WO2001030060A1

WO2001030060A1 - Method and apparatus for separating data and voice signals

Info

Publication number: WO2001030060A1
Application number: PCT/US2000/011063
Authority: WO
Inventors: Walter S. Schopfer
Original assignee: Legerity, Inc.
Priority date: 1999-10-20
Filing date: 2000-04-25
Publication date: 2001-04-26

Abstract

A method and apparatus (200) is provided for matching impedance for the apparatus (200) coupled to a connection (21). The method includes receiving an input signal having a voice and data band from the connection (21), obtaining the data band of the input signal using a transformer (238), and adjusting an impedance of the apparatus (200) to a selected level for the data band of the input signal. An apparatus (200) includes an interface (232) for receiving an input signal comprising a first and second band. The apparatus (200) includes a splitter (222) including a transformer (238) for obtaining the first band from the input signal and a balance network (240) capable of adjusting an impedance of the apparatus (200) to a selected level for the first band of the input signal. The apparatus (200) further includes a first transceiver (20) capable of receiving the second band of the input signal, and a second transceiver (17) capable of receiving the first band of the input signal.

Description

METHOD AND APPARATUS FOR SEPARATING DATA AND VOICE SIGNALS

TECHNICAL FIELD

This invention relates generally to telecommunications, and, more particularly, to a method and apparatus for separating data and voice signals.

BACKGROUND 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.

Subscriber lines generally have a natural characteristic 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 to the far end, it is desirable to match the characteristic impedance of the subscriber line when it is terminated. This impedance is typically symbolized as Z_LOOP, which is a function of frequency and generally 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 subscriber line 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 signal transmitted down the wire is reflected back to the driver.

In a 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 delay 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 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 modern 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 technology. xDSL is making the existing network of telephone 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 telephone 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, all four 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 (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.

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) rate commonly available through POTS.

Although DSL and POTS systems can co-exist on one line, the DSL traffic on the subscriber line is generally separated from the voice signals using a splitter. The DSL data is provided to a data transceiver capable of processing such data, and the voice data is forwarded to a voice transceiver capable of processing voice signals.

An exemplary data transceiver may be a DSL modem, and an exemplary voice transceiver may be a line card capable of supporting POTS f nctionality.

An exemplary splitter that is sometimes utilized to separate data and voice frequencies is a digital subscriber line access module (DSLAM). DSLAMs and other splitters, however, are generally expensive and relatively large in size. The high cost and large size of these splitters are largely due, in part, to the large, expensive, and multi-pole filters utilized in such splitters.

The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. DISCLOSURE OF INVENTION

In one aspect of the present invention, a method is provided for matching impedance for the apparatus coupled to a connection. The method includes receiving an input signal having a voice and data band from the connection, obtaining the data band of the input signal using a transformer, and adjusting an impedance of the apparatus to a selected level for the data band of the input signal. In another aspect of the present invention, an apparatus is provided for separating voice and data band from an input signal. The apparatus includes an interface for receiving the input signal, a transformer, and a balance network. The transformer is capable of obtaining the data band from the input signal. The balance network is capable of adjusting impedance of the apparatus to a selected level for the data band of the input signal.

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:

Figure 1 illustrates a block diagram of a communications system in accordance with the present invention; Figure 2 depicts a stylized block diagram of an embodiment of an apparatus in accordance with the present invention that can be implemented in the communications system of Figure 1; Figure 3 illustrates an exemplary nominal loop impedance model of a German subscriber line; Figure 4 illustrates an embodiment of a method in accordance with the present invention that can be implemented by the apparatus of Figure 2; and

Figure 5 depicts an exemplary return loss chart for the apparatus of Figure 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.

MODE(S) FOR CARRYING OUT THE INVENTION 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' 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 now to the drawings, and in particular to Figure 1 , a block diagram of a communications system

10 is provided. The communications system 10 includes a voice/data transceiver 15. The voice/data transceiver 15 may be a single unit capable of transmitting and receiving both voice and data signals. Alternatively, the voice/data transceiver 15 may be two separate devices, one for handling the voice signals, such as a conventional telephone, and the other for handling the data signals, such as a DSL modem.

The voice/data transceiver 15 is coupled to a voice transceiver 17 and a data transceiver 20 by a connection 21 via a splitter 22. In the illustrative embodiment, the voice/data transceiver 15 is located at a customer premise 30, and the voice and data transceivers 17, 20 are part of a central office 35. The splitter 22 may be located between the customer premise 30 and central office 35, or it may be located at the central office 35. The connection 21 is an ordinary twisted pair connection, as is common in present-day telephone networks. However, other connection types (e.g., wireless, cellular, etc.) are contemplated, depending on the specific implementation.

The voice and data transceivers 17, 20, along with the splitter 22, have been shown as separate devices, although it is envisioned that these transceivers 17, 20 and the splitter 22 may be integrated in a common device, such as a line card, for example. Also, it is contemplated that the voice and data transceivers 17, 20 may not be part of the central office 35, but rather may be installed in a second customer premise (not shown). For purposes of illustration, the transceivers 15, 20 are described as they might be implemented under the ADSL protocol (ANSI

T1.413). It is contemplated that the techniques described herein may be applied to other communication protocols, depending on the specific implementation. In one embodiment, the data transceiver 20 may act as a gateway to a larger communications network (not shown), such as a local or wide area network, or the Internet. Typically, the voice/data transceiver 15 establishes a connection to the communications network (not shown) through the data transceiver 20. During the process of establishing the connection, the transceivers 15 and 20 complete a training process whereby an initial bit loading technique (e.g., water filling, equal energy distribution, etc.) is employed to establish the throughput available for communication between the transceivers 15, 20. Although the present invention is described as it av be implemented in an ADSL transceiver it ι<; contemplated that, in light of this disclosure the invention may be applied to any type of transceiver, including, but not limited to, a modem or some other wired or wireless communication device

Referring now to Figure 2, an apparatus 200 in accordance with the present invention is illustrated The apparatus 200 includes one embodiment of a splitter 222, a voice transceiver 217, and a data transceiver 220 which correspond to the splitter 22 and transceivers 17, 20 of Figure 1 In one embodiment, the apparatus 200 may be a line card that contains the splitter 222, voice transceiver 217, and data transceiver 220 In an alternative embodiment, the splitter 222, the voice transceiver 217, and the data transceiver 220 may be separate devices that are adapted to interface with each other The splitter 222 receives a signal comprising voice and data band from the voice/data transceiver 15 (see Figure 1) over a connection 221 , which m the illustrated embodiment is a subscπber Ime The subscriber Ime 221 may be a Public Switched Telephone Network (PSTN) Ime, a Private Branch Exchange (PBX) line, or any other medium capable of transmitting signals The voice band, as used herein, refers to a POTS voice signal ranging from 0-4 KHz The data band refers to frequencies outside the voice band, and may include, for example, the frequency range employed in xDSL technologies As is descnbed m more detail below, the splitter 222 separates the voice and data signals and also performs unpedance matching functions for the incoming voice and data signals The splitter 222 provides data signals to the data transceiver 220, which comprises an ADSL modem m the illustrated embodiment The voice signals from the splitter 222 are provided to the voice transceiver 217, which in the illustrated embodiment comprises a subscπber Ime interface circuit (SLIC) Specifically, the SLIC 217 is a current-feed SLIC, which provides current to the subscπber Ime 221 and measures a resultmg voltage between its tip and rmg terminals

The SLIC 217 interfaces with the subscπber line 221 through the splitter 222 via the tip and rmg terminals For claπty and ease of illustration, only those portions of the SLIC 217 that are helpful in understanding the instant invention are illustrated herein Moreover, subscπber lmes 221 are not descnbed m detail herein as they are well-known m the art The SLIC 217 includes a first and second dπver 225, 228 The first driver 225 is capable of providmg a voice signal m a downstream direction over the subscriber Ime 221, while the second driver 228 is capable of providmg a voice signal received from the subscriber Ime 221 in an upstream direction In the illustrated embodiment, although not so limited, the SLIC 217 is capable of providmg a variety of useful functions, such as battery feed, over-voltage protection, and rmgmg signal, to the subscriber Ime 221

The splitter 222 of the apparatus 200 mcludes a first and second termmal adapted to couple to the subscπber Ime 221 A first termmal of a resistor, RFA, 232 is coupled to the first termmal of the splitter 222, and a first termmal of a resistor, FRB, 234 is coupled to the second put termmal of the splitter 222 The impedance of the subscriber line 221 is herein denoted as Z_LO_OP> and unpedance seen by an incoming signal from the subscπber Ime 221 is hereinafter referred to as Z.ι_N The value of Z_LOOP, which is determined by individual telephone authorities m various countries, may be in the range of 600-900 ohms for the POTS (/ e , voice) band and m the range of 100-135 ohms for the xDSL (t e , data) band Figure 3 illustrates an exemplary nommal loop unpedance model 310 that represents the unpedance of subscπber lmes in Germany The model 310 mcludes a first resistor 320, a second resistor 325, and a capacitor 330 havmg respective values of 820 ohms, 220 ohms, and 115 nano- farads A first termmal of the first resistor 320 is coupled to a first termmal of the second resistor 325 and to a first termmal of the capacitor 330 A second termmal of the first resistor 320 is coupled to a second termmal of the capacitor 330 Typically, the apparatus 200, if employed on subscπber lmes m Germany, for example, substantially matches the impedance represented by the model 310 of Figure 3 for voice signals, and the apparatus 200 would adjust the ZΓ_N impedance to substantially 100-135 ohms for data signals. It should be noted that the German nominal loop impedance model 310 illustrated in Figure 3 is shown for illustrative purposes only and that the instant invention is not limited in its application to Germany, rather it may be utilized in any country. As expected, the nominal loop impedance may vary from country to country. Referring back to Figure 2, the splitter 222 of the apparatus 200 includes a transformer 238 and a balance network 240. A second terminal of the RFA resistor 232 is coupled to a first terminal of a first secondary winding of the transformer 238. The balance network 240 includes a first, second, and third resistor 242, 244, 246 and a first and second capacitor 248, 250. A second terminal of the first secondary winding of the transformer 238 is coupled to a first terminal of the first resistor 242 and a first terminal of the second resistor 244 of the balance network 240. A second terminal of the first resistor 242 of the balance network 240 is coupled to a first terminal of the first capacitor 248 and to a first terminal of the third resistor 246 of the balance network 240. A second terminal of the second resistor 244 is coupled to a first terminal of the second capacitor 250 of the balance network 240. A second terminal of the second capacitor 250 is coupled to a second terminal of the first capacitor 248, and a second terminal of the third resistor 246 is coupled to a first terminal of a second secondary winding of the transformer 238. A second terminal of the second secondary winding of the transformer 238 is coupled to a first terminal of the RFB resistor 234.

A first terminal of a primary winding of the transformer 238 is coupled a ground node 255. A second terminal of the primary winding of the transformer 238 is coupled to a first terminal of a fourth resistor 258, to an inverting terminal of a downstream data driver 260, and to an inverting terminal of an upstream driver 262 of the ADSL modem 220. A second terminal of the fourth resistor 258 is coupled to a non-inverting terminal of the upstream data driver 262. A non-inverting terminal of the downstream data driver 260 receives a signal from a data source 265, which, in one embodiment, may be a CODEC (not shown) of the ADSL modem 220. It should be appreciated that the data source 265 is illustrated as being connected between the non-inverting terminal of the downstream data driver 260 and the ground node 255 only for ease of illustration. The data source 265, depending on a particular implementation, may be any source capable of providing a data signal to the non-inverting terminal of the downstream data driver 260.

Figure 4 illustrates an embodiment of a method that can be employed by the apparatus 200 of Figure 2 to adjust ZΓ to substantially match the subscriber line loop impedance Z_LOOP- It should be noted that the apparatus 200 is capable of adjusting the ZΓ_N to substantially match the nominal impedance of the subscriber loop 221 for downstream, as well as upstream, transmissions of voice and or data signals. Because voice and/or data can be transmitted on the subscriber line 221, the input signal received by the apparatus 200 may include voice and data band frequencies.

At block 410, the data band is obtained from the input signal using the transformer 238. Although not so limited, in the illustrated embodiment the transformer 238 is a digital transformer designed to handle primarily signal in data frequencies. Accordingly, the data transformer 238 offers little resistance in voice band. The data band frequencies from the input signal are provided to the secondary windings of the transformer 238, which then induces a current onto the primary winding. The current then flows through the fourth resistor 258, creating a voltage drop across the fourth resistor 258. An exemplary value of the fourth resistor 258 may be 20 ohms. The voltage is then amplified by the upstream data driver 262 and sent to the data CODEC (not shown) of the ADSL modem 220 for further processing. At block 420 the impedance of the apparatus 200 is adjusted to a selected level for the data band of the input signal In one embodiment, the selected level is approximately 100-135 ohms for the data band The balance network 240 adjusts Z_ΓM to substantially match the impedance of the subscriber Ime 221 for the data band In the data band, the two secondary windings of the transformers 238 are essentially shorted, and the balance network 240 has a low impedance Accordmgly, m essence, the unpedance of the RFA and RFB resistors 232, 234, along with the impedance of the balance network 240, equals approxunately 100-135 ohms, the approximate impedance range required for the data band The exemplary values of RFA and RFB resistors 232, 234 may be 25 ohms, wherein the balance network 240 accounts for approxunately the remainmg 50-85 ohms of the 100-135 ohms

The balance network 240 is also capable of adjustmg ZΓ_N to substantially match the unpedance of the subscriber line 221 for the voice band The transformer 240, which is adapted to handle prunarily data band frequencies, offers little resistance m voice band The unpedance of the transformer 238 reflects the output unpedance based on the signal on the primary winding along with the turns-ratio factor, which, m the instant case, is pretty low Thus, the unpedance seen by the voice band of the mput signal is set mamly by the balance network 240, as the transformer 238 contributes small, if not negligible, amounts of resistance for frequencies under 4 KHz In the balance network 240, the first resistor 242 may have an exemplary value of 180 ohms, the second resistor 244 may be 120 ohms, the third resistor 246 may have a value of 820 ohms, and the first and second capacitors 248, 250 may have respective values of 115, 22 nano-farads In the illustrated embodiment, the balance network 240 also removes at least a portion, if not all, of the frequencies above 4 KHz (/ e , data band frequencies) and provides a filtered (i e , voice only) signal to the to the SLIC 217 As mentioned above, the apparatus 200 is capable of adjustmg ZΓ for downstream transmissions as well

For downstream transmission of signals in the data band, the output unpedance of the downstream data driver 260 is relatively low so that the secondary windings of the transformer 238 present a low impedance Thus, smce the SLIC 217 feeds the balance network 240 current, the balance network 240, m essence, becomes the output impedance for the apparatus 200 At high frequencies (/ e , data band frequencies), the unpedance of the RFA and RFB resistors 232, 234, along with the unpedance of the balance network 240, amounts to approxunately 100-135 ohms, the desired impedance level for the data band For downstream transmission of voice band signals, the secondary wmdmgs of the transformer 238 are essentially shorted, wherem the balance network 240 once again determines the ZΓ unpedance to substantially match the subscriber loop unpedance, Z_LOOP

Referring now to Figure 5, an exemplary return loss chart for the apparatus 200 is shown for both the voice and data signals Generally, a high return loss is desirable, which mdicates that little of the signal transmitted down the wire is reflected back to the drivers 225, 260 The x-axis represents the frequency of an mcommg audio signal (either voice or data) and the y-axis represents the audio return loss m decibels Dashed lmes 500, 510 indicate exemplary limits for return loss for voice and data signals, respectively, as may be dictated by Germany, for example Curve 520 represents the return loss for voice signals, while curve 530 represents the return loss for data signals As can be seen m Figure 5, particularly with respect to the curve 520, the return loss for voice signals (range of about 200-3400 Hz) is greater than the required 20 dB level In the data band, for frequencies about 30 KHz, the return loss is greater than the required 10 dB Accordmgly, the apparatus 200 accordance with the present mvention is capable of providmg acceptable return loss values for voice and data signals

The present mvention, through the use of the transformer 238, is capable of supporting both voice and data signals on the subscπber Ime 221 The use of the transformer 238 avoids the need for expensive, large, multi- pole filters that are typically employed m splitters to separate the voice and data bands, as well as to match the nommal impedance of the subscriber line 221 An added advantage of the instant invention is that, by avoiding expensive and large filters, the splitter 222 may be mtegrated on the same chip as the SLIC 217 or the ADSL modem 220 Alternatively, the splitter 222, SLIC 217, and the ADSL modem 220 may all be implemented in the apparatus 200, such as a Ime card The particular embodiments disclosed above are illustrative only, as the mvention may be modified and practiced m different but equivalent manners apparent to those skilled in the art havmg the benefit of the teachmgs herem Furthermore, no limitations are mtended to the details of construction or design herein shown, other than as described m the claims below It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered withm the scope and spirit of the invention Accordingly, the protection sought herein is as set forth in the claims below

Claims

1. A method for matching impedance for an apparatus (35) coupled to a connection (21 ), comprising: receiving an input signal having a voice and data band from the connection (21); obtaining the data band of the input signal using a transformer (238); and adjusting impedance of the apparatus (35) to a selected level for the data band of the input signal.

2. The method of claim 1 , wherein the selected level is substantially equal to the impedance of the connection (21) for the data band.

3. The method of claim 2, further including adjusting an impedance of the apparatus (35) to a second selected level for the voice band of the input signal, and wherein the second selected level is substantially equal to the impedance of the connection (21) for the voice band.

4. An apparatus (222) for processing a data band portion of an input signal from a connection (221 ), comprising: a transformer (238) for obtaining the data band from the input signal; a balance network (240) capable of adjusting impedance of the apparatus (222) to a selected level for the data band of the input signal.

5. The apparatus (222) of claim 4, wherein the selected level is substantially equal to the impedance of the connection (221) for the data band.

6. The apparatus (222) of claim 5, wherein the selected level is in a range of 100 to 135 ohms.

7. An apparatus (200) for matching impedance CHARACTERIZED IN THAT, the apparatus (220) comprising: an interface (232) for receiving an input signal comprising a first and second band; a splitter (222), comprising: a transformer (238) for obtaining the first band from the input signal; and a balance network (240) capable of adjusting an impedance of the apparatus (200) to a selected level for the first band of the input signal; a first transceiver (20) capable of receiving the second band of the input signal; and a second transceiver (17) capable of receiving the first band of the input signal.

8. The apparatus (200) of claim 7, wherein the first band is a data band, wherein the second band is a voice band, and wherein the first transceiver (20) is an ADSL modem (220).

9. The apparatus (200) of claim 8, wherein the second transceiver (17) is a subscriber line interface circuit (217) and wherein the subscriber line interface circuit (217) is a current feed subscriber line circuit interface circuit.

0. The apparatus (200) of claim 9, wherein the selected level is in a range of 100 to 135 ohms.