Abstract:
A method and apparatus for selectively filtering signals on a digital subscriber line (DSL) so as to prevent interference resulting from a telephone instrument switching between its on-hook and off-hook states. One embodiment of the invention utilizes an off-hook detector controlling a variable attenuator element. Another embodiment of the invention uses a switch in parallel with a resistive element to implement the variable attenuator element. Yet another embodiment has an inductive element with a saturable core for providing attenuation to compensate for inconsistencies that result from the changing hookswitch status of a telephone instrument.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to data and voice communications over digital subscriber lines and, more particularly, to a method and apparatus for filtering signals in a splitterless asymmetric digital subscriber line (ADSL) system. 
     BACKGROUND OF THE INVENTION 
     Asymmetric Digital Subscriber Line (ADSL) refers to a new modem technology that allows existing twisted pair telephone lines to be converted into a high-performance access path for multimedia and high-speed data communications. An ADSL circuit connects an ADSL modem on each end of the twisted pair telephone line, creating three information channels—a high speed downstream (central office to end user) channel, a medium speed upstream (end user to central office) channel, and a POTS (“Plain Old Telephone Service”) channel. The POTS channel is separated from the ADSL modem by filters, thus guaranteeing uninterrupted POTS, even if the ADSL circuit fails. 
     While description is provided in terms of the POTS channel, telephone voice communications signals, telephone instruments, and the like for the benefit of familiarity, it should be understood that telephone equipment and signals need not be limited to voice communications, but may also include other technologies, for example equipment and signals compatible with regular telephone lines, such as facsimiles machines, voiceband modems (for example, V.90 modems), answering machines, and the like. 
     Two variants of ADSL systems are available today —full-rate ADSL in accordance with the T1E1.413 or ITU G.992.1 standards and “splitterless” ADSL defined by the ITU G.992.2 standard. Full-rate ADSL uses POTS splitters to separate the POTS channel from the ADSL data signals. A POTS splitter is installed at each end of the line and includes a lowpass filter for separating out POTS telephone voice communication signals and a highpass filter for separating out data communication signals. 
     The POTS splitter divides the subscriber line into two separate twisted pairs—one for data communication (ADSL) and one for telephone voice communication signals (POTS). As a result, the existing two-wire internal house telephone wiring is not usable for ADSL. New wiring must be installed from the splitter to the modem, resulting in increased installation cost. 
     Splitterless ADSL can be installed without the need for additional home wiring. In this case, the ADSL modem includes a high-pass filter that rejects the POTS telephone voice communication signal, while every telephone instrument in the house is connected to the telephone line through a low-pass filter that rejects the ADSL data signals. 
     When only one telephone instrument is used, the well-known design of LC filters is adequate to implement the required low pass filter. However, in real life, several telephone instruments are usually connected to each telephone line, each of them in an on-hook or off-hook state. In “splitterless” ADSL, the result of such a configuration is that several lowpass filters are connected in parallel on the same telephone line. Certain electrical properties of a telephone instrument, for example its input impedance, depends on the operational state or hookswitch condition (e.g., whether the telephone instrument hookswitch is on-hook or off-hook). Hookswitch condition can refer to the on-hook or off-hook states of the hookswitch or the transitions of the hookswitch between these states. As a result, certain filter characteristics, for example the frequency response, of a low pass filter connected to a telephone instrument will change when the telephone changes its state. 
     In reality, the behavior is much more complicated. A low pass filter connected to an on-hook telephone has zero impedance at 4 kHz, which will produce distortion in another telephone instrument, should it happen to be off-hook. Several on-hook telephones connected in parallel will create several resonance frequencies (Universal ADSL Technical Group Contribution, Document # [TG/98-121]; “Preliminary Report of the POTS Filter and Power Reduction Ad-hoc;” Bob Beeman; Redmond, Wash.; Apr. 14, 1998; pp. 1-9). 
     FIG. 1 is a block diagram illustrating a splitterless ADSL system of the prior art. Customer premises equipment (CPE)  101  is coupled to central office (CO)  102  by digital subscriber line (DSL)  103 . CPE  101  includes a highpass filter  104 , ADSL modem  107 , computer  108 , lowpass filters  105  and  106 , and telephone instruments  109  and  110 . Computer  108  is coupled to ADSL modem  107 , which is coupled to highpass filter  104 , which is coupled to DSL  103 . Telephone instrument  109  is coupled to lowpass filter  105 , which is coupled to DSL  103 . Telephone instrument  110  is coupled to lowpass filter  106 , which is coupled to DSL  103 . 
     CO  102  includes a POTS (“plain old telephone service”) splitter  111 , ADSL modem  112 , data switch  113 , voice switch  114 , data network  115 , and voice network  116 . DSL  103  is coupled to POTS splitter  111 , which is coupled to voice switch  114  and ADSL modem  112 . Voice switch  114  is coupled to voice network  116 . ADSL modem  112  is coupled to data switch  113 , which is coupled to data network  115 . 
     Voice communications passing through voice switch  114  are passed through POTS splitter  111  and applied to DSL  103  as baseband signals. Data communications passing through data switch  113  are modulated at a frequency range higher than that of the baseband POTS signals and passed through POTS splitter  111  and applied to DSL  103 . Since the data communications are transmitted at a different frequency range than the voice communications, frequency-division-multiplexing (FDM) allows simultaneous transmission of both voice communications (POTS) and data communications over a single DSL  103 . 
     Since data communications are suitably processed by ADSL modem  107  and computer  108 , while voice communications are intended for telephone instruments  109  and  110 , highpass filter  104  and lowpass filters  105  and  106  provide selective filtering of the voice and data communications. Highpass filter  104  passes the higher frequency data communications to ADSL modem  107  and computer  108 , while blocking the lower frequency baseband voice communications. Lowpass filters  105  and  106  pass the lower frequency baseband voice communications to telephone instruments  109  and  110 , respectively, while blocking the higher frequency data communications. 
     Unfortunately, lowpass filters  105  and  106  exhibit a deficiency that can adversely affect the performance of the ADSL system. The frequency response of lowpass filters  105  and  106  changes based on the status of telephone instruments  109  and  110 , respectively. For example, while lowpass filter  105  might properly differentiate between voice communications and data communications when telephone instrument  109  is off-hook (e.g, when telephone instrument  109  is in use), the electrical characteristics of lowpass filter  105  are altered when telephone instrument  109  is returned to its on-hook state (e.g., when the user hangs up). This change in the electrical characteristics of lowpass filter  105  can cause interference with the data communications between ADSL modem  107  of CPE  101  and ADSL modem  112  of CO  102 . Thus, a circuit is needed that will allow telephone instruments, such as telephone instruments  109  and  110 , to change between their off-hook and on-hook states without adversely affecting ongoing data communications over DSL  103 . 
     FIG. 2 is a block diagram illustrating a lowpass filter and telephone instrument of the prior art. Lowpass filter  201  is coupled to telephone instrument  202 . Telephone instrument  202  includes a load  203 , which exhibits a load impedance. Load  203  is coupled in series with hookswitch  204 . When telephone instrument  202  is off-hook, hookswitch  204  is closed, coupling load  203  to lowpass filter  201 . However, when telephone instrument  202  is on-hook, hookswitch  204  is open, disconnecting load  203  from lowpass filter  201 . Thus, the electrical characteristics (e.g., the frequency response) of lowpass filter  201  are affected by load  203 . Therefore, a circuit is needed to compensate for the undesirable interaction between load  203  and lowpass filter  201 . 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method and apparatus for filtering ADSL signals. The filtering technique provided by the invention avoids interference when telephone instruments coupled to a DSL switch between the off-hook and on-hook states. One embodiment of the invention provides a variable inductance in series with a simple lowpass filter used to block high-frequency data communications signals. The variable inductance is controlled by an off-hook detector that senses the status of the telephone instrument hookswitch. 
     Another embodiment of the invention includes a parallel combination of a resistance element and a switch in series with the DSL to provide attenuation and control the overall electrical characteristics of the filter assembly and telephone instrument. Another embodiment of the invention includes an inductive element with a saturable core in series with the DSL. The saturable character of the inductor core results in the inductive element having different electrical properties depending on whether the telephone instrument is on-hook or off-hook. 
     Thus, the invention reduces interference to data communications that can otherwise occur in a splitterless ADSL system if a telephone instrument changes hookswitch state. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating a splitterless ADSL system of the prior art. 
     FIG. 2 is a block diagram illustrating a lowpass filter and telephone instrument of the prior art. 
     FIG. 3 is a block diagram illustrating one embodiment of the present invention. 
     FIG. 4 is a block diagram illustrating one embodiment of the present invention. 
     FIG. 5 is a block diagram illustrating one embodiment of the present invention. 
     FIG. 6 is a flow diagram illustrating one embodiment of the present invention. 
     FIG. 7 is a schematic diagram illustrating one embodiment of the invention. 
     FIG. 8 is a schematic diagram illustrating one embodiment of the invention. 
     FIG. 9 is a schematic diagram illustrating one embodiment of the invention. 
     FIG. 10 is a schematic diagram illustrating one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a method and apparatus for filtering ADSL signals. The filtering technique of the invention avoids interference caused by a telephone instrument changing between on-hook and off-hook states. 
     In accordance with the present invention, disadvantages and problems associated with lowpass filters in splitterless ADSL have been substantially reduced. One embodiment of a telephone filter for splitterless ADSL in accordance with the present invention includes a lowpass filter, an off-hook detector, an additional attenuator, and one or more switches. The telephone filter in accordance with the present invention changes its input impedance and frequency characteristics as a function of DC current in the telephone line. The lowpass filter is a passive LC circuit that provides the necessary attenuation for ADSL data communications signals, for example, about 50-60 dB for the typically lowest ADSL frequency of 30 kHz. 
     The off-hook detector according to one embodiment of the invention is an active circuit that senses DC current flow in the telephone line. During the on-hook state, the DC current in a telephone line is very low, for example, about 10 microamperes. In this state, the lowpass filter is connected to the telephone line through an additional attenuator that has a high and determined input impedance over a wide frequency range from 30 Hz to 10 MHz. 
     While off-hook, a telephone set typically consumes a higher DC current, e.g., 20-70 milliamperes. The off-hook detector of one embodiment of the invention senses this current, activates appropriate switches and connects the lowpass filter to the telephone line. The additional attenuator is disconnected in this situation. During the off-hook state, the telephone filter operates like a lowpass filter that passes a telephone voice communications signal and stops a higher-frequency data communications signal. 
     While on-hook, a telephone filter configured according to the present invention typically has a high input impedance for telephone voice communications and data communications signals. As a result, a telephone filter loaded by the higher input impedance of an on-hook telephone will not cause distortion to another off-hook telephone. 
     In accordance with one embodiment of the present invention, the additional attenuator is a resistor that connects in series with the lowpass filter. This: attenuator is shorted by an electronic or mechanical switch. In accordance with another embodiment of present invention, the additional attenuator is a saturated inductance, that is connected in series with the lowpass filter. This attenuator changes its impedance through the action of DC current flowing through the circuit. 
     FIG. 3 is a block diagram illustrating one embodiment of the present invention. Lowpass filter assembly  301  is coupled to telephone instrument  302 . Lowpass filter assembly includes variable attenuator  305 , off-hook detector  306 , and lowpass filter  307 . Telephone instrument  302  includes load  303  and hookswitch  304 . Load  303  is coupled in series with hookswitch  304 , which is coupled to off-hook detector  306  of lowpass filter assembly  301 . Off-hook detector  306  is coupled to lowpass filter  307 , which is coupled to variable attenuator  305 , which is coupled to DSL  308 . 
     Off-hook detector  306  senses whether hookswitch  304  is in an off-hook or on-hook state. Off-hook detector  306  may detect changes of DSL current to determine the state of hookswitch  304 . Off-hook detector  306  senses the current through DSL  308 . For example, a low-value resistor may be placed in series with DSL  308  and the voltage across the resistor measured by off-hook detector  306 . As another alternative, an optocoupler circuit may be used to sense the current through DSL  308 . The level of attenuation provided by variable attenuator  305  may be controlled based on this measurement of current through DSL  308 . As yet another alternative, off-hook detector  306  may employ mechanical means or sensors to determine the state of the hookswitch  304 . 
     Off-hook detector  306  need not be located between lowpass filter  307  and hookswitch  304 . Rather, off-hook detector  306  may be configured in any manner that provides adequate sensing of the status of hookswitch  304 . For example, off-hook detector  306  may be located between lowpass filter  307  and variable attenuator  305  or somewhere along DSL  308  beyond variable attenuator  305 . 
     FIG. 4 is a block diagram illustrating one embodiment of the present invention. Lowpass filter assembly  401  is coupled to telephone instrument  402 . Lowpass filter assembly  401  includes off-hook detector  403 , lowpass filter  404 , resistance element  405 , and switch  406 . Off-hook detector  403  is coupled to telephone instrument  402  and lowpass filter  404 . Lowpass filter  404  is coupled to the parallel combination of resistance element  405  and switch  406 . Resistance element  405  and switch  406  are coupled to DSL  407 . 
     Off-hook detector  403  controls the operation of switch  406 . When off-hook detector  403  detects an off-hook state, it causes switch  406  to close. When switch  406  is closed, it shorts resistance element  405 , reducing the effective attenuation of DSL  407 . When off-hook detector  403  detects an on-hook state, it causes switch  406  to open. When switch  406  is open, it has the effect of electrically inserting resistance element  405  in series with DSL  407 . Resistance element  405  serves to attenuate signals present on DSL  407 . The increased attenuation provided by resistance element  405  compensates for the effect on lowpass filter  404  from being disconnected from the telephone instrument load by the action of the telephone instrument hookswitch. 
     Off-hook detector  403  need not be located between lowpass filter  404  and telephone 
     instrument  402 . Rather, off-hook detector  403  may be configured in any manner that allows it to detect the status of the telephone instrument hookswitch. For example, off-hook detector  403  may be located between lowpass filter  404  and the parallel combination of resistance element  405  and switch  406  or on DSL  407  somewhere beyond the parallel combination of resistance element  405  and switch  406 . 
     FIG. 5 is a block diagram illustrating one embodiment of the present invention. Lowpass filter assembly  501  is coupled to telephone instrument  502 . Lowpass filter assembly  501  includes lowpass filter  503  and inductive element  504  with saturable core  505 . Inductive element  504  with saturable core  505  is configured in series with DSL  506  and lowpass filter  503 . 
     When telephone instrument  502  is in an on-hook state, negligible current flows through inductive element  504 . Thus, saturable core  505  is not saturated, and inductor  504  introduces significant impedance in the circuit, thereby attenuating signals present on DSL  506  and compensating for the influence of the state of telephone instrument  502  on the electrical characteristics of lowpass filter  503 . 
     When telephone instrument  502  is in an off-hook state, significant DC current flows through inductor  504 , saturating saturable core  505 . When saturable core  505  is saturated, the amount of attenuation it provides is reduced. This reduction in attenuation is offset by the influence of the hookswitch of telephone instrument  502  on the electrical characteristics of lowpass filter  503 . By compensating for the changing status of the telephone instrument impedance, inductive element  504  with saturable core  505  avoids interference to data communications occurring on DSL  506 . 
     FIG. 6 is a flow diagram illustrating a process according to one embodiment of the present invention. The process begins at step  601 . At step  602 , a decision is made as to whether or not the telephone instrument is in an off-hook state. If not, the process remains at step  602 . However, if the telephone instrument is in an off-hook state, the process proceeds to step  603 . In step  603 , the attenuation associated with a lowpass filter is reduced. In step  604 , a determination is made as to whether or not the telephone instrument is still in the off-hook state. If it is, the process remains at step  604 . If not, the process continues to step  605 . In step  605 , the attenuation associated with the lowpass filter is increased and the process returns to step  602 . 
     FIG. 7 is a schematic diagram illustrating one embodiment of the invention. Lowpass filter assembly  701  is coupled to telephone instrument  702 . Lowpass filter assembly includes off-hook detector  703 , low pass filter  704 , two resistance elements  705  and two switches  706 . Off-hook detector  703  is coupled to the telephone device  702  and lowpass filter  704 . 
     Lowpass filter  704  is coupled to the two parallel combinations of resistance element  705  and switch  706 . Two resistive elements  705  and two switches  706  are coupled to DSL  707 . Protection device  711  is connected to the input of lowpass filter assembly  701  for lighting protection. The switches  706  are implemented by optoMOS relay  708 . This relay  708  has low on resistance, for example about 15 ohms and very high off impedance, for example about several megohms. Input light emitting diode (LED)  709  of optorelay  708  is very sensitive. Its turn-on input current is as small as 0.5 milliamperes, for example. Off-hook detector  703  is a simple Schottky diode bridge  710  that is connected in series, for example, into one wire of the twisted pair DSL  707 . 
     When the telephone  702  is on-hook, the current that flows through photo diodes  709  is very small, for example about 10 microamperes, and optorelays  708  are off. In this state, the impedance of the lowpass filter assembly  701  is high, for example, about 3 kiloohms, because of the resistive elements  705 . 
     During the off-hook state of telephone instrument  702 , independently of the polarity of the TIP and RING terminals, a current of about 20-50 milliamperes flows through both LED&#39;s  709 . This current switches optorelays  708  to the on state. Optorelays  708  short resistance elements  705  and change the impedance of the lowpass filter assembly  701  to a nominal 600 ohms in the voice frequency range. In the high frequency range of ADSL, the lowpass filter assembly  701  has high impedance during both off-hook and on-hook states of the telephone instrument  702 . In the on-hook state of the telephone instrument  702 , resistive elements  705  contribute to the high impedance of the lowpass filter assembly  701  because optorelays  708  have very low capacitance, for example about 10 picofarads, between their inputs and outputs. In both the on-hook and off-hook states of the telephone instrument  702 , lowpass filter  704  contributes to the high impedance of the lowpass filter assembly  701  because lowpass filter  704  includes input inductance  710  connected in series with DSL  707 . 
     One advantage of this embodiment of this invention is the high breakdown voltage of optorelays  708 , for example about 400 VDC between output pins and more than 3 KV between input and output. This makes it possible to use this lowpass filter assembly  701  for a home telephone network or other telephone or communication network in accordance with existing standards. 
     FIG. 8 is a schematic diagram illustrating one embodiment of the invention. Inline filter  801  comprises attenuator  802 , lowpass filter  803 , off-hook detector  806 , and switches  808 . Attenuator  802  is coupled to telephone line  809 , which preferably comprises two conductors, and to lowpass filter  803 . Switches  808  are also coupled to telephone line  809  and to lowpass filter  803 . Lowpass filter  803  comprises inductors  804  and capacitor  805 , with one of inductors  804  configured in series with each conductor of telephone line  809  as it extends through inline filter  801 . Capacitor  805  is configured in parallel with the conductors. Alternatively, other types of lowpass filters may be used to provide lowpass filter  803 . Off-hook detector  806  is coupled to lowpass filter  803  and to line  810 , which comprises two conductors and is coupled to telephone equipment, such as a telephone instrument or other station equipment. Line  810  preferably comprises two conductors. Off-hook detector  806  provides control outputs  807  to switches  808 . 
     When the telephone equipment coupled to line  810  is on-hook, off-hook detector  806  detects the on-hook state and provides the appropriate signals on control outputs  807  to switches  808 , causing switches  808  to open. With switches  808  open, attenuator  802  is effectively inserted into the circuit between telephone line  809  and lowpass filter  803 . Attenuator  802  comprises attenuation elements coupled to each conductor of telephone line  809 . For example, these attenuation elements may be resistors or other impedance elements, preferably impedance elements exhibiting a relatively high resistance component as compared with their reactance components. 
     When the telephone equipment coupled to line  810  is off-hook, off-hook detector  806  detects the off-hook state and provides the appropriate control signals on control outputs  807  to switches  808 , causing switches  808  to close. With switches  808  closed, switches  808  short out the attenuation elements of attenuator  802 . Thus, by closing switches  808 , attenuator  802  is effectively removed from the circuit between telephone line  809  and lowpass filter  803 . 
     Thus, the circuit of FIG. 8 automatically controls attenuator  802  and switches  808  to provide compensation for the performance of lowpass filter  803  when the impedance of telephone equipment coupled to line  810  changes in transition between on-hook and off-hook states. 
     FIG. 9 is a schematic diagram illustrating one embodiment of the invention. Inline filter  901  comprises attenuator  902 , lowpass filter  903 , and off-hook detector  906 . Attenuator  902  is coupled to telephone line  909 , which preferably comprises two conductors, and to lowpass filter  903 . Attenuator  902  comprises a saturable inductance element or other impedance element exhibiting substantial inductive reactance in series with each conductor of telephone line  909 . 
     Lowpass filter  903  comprises inductors  904  and capacitor  905 , with one inductor configured in series with each conductor of telephone line  909  as it extends through inline filter  901 . Alternatively, other types of lowpass filters may be used to provide lowpass filter  903 . Capacitor  905  is configured in parallel with the conductors. 
     Off-hook detector  906  is coupled to lowpass filter  903  and to telephone equipment, such as a telephone instrument or other station equipment coupled to line  910 . Line  910  preferably comprises two conductors. Off-hook detector  906  provides control outputs  907  to attenuator  902 . 
     When the telephone equipment coupled to line  910  is in an on-hook state, off-hook detector  906  provides signals to attenuator  902  to prevent current from flowing through secondary windings of the saturable inductive elements, such as saturable core inductors or saturable core transformers. Without this current flowing, the saturable inductive elements of attenuator  902  effectively insert a substantial impedance in series with lowpass filter  903 . This inductance works in conjunction with the elements of lowpass filter  903  to change the characteristics of lowpass filter  903  to compensate for the higher on-hook impedance of the telephone equipment coupled to line  910 . 
     When the telephone equipment coupled to line  910  is in an off-hook state, off-hook detector  906  provides signals to attenuator  902  to cause current to flow through the secondary windings of the saturable inductive elements. When current flows through the secondary windings of the saturable inductive elements of attenuator  902 , the saturable inductive elements become saturated with magnetic flux, thereby effectively reducing the inductance of the primary windings of the saturable inductive elements. With the impedance of the primary windings reduced, lowpass filter  903  provides the appropriate frequency response in conjunction with the impedance of the telephone equipment coupled to line  910 . 
     Thus, the circuit of FIG. 9 provides automatic adjustment of the characteristics of lowpass filter  903  to compensate for changes in impedance of telephone equipment coupled to line  910  when the telephone equipment switches between the on-hook and off-hook states. 
     FIG. 10 is a schematic diagram illustrating one embodiment of the invention. Inline filter  1001  comprises attenuator  1002 , lowpass filter  1003 , diode bridge  1011 , and optocouplers  1012 . Attenuator  1002  is coupled to telephone line  1009  and to lowpass filter  1003 . 
     Diode bridge  1011  is coupled to lowpass filter  1003  and to telephone equipment, such as a telephone instrument or other station equipment, connected to line  1010 . Line  1010  preferably comprises two conductors. Diode bridge  1011  comprises four diodes, preferably Schottky diodes or other diodes having low forward voltage ratings. The diodes are configured such that two diodes, one of each polarity orientation, are coupled to lowpass filter  1003  and two diodes, one of each polarity orientation, are coupled to one conductor of line  1010 . The remaining terminals of these four diodes are coupled anode-to-anode and cathode-to-cathode to form two nodes at which control outputs are provided to the LED or input portions of optocouplers  1012 . Additional components, for example current limiting resistors may be provided in series with the LED or input portions of optocouplers  1012 . 
     When the telephone equipment coupled to line  1010  is in the off-hook state, current flows through line  1010  and, therefore, through diode bridge  1011 . Thus, diode bridge  1011  causes current to flow through the LED or input portions of optocouplers  1012 . The LED or input portions of optocouplers  1012  provide an optical signal to the photodiode portions of optocouplers  1012 . The photodiode portions of optocouplers  1012  are coupled to MOSFET switches of optocouplers  1012 . The MOSFET switches close when optical signals are received at the photodiodes portions of optocouplers  1012 . Examples of optocouplers  1012  with which the invention may be practiced include optoMOS relays and other optoelectronic devices. 
     Attenuator  1002  comprises attenuation elements, for example resistors or other impedance elements. When the MOSFET switches of optocouplers  1012  close, they short out the attenuation elements of attenuator  1002 , thereby effectively removing the attenuation of attenuator  1002  from the circuit. 
     When telephone equipment coupled to line  1010  is placed in an on-hook state, current stops flowing through line  1010  and through diode bridge  1011 . Thus, current stops flowing through the LED or input portions of optocouplers  1012 . Consequently, the LED or input portions of optocouplers  1012  stop emitting optical energy. When no optical energy is received by the photodiode portions of optocouplers  1012 , the MOSFET switches of optocouplers  1012  stop conducting and, therefore, stop shorting out attenuator  1002 . As a result, attenuator  1002  is effectively inserted in the circuit. 
     Lowpass filter  1003  preferably comprises inductors  1004  and capacitor  1005 , with one inductor  1004  configured in series with each conductor of telephone line  1009  as it extends through inline filter  1001  and with capacitor  1005  configured in parallel across the conductors. When attenuator  1002  is effectively inserted in the circuit, the characteristics of lowpass filter  1003  are adjusted to compensate for the difference in impedance of the telephone equipment between the on-hook state and the off-hook state. 
     While this invention is useful in an ADSL context, it may also be practiced with other types of communications lines, especially communication lines where additional signals are frequency division multiplexed at higher frequencies over low-frequency signals. For example, to avoid the need to install additional wiring in a location such as a home or office, additional signals, such as data signals, may be multiplexed at higher frequencies over existing telephone wiring. The invention may be applied to prevent interference to telephone equipment from the new signals. In areas where telephone cable plant facilities are scarce, an added main line (AML) system may be used to multiplex a virtual second telephone line on the same physical telephone line. The present invention may be applied to prevent interference between the AML system and the existing telephone equipment. In general, the invention may be applied to any type of network or wiring where filtering is to occur in the presence of dynamic electrical characteristics.