Patent Publication Number: US-6990191-B2

Title: Method and apparatus for handling voice and data signals

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to telecommunications, and, more particularly, to a method and apparatus for handling Plain Old Telephone System (POTS) and x-digital subscriber line (xDSL) signals. 
   2. Description of the Related Art 
   In communications systems, particularly telephony, it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire, bi-directional communication channel. A line card generally connects the subscriber station to the central switching office. The functions of the line card range from supplying talk battery to performing impedance matching to handling ringing signals, voice signals, and testing signals. 
   Equipment connected to telephone lines generally have a standardized impedance. To drive a signal on a subscriber line while minimizing signal reflection from the far end of the subscriber line and maximizing the signal power coming out the far end, it is desirable to match the standardized impedance. This impedance is typically symbolized as Z LOOP , which may be a function of frequency and decreases as frequency increases. For POTS lines, the value of Z LOOP  is determined by individual telephone authorities in various countries and, although somewhat variable, is in the range of 600–900 ohms and may or may not include some type of capacitive element. The extent to which a signal driver is matched to the standardized impedance in these systems is measured with a parameter known as “return-loss.” Perfect matching will have an infinite return-loss. This indicates that none of the incident signal transmitted from a source with the standard impedance is reflected back to the source. 
   In an electronic Plain Old Telephone System (POTS), the impedance matching function has generally been performed by line cards using a variety of well-known impedance matching filter loops. The function of the impedance matching filter loop in POTS-only applications is to take the input signal, modify it through a programmable gain and phase element, and feed it back to the output so that the input signal sees a different response than it would without the presence of the impedance matching filter. The above-described impedance matching process is generally effective in accomplishing the intended purpose, at least as it pertains to a POTS-only system. 
   The Plain Old Telephone System, designed primarily for voice communication, provides an inadequate data transmission rate for many modem applications. To meet the demand for high-speed communication, designers have sought innovative and cost-effective solutions that would take advantage of the existing network infrastructure. Several technological solutions proposed in the telecommunications industry use the existing network of telephone wires. A promising one of these technologies is the xDSL (digital subscriber line) technology. 
   xDSL is making the existing network of subscriber lines more robust and versatile. Once considered virtually unusable for broadband communications, an ordinary twisted pair equipped with DSL interfaces can transmit video, television, and very high-speed data. The fact that more than six hundred million subscriber lines exist around the world is a compelling reason for these lines to be used as the primary transmission conduits for at least several more decades. Because DSL utilizes telephone wiring already installed in virtually every home and business in the world, it has been embraced by many as one of the more promising and viable options. 
   There are now at least three popular versions of DSL technology, namely Asymmetrical Digital Subscriber Line (ADSL), Very High-Speed Digital Subscriber Line (VDSL), and Symmetric Digital Subscriber Line (SDSL). Although each technology is generally directed at different types of users, they all share certain characteristics. For example, DSL systems utilize the existing, ubiquitous telephone wiring infrastructure, deliver greater bandwidth, and operate by employing special digital signal processing. Because the aforementioned technologies are well known in the art, they will not be described in detail herein. 
   DSL and Plain Old Telephone System technologies can co-exist in one line (e.g., also referred to as a “subscriber line”). Traditional analog voice band interfaces use the same frequency band, 0–4 Kilohertz (KHz), as telephone service, thereby preventing concurrent voice and data use. A DSL interface, on the other hand, operates at frequencies above the voice channels, from 25 KHz to 1.1 Megahertz (MHz). Standards for certain derivatives of DSL are still in definition, and, therefore, are subject to change. Thus, a single DSL line is capable of offering simultaneous channels for voice and data. It should be noted that the standards for certain derivatives of ADSL are still in definition as of this writing, and therefore are subject to change. 
   DSL systems use digital signal processing (DSP) to increase throughput and signal quality through common copper telephone wire. It provides a downstream data transfer rate from the DSL Point-of-Presence (POP) to the subscriber location at speeds of up to 1.5 megabits per second (MBPS). The transfer rate of 1.5 MBPS, for instance, is fifty times faster than a conventional 28.8 kilobits per second (KBPS). 
   Although DSL and POTS systems can co-exist on one line, the DSL traffic passing through the POTS circuitry impairs the functionality of the impedance matching filter of the POTS circuitry. This is because decimators and analog-to-digital converters that are ordinarily utilized in a POTS-only system cannot process the high frequencies of the data band, thus causing the performance of the POTS impedance matching filter to degrade. Additionally, the impedance required at data frequencies is much lower than at voice frequencies. 
   The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
   SUMMARY OF THE INVENTION 
   In one aspect of the present invention, a method is provided for impedance matching for an apparatus capable of supporting voice and data. The method includes receiving an input signal having at least one of a voice component, data component, and DC component, and filtering at least a portion of the data component and DC component of the input signal to provide a filtered signal. The method further includes adjusting an input impedance of the apparatus to a first preselected value for the voice band in response to the filtered signal and adjusting the input impedance of the first apparatus from the first preselected value to a second preselected value. The method includes adjusting at least one of a magnitude and phase of the filtered signal to adjust the input impedance to a third value. 
   In another aspect of the present invention, an apparatus is provided for impedance matching. The apparatus includes circuitry adapted to receive an input signal having at least one of a voice, data, and DC component. The apparatus includes a first and second filter. The first filter is adapted to filter at least a portion of the data component of the input signal to provide a filtered data signal, and the second filter adapted to filter at least a portion of the DC component of the filtered data signal to provide a filtered signal. The apparatus includes a first, second, and third impedance blocks. The first impedance block is adapted to adjust an input impedance of the apparatus to a first preselected value for the voice band in response to the filtered signal. The second impedance block is adapted to adjust the input impedance of the first apparatus from the first preselected value to a second preselected value. The third impedance block is adapted to adjust at least one of a magnitude and phase of the filtered signal to adjust the input impedance to a third value. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
       FIG. 1  illustrates a communications system in accordance with the present invention; 
       FIG. 2  depicts a block diagram of an embodiment of a line card in accordance with the present invention that can be implemented in the communications system of  FIG. 1 ; 
       FIG. 3  depicts an exemplary DC feed curve that may be employed by the line card of  FIG. 2 ; 
       FIG. 4  illustrates an embodiment of an impedance matching module that may be employed by the line card of  FIG. 2 ; and 
       FIG. 5  illustrates a method in accordance with the present invention that may be implemented by the line card of  FIG. 2 . 
   

   While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
   Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
   Referring first to  FIG. 1 , a simplified block diagram of a communications system  5  in accordance with the present invention is provided. The communications system  5  includes a line card  10  that interfaces with a telephonic device  12  over a subscriber line  20 . In an actual implementation, the line card  10  interfaces with a plurality of subscriber lines  20 , but for clarity and ease of illustration, only one is shown. A subscriber line interface circuit (SLIC)  30  is coupled to the subscriber line  20 . Hereinafter, signals received by the line card  10  over the subscriber line  20  are referred to as upstream signals, and signals transmitted by the line card  10  on the subscriber line  20  are referred to as downstream signals. 
   The SLIC supplies an analog upstream signal to a coder/decoder (CODEC)  40 . The CODEC  40  receives the analog upstream signal from the SLIC  30  and generates a digital upstream signal that is subsequently passed to a digital signal processor  50 . The DSP  50  also provides a digital signal for eventual transmission on the subscriber line  20 . The CODEC  40  receives the digital signal, converts it to an analog signal, and provides the analog signal to the SLIC  30 , which sends the analog signal over the subscriber line  20 . 
   In the illustrated embodiment, the line card  10 , in addition to supporting plain old telephone service (POTS), is adapted to implement an asynchronous digital subscriber line (ADSL) modem for high bandwidth data transfer. The ADSL protocol is described in ANSI T1.413 Issue 2, entitled, “Interface Between Networks and Customer Installation—Asymmetric Digital Subscriber Line (ADSL) Metallic Interface.” The SLIC  30  of the line card  10  is capable of performing a variety of functions, such as battery feed, overload protection, polarity reversal, on-hook transmission, and current limiting. Only relevant portions of the SLIC  30 , CODEC  40 , and DSP  50  are described herein, although those of ordinary skill in the art will appreciate that these devices may perform other functions that are not described in this disclosure. 
   The telephonic device  12  may comprise a telephone or any other device capable of providing a communication link between at least two users. In one embodiment, the telephonic device  12  may be one of a variety of available conventional telephones, such as wired telephones and similar devices. In an alternative embodiment, the telephonic device  12  may be any device capable of performing a substantially equivalent function of a conventional telephone, which may include, but is not limited to, transmitting and/or receiving voice and data signals. Examples of the telephonic device  12  include a data processing system (DPS) utilizing a modem to perform telephony, a television phone, a DPS working in conjunction with a telephone. 
     FIG. 2  illustrates one embodiment of the line card  10  in accordance with the present invention. Specifically, the line card  10  includes the SLIC  30 , which, in the illustrated embodiment, is a voltage-feed SLIC. The line card  10  also includes the CODEC/DSP  40 ,  50 , which in the illustrated embodiment are shown as a subscriber line audio-process circuit (SLAC)  215  that integrates the functions of both the CODEC and DSP  40 ,  50 . The data band circuits are generally not involved in the impedance setting function and are not shown. The line card  10  may be located at a central office or a remote location somewhere between the central office and the telephonic device  12  (see  FIG. 1 ). The line card  10  interfaces with the telephonic device  12  through tip and ring terminals  237 ,  239  at the SLIC  30 . The combination of the telephone device  12  and the subscriber line  20  is generally referred to as a subscriber loop. 
   The standard impedance of the subscriber line  20  is herein denoted as Z LOOP , and impedance seen by an incoming signal from the subscriber line  20  is hereinafter referred to as Z IN . The value of Z LOOP , which is determined by individual telephone authorities in various countries, may be in the range of 600–900 ohms for the POTS band and in the range of 100–135 ohms for the xDSL band. The SLIC  30  is adapted to be coupled to first and second (RFz) resistors  217 ,  219 , which, as described below, are utilized to define the input impedance. 
   The line card  10 , which may be capable of supporting a plurality of subscribers lines  20 , performs, among other things, two fundamental functions: DC loop supervision and DC feed. The purpose of DC feed is to supply enough power to the telephone device  12  at the customer end. The purpose of DC loop supervision is to detect changes in DC load, such as on-hook events, off-hook events and rotary dialing, or any other event that causes the DC load to change. In the interest of clarity and to avoid obscuring the invention, only that portion of the line card  10  that is helpful to an understanding of the invention is illustrated. 
   The voltage-feed SLIC  30  is a high voltage bipolar SLIC that drives voltages to the subscriber line  20  and senses current flow in the subscriber line  20 . The SLIC  30  includes first and second differential line drivers  230 ,  235  that interface with the subscriber line  20  via tip and ring terminals  237 ,  239 . The tip terminal  237  is coupled to a first terminal of a first sensing resistor (R ab )  240  and to an inverting terminal of the first line driver  230 . A second terminal of the first sensing resistor  240  is coupled to an output terminal of the first line driver  230 . The ring terminal  239  is coupled to a first terminal of a second sensing resistor (R bd )  245  and to an inverting terminal of the second line driver  235 . A second terminal of the second sensing resistor  245  is coupled to an output terminal of the second line driver  235 . 
   The SLIC  30  includes a sum block  250  and a current-sensing circuit  260 . The sum block  250  includes a first output terminal coupled to a non-inverting terminal of the first line driver  230 , and a second (inverted) output terminal coupled to a non-inverting terminal of the second line driver  235 . The sum block  250  is capable of receiving a DC feed signal (as well as ringing signals) from a DCIN terminal  265 , a voice signal, a metering signal, and a data signal and is capable of adding one or more of the received signals and providing it to the first and second line drivers  230 ,  235 . The signals into the SUM block  250  may be subjected to different levels of gain for optimal performance. The signal from the DCIN terminal  265  is low-pass filtered. 
   The current-sensing circuit  260  produces a current proportional to the loop current through the current sensing resistors  240 ,  245 , subtracts a current proportional to a current from a cancellation terminal (CANC)  270 , and provides the resulting current to an IMT terminal  275  of the SLIC  30 . Although not so limited, in the instant embodiment, the constant of proportionality (KCN) for the current from the cancellation terminal (CANC)  270  is unity, and the constant of proportionality for the metallic line current is 0.001. Those skilled in the art will appreciate that only those portions of the SLIC  30  deemed relevant to the invention are disclosed herein. The SLIC  30  may employ resistors or other circuitry that is not illustrated in  FIG. 2 . 
   The SLIC  30  includes a first impedance matching loop  262  that adjusts a nominal value of the input impedance (Z IN ) to substantially match the impedance of the subscriber line  20 . The first impedance matching loop  262  includes a nominal Z block  263  that receives the output signal of the current sensing circuit and provides a selected amount of “fixed” gain and phase to adjust a nominal value of the input impedance, Z IN . In the illustrated embodiment, the nominal Z block  263  sets the nominal value of the input impedance to a fixed value of 100 ohms plus 800 ohms in parallel with 34 nF, which includes the resistance provided by resistors  217 ,  219 ,  240  and  245 . 
   The SLIC  30  is connected to the SLAC  215 , an external resistor  280 , as well as a capacitor  281 . A first terminal of the resistor  280  is coupled to the IMT terminal  275  of the SLIC  30 , as well as to the VIN terminal  285  of the SLAC  215 . A second terminal of the resistor  280  is coupled to a reference voltage node  282 , as well as to a terminal of the capacitor  281 . In one embodiment, the reference voltage  282  is in the range of about 1.4 volts. The external resistor  280  and the capacitor  281  form a single-pole low pass filter  283  that is capable of filtering at least a portion, if not all, of the signals above the voice band, such as data signals and metering signal. The external resistor  280  and the capacitor  281  convert the current flowing from the IMT terminal  275  to a proportional voltage signal for the SLAC  215 . Although not necessary, the resistor  280  is external in the illustrated embodiment because in some embodiments it may be useful for the drive value of the resistor to be relatively precise and because each line card  10  may require different values. 
   A discrete network  288  couples the SLIC  30  to the SLAC  215  via the CANC terminals  270 ,  290 . The discrete network  288  includes a first and second resistor  292 ,  294  and a capacitor  296 . A first terminal of the first resistor  292  is coupled to the CANC terminal  270  of the SLIC  30  and a second terminal of the first resistor  292  is coupled to a first terminal of the second resistor  294 . The second terminal of the second resistor  294  is coupled to the CANC terminal  290  of the SLAC  215 . The capacitor  296  is coupled between the second terminal of the first resistor  292  and the reference voltage node  296 . The discrete network  288  acts as a low pass filter and converts the voltage output signal from the SLAC  215  to a current and provides it to the SLIC  30 . 
   The SLAC  215  interfaces with the telephonic device  12  through the SLIC  30  and over the subscriber line  20 . The SLAC  215  includes two feedback loops: a DC cancellation loop  298  and a DC feed loop  300 . In the illustrated embodiment, the two loops  298 ,  300  are implemented within a digital signal processor (DSP). Only those portions of the SLAC  215  deemed relevant to the instant invention are described herein, albeit the SLAC  215  may perform a variety of other functions that are not illustrated in  FIG. 2 . 
   The DC cancellation loop  298  includes an analog-to-digital converter  305 , DC cancellation logic  315 , a current limiter  317 , and a digital-to-analog converter  319 . In the illustrated embodiment, to reduce hardwire complexity, the voice and DC components of the input signal from the VIN terminal  285  share the same analog-to-digital converter  305 . The analog-to-digital converter  305  and digital-to-analog converter  319  include a decimator and interpolator, respectively. The analog-to-digital converter  305  in the illustrated embodiment is capable of providing two output signals, the first output signal is sampled at a 4 KHz frequency and provided as a digital signal to the DC cancellation logic  315 , as well as to a switch hook detection logic  320 . The second output signal of the analog-to-digital converter  305 , comprising of voice and/or data (residual) components, is sampled at 32 KHz and provided to a CODEC (not shown). A residual data component may exist at the output of the analog-to-digital converter  305  since the single-pole low pass filter  283  may not remove all of the data signal; however, this signal is removed by the decimator. 
   The DC cancellation logic  315  receives the digital signal from the analog-to-digital converter  305 , filters high frequencies, and provides a substantially DC signal. The DC signal is provided as an input to the DC feed logic  321 , as well as to the current limiter  317 . The output of the current limiter  317  is converted to an analog signal and then provided back to the SLIC  30  via the CANC terminal  270 . The output of the current limiter  317  is also provided to the switch hook detection logic  320  for switch hook detection during pulse dialing. The current provided to the CANC terminal  270  of the SLIC  30  is used to cancel the DC component of the signal from the current sense circuit  260 . Thus, during a “stable” state (i.e., no transients present), the signal at the VIN terminal  285  of the SLAC  215  is essentially DC free. 
   The DC feed loop  300 , in addition to the analog-to-digital converter  305  and DC cancelation logic  315 , includes DC feed logic  321  and a digital-to-analog converter  322 . In the illustrated embodiment, the digital-to-analog converter  322  may also interpolate. The output from the digital-to-analog converter  322  is provided to a DCIN terminal  265  of the SLIC  30  via VHL terminal  323  of the SLAC  215 . The DC feed logic  321  is capable of providing high DC voltage to the subscriber loop so that sufficient current (20–60 mA) can be driven through a resistance as high as 2K ohms. When the DC conditions on the subscriber loop change suddenly, the DC feed logic  321  adapts to the change, thereby allowing normal transmission to continue. Examples of sudden changes in DC conditions include on-hook, off-hook, rotary dialing, and tone signaling. When the telephonic device  12  goes off-hook, the loop impedance drops almost instantly to a value below 2K ohms. In short subscriber loops, the loop impedance may be less than 200 ohms. For the line card  10  to function and transmit information properly, the DC conditions on the subscriber loop should be stabilized quickly, and in some cases, within milliseconds. 
     FIG. 3  illustrates an exemplary DC feed curve that may be adapted for use by the DC feed logic  321 . A dashed line  328  provides the upper limits for the electrical power, and a dashed line  329  provides the lower limits for the electrical power provided to the subscriber loop. A Y-axis  330  represents voltage, and an X-axis  335  represents current. As can be seen in  FIG. 3 , although not so limited, the DC feed curve includes an anti-saturation region, a resistance feed region, and a current limit region. 
   The SLAC  215  includes a second matching loop  324  that includes an impedance matching block  325 . The impedance matching block  325  receives the signal from the VIN terminal  285  of the SLAC  215  and provides an output signal to an VOUT terminal  326  of the SLAC  215 . The signal from the VOUT terminal  326  is provided to a VIN terminal  327  of the SLIC  30 . The sum block  250  receives the signal from the VIN terminal  327  of the SLIC, sums the signal with other signals, such as DC feed signal, the data signal, and metering signal, and provides the resulting signal to the drivers  230 ,  235  of the SLIC  30 . 
   When the line card is in a “stable” state (i. e., no transients), the signal at the VIN terminal  285  of the SLAC  215  comprises primarily a voice signal, although it may include residual metering or data signals that are not removed by the single-pole low pass filter  283 . This single-pole low pass filter  283  provides adequate performance by attenuating the data and metering signals to acceptable levels. Aside from being more cost effective than higher order low-pass filters, the single-pole low pass filter  283  also provides an added advantage in that it is less likely to make the line card  10  unstable. 
   Referring now to  FIG. 4 , a specific embodiment of the impedance matching module  325  is shown. As can be seen with reference to  FIG. 4 , the impedance matching module  325  comprises three loops: an analog impedance scaling network (AISN) loop  350  and two Z-filter loops  355 ,  356 . The AISN loop  350  includes an AISN block  358  that is coupled between the VIN and VOUT terminals  285 ,  326  of the SLAC  215 . The signal present at the VIN terminal  285  of the SLAC  215  is low-pass filtered to prevent higher frequencies of the data band from impairing the function of the impedance matching module  325 , although some frequencies above the voice band range that might not have been filtered may be present at the VIN terminal  285 . The AISN block  358  may be a programmable impedance matching filter that is capable of varying the nominal value of the input impedance that is set by the nominal-Z block  263  (see  FIG. 2 ) of the SLIC  30 . Typically, the AISN block  358  is effective in adjusting the input impedance for frequencies throughout the voice band. This impedance is relatively constant in the voice band. 
   The fixed analog gain forms an impedance which is equal to the product of the current sense gain (KIMT) times the impedance of the parallel combination of the external resistance (RIMT) and capacitance (CIMT) times the feedback (NOMINAL-Z) gain. Taking into account the external fuse resistance (RF), equation (1) below illustrates the impedance: 
               Z     I   ⁢           ⁢   N       (   5   )       =       R   ⁢           ⁢   F   ⁢           ⁢   Z     +     K   ⁢           ⁢   I   ⁢           ⁢   M   ⁢           ⁢   T   *   K   ⁢           ⁢   Z   *       R   ⁢           ⁢   I   ⁢           ⁢   M   ⁢           ⁢   T       1   +     R   ⁢           ⁢   I   ⁢           ⁢   M   ⁢           ⁢   T   *   C   ⁢           ⁢   I   ⁢           ⁢   M   ⁢           ⁢   T   *   S                     (   1   )             
 
where the exemplary values are as follows: RFZ=100 Ω; KIMT=0.001; NOMINAL-Z=8; RIMT=100 KΩ; and CIMT=270 pF (+6 pF of stray capacitance). This leads to an input impedance of 100 Ω+(800 Ω∥35.5 F).
 
   The programmable analog gain (AISN) inside the SLAC  215  has substantially the same effect as NOMINAL-Z. In the illustrated embodiment, the gain varies from 15/16 to 15/16 in 1/16 steps inside the SLAC  215 . This is passed through a gain of KIN=5 inside the SLIC  30 . The effect is to increase or decrease KL by as much as 4.6875. This allows the 800 Ω resistor to be lowered to 331.25 Ω or raised to 1268.75 Ω. The corner frequency is unchanged. 
   The Z-filter loop  355  of the impedance matching module  325  receives a signal from the analog-to-digital converter  305 . The analog-to-digital converter  305  converts the signal from the VIN terminal  285  of the SLAC  215  to a digital signal and delivers it to the high pass filter  370 . In the illustrated embodiment, the analog-to-digital converter  305  also, through the process of decimation, reduces the frequency of the bits of the signal and provides a more accurate signal to the input terminals of the Z-filters  375 ,  376 . The CANC input terminal  270  of the SLIC  30  is used to remove most of the DC feed component. The high-pass filter  370  substantially reduces the residual DC component, if one is present, from the received signal and provides primarily a voice-only-signal to an input terminal of the Z-filters  375 ,  376 . 
   The Z-FIR filter  375  may be a programmable impedance matching filter that allows a user to program the gain factor, the phase shift, or a combination thereof to vary Z IN . Typically, the Z-FIR filter  375  provides a frequency variable input impedance to the line card  10 . The Z-FIR filter  375  also attempts to account for delays in the input signal caused by digital processing, for example, during decimation and interpolation. The programmable Z-FIR filter  375  further modifies the analog gains with a frequency variable gain. This allows control of the midband frequency characteristics. For very high frequencies, the delay in the digital interpolation and decimators reduces the effectiveness of the Z-FIR filter  375 . At low frequencies, the limited length of the Z-FIR filter  375  limits the filter; however, the Z-IIR filter  376  then takes over. 
   The Z-IIR filter  376  provides a single pole low pass filter for responses which demand a long exponential time response. This has the effect of applying what looks like a large capacitor in series with the rest of the network. 
   The output from the Z-FIR filter  375  and Z-IIR filter  376  is added and provided to a first signal adder  380 , which adds the filter  375 ,  376  output to a downstream voice signal. The output of the first signal adder  380  is provided to a digital-to-analog converter  383  that converts the received signal and provides an analog output signal to a second signal adder  385 . The second signal adds the signal from the AISN block  358  and the digital-to-analog converter  383  and provides a resulting signal to the VOUT terminal  326  of the SLAC  215 . The signal from the VOUT terminal  326  of the SLAC  215  is delivered to the subscriber line  20  through the SLIC  30 . 
   The input impedance, Z IN , produced by the nominal Z block  263 , AISN block  358 , and Z-filters  375 ,  376  is calculated according to equation (2):
 
 Z   IN   =Kimt*R   280 *(NOMINAL- Z+KIN* ( Kaisn +Nominal- zfil ))+ R   217   +R   219   +R   240   +R   245   (2)
 
where Kimt is the constant of proportionality for the metallic line current, R 280 , R 217 , R 219 , R 240 , and R 245  correspond to the resistors shown in  FIG. 2  having respective values of 100000, 35, 35, 15, and 15 ohms, NOMINAL-Z is a fixed amount of injection gain in the SLIC  30  from IMT terminal  275  to the output of SLIC  30 , KIN is a no-load gain from VIN terminal  327  of the SLIC to the output of the SLIC  30 , Kaisn is an impedance adjustment factor of the AISN block  358 , and Nominal-zfil is the frequency variable gain provided by the Z-filter  375 .
 
   For example, if a 600-ohm resistive impedance is required for the voice band, then the output of the AISN block  358  is adjusted so that Z IN  substantially equals 600 ohms. To obtain the desired 600 ohms resistive impedance, an exemplary value of Kaisn may be −0.6, as shown below. Furthermore, for illustrative purposes, assume that Kimt=0.001, NOMINAL-Z=8, and KIN=5. Substituting the exemplary values provided above into equation (2) yields: Z IN =0.001*100000*(8+5*(−0.6+0))+35+35+15+15=600. There may be a capacitive element due to CIMT ( 281 ). This can be neutralized by a suitable choice of the digital Z-filter. Like-wise, other values of resistive impedance may be obtained by adjusting the Kaisn and/or Nominal-zfil values. 
   The input impedance, Z IN , for the data band is primarily governed by the values of resistors  217 ,  219 ,  240 , and  235 . This is because the low pass filter  283  between the IMT terminal  275  of the SLIC  30  and VIN terminal  285  of the SLAC  215 , isolates the nominal Z block  263  (see  FIG. 2 ), AISN block  358 , and Z-filter block  375  for the higher frequencies of the data band. When the data band is substantially isolated from the nominal Z block  263  and the impedance matching module  325 , the natural impedance of the line card  10  substantially equals the impedance of the data band. The data impedance is therefore set primarily by the resistors  217 ,  219 ,  240 , and  245 . Exemplary values of the resistors  217 ,  219 ,  240 , and  245  are 35, 35, 15, and 15 ohms, respectively. 
   The method of  FIGS. 5  can be implemented by the line card  10  of  FIG. 2  to adjust Z IN  to substantially match Z LOOP  for signals in the voice, as well as the data, band. The method of  FIG. 5  begins at block  710 , where the line card  10  receives an input signal having at least one of a voice, data, and DC component. At block  720 , the low-pass filter  283  filters at least a portion of the data component and DC component of the input signal to provide a filtered signal. At block  730 , the nominal Z block  263  of the SLIC  30  adjusts the input impedance of the line card  10  to a first preselected value for the voice band in response to the filtered signal. For example, the nominal Z block  263  may adjust the input impedance of the line to 900 ohms, or to any other desirable level. An exact match is not required, as the AISN block  358  is utilized in accordance with the present invention to arrive at a more precise value. 
   At block  740 , the AISN block  358  adjusts the input impedance of the first apparatus from the first preselected value to a second preselected value. Thus, for example, the second preselected value may be a more precise value of the first preselected value. In the illustrated embodiment, the AISN block  358  is implemented in an analog circuit in the SLAC  215 . The AISN block  358  operates in combination with the nominal Z block  263  to arrive at or near the desired input impedance value. The AISN algorithm of the AISN block  358  includes a finite number of gain steps to arrive at the desired range of the input impedance. For this reason, it is desirable to utilize the nominal Z block  263  to first arrive in proximity to the first preselected value of the input impedance and then use any of the remaining bits available to the AISN algorithm to obtain the second preselected value of the input impedance. 
   At block  750 , the Z-filter block  375  adjusts the input impedance further in response to detecting at least one of attenuation and delay in the filtered signal. That is, the Z-filter block  375  is capable of modifying the gain and phase characteristics of the input signal by a selected amount to adjust Z IN  to substantially equal Z LOOP  for the POTS (i.e., voice) band. 
   The AISN and Z-filter blocks  358 ,  375  may be implemented within a mixed signal integrated circuit. Furthermore, in accordance with the present invention, the two blocks may be implemented as hardware, software, or a combination thereof. 
   The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.