Abstract:
A plain old telephone system (POTS) device, for example but not limited to, a telephone, facsimile machine, analog modem, caller identification (ID) system, speaker phone, cordless phone, etc., employs an on-board POTS filter to enable reliable and efficient decoupling of a POTS channel from one or more xDSL channels on a telephone connection, while permitting and not interfering with the xDSL channels. The POTS device, which connects to a subscriber loop associated with the public service telephone system, includes a subscriber loop connection switch (for example, a hook switch), a POTS transmitter, receiver, or transceiver, and a low pass filter interconnecting the subscriber loop connection switch with the transmitter, receiver, or transceiver. The switch effectively isolates the POTS filter from the telephone line when the POTS device is disconnected, or is on hook, and interfaces the telephone line through the POTS filter to the POTS transceiver when the POTS device is connected, or is off hook. The arrangement enhances DSL device performance, network stability, sidetone, ringer load, and termination impedance and allows greater flexibility in the choice of filter elements, in that passive and active elements can be utilized and in that filters of higher order can be employed.

Description:
For this application and pursuant to 35 U.S.C. Section 119, the inventors herein claim priority to and the benefit of the filing date of copending commonly assigned provisional application, “ADSL Filter Included Within An ADSL-Ready Phone, filed Feb. 16, 2000, and assigned Ser. No. 60/182,805. The foregoing document is incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the art of data communications and telephony. More particularly, the invention relates to a plain old telephone system (POTS) device, or a device that operates in connection with the Public Service Telephone Network (PSTN), for example but not limited to, a telephone, facsimile machine, analog modem, caller ID system, speaker phone, cordless phone, etc., that employs an on-board POTS filter to enable reliable and efficient decoupling of a POTS channel from one or more xDSL channels on a telephone connection, while reducing interference imposed upon the xDSL channels. 
     BACKGROUND OF THE INVENTION 
     Recently, in the art of telephony, 2-wire copper telephone connections have been utilized for communicating more than one simultaneous communications channels, such as digital data and analog voice signals. For example, a high speed digital subscriber line (XDSL, e.g., asymmetric DSL (ADSL), symmetric DSL (SDSL), rate adaptive digital subscriber line (RADSL), very high speed ADSL (VADSL), etc.) channel and a plain old telephone system (POTS) channel can be established over a single physical 2-wire connection. The signals are typically separated in frequency. The POTS channel usually exhibits a frequency spectrum of about 0 KHz to about 4 KHz, whereas the XDSL channels exhibit a frequency spectrum of about 20 KHz to about 500 KHz. 
     A POTS splitter, also known as a POTS filter, a telephone filter, a microfilter, or a CP (customer premises) filter, has traditionally been utilized to decouple the channels, or separate the POTS channel from the other channels, due to non-linearities inherent in most, if not all central office powered telephones. The POTS filter is usually implemented in series with the telephone in order to reduce the DSL power imparted on the telephone electronics, which imposes interference upon the POTS and the DSL signals. 
     A POTS splitter is typically a passive or active one-to-two port device. The POTS splitter is installed between a telephone jack and the telephone itself and is also connected to a DSL device. It includes a POTS filter (a one-to-one port device) situated between the telephone line and the telephone that is designed to minimize high frequency transients produced by on-hook/off-hook transitions, so as to prevent tainting or slowing of the high speed data on the DSL channel. Also, it is usually configured to provide a high impedance to the telephone line in the DSL frequency band in order to prevent DSL power from being imparted on the POTS communications device that is connected to the line. In essence, the filter reduces incident DSL signal power to reduce DSL signal intermodulation (IMD), which undesirably creates audio noise heard in the receiver of a telephone, and interferes with the DSL signal. 
     Because of the foregoing requisite functionality, POTS splitters are usually expensive devices and are oftentimes installed on a physical wall of a customer premises (CP), such as on an outside wall of a building. Furthermore, generally, POTS splitters require installation by a skilled telephone company worker, not the premise owner, resulting in undesirable installation time, labor, and expense. Distributed microfilters per phone are an increasingly used alternative to the outside wall splitter for the reason cited. 
     The specification of POTS splitters and filters has been the subject of several industry standards bodies. For example, see American National Standards Institute, ANSI T1.413-1995, Sections 8 and 10, regarding ADSL/POTS splitters. Moreover, as an example of a possible implementation of a POTS splitter, see J. Cook, P. Sheppard, “ADSL and VADSL Splitter Design and Telephony Performance,”  IEEE Journal on Selected Areas in Communications , December 1995. 
     Commonly assigned U.S. Pat. No. 5,848,150 to T. J. Bingel, who is also one of the inventors herein, describes a passive one-to-one port POTS filter for enabling decoupling of a first channel, such as a POTS channel, from a second channel, such as an xDSL channel, that is communicated simultaneously with the first channel on a telephone connection, while permitting and not appreciably interfering with the second channel. The POTS filter can be connected between the telephone jack and a telephone or can be situated within the telephone. Each POTS filter has a passive automatic control mechanism and a passive first channel filter (e.g., a POTS filter). The automatic control mechanism is configured to isolate the first channel filter when a respective first communications device is inactive (on-hook). Further, the distributed filter is configured to interface communications on the first channel (e.g., POTS channel) on the telephone connection with the respective first communications device when the first communications device is active (off-hook). Moreover, a second communications device (for example, a modem) is connected to the telephone connection and communicates signals over a second channel (xDSL, e.g., ADSL, SDSL, RADSL, VADSL, etc.), simultaneously with the first channel. As a result of the invention, an inexpensive passive distributed filter system associated with the POTS channel prevents on-hook/off-hook transitions from interfering with the second channel communications. 
     Although meritorious and effective to an extent, the passive distributed filter system implements a control mechanism, and it would be desirable to implement such a filtering function with less complexity and requisite circuitry. Furthermore, the filter usually contains inductors in series with the telephone line and capacitors in parallel with the telephone. The presence of these reactive elements introduces concerns about excessive POTS attenuation, reduced PSTN network stability, poor telephone sidetone, increased ringer loading, and degraded termination impedance, especially with multiple telephones on the telephone line (a typical scenario). Thus, a heretofore unaddressed need exists in the industry for an improved filtering systems and methods. 
     SUMMARY OF THE INVENTION 
     The present invention provides a plain old telephone system (POTS) device, for example but not limited to, a telephone, facsimile machine, analog modem, caller identification (ID) system, speaker phone, cordless phone, etc., that employs an on-board POTS filter to enable reliable and efficient decoupling of a POTS channel from one or more XDSL channels on a telephone connection, while permitting and not interfering with the XDSL channels. In general, the POTS filter is situated after the telephone connection switch (for example, a hook switch). The switch effectively isolates the POTS filter from the telephone line when the POTS device is disconnected, or is “on hook,” and interfaces the telephone line through the POTS filter to the POTS transmitter, receiver, or transceiver when the POTS device is connected, or is “off hook.” 
     In architecture, the POTS device, which connects to a subscriber loop associated with the public service telephone network (PSTN), includes a subscriber loop connection switch (for example, a hook switch), POTS electronics (including a transmitter, receiver or both (transceiver)), and a low pass filter (LPF) interconnecting the subscriber loop connection switch with the POTS electronics. The switch effectively isolates the POTS filter from the telephone line when the POTS device is disconnected, or is on hook, and interfaces the telephone line through the POTS filter to the POTS electronics when the POTS device is connected, or is off hook. The arrangement enhances DSL device performance, network stability, sidetone, ringer load, and termination impedance and allows greater flexibility in the choice of filter elements, in that passive and active elements can be utilized and in that filters of higher order can be employed. 
     The invention can also be conceptualized as providing a method for a POTS device that is connected to a subscriber loop. Broadly stated, the method can be summarized by the following steps: coupling a subscriber loop connection switch to a POTS transmitter, receiver, or transceiver; and filtering out high frequency signals from voice band signals communicated between the loop connection switch and the transmitter, receiver, or transceiver. 
     The invention has numerous advantages, a few of which are delineated hereafter, as merely examples. 
     An advantage of the invention is that it provides a way of achieving telephone XDSL compatibility without an external CP telephone filter. That is, the invention teaches a method for telephone manufacturers to design and produce DSL compatible telephones. 
     Another advantage of the invention is that it can be used in any data communication device that connects to a single physical connection having at least two communications channels (e.g., a high speed xDSL link in combination with a POTS link), where the channels are separated by frequency, and that includes a telephone line connection/disconnection switch (such as a hook switch). 
     Another advantage of the invention is that it can be used to effect economical deployment of a XDSL communications channels simultaneously in combination with a POTS communications channel on a telephone connection. 
     Another advantage of the invention is that it is simple in design, efficient in operation, and easily and economically implemented and manufactured on a mass scale. 
     Another advantage of the invention is that passive components, active components, or a combination of both can be utilized in the filter. 
     Another advantage of the invention is that it is very desirable for typical homeowners and consumers in that it does not require sophisticated installation. 
     Another advantage of the invention is that it meets surge and safety requirements of industry standard UL1950, Third Edition, and applicable requirements of FCC, Part 68. 
     Other systems, methods, features, and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included herein within the scope of the present invention and protected by the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be better understood with reference to the following drawings. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating principles of the present invention. Furthermore, in the drawings, like reference numerals designate corresponding part throughout the several views. 
     FIG. 1 is a circuit diagram of a POTS device employing an on-board POTS filter in accordance with the present invention; 
     FIG. 2 is a physical view of a distributed POTS filter (DPF) system utilizing a plurality of POTS devices of FIG. 1 at a customer premises (CP); 
     FIG. 3 is a functional view of the DPF system of FIG. 2; 
     FIG. 4A is a circuit diagram of a possible implementation of a POTS filter of FIG. 1; 
     FIG. 4B is a circuit diagram of another possible implementation of a POTS filter of FIG. 1; and 
     FIG. 4C is a circuit diagram of yet another possible implementation of a POTS filter of FIG. 1, which provides improved sidetone. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A DSL-ready POTS device in accordance with the present invention is illustrated in FIG.  1  and is generally denoted by reference numeral  10 . The DSL-ready POTS device  10  can be, for example but not limited to, a telephone, facsimile machine, analog modem, caller identification (ID) system, speaker phone, cordless phone, etc. The DSL-ready POTS device  10  can be directly connected to a subscriber loop  12  having a POTS channel and one or more other coexisting channels, such as high speed digital subscriber line channels (XDSL, e.g., T1, ADSL, SDSL, RADSL, VADSL, etc.) to effectively decouple the POTS channel from the loop  12 , while minimizing adverse effects to the one or more DSL channels. 
     In architecture, as shown in FIG. 1, the DSL-ready POTS device  10  includes a telephone line connection switch  13 , for instance a hook switch in the case of a conventional telephone, that connects the subscriber loop  12  to a one-to-one port POTS filter  17 , when appropriate, as well as disconnects the subscriber loop  12  and the POTS filter  17 , when appropriate. The POTS filter  17  preferably exhibits the following approximate signal characteristics: −3 dB cutoff frequency of 4 KHz to 25 KHz; a stop band of approximately 25 KHz to 1.104 MHz; stop-band attenuation of at least 20 dB; and a passband of 0 (DC) to −3 dB cutoff frequency. Some POTS devices  10  will have a ringer  15 , such as in the case of a conventional telephone, and as illustrated by phantom lines in FIG. 1, the ringer  15 , if present, would connect directly (unswitched) to the subscriber loop  12 . A transmitter, receiver, or both (i.e., a transceiver), denoted collectively by reference numeral  18  is connected to the POTS filter  17 . The transmitter, receiver, or transceiver  18  implements the modulation and other signal processing functions, as appropriate. 
     It is possible that the filter  17  could be situated somewhere within the electronics block  18 . The key is placing it after a switch that connects and disconnects it relative to the subscriber loop  12 . Some circuit elements can be interposed between the switch  13  and the POTS filter  17 , for example but not limited to, a polarity guard, which is well known in the art and which enables the tip and ring lines to be interchanged without hazard or disfunction. 
     In operation, the telephone line connection switch  13  automatically either isolates or interfaces the POTS filter  17  (in the telephony voice spectrum), based upon the connection status (e.g., off-hook or on-hook, respectively) of the POTS device  10 . There are numerous advantages of this arrangement of the POTS filter  17  relative to the switch  13 . Because the POTS filter  17  is situated beyond the switch  13  within the DSL-ready POTS device  10 , the POTS filter  17  is physically separate from the telephone line when the POTS device  10  is in a disconnected status and does not degrade the DSL device performance. In fact, the arrangement improves network stability, sidetone, ringer load, and termination impedance. The POTS filter  17  is not subjected to ringing voltage (except perhaps during ring trip). Therefore, greater flexibility is afforded in the choice of filter elements, in that passive and active elements can be utilized and in that filters of higher order (higher precision) can be employed. With this implementation, there could be a reduction in complexity and cost, as compared to prior art CP filter implementations, depending upon the selected filter design. Finally, the arrangement extends DSL loop reach. 
     Note that the POTS filter  17  of FIG. 1 is a one-to-one port device, unlike conventional POTS splitters, which are typically one-to-two port devices with one port receiving the combined signal and the other two each outputting a respective channel. However, the functionality of the POTS filter  17  and the POTS splitter are similar in that they both parse out a POTS channel from a communications connection having more than one communications channel. Accordingly, the POTS filter  17  of FIG. 1 can be implemented with a POTS splitter, if desired, by using only two of its ports. Many designs of POTS splitters are known in the art. 
     A distributed POTS filter (DPF) system  28  that utilizes a plurality of DSL-ready POTS devices  10  (FIG. 1) is shown in FIG.  2 . Referring to FIG. 2, a customer premises (CP)  31  is shown with a plurality of the DSL-ready POTS device  10  connected directly to CP wiring  16 . Any number of DSL-ready POTS devices  10  can be supported, up to the ringer equivalence number (REN) limit. The CP wiring  16  can be connected to the subscriber loop  12  by way of a conventional network interface (NI)  37 . The network interface  37  is well known in the art. 
     One or more data terminal equipment (DTE) devices  39  can be interfaced with the CP wiring  16  by way of any suitable communications device  41 , for instance, a data channel modem, or digital modem, in order to permit communications over one or more of the XDSL channels on the loop  12 . These xDSL channels can exist concurrently with the POTS communications channel on the telephone connection  12  without any appreciable interference to each, in accordance with the invention. A local connection  43  interconnects each DTE  39  with a communications device  41 . In the preferred embodiment, the communications device  41  is a conventional digital subscriber line (DSL) modem. A router could also be employed as the communications device  41 . 
     FIG. 3 is an abstract functional view of the DPF system  28  (FIG. 2) showing that the DPF system  28  can be viewed as implementing a plurality of parallel distributed POTS filters  17 , each of which is situated within a respective DSL-ready POTS device  10 . As shown, these distributed POTS filters  17  are in parallel with the DSL channel communications device, the DTE  39 , relative to the CP wiring  16 . 
     The POTS filter  17  of FIG. 1 can be configured in many ways and many implementations are known in the art. A possible implementation (a nonlimiting example) of the POTS filter  17  is shown in FIG.  4 A and generally denoted by reference numeral  17 ′. The POTS filter  17 ′ is essentially a 2nd-order low pass filter (LPF) and is well known in the art. In architecture, referring to FIG. 4A, the POTS filter  17 ″ includes, on the tip side, an inductor L 1  connecting the tip line  21   a  from the switch  13  (FIG. 1) to the tip line  23   a , which is connected to the transmitter and/or receiver block  18 . On the ring side, an inductor L 2  connects ring line  21   b  from the switch  13  to the ring line  23   b . A capacitor C 1  is connected between lines  23   a ,  23   b . In operation, the circuit exhibits high impedance to high frequency signals (DSL signals) and low impedance to low frequency signals (POTS signals). In the preferred embodiment, inductor L 1 , inductor L 2 , and capacitor C 1  have the following circuit values: 8mH, 8mH, and 56 nF, respectively. 
     Another possible implementation of the POTS filter  17  is shown in FIG.  4 B and generally denoted by reference numeral  17 ″. The POTS filter  17 ″ is essentially a 4th-order LPF and is well known in the art. In architecture, referring to FIG. 4B, the POTS filter  17 ″ includes, on the tip side, an inductor L 1  in series with an inductor L 3  connecting the tip line  21   a  from the switch  13  (FIG. 1) to the tip line  23   a , which is connected to the transmitter and/or receiver block  18  (FIG.  1 ). On the ring side, an inductor L 2  in series with an inductor L 4  connects ring line  21   b  from the switch  13  to the ring line  23   b . A capacitor C 1  is connected between nodes  51   a ,  51   b . A capacitor C 2  is connected between the nodes  23   a ,  23   b . In operation, the circuit exhibits high impedance to high frequency signals (DSL) and low impedance to low frequency signals (POTS signal). In the preferred embodiment, inductors L 1 -L 4  and capacitors C 1 -C 2  have the following circuit values: 8mH, 8mH, 8mH, 8mH, 56 nF, and 56 nF, respectively. 
     Still another possible implementation of the POTS filter  17  is shown in FIG.  4 C and generally denoted by reference numeral  17 ″. The POTS filter  17 ″ of FIG. 4C is basically another LPF and is preferred in that it has demonstrated excellent sidetone performance. Many conventional POTS filters demonstrate very poor sidetone performance. In general, the improved POTS filter  17 ″ of FIG. 4C utilizes a tuning technique (tuning circuits C 3 , R 3  and C 4 , R 4 ) to improve sidetone, but is still a passive POTS filter device. The tuned frequency and quality factor Q are optimized to improve sidetone in the region most required, in this case, are optimized in the range between about 1 KHz and about 3 KHz. Note that commonly assigned U.S. Pat. No. 5,848,150 to T. J. Bingel, which is incorporated by reference, describes the POTS filter  17 ″ in further detail. 
     More specifically, as shown in FIG. 4C, the POTS filter  17 ″ is implemented by modifying a balanced 5th-order 0.01 dB-ripple Chebyshev filter (note that the 5 th  reactance component is based upon a 27 to 33 nF capacitance associated with the second channel communications device and which appears between connections  21   a ,  21   b ) with tuning circuit components, comprised of a capacitor (C 3 , C 4 ) in series with a de-Q resistor (R 3 , R 4 , respectively) on each balanced side. 
     In architecture, referring to FIG. 4C, the POTS filter  17 ″ includes, on the tip side, an inductor L 1  connected to the tip line  21   a  from the automatic control mechanism  14  (FIG.  1 ). A series combination of a capacitor C 3  and a resistor R 3  is connected between the node  51   a  and a node  23   a . Also, between the nodes  51   a  and  23   a  is an inductor L 3 . In the preferred embodiment, inductor L 1 , inductor L 3 , resistor R 3 , and capacitor C 3  have the following circuit values: 8mH, 8mH, 100 Ω, and 1 μF, respectively. 
     On the ring side of the circuit, an inductor L 2  is connected to the ring line  21   b . An inductor L 4  is situated between the nodes  5   b  and  23   b . Also, between these nodes  51   b  and  23   b  is situated a series combination of a capacitor C 4  and a resistor R 4 . In the preferred embodiment, inductor L 2 , inductor L 4 , resistor R 4 , and capacitor C 4  have the following circuit values: 8mH, 8mH, 100 Ω, and 1 μF, respectively. 
     The inductors L 1  and L 2 , as well as the inductors L 3  and L 4 , may be implemented as mutually coupled inductors. In other words, the inductors L 1  and L 2  may be implemented with a first transformer, and/or the inductors L 3  and L 4  may be implemented with a second transformer. Use of these transformers may reduce the cost of the POTS filter  17 ″. 
     Several capacitors are connected between the tip and ring sides. In particular, a capacitor C 2  is connected between tip and ring lines  23   a ,  23   b . In the preferred embodiment, capacitor C 2  is 27 nF. Further, a capacitor C 1  is connected between the tip and ring lines  51   a ,  51   b . In the preferred embodiment, the capacitor C 1  is 56 nF. 
     When the improved POTS filter  17 ″ is operational, the inductors L 1 , L 2  provide high impedance to high frequencies, particularly those frequencies in the xDSL band. The capacitors C 3 , C 4  resonate with the inductors L 3 , L 4  in order to parallel resonate at the geometric mean of about 1 KHz and about 3 KHz. This feature improves telephone sidetone performance in the 1 to 3 KHz band by improving impedance (restoring the resistance and capacitive reactance) presented to the telephone  33  as compared to a pure L-C (inductor/capacitor) filter structure in prior art embodiments. 
     It should be emphasized that, although no active filters  17  have been specifically shown and specifically described, the filter  17  could be an active filter if it can receive electrical power locally, at least in part. As an example, consider a facsimile machine, which is typically plugged into an electrical outlet at the CP  31 . The filter  17  could receive electrical power, directly or indirectly, from the outlet, and utilize the power to implement and enhance filtering functionality, as is known in the art. 
     It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. For example, although not an optimal configuration, the filter  17  (FIG. 1) may be implemented as an unbalanced circuit (for example, in the filter  17 ′, either L 1  or L 2  would be eliminated). All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.