Abstract:
Various embodiments are configured to transform characteristics of a communication signal. One embodiment is a method comprising decreasing amplitude of a first detected portion of the communication signal so that the decreased amplitude is in close proximity to a predefined specification; and increasing amplitude of a second portion of the communication signal so that the increased amplitude is in close proximity to the predefined specification, thereby resulting in a transformed communication signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a divisional of U.S. utility application entitled, “SUBSCRIBER LINE DRIVER AND TERMINATION,” having Ser. No. 09/439,933, filed Nov. 12, 1999 now U.S. Pat. No. 6,782,096, which is entirely incorporated herein by reference. 
   This document claims priority to and the benefit of the filing date of co-pending commonly assigned Provisional Application entitled, “SUBSCRIBER LINE DRIVER AND TERMINATION,” having Ser. No. 60/108,044, filed Nov. 12, 1998. The foregoing provisional application is hereby incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates generally to the art of data communications. The preferred embodiment generally relates to the art of telephony, and more particularly, to a communication signal driver system (and associated methodology) for connection between a telephony switching unit, which may be located at a central office (CO), at a private branch exchange (PBX) or the like, and customer premises equipment via an existing telephony connection (e.g., copper wire twisted-pair, digital subscriber loop or the like). 
   BACKGROUND OF THE INVENTION 
   With the increasing bandwidth demands from the advent of the Internet, service providers have looked for ways to increase data performance over the copper wire twisted-pair local loop transmission lines that connect the telephone central offices (COs) to the customer premises (CPs). The customer premises equipment (CPE) is connected to the CO switches over transmission lines known as “local loops,” “subscriber loops,” “loops,” or the “last mile” of the telephone network. Historically, the public switched telephone network (PSTN) evolved with subscriber loops connected to a telephone network with circuit-switched capabilities that were designed to carry analog voice communications. Digital service provision to the customer premises is a more recent development, with the evolution of the telephone network from a system just designed to carry analog voice communications into a system which could simultaneously carry voice and digital data. 
   Because of the prohibitive costs of replacing or supplementing existing subscriber loops, technologies have been implemented that utilize existing subscriber loops to provide easy and low cost customer migration to digital technologies. Subscriber loops capable of carrying digital channels are known as digital subscriber lines (DSLs). Logical channels within a subscriber line which carry digital signals are known as DSL channels, while logical channels within a subscriber line which carry plain old telephone service (POTS) analog signals are known as POTS channels. Furthermore, to provide customers with additional flexibility and enhanced services, frequency-division multiplexing and/or time-division multiplexing techniques have been designed to fill a subscriber loop with multiple logical channels. These newer DSL technologies provide digital service to the customer premises without significantly interfering with the existing POTS equipment and wiring. The newer DSL technologies accomplish this functionality by frequency-division multiplexing (FDM) their digital signal above (at higher frequencies than) the 0 KHz to 4 KHz baseband of standard, analog POTS signals. Multiplexing techniques and terminology are common to those skilled in the art, and are not described herein. 
   Several variants of new DSL technology exist (e.g., ADSL, SDSL, RADSL, VADSL, MVL™, Tripleplay™, etc.), with this group generally referred to as xDSL. Communications systems carrying xDSL usually multiplex xDSL signals and a POTS signal onto a single physical local loop. 
   Historically, the POTS subscriber loop was designed with the functions needed to communicate both analog, voice-conversation signals and subscriber loop signaling. The CO switch uses subscriber loop signaling to notify the customer premises about events in the telephone network, while customer premises equipment (CPE) use subscriber loop signaling to inform the CO to perform actions for the customer. Some examples of subscriber loop signaling include: the CO switch signaling to the CPE that an incoming call has arrived by ringing the phone, the CPE (e.g., a telephone) signaling to the CO switch that the CPE is initiating a call by an on-hook to off-hook transition of the telephone handset, and the CPE signaling to the CO switch that a call should be connected to a location by sending the phone number of the location. 
   Although the transmission of both digital and analog POTS signals over a subscriber loop offers many potential advantages for customers, several practical problems must be solved in implementing DSL solutions. One significant problem resulting from the POTS subscriber loop signaling functions is the generation of high-frequency interference, known in the art as noise, into DSL channels. For instance, the on-hook/off-hook signal and the pulse-dialing signal are square waveforms which have high-frequency components and harmonics, and theoretically require infinite frequency bandwidth. This high-frequency noise may degrade the signal to noise (S/N) ratio of the DSL channel. The S/N ratio is commonly known to those skilled in the art, but can be simply described as the ratio of the transmit signal amplitude to the noise amplitude, expressed in decibels (dB). Thus, a heretofore unaddressed need exists in the industry for a way to prevent or substantially minimize the adverse affects on the DSL channel S/N ratio caused by noise introduced by the POTS subscriber loop functions. 
   Another practical problem facing the industry effort to implement DSL technology on the existing PSTN system is the large voltage magnitude change occurring on the subscriber loop during transitions between on-hook and off-hook conditions, as is well known in the art. Some embodiments of prior art DSL technology require a change in the input impedance of the DSL device upon sensing of a transition between on-hook and off-hook conditions. Thus, a heretofore unaddressed need exists in the industry for a way to prevent or substantially minimize the adverse affects of the on-hook/off-hook transition. 
   Another practical problem facing the industry effort to implement DSL technology on the existing PSTN system is the unpredictable nature of the subscriber loop transmission system impedance. Signal attenuation (decrease in signal strength) and signal distortion (changes in the signal shape) are caused by real and reactive impedance losses incurred on the subscriber loop as the signal is transmitted between the CO and the CPE. Each subscriber loop, consisting of a copper wire twisted-pair circuit connecting the CO to the CPE, is unique. That is, each subscriber loop differs in length, and often these subscriber loops are constructed with varying copper wire gauge sizes. Therefore, the actual circuit impedance of any given subscriber loop is unique and different from other subscriber loops. DSL technology utilizes FDM to shift the frequency of the communication signal into the 25 KHz to 1 MHz frequency range. As is well known in the art, subscriber loop circuit impedance is not a constant, but rather a variable over the frequency spectrum because the subscriber loop impedance is complex (having reactive impedance components as well as resistive impedance components). Therefore, signal attenuation also varies with the frequency of a transmission signal. That is, some frequencies will be attenuated more or less than other frequencies. 
   The presence of bridged taps connected to the subscriber loop introduces another unpredictable impedance component. Bridged taps are unused copper wire twisted-pair lengths connected at various points of the subscriber loop. Bridged taps constitute parallel circuits which alter the impedance of the subscriber loop circuit, and effectively reduce the transmit signal strength. 
   Finally, the wiring of the customer premise and the various types of customer equipment and devices, including multipoint communication, connected to the subscriber loop is unique. These differences at the customer premise also impact the overall impedance of the subscriber loop transmission system. 
   For the purpose of establishing the transmitter frequency domain specifications and limits, current practice typically models the subscriber loop impedance as a resister, R L , that is representative of the characteristic impedance of the subscriber loop transmission line. At the remote end of the transmission line, the receiver equipment is typically modeled as a terminating resister, R R , usually of the same value as R L . Transmission of signals onto subscriber loops has been provided by a voltage signal source, V s , and a series resister, R T . Current practice is to transmit at the subscriber loop transmission line input a transmit signal spectral shape of V S  that is designed to be the same as a voltage power spectral distribution (PSD) standard. The PSD standard specifies maximum signal strength (amplitude) and frequency bandwidth boundaries for a DSL channel. 
   Design of the transmit signal spectral shape of V S  necessarily requires certain assumptions about the subscriber loop transmission system. Traditional transmission line theory teaches that for optimum communication, the subscriber loop transmission system should have R T =R L =R R . As an example, it is customary in some DSL technologies to select R L =135 ohms for transmission signals in the band from approximately DC to 192 kHz. This 135 ohm value is a reasonable best choice for a simplistic resistive compromise model. Because the prior art model is resistive, the design transmit signal is the same as the design PSD of V S . 
   However, the prior art assumptions may be wholly inadequate in representing the wide range of subscriber loop transmission lines found in practice. R T  is not ideal (R T ≠R L ≠R R ) since each individual subscriber loop is unique. Also, R L  is not ideal because customer premises wiring are often different and because of bridged taps on the subscriber loop. In practice, the actual subscriber loop transmission system impedance can vary in magnitude from well over 200 ohms to less than 50 ohms, and the actual impedance is complex. The result in practice is that the actual transmit signal on any given transmission line can vary dramatically, and this variance is usually such that the transmit signal amplitude is lower than permitted in part of or all of the transmission band as defined by the PSD standard. It can be shown, for example, that the actual transmit signal amplitude can be 12 dB lower than the PSD standard in part of the band, and even average power can be 6 dB lower than allowed. This means that 6 dB or more of potential transmit signal power is being sacrificed, and that the receive signal S/N ratio is thus 6 dB lower than the S/N that could be realized with an optimized transmit signal. 
   Another problem involves instances where the actual transmit signal voltage exceeds the PSD standard. If the actual transmit signal voltage exceeds the PSD standard, undesirable interference or noise is induced onto other subscriber loops sharing the same underground cable or overhead wire. 
   Thus, a heretofore unaddressed need exists in the industry for a way to provide for a transmit signal which conforms to a defined PSD standard regardless of the actual impedance characteristics of the transmission system. 
   SUMMARY OF THE INVENTION 
   The present invention provides a subscriber line driver (SLD) for transforming the characteristics of a communication system signal. The signal is transformed by the SLD which increases (amplifies) portions of the signal to a predefined specification, decreases (attenuates) portions of the signal to a predefined specification, and/or frequency modulates or filters the transmit signal frequencies to fit within the communication channel frequency bandwidth as defined by the frequency band of the predefined specification. After modification by the SLD, the transformed communication signal is injected (transmitted) into a communication transmission line. The SLD may operate in a continuous and automatic mode. An SLD may be applicable to a variety of communication systems, for example but not limited to, a public telephony system, a private branch exchange (PBXs), a coaxial cable system, a fiber optic system, a microwave system or a radio communication system. In the preferred embodiment, the SLD operates on a telephony system subscriber loop which is operated as a digital subscriber loop (DSL) having a plain old telephone system (POTS) channel and at least one DSL channel. 
   The method of the preferred embodiment of the SLD comprises the following steps. The direction of travel of a communication signal is sensed when in the transmit signal direction, where the transmit direction is defined as traveling in a direction out to the communication system, here a subscriber loop. The SLD transforms the communication signal traveling in the transmit direction such that the transformed communication signal conforms to a predefined specification. 
   The preferred embodiment of the SLD comprises at least two functional components, a transmit signal equalizer and a current driver connected to the output of the transmit signal equalizer. In the preferred embodiment, the current driver injects (transmits) the transformed communication signal into the subscriber loop. Another embodiment of the SLD utilizes a voltage driver (rather than the current driver). A voltage feedback loop can be added to the SLD circuitry which further optimizes the transformed communication signal. 
   The SLD has an infinite input impedance at all frequencies. Addition of a parallel resister connected to a tip wire and a ring wire of the telephony system can enable the design engineer to set the transmission system terminating impedance to any desired value. 
   Another embodiment of the SLD modifies the transmit signal to conform to a first predefined specification, and also modifies the receive signal to conform to a second predefined specification. This embodiment of the SLD may have any of the methods, features and options of the SLD embodiments previously described. 
   This invention also provides for a telephony system central office (CO), comprising at least one telephony switching unit, at least one digital equipment unit and at least one subscriber line driver (SLD). The telephony switching unit is ultimately connected to a telephony transmission system on one side and to at least one telephony subscriber loop or DSL on the other side. At least one subscriber line driver (SLD) would be connected between one terminal of the digital equipment unit and one subscriber loop or DSL. The SLD would receive a communication signal from the digital equipment unit, and would transform the communication signal into a transformed communication signal so that the transformed communication signal conforms to a predefined specification. 
   This invention also provides for a private branch exchange (PBX), comprising at least one telephony switching unit, at least one digital equipment unit and at least one subscriber line driver (SLD). The telephony switching unit is ultimately connected to a telephony transmission system on one side and to at least one of telephony subscriber loop or DSL on the other side. At least one SLD would be connected between one terminal of the digital equipment unit and one subscriber loop or DSL. The SLD would receive a communication signal from the digital equipment unit, and would transform the communication signal into a transformed communication signal so that the transformed communication signal conforms to a predefined specification. 
   Other features and advantages of the present invention will become apparent to one skilled in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be better understood with reference to the following drawings. The elements of the drawings are not necessarily to scale relative to each other, emphasis instead being placed upon clearly illustrating the principles of the present invention. Furthermore, like reference numerals designate corresponding parts throughout the several views. 
       FIG. 1  is a block diagram of an existing telephony system of the prior art. 
       FIG. 2  is a block diagram of an SLD of an embodiment of the present invention located on the premises of a transmitting company, organization, and/or individual. 
       FIG. 3A  is a graph illustrating ideal transmit signal amplitude spectra for a POTS channel and two DSL channels. 
       FIG. 3B  is a graph illustrating non-ideal transmit signal amplitude spectra for a POTS channel and two DSL channels. 
       FIG. 3C  is a graph illustrating the modification of a non-ideal transmit signal by the preferred embodiment of the SLD of  FIG. 2 . 
       FIG. 3D  is a graph illustrating the modification of a non-ideal transmit signal by another embodiment of the SLD of  FIG. 2 . 
       FIG. 3E  is a graph illustrating the modification of a non-ideal transmit signal by the another embodiment of the SLD of  FIG. 2 . 
       FIG. 4  is a block diagram illustrating an SLD located at the telephone company central office and an SLD located at the customer premises. 
       FIG. 5A  is a block diagram illustrating two components of a first embodiment of the SLD of  FIG. 4 , a transmit signal equalizer and a current driver. 
       FIG. 5B  is a block diagram illustrating two components of a second embodiment of the SLD of  FIG. 4 , a transmit signal equalizer and a voltage driver. 
       FIG. 6  is a block diagram illustrating electrical components of the SLD of  FIG. 4 ; a transmit signal equalizer, a current driver, an amplifier and a parallel resister (R o ). 
       FIG. 7A  is a block diagram illustrating a central office with a SLD located at the premises of the central office. 
       FIG. 7B  is a block diagram illustrating a private branch exchange (PBX) with a SLD located at the premises of the PBX. 
       FIG. 8  is a diagram illustrating a transmitter with a SLD. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a block diagram illustrating an existing telephony system  20  which includes a telephone company central office (CO)  22  connected to a customer premises (CP)  24  via a subscriber loop  26 . The subscriber loop  26  may be any suitable connection for passing electrical signals, but is typically a copper wire twisted-pair, as is well known in the art, that was originally designed to carry a 0-4 KHz analog voice channel. Located within the CO  22  is the CO telephony switching unit  28  which transmits communication signals received from the outside world to the CP  24  via the subscriber loop  26 , or which receives communication signals from the customer premises equipment (CPE)  29  via the subscriber loop  26  for transmission to designated locations in the outside world. In the context of this disclosure describing the existing telephony system, “outside world” means any telephone or communications device connected to or having access to the global telephone network, the public switched telephone network (PSTN) and/or a private telephony system, and where designated locations in the outside world are identified by telephone numbers or some other identification manner commonly employed by the art. CO digital equipment  21  and CP digital equipment  52  may be added at the central office and the customer premises to facilitate transmission of digital data. When the copper wire twisted-pair is used for digital transmission, the twisted-pair is often referred to as a digital subscriber loop (DSL). “Central office” or “CO” means any site where a subscriber loop  26  connects into a telephony switching unit, such as a public switched telephone network (PSTN), a private branch exchange (PBX) telephony system, or any other location functionally connecting subscriber loops to a telephony network. 
     FIG. 2  is a block diagram illustrating the relative location of the preferred embodiment of the subscriber line driver (SLD) at the transmit signal site. The preferred embodiment of the SLD continuously and automatically modifies a non-ideal communication signal amplitude spectra  238 , which will be further described in detail hereinafter in  FIG. 3A  through  FIG. 3E , received from the transmit signal equipment  128 , to fit within the frequency bandwidth and within the maximum amplitude of the PSD standard  40  ( FIG. 3A ) prior to injecting (transmitting) the transformed communication signal into the communication connection  126 . The communication signal is then delivered to the receive signal equipment  129 . 
   The method of the preferred embodiment of the SLD comprises the following steps. The direction of travel of a communication signal is sensed when in the transmit signal direction, where the transmit direction is defined as traveling in a direction out to the subscriber loop. The SLD transforms the communication signal traveling in the transmit direction such that the transformed communication signal conforms to a predefined specification or a predefined difference threshold. This method is described in detail hereinafter. 
     FIG. 3A  illustrates examples of an ideal communication signal amplitude spectra  32  consisting of three communication signals multiplexed into three separate channels. The three signals would be transmitted into, or injected into, a communications system, for example but not limited to, a DSL subscriber loop. The vertical axis of the spectra is the signal strength or amplitude measured in dB, where dB is commonly known in the art as decibels (dB). The horizontal axis of the spectra is signal frequency measured in Hertz (Hz). The same axis definitions will apply to  FIG. 3B  through  FIG. 3E . 
   In  FIG. 3A , the analog voice communication signal occupies the plain old telephone system (POTS) channel  34 . As is well known in the art, the POTS channel typically occupies a bandwidth from about 0 to 4 KHz. Two additional channels may be used in the DSL industry to transmit digital data. In this embodiment of the DSL system, channel A  36  occupies a bandwidth of 30 KHz to F 1  KHz, and channel B  38  occupies a bandwidth of F 2  KHz to F 3  KHz. Channel A  36  and channel B  38  each contain an ideal communication signal of a two channel DSL system. The communication signals may be comprised of either analog or digital data. F 1 , F 2  and F 3  are communication bandwidth frequency boundaries of a PSD standard  40  selected by the system design engineer. The 30 KHz lower frequency of the channel A  36  bandwidth is a typical value encountered in the art, but which may be adjusted to a different value by the system design engineer. 
   Shown in  FIG. 3A  with a dashed line is the power spectral distribution (PSD) standard  40  for a channel A and channel B. A PSD standard  40  defines the allowable PSD frequency range (bandwidth) and the maximum signal strength (amplitude) for a communication channel at the sending (transmitting) location. If the transmitted communication signal amplitude exceeds the PSD standard  40 , then undesirable interference or noise could be induced onto other subscriber loops sharing the same underground cable or overhead wire. If a transmitted communication signal amplitude is less that the PSD standard  40 , the transmitted communication signal is under-powered resulting in a less than optimal S/N ratio. If the bandwidth of a transmitted communication signal lies outside of the frequency boundaries of the PSD standard  40 , then the transmitted communication signal may overlap onto and interfere with other communication channels. The transmitted communication signals of channel A  36  and channel B  38  as shown in  FIG. 3A  are nearly ideal. That is, the two transmitted communication signals occupy the greatest region of the PSD  40  standard without exceeding the amplitude and bandwidth limits as defined by the PSD standard  40 . 
   Often, on a prior art two channel DSL system, a communication signal in one channel is traveling in the opposite direction of a communication signal in the other channel. Direction of signal travel depends upon the application of the DSL system user. As an illustrative example, the communication signal of channel A  36  could be transmitted at the CO digital equipment  21  ( FIG. 1 ) into the subscriber loop  26  for transmission to the CP digital equipment  52 . Similarly, the communication signal of channel B  38  could be transmitted at the CP digital equipment  52  into the subscriber loop  26  for transmission to the CO digital equipment  21 . (For the remainder of the disclosure of the preferred embodiment, for illustrative purposes only, the communication signal transmission location of channel A  36  will be designated as the CO  22  and the communication signal transmission location of channel B  38  will be designated as the CP  24 .) In actual practice of the prior art, signals may be transmitted from or received by both the CO digital equipment  21  and the CP digital equipment  52 . 
   Often, signal transmission direction in a channel changes direction regularly, as in the POTS channel. For example, during a telephone voice conversation between two people over the PSTN, the speaker determines the transmission location of the communication signal and the listener determines the location of the received signal. As a conversation proceeds between the two people, the direction of travel of the communication signal regularly changes depending upon which party is doing the talking. Direction of travel of the communication signals of a DSL system can also be regularly changing. 
     FIG. 3B  is illustrative of non-ideal communication signal amplitude spectra  132  which may be encountered with the prior art DSL technologies. The transmitted communication signal  136  of channel A is illustrated in  FIG. 3B  as degraded below the maximum signal strength allowed by the PSD standard  40  due to effects of the actual impedance of the subscriber loop, the presence of bridged taps, wiring of the customer premises, and/or the various types of customer equipment as previously described in the Background section of this disclosure. For further illustrative purposes, a part of the communication signal channel B  138  has been degraded below the maximum signal strength allowed by the PSD standard  40 , while part of the communication signal channel B  138  exceeds the maximum signal amplitude allowed by the PSD standard  40 . Also, the higher frequencies of communication signal channel B  138  are greater than the high frequency (F 3 ) bandwidth limit of the PSD standard  40  due to the reactive components of the transmission system. 
     FIG. 3C  is an enlarged view illustrating the non-ideal communication signal amplitude spectra  238  of channel B ( FIG. 3B ) before processing by the SLD. Transmitting this non-ideal communication signal amplitude spectra  238  into a subscriber loop will cause a variety of problems, as previously discussed in the Background section. The preferred embodiment of the SLD  30  senses the direction of travel of a communication signal and selects the signal if traveling in the transmitting direction, defined as traveling in a direction out to the subscriber loop. Once a communication signal has been selected, the SLD  30  would continuously and automatically amplify a digital signal to transform the communication signal into a transformed communication signal such that the transformed communication signal conforms to a predefined specification. This specification would not be greater than the maximum amplitude allowed by the PSD standard  40 . Here, in this illustrative example, the lower frequency portion  238   a  of the non-ideal communication signal amplitude spectra  238  exceeds the maximum amplitude of the PSD standard  40 . If the communication signal portion  238   a  is injected (transmitted) into the subscriber loop, undesirable interference could be induced in adjacent subscriber loops, as previously described in the Background section. That portion of the communication signal  238   a  would be reduced (attenuated) by the preferred embodiment of the SLD  30  to an amplitude value in close proximity to the maximum amplitude of the PSD standard  40 , as shown by the transformed communication signal  338 . Here, close proximity can be defined as the amplitude of the transformed communication signal  338  being below, at, or above the PSD standard  40 , or another predefined standard, such that the error (difference) between the PSD standard  40  and the transformed communication signal  338  is within some predefined difference threshold. 
   Here, in the illustrative example of  FIG. 3C , the mid-range portion  238   b  of the non-ideal communication signal amplitude spectra  238  is less than the maximum amplitude of the PSD standard  40 . If the communication signal portion  238   b  is transmitted into the subscriber loop, the S/N ratio will not be maximized, as previously discussed in the Background section. The mid-range portion  238 b of the non-ideal communication signal amplitude spectra  238 , which is below the maximum amplitude of the PSD standard  40 , would be increased (amplified) by the preferred embodiment of the SLD  30  to a value in close proximity to the maximum amplitude of the PSD standard  40 , as shown by the transformed communication signal  338 . 
   Another embodiment of the SLD  30  may have the additional feature of providing for frequency modulation, frequency shifting, or filtering a non-ideal communication signal to conform the transformed communication signal to a predefined frequency band specification that is within the frequency bandwidth limits specified by the PSD standard  40 . As shown in the illustrative example of  FIG. 3C , the highest frequency portion  238   c  of the non-ideal communication signal amplitude spectra  238  exceeds the high frequency limit F 3  of the PSD standard  40 . If the communication signal portion  238   c  is transmitted into the subscriber loop, undesirable interference could be induced in adjacent DSL channels, as previously described in the Background section. This embodiment of SLD  30  would frequency shift or filter the non-ideal communication signal amplitude spectra  238  to fit within the frequency boundaries of the PSD standard  40 , as shown by the transformed communication signal  338 . 
     FIG. 3D  depicts an illustrative non-ideal communication signal amplitude spectra  438  before processing of a DSL channel. Another embodiment of the SLD  30  acts upon the non-ideal communication signal amplitude spectra  438  to conform the non-ideal communication signal amplitude spectra  438  to a predefined specification which is equal to a percentage of the PSD standard  40 , as shown by the transformed communication signal  538 . For illustrative purposes,  FIG. 3D  shows the transformed communication signal  538  to be approximately 85 percent of the PSD standard  40 . The SLD  30  continuously and automatically determines the amount of amplification at any specific frequency of the non-ideal communication signal amplitude spectra  438  and selects the degree of amplification necessary to conform the non-ideal communication signal amplitude spectra  438  to the predefined specification of the PSD standard  40 . For example, the degree of amplification of the lower frequencies of the non-ideal communication signal  438  is seen to be about ten to fifty percent. The degree of amplification of the higher frequencies of the non-ideal communication signal  438  is seen to be as great as five hundred percent. 
     FIG. 3E  depicts an illustrative non-ideal communication signal amplitude spectra  438  before processing by the SLD  30 . Another embodiment of the SLD  30  modifies a non-ideal communication signal amplitude spectra  438  by simply amplifying the non-ideal communication signal amplitude spectra  438  by some fixed amount as determined by the predefined specification, as shown by the transformed communication signal  638 . For illustrative purposes,  FIG. 3E  shows the fixed amount of amplification applied to the non-ideal communication signal amplitude spectra  438  to be approximately thirty percent of the non-ideal communication signal. 
     FIG. 1  shows an existing telephone central office  22  and the customer premises  24  without the SLD  30  ( FIG. 2 ). Digital signal transmission and signal receiving equipment is depicted as the CO digital equipment  21  and the CP digital equipment  52 .  FIG. 4  shows a more detailed telephone system with installation of a telephony system embodiment of the SLD  30 . One skilled in the art will realize that the telephone system illustrated in  FIG. 4  can be replaced with other types of communication systems where transmit signal processing by the SLD would be beneficial. Other types of communication systems could include, but are not limited to, private telephony systems, coaxial cable systems, fiber optic systems, microwave systems or radio communication systems. 
     FIG. 4  is now described in greater detail. Three communication equipment components of the telephony system CO  22  are shown, the telephony switching unit  28 , digital equipment  21  and the SLD  30   a . (More communication equipment components, unrelated to the operation of the SLD  30   a , would likely be located at the telephone company CO  22 , but are not shown in  FIG. 4 .) Three communication equipment components of the customer premises  24  are shown, a telephone  54 , the SLD  30   b , and the CP digital equipment  52 . Examples of the CP digital equipment  52  could be, but are not limited to, a computer, or a television set-top-box. For illustrative purposes for the preferred embodiment of this SLD, and as previously noted during the discussion of  FIG. 3A , the communication signal transmission location of channel A  36  of the DSL system will be designated as the CO  22  and the communication signal transmission location of channel B  38  will be designated as the CP  24 . One skilled in the art will realize that the transmission location of the communication signals could be at either, or both, the CO  22  or the CP  24 . Also, one skilled in the art will realize that any data channel could be applicable to the illustrative example of  FIG. 4  and to the application of the SLD. 
   When a communication signal is transmitted from the CO  22  to the CP  24  over channel A, the transmitted communication signal may not be ideal (channel A  136  of  FIG. 3B ). The preferred embodiment of the SLD  30   a , located at the CO  22 , will continuously and automatically transform (amplify, attenuate and/or frequency modulate) a communication signal from the CO digital equipment  21  to conform to a predefined specification which does not exceed the signal strength or the frequency bandwidth of the PSD standard  40  (channel A  36  of  FIG. 3A ). The SLD then transmits the transformed communication signal of channel A onto the DSL  226  for transmission to the CP  24 . When the communication signal is received at the CP  24 , then becoming the receive signal, the receive signal is delivered to the CP digital equipment  52 . One skilled in the art will realize that the receive signal will pass through the SLD  30   b  unaffected, or entirely bypass the SLD  30   b , depending upon the actual circuitry configuration of the digital signal processing equipment. That is, the preferred embodiment of the SLD will sense the direction of travel of the communication signal and selectively operate only in the communication signal transmission direction. 
   Similarly, when a communication signal is transmitted from the CP  24  to the CO  22  over the channel B, the communication signal may not be ideal (channel B  138  of  FIG. 3B ). The preferred embodiment of the SLD  30   b , located at the CP  24 , will transform (amplify, attenuate and/or frequency modulate) a communication signal from the CP digital equipment  52  to conform to a predefined specification which does not exceed the signal strength or the frequency bandwidth of the PSD standard  40  (channel B  38  of  FIG. 3A , or channel B  338  of  FIG. 3C ). The SLD  30   b  then transmits the transformed communication signal of channel B onto the DSL  226  for transmission to the CO  22 . When the communication signal is received at the CO, then becoming a receive signal, the receive signal is delivered to the CO digital equipment  21 . One skilled in the art will realize that the receive signal will pass through the SLD  30   a  unaffected, or entirely bypass the SLD  30   a , depending upon the actual circuitry configuration of the digital signal processing equipment. 
   As shown in  FIG. 4 , and which is well known by those skilled in the art, the analog telephony signal transmitted on the POTS channel  34  ( FIGS. 3A and 3B ) between the CO telephony switching unit  28  and the telephone  54  over the DSL  226  is transmitted without interacting with the DSL,  30   a  or  30   b , which is transmitting over channels A and B. 
     FIG. 5A  is a block diagram showing two of the components of the preferred embodiment of the SLD  30 , a transmit signal equalizer  60  and a current driver  62 . The transmit signal equalizer  60  detects the incoming communication signal (not shown), and transforms (amplify, attenuate and/or frequency shift) the communication signal to conform to a predefined specification. The current driver  62  then transmits the transformed communication signal into the communication connection  126 . One skilled in the art will recognize that the degree of communication signal distortion and the amount of amplification and frequency modulation required to transform the communication signal will dictate the complexity of the transmit signal equalizer  60 .  FIG. 5B  is a variation of the SLD  30  wherein a voltage driver  64  is used to inject the transformed communication signal into the communication connection  126 . 
     FIG. 6  shows two enhancements of the SLD  30  of  FIG. 5A . The first enhancement is a voltage feedback loop wherein an amplifier  66  provides signal feedback to the transmit signal equalizer  60 . The feedback loop detects a communication signal that may not be ideal (Channel B  138  of  FIG. 3B ) and provides for the continuous and automatic adjustment of the communication signal after the current driver  62  injects the transformed communication signal into the subscriber loop. The SLD  30  has the capability to provide a transformed communication signal PSD that is ideal regardless of the transmission channel impedance. Also, the SLD  30  has the capability to provide a transformed communication signal PSD that is ideal regardless of other multipoint transceivers. Once the SLD  30  transmit signal equalizer  60  has been calibrated for a particular DSL circuit, there is no need for continuing recalibration under practical applications. Here in  FIG. 6 , the subscriber loop is shown as a twisted pair copper wire local loop  326  of a telephony system or a DSL system consisting of a Tip  70  line and a Ring  72  line. The twisted pair copper wire local loop  326  is referenced in  FIG. 1  as the telephony system subscriber loop  26  and in  FIG. 4  as the DSL  226 . As is well known by those skilled in the art, all of the above expressions describing telephony and DSL communication systems may be equivalent. 
   The second enhancement of the SLD  30  shown in  FIG. 6  is the addition of a parallel resistor  68  of some finite impedance. The SLD  30  enjoys an infinite input impedance, often defined in the prior art as RR. Note especially that with the SLD  30 , an infinite input impedance RR is true for all frequencies. An infinite input impedance of the SLD  30  in the POTS band is desirable, as there would be no loading of the POTS band. And, although tradition of the prior art implies that for practical applications the terminating impedance of a transmission line should be assumed to be the “characteristic impedance” of the transmission line, one skilled in the art will realize that this is an incorrect conclusion based on “maximizing power transfer.” In actuality, the ideal signal transmission optimization technique is to maximize the receive signal level as long as loss vs. frequency is within the tolerances of the receive signal equipment (can be read with acceptable bit error tolerances) and potential signal reflection on the transmission line is suitable. Although tradition of the existing prior art indicates the frequency band above 25 kHz should be terminated from 100 ohms to 135 ohms, empirical tests show that that termination at 1000 ohms or higher, or even at an infinite impedance, would provide for superior voltage signal transmission. One skilled in the art will recognize that the simple addition of a parallel resister  68  shown in  FIG. 6  can enable the design engineer to set the transmission system terminating impedance to any desired value without compromising the other attributes of the subscriber loop or the SLD  30 . 
   Another benefit is provided by the infinite input impedance of the SLD  30 . “Splitter-less” DSL technologies, well known in the art, require a subscriber loop transmission system with a relatively low RT and RR in the DSL frequency bands while having a relatively high RR in the POTS frequency band. For example, a desirable DSL transmission system RT and RR would be 100 ohms at 26 kHz and above, and for the POTS perhaps 1200 ohms at 4 kHz and below. This desirable DSL transmission system is very difficult, and perhaps impossible, to achieve with the prior art. The SLD  30  provides a way to implement specified impedances on a DSL system which provides for desirable impedances on both a POTS channel and splitter-less DSL channels. Also, the infinite input impedance of the SLD  30  minimizes the adverse affects of the POTS on-hook/off-hook transition on the DSL channel. 
   Yet another practical benefit from the SLD  30  is optimizing a DSL transmission system when two or more transceivers are placed at one or both ends of the subscriber loop, as in multipoint communication. The transmitted communication signal amplitude would, in the absence of the SLD  30 , be significantly reduced due to the lowered net load impedance seen by that transmitter. For two transceivers, the transmitted communication signal could be reduced by as much as 4 dB. Similarly, the effective RR now becomes the parallel combination of the RR of the two transceivers, and the receive signal is reduced. Thus, the SLD provides for a transmitted communication signal which is not affected by the presence of multipoint operation, thereby optimizing the receive signal. 
   The inclusion of an SLD  30  into a larger system may be considered as an improvement to the larger system. When an SLD is incorporated into a CO  22 , as shown in  FIG. 7A , the CO  22  is improved in that the CO  22  may now transmit transformed communication signals from CO  22  digital equipment  21  which have been modified to conform to a predefined specification. Similarly, an SLD can be incorporated into a PBX  23  as an improvement, as shown in  FIG. 7B . In both the CO  22  and the PBX  23 , at least one SLD may be installed at the CO  22  or PBX  23 , with one SLD  30  being ultimately located at some point between the digital equipment  21  or  121  and communication system transmission line, such as, but not limited to, a DSL  226 . 
   The SLD  30  may be considered as an improvement to a transmitter  130  system. The SLD  30 , when incorporated into the transmitter  130 , would transform communication signals to conform to a predefined specification. A transmitter  130  with an SLD  30  is shown in  FIG. 8 . The SLD  30  is ultimately connected to a communication system transmission line, such as, but not limited to, a DSL  226 . 
   It should be emphasized that the above-described embodiments of the present invention, particularly, and “preferred” embodiments or configurations, are merely possible examples of implementation, 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 form the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the present invention.