Abstract:
A method for reducing audible noise in a telephone from a splitterless asymmetric digital subscriber line modem. The modem examines the shape of the power spectrum of fast retrain signals both when the telephone is on-hook and off-hook. By comparing the spectra, in particular the slopes of the spectra, the modem may determine the quality of the telephone that shares the telephone loop with the modem. The modem cuts back its transmitted power depending on the quality of the telephone by an amount that suppresses the audible noise in the telephone. In this manner, a customer may simultaneously use the telephone and the modem on the same telephone loop.

Description:
FIELD OF INVENTION 
     The present invention relates to communications in digital subscriber lines. More specifically, it relates to a method for reducing audible noise from a splitterless asymmetric digital subscriber line modem. 
     BACKGROUND OF THE INVENTION 
     Digital Subscriber Line (“DSL”) is a developing modem technology that allows existing copper telephone lines to carry high bandwidth information. Familiar twisted-pair telephone lines are then able to carry high speed data communication to and from a customer site in addition to retaining a plain old telephone service (“POTS”) channel for voice communication. In DSL, digital data is transmitted to the customer site directly without being converted to an intermediary analogue signal. At the customer site, a DSL modem receives the downstream digital data for a customer from a central office for a telephone company and transmits upstream digital data from the customer to the central office. 
     One configuration of DSL is generally termed Asymmetric Digital Subscriber Line (“ADSL”). ADSL allows high-speed data transport to the customer site but only medium-speed data transfer from the customer site, whence the asymmetry. ADSL also allows the simultaneous use of the ADSL modem and POTS on the same telephone line. In this configuration, at a point where the twisted-pair telephone line enters the customer site, the POTS channel is split off from the ADSL modem by filters. One problem with this ADSL, however, is that a technician from the telephone company has to visit the customer site and install the splitter, referred to as “the truck roll.” 
     To overcome this problem, another configuration of ADSL forgoes the requirement of a splitter at the customer site. This configuration is generally called “splitterless ADSL” and also goes under the name of “G.Lite.” Splitterless ADSL is described in the International Telecommunications Union-Telecommunication Standardization Sector (“ITU-T”, formerly known as the CCITT) standard identified by “G.992.2” which is incorporated herein by reference. ITU-T standards can be found on the World Wide Web at the Universal Resource Locator (“URL”) “www.itu.ch.” Splitterless ADSL modems include those manufactured by  3 Com Corporation of Santa Clara, Calif., Lucent Technologies of Murray Hill, N.J., Texas Instruments of Dallas, Tex., and others. 
     In splitterless ADSL, an ADSL modem directly shares the same twisted-pair telephone line as a POTS telephone without the intervention of a splitter. Sharing the same twisted-pair telephone line, however, may result in some of the ADSL signal bleeding over into the audible frequency range of the POTS telephone. Audible noise effects, such as a buzzing in the earpiece or speaker, are heard on the POTS telephone when off-hook. The noise effect is the response of the internal circuitry of the telephone to the ADSL signal. Moreover, each telephone has a different response to the ADSL signal: some telephones can be rather quiet, such as many of the cordless telephones, while other telephones produce a severely loud effect making the telephone itself unusable. 
     It is desirable, therefore, to reduce the audible noise in the telephone from the ADSL modem without the introduction of a splitter. It is also desirable to achieve noise reduction for a wide range of telephone models. This may allow customers to use their telephones while the ADSL modem is transferring data. 
     SUMMARY OF THE INVENTION 
     In accordance with preferred embodiments of the present invention, some of the problems associated with audible noise in a POTS telephone from a splitterless ADSL modem are overcome. A method for reducing noise from an ADSL modem is provided. One aspect of the invention includes a method for reducing audible noise in a telephone from a splitterless Asymmetric Digital Subscriber Line modem, where the telephone and the modem share a telephone loop, to enable simultaneous use of the telephone and the modem. The method includes measuring a first spectral characteristic for the telephone loop when the telephone is on-hook. The first spectral characteristic is measured by the modem. The method further includes measuring a second spectral characteristic for the telephone loop when the telephone is off-hook. The second spectral characteristic is measured by the modem. A power cutback value is calculated for the modem. The power cutback value is determined from the first spectral characteristic and the second spectral characteristic. Transmitted power from the modem is limited by an amount equal to the power cutback value. 
     For example, the method and system of the present invention may provide for reduction of audible noise in the telephone from a G.Lite modem. The present method and system may achieve the noise reduction for a wide range of telephone models. The method and system described herein may allow customers to use their telephones while the G.Lite modem is transferring data. 
    
    
     The foregoing and other features and advantages of preferred embodiments of the present invention will be more readily apparent from the following detailed description, which proceeds with references to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention are described with reference to the following drawings, wherein: 
     FIG. 1 is a block diagram illustrating a splitterless ADSL system; 
     FIG. 2 is a diagram illustrating the available bandwidth for G.Lite modems; 
     FIG. 3 is a flow diagram illustrating a method for reducing noise in a telephone from a splitterless ADSL modem; and 
     FIG. 4 is a graph illustrating an envelope of a typical on-hook C-REVERB-FR1 signal. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a block diagram illustrating a splitterless ADSL system  10 . The system  10  allows digital communications from a data network  12  and analog telephone signals from a Public Switched Telephone Network  14  (“PSTN”) to be combined and sent to a customer site  24  over a customer loop  22  comprising twisted-pair copper wire. The PSTN  14  includes those public switched telephone networks provided by AT&amp;T, Regional Bell Operating Companies (e.g., Ameritech, U.S. West, Bell Atlantic, Southern Bell Communications, Bell South, NYNEX, and Pacific Telesis Group), GTE, and others. 
     The digital data communications are received by a concentrator  16  at a telephone company&#39;s central office (“CO”). For example, data network  12  may be the Internet and the concentrator  16  may be a network hub that receives Internet Protocol packets and routes them to a particular network device connected to the network hub. Alternatively, the concentrator may  16  may be a DSL Access Multiplexer (“DSLAM”) that links many ADSL telephone lines to the data network  12 . The data intended for the customer site  24  is sent from the concentrator  16  to an ADSL Transceiver Unit  18  (“ATU”) where it is converted to an ADSL signal. 
     A functional description of the conversion of data to an ADSL signal in the ATU  18  is to be found in ITU-T G.992.2. The ATU  18  takes the data from the concentrator  16  and frames the data. Thereafter, the ATU applies a cyclic redundancy check, scrambling, forward error correction, coding and interleaving. The data stream is then tone ordered and combined into a data symbol that is input to a constellation encoder. After constellation encoding, the data is modulated to produce an analog signal for transmission across the customer loop  22 . The G.Lite signal permits high-speed downstream transfer of digital data from the central office to the customer site  24  at rates up to 1.536 Megabits per second (“Mbps”). 
     The G.Lite high-speed downstream signal occupies a frequency range of approximately 138 to 552 kiloHertz (“kHz”). Analog voice signals from the PSTN  14 , in contrast, occupy a low frequency range of zero to 4 kHz. The high frequency G.Lite signal from the ATU  18  and the low frequency analog signal from the PSTN  14  are combined in a frequency splitter  20  in the central office. The combined signal is sent over the customer loop  22  to the customer site  24 . 
     At the customer site  24 , the customer loop  22  from the central office connects to copper wiring within the customer site  24 . For example, the copper wiring may be an internal home wiring system having multiple standard wall telephone outlets. A POTS telephone  30  and an ADSL modem  26  may be attached to the same internal copper wiring. Importantly, in the G.Lite version of ADSL, there is no splitter at the customer site for isolating the POTS analog signal from the high-frequency G.Lite signal. Both telephones  30  and ADSL modems  26  may be plugged into the same standard telephone outlets, avoiding the expense and delay of a visit by a technician from the telephone company for the purpose of installing the splitter. The ADSL modem  26  demodulates the G.Lite signal into data which is forwarded to customer premise equipment  28 , e.g. a personal computing, multimedia, or video device. 
     Similarly, in the upstream channel from the customer site  24  to the central office, the ADSL modem  26  may receive data from the customer premise equipment  28  and convert it to an ADSL signal which is placed on the customer loop  22 . At the same time, the telephone  30  may be off-hook and sending analog voice signals to the PSTN  14 . The G.Lite upstream signal occupies a frequency range of approximately 26 to 134 kHz medium-speed upstream data transfer. Analog voice signals from the telephone  30  are in the low frequency range of zero to 4 kHz. The high frequency G.Lite signal from the ADSL modem  26  and the low frequency analog signal from the telephone  30  are both sent over the customer loop  22  to the central office. The G.Lite signal permits medium-speed upstream transfer of digital data from the customer site  24  to the central office at rates up to 512 kilobits per second (“kbps”). 
     At the central office, the splitter  20  passes the low-frequency analog signals to the PSTN  14  and passes the high-frequency G.Lite signal to the ATU  18 . The G.Lite signal is demodulated by the ATU  18  into data, passed to the concentrator  16 , and sent to the data network  12 . 
     An operating environment for the modems and ATUs of the present invention include a processing system with at least one Central Processing Unit (“CPU”) and a memory. In accordance with the practices of persons skilled in the art of computer programming, the present invention is described below with reference to acts and symbolic representations of operations or instructions that are performed by the processing system, unless indicated otherwise. Such acts and operations or instructions are referred to as being “computer-executed” or “CPU executed.” 
     It will be appreciated that acts and symbolically represented operations or instructions include the manipulation of electrical signals or biological signals by the CPU. An electrical system or biological system represents data bits which cause a resulting transformation or reduction of the electrical signals or biological signals, and the maintenance of data bits at memory locations in a memory system to thereby reconfigure or otherwise alter the CPU&#39;s operation, as well as other processing of signals. The memory locations where data bits are maintained are physical locations that have particular electrical, magnetic, optical, or organic properties corresponding to the data bits. 
     The data bits may also be maintained on a computer readable medium including magnetic disks, optical disks, organic memory, and any other volatile (e.g., Random Access Memory (“RAM”)) or non-volatile (e.g., Read-Only Memory (“ROM”)) mass storage system readable by the CPU. The computer readable medium includes cooperating or interconnected computer readable medium, which exist exclusively on the processing system or be distributed among multiple interconnected processing systems that may be local or remote to the processing system. 
     Discrete Multi-Tone modulation 
     The ADSL modem  26  and the ATU  18  modulate data for transmission as a G.Lite signal and demodulate a received G.Lite signal back into data. An ADSL transmitter typically employs a multi-carrier modulator, utilizing Discrete Multi-Tone (“DMT”) modulation. The DMT modulator may be implemented by mapping the data values to symbols specified by the magnitude and phase of each carrier, and then performing an inverse discrete Fourier transform (“IDFT”). The time domain sequence is then transmitted over the customer loop  22 , which includes the effects of the subscriber line twisted pair wire and the analog components and amplifiers of the analog front ends of the ADSL transceivers. An ADSL receiver includes a DMT demodulator. The demodulator may be implemented utilizing a discrete Fourier transform (“DFT”), for which there are many algorithms, to recover the magnitude and phase of the symbols on each carrier. It is understood that in a typical ADSL configuration, each end of the communication channel uses both a transmitter and receiver, making an ADSL transceiver. The downstream transmission direction (from the central office to the customer site  24 ) uses a wider bandwidth than the return direction, or upstream direction. 
     As is known in the art, DMT is a method for dividing the available G.Lite bandwidth into 128 sub-channels from 0 to 552 kHz. Essentially, DMT splits the available bandwidth into a large number of sub-channels, each associated with a “bin” that is 4.3125 kHz wide. The frequency response of the customer loop  22  means that some bins have a higher data transport capacity compared to other bins. Data transport capacity is limited by noise on the customer loop  22 , including external radio-frequency sources, thermal noise, and cross-talk from other customer loops. By measuring the frequency response of the customer loop  22 , the system  10  may determine the data carrying capacity of each sub-channel. In this manner, DMT may allocate data so that the throughput of each sub-channel is maximized. 
     FIG. 2 is a diagram illustrating the available bandwidth  50  for G.Lite modems. Bins  0  to  5 , corresponding to 0 to 25.875 kHz, are reserved for POTS  52 . Although the POTS  52  bandwidth is typically only about 4 kHz, DMT has large sidelobes that may bleed down to lower frequencies. Upstream data  54  is transmitted in bins  6  to  31 , corresponding to 25.875 kHz to 138 kHz. With the inclusion of echo canceling, these bins may also permit medium speed downstream transfer. Downstream data  56  is transmitted in bins  32  to  127 , corresponding to 138 kHz to 552 kHz. Finally, bin  128  corresponds to the Nyquist frequency  58  and no data is transmitted in this bin. 
     A Fast Retrain procedure is defined for G.Lite modems to adapt transmission characteristics to changing customer loop  22  conditions e.g. caused by telephone  30  on/off hook  20  transitions. As known to those skilled in the ADSL art, during the Fast Retrain procedure the ATU  18  sends a predetermined series of tones to the ADSL modem  26 . The modem  26  at the other end of the customer loop  22  receives the tones and measures the power of the signal received in each bin. From these measurements, the modem  26  is able to determine the frequency response of the customer loop  22 . From this, the modem  26  determines the signal-to-noise ratio (“SNR”) in each bin and hence derives a value for the maximum data throughput for that bin. The modem  26  may then independently select a downstream modulation scheme from a set of previously negotiated modulation schemes to optimize the throughput for the downstream transmission bins. Similarly, the modem  26  sends a predetermined series of tones which are received by the ATU  18 . The ATU  18  may also determine the maximum data throughput for each bin. The ATU  18  may then independently select an upstream modulation scheme from a set of previously negotiated modulation schemes to optimize the throughput for the upstream transmission bins. 
     Audible Noise Reduction 
     The ADSL modem  26  generates audible noise in the telephone  30  when the telephone is off hook. The DMT transmissions from the modem  26  reach the telephone  30  through the local telephone loop and interact with the electronics in the telephone  30 . This interaction with the telephone  30  electronics in the absence of a splitter at the customer site is the source of the audible noise. Unless the power of the modem  26  is cut back, the customer may be unable to simultaneously use the telephone  30  whilst transferring data from the modem  26  because the audible noise is overwhelming. 
     The level of audible noise generated in the telephone  30  is dependent on the model of telephone. Some telephones are noisier than others. The degree of power cutback of the modem  26  required to suppress the audible noise is dependent on the model of telephone. Quieter models of telephones include: General Electric models 2-9221 and 2-9295; Lucent/AT&amp;T models 210 and 9200; Bell South model 33012; and White-Westinghouse model WNCP-440. Once the transmitted power from the modem  26  is cut back by the required amount, the user may use the telephone  30  and the modem  26  simultaneously. 
     FIG. 3 is a flow diagram illustrating a Method  80  for reducing audible noise in a telephone  30  from a splitterless Asymmetric Digital Subscriber Line modem  26 . The telephone  30  and the modem  26  share a telephone loop  22 . At Step  82 , a first spectral characteristic is measured, by the modem  26 , for the telephone loop  22  when the telephone  30  is on-hook. A second spectral characteristic for the telephone loop  22  is measured, by the modem  26 , when the telephone  30  is off-hook at Step  84 . At Step  86 , a power cutback value for the modem  26  is computed. The power cutback value is determined from the first spectral characteristic and the second spectral characteristic. At Step  88 , transmitted power from the modem  26  is limited by an amount equal to the power cutback value. In this manner, audible noise in the telephone  30  from the modem  26  is substantially reduced, enabling simultaneous use of the telephone  30  and the modem  26 . 
     The power cutback required to suppress the audible noise depends on the quality of telephone  30  in the customer loop  22 . The modem  26  may distinguish between a noisy telephone and a quiet telephone. In one exemplary preferred embodiment of the present invention, the modem  26  ascertains the quality of the telephone  30  with which it shares a local telephone loop  22  by monitoring Fast Retrain signals from the ATU  18 . Fast Retrain, as mentioned above, are a series of tones, power levels, and procedures that allow G.lite modems to modify their transmission characteristics before entering a data transfer mode. The modem  26  monitors Fast Retrain signals while the telephone  30  is on-hook and again while the telephone  30  is off-hook. A comparison of both signals may indicate how the presence of the telephone  30  influences transmission characteristics of the local customer loop  22  and provides an indication of the quality of the telephone  30 . 
     A Fast Retrain procedure may be initiated by either the ATU  18  or the ADSL modem  26 . In the present invention, a Fast Retrain procedure may be initiated by the ADSL modem  26  under two circumstances. A first circumstance is when the modem  26  is starting up. The modem  26  requests an entry to Fast Retrain for the purpose of measuring a spectral characteristic of the customer loop  22  when the telephone  30  is not in the circuit, i.e. the telephone  30  is on-hook. In one exemplary preferred embodiment of the present invention the spectral characteristic is the slope of a C-REVERB-FR1 signal&#39;s envelope when the telephone  30  is on-hook. This entry is made from a handshake protocol, known to those skilled in the art as “G.hs,” which is running on a CPU in the modem  26 . C-REVERB-FR1 is described in Section 12 of ITU-T standard G.992, and G.hs is described in ITU-T standard “G.994.1” which is incorporated herein by reference. 
     The second circumstance for Fast Retrain is when the ATU  18  and modem  26  are in data transfer mode, also known to those skilled in the G.992 art as “Showtime” mode. If the modem  26  detects the telephone  30  going off-hook, it initiates the Fast Retrain procedure, according to the guidelines in Section 12.3 of G.992, for the purpose of measuring a spectral characteristic of the customer loop  22  when the telephone  30  is in the circuit, i.e. the telephone  30  is off-hook. In another exemplary preferred embodiment of the present invention this spectral characteristic is also the slope of the C-REVERB-FR1 signal&#39;s envelope, only now in the off-hook condition. Upon detection of the telephone  30  going off-hook, the modem  26  stops sending data and begins sending a R-RECOV signal. R-RECOV is a single tone signal corresponding to sub-carrier bin  20  without a cyclic prefix. R-RECOV is transmitted by the modem  26  at nominal power (−38 deciBels below one milliWatt per Hertz (“dBm/Hz”)). When the ATU  18  detects the R-RECOV signal, it stops sending data and begins sending a C-RECOV signal. C-RECOV is a single tone signal corresponding to sub-carrier bin  68  without a cyclic prefix accompanied by a pilot sub-carrier in bin  64 . The C-RECOV signal is transmitted by the ATU  18  at nominal power (− 40   dBm/Hz). C-RECOV is sent for exactly 64 symbols, at which point the ATU  18  begins sending C-REVERB-FR1. C-REVERB-FR1 is associated with C-REVERB1, which is a wide-band signal described in Section 11.7 of G.992. The C-REVERB1 signal modulates a contiguous range of sub-carrier bins. 
     The modem  26  controls the length of the C-REVERB-FR1 signal. The ATU  18  continues to send the C-REVERB-FR1 signal until it measures the R-REVERB-FR1 for at least 32 symbols. The modem  26  continues to send R-RECOV while it measures the slope of the envelope of the C-REVERB-FR1 signal. In practice, the modem  26  only needs 64 symbols worth of the C-REVERB-FR1 signal to effectively characterize the slope of the envelope, at which point it would transition to sending the R-REVERB-FR1 signal in order to continue the Fast Retrain process with the ATU  18 . 
     FIG. 4 is a graph illustrating an envelope  102  of a typical on-hook C-REVERB-FR1 signal. This signal was measured by the modem  26  on a 24 American Wire Gauge (“AWG”) telephone line that is 4000 feet long. In this Figure, the upstream bins are from  6  to  31  and the R-RECOV tone  100  can be seen at bin  20 . The envelope  102  represents a distribution of power measured by the modem  26  upon receiving the C-REVERB-FR1 tones from the ATU  18 . The power is measured logarithmically with respect to a nominal power value. 
     The slope of the envelope  102  may be measured in several ways known to those skilled in the art. In one exemplary preferred embodiment, logarithmic power values may be measured for multiple bins and a straight-line fit performed to estimate the slope of the envelope  102 . In another exemplary preferred embodiment, logarithmic power values may be measured for only two bins and a slope estimated from the two measurements. For example, the two bins may be chosen to be bin  51  corresponding to a lower frequency in the envelope  102  and bin  120  corresponding to a higher frequency in the envelope  102 . An estimate for the slope of the envelope  102  may be derived from Equation 1.                  log        (     Average                 Power                 in                 Bin                 120     )       -     log        (     Average                 Power                 in                 Bin                 51     )           120        -        51             (   1   )                                
     The average power in each bin is calculated by squaring the real and imaginary values produced by a Fast Fourier Transform (“FFT”) process, and adding the two values together. Accumulating these values over 64 symbols for both bin  51  and bin  120 , and dividing each of these results by 64 yields the average power in each of these bins. For example, the slope of the envelope  120  for the on-hook telephone  30  measured by the modem  26  on a 24 American Wire Gauge (“AWG”) telephone line that is 4000 feet long is typically found to be approximately −0.012 deciBels per bin (“dB/bin”). 
     In another exemplary preferred embodiment of the present invention, an estimate of the quality of the telephone  30  may be made by computing a difference between the slope of the C-REVERB-FR1 signal while the telephone  30  is on-hook and the slope of another C-REVERB-FR1 signal while the telephone  30  is off-hook. The difference in slope between the on-hook and off-hook values is typically independent of the length of the loop  22  because we are taking a difference between logarithms of average power. We have determined that if the slope difference satisfies Equation 2, then telephone  30  is typically a noisy telephone. 
     
       
         Slope(on-hook)—Slope(off-hook)&gt;0.036 dB/bin  (2) 
       
     
     Equivalently, expressed in other units, the slope difference is 8.4×10 −3  dB/kHz. 
     Equation 2 sets an approximate threshold for determining the quality of the telephone  30  on the customer loop  22  with the modem  26 . In one exemplary preferred embodiment of the present invention, a noisier telephone  30 , i.e. where the slope satisfies Equation 2, requires a cutback of approximately 24 dB in power transmitted by the modem  26  in the upstream bandwidth  54  to suppress the audible noise. The power cutback value is measured with respect to the nominal transmitted power value of −38 dBm/Hz for the modem  26 . In another exemplary preferred embodiment, a quieter telephone  30 , i.e. where the slope does not satisfy Equation 2, only requires a power cutback of approximately 12 dB by the modem  26 . The power cutback may be applied during an R-REVERB-FR3 period of Fast Retrain. However, it should be understood that the present invention is not restricted to these power cutback values and that other values are possible. 
     It should be understood that the programs, processes, methods, systems and apparatus described herein are not related or limited to any particular type of computer apparatus (hardware or software), unless indicated otherwise. Various types of general purpose or specialized computer apparatus may be used with or perform operations in accordance with the teachings described herein. 
     In view of the wide variety of embodiments to which the principles of the invention can be applied, it should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the present invention. For example, the Steps of the flow diagrams may be taken in sequences other than those described, and more or fewer elements or components may be used in the block diagrams. 
     The claims should not be read as limited to the described order or elements unless stated to that effect. In addition, use of the term “means” in any claim is intended to invoke 35 U.S.C. §112, paragraph 6, and any claim without the word “means” is not so intended. Therefore, all embodiments that come within the scope and spirit of the following claims and equivalents thereto are claimed as the invention.