Abstract:
A hybrid circuit models a plurality of attributes of a communication system including a subscriber line coupled to a line interface circuit. The plurality of attributes includes subscriber line impedance, and impedance of a coupling transformer. The attributes are based on other factors, such as whether or not bridge taps exist at or near the line interface circuit on the subscriber line. The plurality of attributes are modeled by a specific arrangement of resistive and capacitive elements to substantially duplicate the collective transformative effects of the attributes on a transmission signal being sent out on the subscriber line. The transmission signal is transformed and provided to an output, where it is subtracted from a composite signal representing a combination of the actual transformed transmission signal and a receive signal. The subtraction yields an isolated receive signal, which is later processed to recover the full receive signal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to data communications, and more particularly to a circuit for matching a digital transmission on a subscriber line, where the matched transmission is used for echo-cancellation. 
     FIG. 1 shows a communication system  10  which includes a subscriber line interface circuit  100  coupled to one end of a subscriber line  110 . The subscriber line  110  is a communication medium for carrying voice and/or data signals. One example of a subscriber line  110  is a conventional telephone line comprised of a twisted-pair of copper wires. The subscriber line  110  includes a tip line  112  and a ring line  114 . According to one configuration for which the present invention is suited, the subscriber line  110  is a two-way communication medium, whereby the tip and ring lines  112  and  114  together carry both signals being transmitted and signals being received. The outbound transmission signals and inbound receive signals overlap each other in both time and frequency domains on the communication medium. In effect, the communication medium carries a composite signal representing a combination of the transmission and receive signals at each moment in time and at the same frequency. 
     The line interface  100  includes a transmission path  122  and a receive path  124 , each of which having lines corresponding to the tip line  112  and the ring line  114 . The line interface  100  is coupled to the subscriber line  110 , typically by a coupling transformer  115 . A driver  120  drives transmission signals onto the subscriber line  110 . The driver is preferably a low-impedance driver. A pair of isolation resistors R T  and R L  are matched to an impedance Z SL  of the subscriber line at the transformer  115 . The impedance Z SL  is based on an impedance of the subscriber line Z LOOP  as it is converted through the transformer  115 , as seen through the turn ratio of the coils of the transformer  115 . Secondarily, Z SL  is also based on other attributes, such as capacitance, for example, of the transformer  115  and other potential circuit components of the line interface  100 , which are not shown in FIG.  1 . 
     Point A in FIG. 1 represents a point between the output of the driver  120  and the isolation resistors, R T  and R L . The driver  120  has a very low output impedance, approaching zero, so that transmission signals on the output of the driver  120  are not affected by signals being received. Thus, signals on the driver  120  side of isolation resistors R T  and R L  are largely, if not exclusively, transmission signals. On the subscriber line side of the terminal resistors R T  and R L , several attributes of the communication system  10  transform the transmission signals. The attributes include an impedance of the subscriber line  110  and an impedance of the coupling transformer  115  At that point, the transmission signals are also combined with, and affecting, the signals being received. Point B in FIG. 1 represents a point on the subscriber line interface circuit where transformed transmission signals are combined with receive signals. 
     As both receive and transmit signals are present on the subscriber line, and have overlapping spectral content, signals being received must be isolated from transmission signals at the receiving end, i.e. at the line interface circuit  100 . However, such a procedure is very complex, due to the difficulty of determining the signal being transmitted and its effect on the signal being received. This difficulty exists because the transmission signals are transformed from a known signal at the point where they are output from the driver  120 , to a transformed signal at the point where they reach a transformer coupled to the subscriber line  110 , influenced by a plurality of transforming attributes. Most of the transformation is related to the attributes of the communication system  10  described above. 
     Signals arriving at the line interface  100  have attenuated extensively, and thus make up a smaller relative portion of the combined signal present on the subscriber line  110 . Therefore, some line interfaces employ a device known as a hybrid circuit  130  to approximate the transformation of the transmission signals. The hybrid circuit  130  is configured to produce a simulated transformed transmission signal in order to remove any transformed transmission signals from the receive signals. 
     Operation of the line interface  100  shown in FIG. 1 occurs as follows. The hybrid circuit  130  receives a pure transmission signal from the driver, and transforms it based on approximated characteristics of the subscriber line at the line interface  100 . A transformed transmission signal, representing a transmission signal that would occur at a point where it is combined with a receive signal, is passed to a subtractor  150 . A composite signal, having both transmission signals and receive signals, is coupled and filtered by a filter  140 , to remove aliasing or interfering frequencies. At the subtractor  150 , the signal provided by the hybrid circuit  130  is subtracted from the composite signal provided by the filter  140 , to theoretically yield only a receive signal. The recovered receive signal is then passed on for digital signal processing. The anti-aliasing filter  140  is normally provided separately from the hybrid circuit  130  to ensure a receive signal does not exhibit aliasing when the digitization process occurs in the DSP. 
     Conventional hybrid circuits, therefore, generally take a “known” signal being transmitted, and approximate how that signal will change in the presence of signal-transforming characteristics of the subscriber line. An approximated transmission signal is needed so that it may be removed from a receive signal. Accordingly, a hybrid circuit should model the transformative characteristics of the subscriber line on a transmission signal as accurately as possible. Conventional hybrid circuits are limited in how well they model those transformative characteristics of a given subscriber line. 
     SUMMARY OF THE INVENTION 
     The present invention is a circuit and method for modeling a plurality of attributes of a communication system, and their transformative effects on outbound transmission signals. The communication system includes both a line interface device that transmits outbound transmission signals and receives inbound receive signals, and a subscriber line that carries combined transmission and receive signals that overlap in time and frequency domains. The plurality of attributes collectively transform an outbound transmission signal such that, at a point where it is combined with a receive signal on the subscriber line, the outbound transmission signal is significantly altered. The attributes include impedances from various sources on the subscriber line and circuits of a line interface circuit. The present invention accurately models the plurality of attributes in order to substantially duplicate the transformative effects of the attributes on the outbound transmission signal, and provide an accurate copy of a transformed transmission signal for removal from the receive signal. 
     In an embodiment of the invention, a circuit includes an input/output section that couples an outbound transmission signal to provide a copy of a transmission signal at an input of the circuit, and a filter section, connected to the input, for filtering aliasing frequencies from the transmission signal. The circuit further includes a modeling section, connected to the filter section. The modeling section is configured to model a plurality of attributes of the communication system, wherein the model transforms the copy of the transmission signal to substantially duplicate the transformative effects of the plurality of attributes on the outbound transmission signal, and for providing a transformed transmission signal to an output of the circuit at the input/output section. 
     In another embodiment of the invention, a method includes the steps of coupling an outbound transmission signal to provide a copy of a pure transmission signal that is unaltered by transformative attributes of the communication system. The method further includes filtering the copy of the transmission signal to remove aliasing frequencies, and transforming the filtered copy of the transmission signal based on a model of a plurality of attributes of the communication system, wherein the model substantially duplicates the transformative effects of the plurality of attributes on the outbound transmission signal. The transformed copy of the transmission signal is then provided as an output, for subtraction from the receive signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a line interface to illustrate the function of a hybrid circuit. 
     FIG. 2 is a circuit schematic diagram of an improved hybrid circuit according to one preferred embodiment of the present invention. 
     FIG. 3 is a circuit schematic diagram of an improved hybrid circuit according to a second preferred embodiment of the present invention. 
     FIG. 4 is a chart illustrating improved echo cancellation effects of a hybrid circuit according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is an improved hybrid circuit and method thereof of the same, where a plurality of attributes of a communication system are modeled by a specific arrangement of resistors and capacitors. The arrangement models the attributes from a point where the transmission signal is provided to the subscriber line, to a point where the transmission signal is combined with a receive signal. The arrangement thus substantially duplicates the transformative effects of the attributes on a transmission signal provided on the subscriber line. The modeled attributes are then applied to a coupled transmission signal as it is being transmitted onto a subscriber line. The hybrid circuit according to the present invention provides a transformed transmission signal which may be removed from a receive signal, to recover a clean receive signal. 
     Generally, a method and circuit for the hybrid circuit  130 , in accordance with the invention, accurately generates a transfer function of a transmission signal between points A and C in FIG. 1, such that it matches a transfer function of the transmission signal between points A and B. 
     FIG. 2 is a circuit schematic diagram of one embodiment of a hybrid circuit  200  according to the invention. According to FIG. 2, and with reference to the communication system  10  shown in FIG. 1, a hybrid circuit  200  is illustrated for a subscriber line  110  that has no bridge taps near the line interface  100 . The hybrid circuit  200  includes a signal input  201  having a first line  202  and a second line  204 . According to one exemplary embodiment, the signal input is a differential input, the first line  202  carries a positive signal, and the second line  204  carries a negative signal. In an alternative embodiment, however, the signal input  201  may be a single-ended connection to the subscriber line  110 , which may or may not be carrying a differential pair of signals. 
     In the exemplary embodiment, the signal input  201  receives a transmission signal from the line interface  100  connected to the subscriber loop  110 , and provides the signal as a differential signal on the first input line  202  and the second input line  204 . The first input line  202  and the second input line  204  correspond to respective tip and ring lines of a common two-wire, twisted pair cable that makes up a subscriber line  110 . According to the invention, transmission signals, as well as signals being received, are transmitted according to a digital subscriber line (DSL) protocol. There are several varieties of DSL, which are collectively known as xDSL. In a preferred embodiment, the transmission protocol is symmetrical DSL (SDSL), in which the speed of transmission signals is the same as the speed of signals being received. Another symmetrical DSL for which the present invention is suited is high bit rate DSL (HDSL). 
     The signal input  201  is coupled to an input/output (I/O) section  230  of the hybrid circuit  200 . The I/O section  230  is made up of resistive elements configured to remove a DC component of the transmission signal on the input  201 , and restore that DC component to a transformed transmission signal on an output  241 . 
     The signal input  201  and I/O section  230  are coupled to a filter section  210 . The filter section  210  is adapted to remove any frequencies outside the frequency band of the transmission signal, in order to minimize alias signals that might occur upstream during eventual digital processing of receive signals. The filtered AC component of the transmission signal is provided to a modeling section  220 . The modeling section  220  is configured to model a plurality of attributes of the subscriber line  110  and line interface  100  that transform a transmission signal from a point where the transmission signal is provided to the subscriber line, i.e. point A in FIG. 1, to a point where the transmission signal is combined with a receive signal, i.e. point B in FIG.  1 . The plurality of attributes are modeled so as to substantially duplicate a collective transformative effect of the attributes on the transmission signal. In an exemplary embodiment, the modeling section  220  is an arrangement of resistive and capacitive elements configured to model a transfer function of the communication system  10  from the output of the driver  120  to the opposite side of the isolation resistors R T  and R L . 
     The modeling section  220  of the hybrid circuit  200  is further configured to provide a transformed transmission signal, representing the AC component of the transmission signal as transformed by the specific modeled attributes of the communication system  10 . The transformed transmission signal is provided back to I/O section  230  for restoration of the DC component to the signal. The restored and transformed transmission signal is then provided to an output  241 , which includes a first line  242  and a second line  244  for carrying a differential signal. 
     According to a specific exemplary embodiment shown in FIG. 2, the filter section  210  includes a resistor R 1  connected to the first input line  202 , a resistor R 2  connected to the second input line  204 , and a capacitor C 1  connected to the first and second resistors R 1  and R 2 , opposite the first and second input lines  202  and  204 , respectively. In an exemplary embodiment, R 1  and R 2  are 121Ω, 1% tolerance resistors, and C 1  is a 5.6 nF, 5% tolerance capacitor. 
     In the specific exemplary embodiment of the hybrid circuit  200 , the modeling section  220  includes a first combination of a resistor R 3 , a resistor R 6 , a resistor R 8  and a capacitor C 6 , connected in series, and wherein the capacitor C 6  is connected to the first output line  242 , and a second serial combination of a resistor R 4 , a resistor R 7 , a resistor R 9  and a capacitor C 7 , connected in series, and wherein the capacitor C 7  is connected to the second output line  244 . The modeling section  220  further includes a capacitor C 3  connected in parallel with the resistor R 6 , a capacitor C 4  connected in parallel with the resistor R 7 , a resistor R 5  connected between resistors R 3  and R 6 , and a capacitor C 2  connected between resistors R 4  and R 7  and connected to the resistor R 5 . The modeling section  220  further includes a resistor R 10  and a capacitor C 5 . The resistor R 10  is connected to a node between the resistor R 8  and the capacitor C 6 . The capacitor C 5  is in turn connected to a node between the resistor R 9  and the capacitor C 7 , and connected to the resistor R 10 . 
     In the specific exemplary embodiment illustrated with reference to FIG. 2, R 3  and R 4  are 27.4Ω resistors, R 5  is a 511Ω resistor, R 6  and R 7  are 16.9Ω resistors, R 8  and R 9  are 93.1Ω resistors, and R 10  is a 1.3 kΩ resistor. All resistors in the modeling section  220  preferably are 1% tolerance. The capacitor C 2  is a 5.6 nF capacitor, C 3  and C 4  are 4.7 nF capacitors, C 5  is a 10 nF capacitor, and C 6  and C 7  are 6.8 nF capacitors. All capacitors in the modeling section  220  are preferably 5% tolerance. 
     The input/output (I/O) section  230  of the hybrid circuit  200  includes a resistor R 11  connected to the first output line  242 , a resistor R 12  connected between the resistor R 11  and the second output line  244 , a resistor R 13  connected to a node between the resistors R 11  and R 12 , and connected to the first input line  202 , and a resistor R 14  connected to the node between the resistors R 11  and R 12 , and connected to the second input line  204 . In the exemplary embodiment, R 11  and R 12  are 2.74 kΩ resistors, and R 13  and R 14  are 5.11 kΩ resistors, all of which preferably having 1% tolerance. 
     FIG. 3 shows an alternative exemplary embodiment of a hybrid circuit  300  according to the present invention. Hybrid circuit  300  is adapted for a case where a subscriber line includes one or more taps to other subscriber lines near the line interface. The circuit  300  includes a signal input  301  having a firsts line  302  and a second line  304 , which are preferably arranged as a differential pair of wires, but which may be a single-ended input connection to a differential subscriber line  110 . The signal input  301  corresponds to the tip and ring lines of the subscriber line  110 . The circuit  300  includes a filter section  310  for removing aliasing frequencies, as discussed above, and a modeling section  320  for modeling a plurality of attributes of the communication system  10  to substantially duplicate the collective transfornative effects thereof on the transmission signal. The modeling section  320  provides a transformed transmission signal to an output  341 . The circuit  300  further includes an input/output section  330  for removing and restoring a DC component of the transmission signal before and after it is transformed, respectively. The output may also be a differential pair of wires, and includes a first line  342  and a second line  344 . 
     According to the embodiment shown in FIG. 3, the filter section  310  includes a resistor R 14  connected to the first input line  302 , a resistor R 15  connected to the second input line  304 , and a capacitor C 8  connected to opposite sides of the resistors R 14  and R 15  from the input lines  302  and  304 . In an exemplary embodiment, R 14  and R 15  are 182Ω, 1% tolerance resistors, and C 8  is a 1.0 nF, 5% tolerance capacitor. 
     In the alternative exemplary embodiment of the hybrid circuit  300 , the modeling section  320  includes a first combination of a resistor R 16  and a capacitor C 9 , connected in parallel, and a second combination of a resistor R 17  and a capacitor C 10 , coupled in parallel. The modeling section  320  further includes a third combination of a resistor R 18 , a capacitor C 12 , and a resistor R 20  connected in series, wherein the resistor R 18  is connected to the first combination, and a fourth combination of a resistor R 19 , a capacitor C 13 , and a resistor R 23 , connected in series, wherein the resistor R 19  is connected to the second combination. A fifth combination of a resistor R 24  and a capacitor C 11  is connected in series, wherein the resistor R 24  is connected to a node between the resistor R 18  and the capacitor C 12 , and wherein the capacitor C 11  is connected to a node between the resistor R 19  and the capacitor C 13 . 
     In the exemplary alternative embodiment  300 , R 16  and R 18  are 681Ω resistors, R 18  and R 19  are 86.6Ω resistors, and R 24  is a 357Ω resistor. The resistors R 20  and R 23  are 196Ω resistors. As illustrated in FIG. 3, C 9  and C 10  are 1 nF capacitors, C 11  is a 2.2 nF capacitor, and C 12  and C 13  are 10 nF capacitors. All resistors in the modeling section  320  are preferably 1% tolerance, and all capacitors are 5% tolerance. 
     The I/O section  330  includes a first combination of a resistor R 21  and a resistor R 22 , connected in series. The I/O section further includes a second combination of a resistor R 25  and a resistor R 26 , connected in series. The resistor R 25  is connected to the first input line  302  and the resistor R 26  is connected to the second input line  304 . The resistor R 21  is connected to the first output line  342 , and the resistor R 23  is connected to the second output line  344 . A node between the resistor R 21  and the resistor R 22  is connected to a node between the resistor R 25  and the resistor R 26 . In an example circuit  300 , resistors R 25  and R 26  are 511Ω, 1% tolerance resistors. Resistors R 21  and R 22  are 1.75 kΩ resistors, also 1% tolerance. 
     The hybrid circuits described above may be included in a single circuit, for switching between a circuit according to either embodiment. Each circuit,  200  and  300  respectively, is modeled for specific impedance conditions and subscriber line characteristics, based in part on an existence and number of bridge taps from the subscriber line to which they are coupled, for example. A measurement of the impedance may be made on the subscriber line, and a switch is then activated to energize one or the other of the hybrid circuits  200  or  300 , in order to optimize the signal approximation function of the hybrid circuit and improve receive signal quality. 
     FIG. 4 illustrates a benefit of a hybrid circuit  200  relative to prior art hybrid circuits. Again, the main function of general hybrid circuit is to characterize signals being transmitted, so that they may be removed from receive signals which they overlap on a transmission medium. The better the characterization, the more of the signal being transmitted is removed. Even after removal, residual frequency response of a signal being transmitted remains, creating “echo” signals along with the signal being received. In FIG. 4, the amount dB of echo cancellation by a circuit according to the present invention is far greater than typical hybrid circuits, in the frequency range of 1 to 1000 kHz. 
     Other embodiments, combinations and modifications of this invention will occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.