The echo signal from amplifier 122 is then subtracted from the sub- scriber and echo signal from amplifier 128 thereby leaving only the subscriber signal at terminal 124. The incoming subscriber signal is then applied through amplifier 130 and the active low pass filter 132 to PCM Codec 18.

In the test mode the photo SCRs 50 can be opened to isolate the subscriber for measuring the impedance of the subscriber and two wire loop. Alternatively, the photodiodes can be

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closed to short the transformer 28 whereupon the line inter¬ face circuit can be tested.

The described line interface circuit is small, economical, arid reliable. The switch is isolated from the subscriber loop for overvoltage protection. The ringing function is readily implemented without the need for mechanical relays. The other BORSHT including testing, battery power, signalling and 2-4 wire conversion are readily implemented, also.

While the invention has been described with reference to a specific embodiment, the description is illustrative of the invention and is not to be construed as limiting the inven¬ tion. Various modifications and applications may occur to those skilled in the art without departing from the true spir¬ it and scope of the invention as defined by the appended claims.

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