IE46568B1 - Subscriber line or trunk circuit - Google Patents

Description

The present invention relates to subscriber line/ trunk circuits such as used in telephone exchanges, and especially to a voltage generator for use in such circuits.

In such circuits it is necessary to supply various 5 signals to the subscriber's instrument connected to the line circuit, or to a remote exchange connected to the line circuit, and the invention enables this to be effected.

According to the present invention there is provided a telephone subscriber's line circuit or a trunk circuit, including means for coupling a two-wire line circuit or trunk circuit to a four-wire line which coupling means includes analog-to-digital conversion means for sensing the analog voltages on the line circuit and for deriving therefrom a digital output signal which is applied to the four-wire line, means whereby a further digital output signal is derived from the said analog voltages, signal processing means to which said further digital output signal is applied for comparison by the processing means with a reference, said processing means deriving a 46368 - 2 pulse duration modulated feedback control signal as a result of said comparison, and regulator means coupled to the two-wire line to which said feedback control signal is applied for regulating the amplitude of the analog voltages on said two-wire line in accordance with the said feed back control signal.

An embodiment of the invention will now be described with reference to the accompanying drawings wherein : Fig. 1 is a line circuit of known type illustrative of a typical interface between a subscriber telephone set and a digital local exchange.

Fig. 2 is a block diagram showing the implementation of the two-wire to four-wire conversion using digital filtering techniques internal to a microprocessor, as used in a circuit to which the invention is applicable.

Fig. 3 is a block circuit diagram illustrative of greater detail of the line circuit excluding the microprocessor system.

Fig. 4 is a schematic diagram of the voltage generator used in a circuit such as Fig. 3.

Features of the line circuit, which is, in effect an interface between a telephone line and an exchange are claimed in Patent Specification No. 435/78, from which the present application has been divided.

Fig. 1 shows a known subscriber line circuit which provides an interface between an analog telephone line and the digital portions of a local exchange, such as its switching matrix. The incoming analog signals on a two-wire analog line 10 from a typical subscriber telephone set are converted to a four-wire PCM encoded signal for connection to a digital local switching matrix via speech line 12 and signalling line 14. Input signals on line 16 and output signals on line 18 are isolated from each other by a two-wire to four-wire converter circuit 20, a circuit which does not compensate for the imperfect impedance match 4!656g at the two-wire line interface. A compromise balance circuit 21, typically 900 ohms and two microfarads in series, is used to attempt to balance the wide variation in two-wire line impedances. Due to its less than perfect action some of the signal 16 is transmitted back on line 18. In a typical connection between two line circuits via the switching matrix their signals can cause an instability or near singing condition causing poor perceived transmission by the subscriber. The prior art seeks to resolve this problem by inserting an extra 2 db of loss in the four-wire path.

Switches 22, 24, 26, 28 and others are for switching in direct voltage for tip and ring signals, test signals, dc power supply, such line monitoring signals as test signals via lines 30 and 32, a talking battery or other direct voltage via line feed loop detector 46, and a ringing supply via line 36 to a ring trip detector 38 and return direct current via line 34. These switches, detectors, connections and lines when multiplied by the number of telephone subscriber sets in a typical telephone system are expensive and, in fact, the line circuits typically represent as much as 80% of the equipment cost of the exchange; hence simplification thereof represents a substantial cost saving. These elements are eliminated in the arrangement described herein. A coder-decoder (codec) 40 may perform the analog-to-digital and digital-to-analog conversion. One such technique encodes the analog signal into digital format using linear A/D conversion techniques, whereafter the digital signal is companded. Analog filtering uses filters 42 and 44 which are expensive bulky audio components. A line feed loop detector 46 provides the D.C. talking battery to the line and supplies a sensing signal to signalling logic, relay drivers and power switches 48, which serve to couple the digital output of the codec 40 to the speech lines 12 and the various signal tones to the signalling lines 14. 6 5 6 8 - 4 Referring now to Figure 2, the two-wire to fourwire portion and the analog-digital conversion portions of the line circuit for interfacing a two-wire analog line or trunk to a digital system is illustrated generally at 100.

The two-wire to four-wire conversion, which is effected without the use of the conventional type of two-wire to four-wire conversion involves some imperfections as a result of which the return signal reaches Fig. 2 over the line 13 finds its way into the go signal output via line 12, albeit with a reduced amplitude. Such an undesired signal return is cancelled out at least partially by a subtraction technique which is implemented under the control of a microprocessor indicated at 126.

This is, in effect a digital filtering technique applied to the intelligence signal to be conveyed over the line 12.

All measurement and testing of analog signals associated with the speech path from a subscriber's set are monitored and measured by the microprocessor-implemented line circuit. The two-wire line 102 couples input analog speech signals via a summating impedance 122 to an analog-digital converter 104, which may include a voltage controlled oscillator foe converting the incoming analog signal to non-amplitude sensitive dc outputs. The dc encoded output is processed by microprocessor 126, within which digital filters 106 and 124 and summation circuit 110 are implemented digitally. After filtering by digital filter 106, which processes the output of the coder 104 according to a stored and/or programmable algorithm, a coded signal output on line 108 is derived. This signal is summed at a junction 110 with an inverted version of the output on line 13, which inverted version is applied to the junction formed oy the summation circuit 110 via the line 112. This is implemented by microprocessor 126, e.g. an Intel 8080 augmented with a high speed arithmetic capability for digital filtering, such that the microprocessor 126, and in particular the digital filters 124 and 106 implemented therein perform all signal processing. In addition, the G 5 β 8 - 5 digital filter provides for variation of and compensation for subscriber line transmission parameters under program control. Further ac signalling may be detected by digital filtering techniques.

The digital input signal corrected by subtraction of the unwanted signals is coupled from the summation circuit 110 via line 114 to the digital portions of the system which may be a digital switching matrix such as that disclosed in Patent Specification No.

The large audio components associated with the two-wire to four-wire conversion have been eliminated particularly the audio transformer associated with the function.

Since the digital filter 106 can compensate for line loss, the current sensitive equalizer often needed at a subset may be moved from the subset to the exchange, since a selective frequency attenuation is added into the line circuit under microprocessor control to insure that the total loss will be equal to all transmitted frequencies and distances from the exchange. Functions heretofore normally requiring separate access, i.e. talking batteries, ringing voltages, test measurements, tone dial pulse coding etc. are, in the present arrangement performed without separate access as shown in Fig. 3. Thus, the line/trunk circuit is quite separate from the switching matrix and modularity thereof on a per line basis is provided, resulting in a standard interface independent of the type of circuit from which signals must be processed. The programmability of the microprocessor provides the flexibility for program modification for adapting to different characteristics and requirements of the line/ trunk circuit vis-a-vis the input lines, such as line 102. The digital return signal is coupled via line 116 to a digital-analog decoder 118, which may be, for instance, of the well known weighted resistive network type. After filtering by filter 120, the analog signal is coupled through summing impedance 122 onto the two-wire line 102. - 6 Microprocessor 126 includes a memory for program storage accessible as needed, and accessible over a high speed data bus (not shown). A permanent storage of data which may be accomplished by means of a read-only memory incorporated within the microprocessor provides for the storage of programs not readily transferred to the line circuit on ar. as-needed basis. However, the storage requirement is minimized by the use of the high speed data bus, since the memory associated with each line circuit must exist for all line circuits; hence any cost savings associated with reduced memory capacity results in a substantial cost saving when considered in the light of the system as a whole. Typically, however, the central storage of non-easily accessible programs is associated with a sixty-four line block of line circuits; so one central memory serves sixty-four two-wire lines, such as line 102, in an actual telephone system implementation.

Referring now to Fig. 3, a hardware implementation of the more generalized subscriber line circuit of Fig. 2 is shown generally at 20G. The key element is a generator 202 for generating the line circuit alternating and direct voltages as required under microprocessor control. Essentially the incoming analog signals, including the tip and ring signals on lines 204 and 206 of the two-wire line 25102, are coupled through a suitable overvoltage protection circuit 208 to the coder 104 (shown in Fig. 3 as 218).

This senses the instantaneous voltage on the lines 204 and 206, respectively,- digitizes the sensed voltage and routes the digitized voltage through the microprocessor, where a 30pulse duration modulated drive signal is generated and fed back via driver 210 to the generator 202 to generate the required talking battery and ringing voltages, in addition, control is provided to switches as will be described below and with a resultant minimization of system hardware ^requirements. In effect, the coder is an efficient means to control a voltage source having dc continuity and ac and dc isolation characteristics even though the coder is 568 - 7 transformer coupled. This is accomplished by modulating the signal including the dc component and transformer coupling the output. Demodulation is effected by recovering the digital signal in the microprocessor, wherein a digital control signal for driving the generator 202 is derived.

All testing is done by appropriately controlling the signal generator and switches.

The generator 202 is coupled to the remainder of the illustrated circuitry by a ferrite transformer, which produces dc isolation. The tip and ring lines are coupled via lines 212 and 214, respectively, to the digital-toanalog decoder 216 and to analog-to-digital coder 218 via lines 220 and 222, without the heretofore required two-wire to four-wire converter. A highly efficient feedback control with pulse-width duration modulation is provided by the digital voltage control output from the microprocessor on line 224, which signal is applied as the base drive to transistor amplifier 226, the pulsed output of which is transformer coupled across a small ferrite transformer 228 to the generator 202. A polarity control signal is provided via lines 230 from the microprocessor and is transformer coupled to signal generator 202. A battery isolating impedance 231 is provided and a line matching impedance 232.

The microprocessor-generated pulse duration modulated control signal is preferably in the range of 50 to 100 kHz. The signal generator control signal on line 224 is derived by first measuring the output of signal generator 202 in the coder 218, wherein the incoming analog signal on lines 220 and 222 is converted to a non-analog sensitive digital output and transformer coupled via transformer 234 to the microprocessor. The instantaneous value of this signal is then compared with a reference value stored internally in the microprocessor such that any deviation therefrom generates a corrective increase or decrease, as the case may be, within the microprocessor to - 8 vary the pulse duration of the signal output from the microprocessor on line 224. Thus, tlie microprocessor functions as a feedback regulator circuit to vary tlie pulse duration of the base drive for driver 226 to generate the desired signal according to the internal reference stored in the microcomputer. One way to generate such a pulse duration signal is by counting down a preset value stored in a counter associated with the microprocessor. When such stored value reaches zero, the pulse, to be transmitted on line 224, is terminated. Of course, the preset counter is controlled by the digital feedback increase/decrease information derived by the microprocessor from the measured dc output. Other techniques for controlling the preset values stored in such a counter, such as a look-up table stored in the microprocessor memory may also be used. The high frequency operation of this power supply enables the generation of relatively smooth waveforms and the consequent use of small ferrite-type transformers and small capacitors, thus avoiding the heavy and bulky battery feed coils and audio transformers of the prior art. An isolated dc-to-dc converter 236 of conventional design may serve as the power supply for decoder 216 and coder 218. Bipolar signals are obtained by a floating isolated bridge which reverses the polarity of the secondary output of the switching regulator.

The incoming speech signals thus pass from the two-wire line 102 through coder 218 to the microprocessor and to the receiver output for coupling to a suitable digital switching matrix, while the decoder 216 output is isolated from the transmissions through coder 216 but passes through the circuit to lines 204 and 206 to enable the normal two-way telephone communication.

Switches SI to S7 are used for testing. They provide for grounding either/or both sides of the line through SI and S2, measuring current output (the voltage across 231) through S7, S4 and S3, and disconnecting impedance 232 through S5 and S6, when line linkage type _ 9 measurements must be made. Test voltages are generated by the signal generator.

While switches si to S7 are illustrated without control inputs thereto for simplicity of description, it is to be understood that such control inputs are coupled to those switches from microprocessor 126. Switches SI to S7 may be implemented in like manner as switches 320, 322, 324 and 326 described hereinafter with reference to F. 4, with any desired switching sequence being obtainable from the microprocessor.

Referring now to Fig. 4, the generator 202 is illustrated. The circuit provides a floating isolated bridge for switching the circuit on the secondary output 302 of ferrite transformer 300. For a given value of.Vcc applied via the primary 304 of transformer 300 to drive transistor 306 of driver 226, and for a generation of a fixed direct voltage at the output of signal generator 202, the pulse duration coupled to the base of transistor 306 is constant. Should the load change on the output (tip and ring lines 204 and 206, respectively), the sensing of such change causes the pulse duration of the base drive to vary correctively, as afore described. Detection of this cnange enables changes in hook status of the subscriber telephone to be detected. Further, for ring-trip detection, the average value of the pulse duration (the equivalent dc value) may be used to measure changes in line direct current. Switches 308 and 310 may be used to ground either side of the line (tip or ring) for test purposes, while inductance 312 isolates the relatively low impedance power supply and signal generator from the line. Monitoring of gain stability is accomplished internally in the microprocessor by sensing the line voltages.

Operationally, the primary 304 stores energy in 1 τ accordance with the well known relationship E = /2Li .

When transistor 306 is OK (the indicated positive polarity dots become negative polarity), diode 314 does not conduct. When transistor 306 is OFF, diode 314 conducts, charging - 10 shunt capacitor 316 and transferring the energy stored in primary 304 to the secondary 302 i.e., to capacitor 316.

Capacitor 316 also serves as a ripple voltage filter.

Energy transfer from primary 304 to secondary 302 is controlled by the switching of transistor 306 while the amount of energy transferred, i.e., the effective output voltage, is controlled by the duty cycle of the switching of transistor 306, which in turn is controlled by the pulse duration modulated signal applied to the base thereof from the microprocessor. A regulated energy feedback power supply is thus provided, but at a high enough frequency ί100 kHz) to avoid the costly and bulky audio transformers and relays of the prior art.

Switches 320, 322, 324 and 326, which may be VMOS, DHOS, bipolar or other semiconductor switches of known configuration, are driven by isolated transformer-coupled microprocessor-controlled switching pulses under the control of the microprocessor programming. To generate an ac signal a halfwave rectified signal and appropriate switch is thus employed. When switches 320 and 322 are ON, switches 324 and 326 are OFF and vice versa.

Illustratively, when switches 320 and 322 are ON, the negative polarity from capacitor 316 is coupled to the tip line 204 and the positive polarity from capacitor 316 to the ring line 206. Conversely, when switches 324 and 326 are ON, the tip has coupled thereto the positive polarity from capacitor 316 and the ring has coupled thereto the negative polarity from capacitor 316. An ac signal from the signal generator under microprocessor control is thus generated from a dc signal. The effect is significant, since the heretofore required ac power supply and switches for switching the ac power into the circuit of the prior art are eliminated. The present circuit generates ail of the subscriber line alternating and direct voltages required for tones, operation and testing.

Claims (5)

CLAIMS :

1. A telephone subscriber's line circuit or a trunk circuit, including means for coupling a two-wire line circuit or trunk circuit to a four-wire line which coupling means includes analog-to-digital conversion means for sensing the analog voltages on the line circuit and for deriving therefrom a digital output signal which is applied to the four-wire line, means whereby a further digital output signal is derived from the said analog voltages, signal processing means to which said further digital output signal is applied for comparison by the processing means with a reference, said processing means deriving a pulse duration modulated feedback control signal as a result of said comparison, and regulator means coupled to the two-wire line to which said feedback control signal is applied for regulating the amplitude of the analog voltages on said two-wire line in accordance with the said feed back control signal.

2. A circuit as claimed in claim 1, and in which the signal processing means is a microprocessor.

3. A circuit as claimed in claim 1 or 2, and in which said regulator means includes switching circuit means having a duty cycle controlled by said feedback control signal and having a switched output signal, and transformer means for transforming energy from the primary thereof to the secondary thereof in response to said switched output signal such that the amount of energy transferred from the primary to the secondary of said transformers is determined by the duty cycle of the switching circuit means.

4. A circuit as claimed in claim 3, and including means for testing the two-wire line with test voltages also generated by the electrical circuit. 5. A circuit as claimed in claim 3 or 4, and in which said transformer is a ferrite transformer and said isolation means include a floating bridge circuit. θ. A circuit as claimed in claim 5, and in which said floating bridge circuit includes a plurality of switches - 12 controlled by the microprocessor for generating an output alternating voltage. 7. A telephone subscriber's line circuit or a trunk circuit as claimed in claim 1, and substantially as

5. Described with reference to Figs. 3 and 4 of the accompanying drawings.