Abstract:
An asymmetric modem communications system achieves high speed data transfers through a telephone network that includes both digital and analog communications mediums. In general, the system includes means for concurrently communicating first and second signals, respectively, in opposite directions along the connection between the communications devices and modems for modulating the first and second signals with different modulation techniques. The communications occur in full duplex manner. In a possible implementation, a digital modem is interfaced to a digital network. The digital network is connected with a coder/decoder (codec). The codec is interfaced with a two-wire analog telephone connection, sometimes referred to as a copper loop. The telephone connection is interfaced with an analog modem. Both the digital and analog modems have a transmitter and a receiver. The digital modem has a transmitter that pulse modulates digital data and a receiver that receives and demodulates signals in accordance with the standard V.34 communications protocol (employs quadrature amplitude modulation/demodulation). The analog modem has a transmitter that transmits and modulates signals in accordance with the V.34 communications protocol and a receiver that demodulates the pulse levels into digital data. With the foregoing configuration, asymmetric data communications are realized. Specifically, the analog modem communicates to the digital modem using the V.34 communications protocol at a data rate of between 33,600 b/s and 2400 b/s, inclusive, while the digital modem communicates to the analog modem at a data rate of between 64,000 b/s and 2400 b/s, inclusive.

Description:
This application is a continuation of copending and commonly assigned U.S. Patent Application entitled “Asymmetric Modem Communications System and Method,” and assigned Ser. No. 08/696,776, filed Aug. 13, 1996, now U.S. Pat. No. 5,991,273. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to data communications and, more particularly, to an asymmetric modem communications system and method for achieving high speed data transfers through a telephone network that includes both digital and analog communications mediums. 
     BACKGROUND OF THE INVENTION 
     A telephone network is often used as an interface between a digital modem and an analog modem. Generally, a digital modem is a device that communicates digital data by using digital signals that replicate analog waveforms. An analog modem is a device that communicates digital data by encoding the data on analog waveforms. 
     FIG. 1 shows a typical telephone network  99  for interconnecting a digital modem  101  and an analog modem  102 . The digital modem  101  is usually interconnected with a digital network  113  via digital connections  112   a ,  112   b . For instance, the digital modem  101  may be interconnected to a digital network  113  in the form of a public switch telephone network (PSTN) via a Local Exchange Carrier (LEC) subscriber loop. The digital network  113  may comprise, among other things, a T1 carrier system, a basic rate or primary rate Integrated Services Digital Network (ISDN), a fiber optic cable network, a coaxial cable network, a satellite network, or even a wireless digital communications network. Communications over the digital network  113  are conducted in accordance with a pulse code modulation (PCM) scheme. Channel capacity through these digital facilities is typically between 56 and 64 kilobits per second (kb/s). Coding of the signals is also employed so that compression and a constant signal/distortion performance over a wide dynamic range is achieved for optimal transmission of voice signals. A commonly used coding technique is a nonlinear mu-law coding. 
     The digital network  113  is in turn interconnected with another LEC subscriber loop that includes a coder/decoder (codec)  106 . The codec  106  is interconnected with the digital network  113  via digital connections  114   a ,  114   b . The codec  106  is often situated at a telephone company office or along a street near the analog modem subscriber in an SLC device. The codec  106  provides an interface between the digital network  113  and an analog telephone connection  118 , sometimes referred to as a copper loop. For communications in the direction from the digital network  113  to the analog modem  102 , the codec  106  includes a mu-to-linear-analog converter  109 , which includes digital-to-analog (DAC) conversion functionality. The converter  109  converts nonlinear mu-law levels to a linear analog signal. For communications in the direction from the analog modem  102  to the digital network  113 , the codec  106  includes a linear-analog-to-mu converter  107 , which includes analog-to-digital (ADC) conversion functionality. The converter  107  converts the linear analog signal to nonlinear mu-law levels. 
     A hybrid  103  is in communication with the DAC and ADC via respective LPFs  111 ,  105 . The hybrid  103  serves to separate the bidirectional analog signals from the analog telephone connection  118  into unidirectional transmit and receive analog signals sent to and received from the ADC  107  and the DAC  109  respectively. 
     Furthermore, the analog modem  102  is connected to the analog telephone connection  118  and communicates analog signals therewith. Thus, communications occur between the digital modem  101  and the analog modem  102  by way of the digital network  113  and the codec  106 . 
     Researchers have been attempting to increase the speed at which data can be transferred through the telephone network between the digital and analog modems  101 ,  102 . U.S. Pat. No. 5,394,437 to E. Ayanoglu et al. describes a high speed analog modem  102  that is synchronized to the DAC and ADC clocks of the codec  106 . Further, a pulse level modulation scheme is utilized to communicate data along the telephone connection  118 . With pulse level modulation, a plurality of voltage levels are communicated along the analog telephone connection  118 . This system permits data transfer rates above 40 kb/s. 
     Although the aforementioned system is meritorious to an extent in terms of increasing data transfer rates, it suffers from various undesirable problems and disadvantages. 
     A primary disadvantage of the Ayanoglu system involves echo problems. Generally, there is sensitivity to quantized echoes because detection occurs at the codec quantizer, and there is an inability to provide echo cancellation prior to detection. More specifically, echo cancellation at the analog modem  102  is not a problem given its exceptional linearity. However, the echo at the codec is a major problem due to the mu-law coding and limited hybrid quality. On a poor subscriber loop, the receive signal is attenuated. The echo is increased due to the impedance mismatch. In fact, the echo level can exceed the receive signal level. Accordingly, both the analog modem  102  and the digital modem  101  will attempt to utilize all PCM levels. When the digital modem  101  echo results from one of the upper compander levels and the analog modem  102  has transmitted on one of the lower levels, then the echo will control the channel bank encoder step size. In this case, it is difficult to resolve the symbols from the analog modem  102 . 
     Another disadvantage of the Ayanoglu system is that it requires complex timing synchronization with the codec. 
     Hence, there exists a need in the industry for systems and methods for increasing the speed of data transfers through a telephone network  99 , which comprises both a digital and analog communications mediums, between a digital modem  101  and an analog modem  102 . 
     SUMMARY OF THE INVENTION 
     The invention provides for an asymmetric modem communications system and method for achieving high speed data transfers through a telephone network that includes both digital and analog communications mediums. In general, the system includes means for concurrently communicating first and second signals, respectively, in opposite directions along the connection between the communications devices and means for modulating the first and second signals with different modulation techniques. The communications occur in full duplex manner. 
     In a possible implementation, a digital modem is interfaced to a digital network. The digital network is connected with a coder/decoder (codec). The codec is interfaced with a two-wire analog telephone connection, sometimes referred to as a copper loop. Finally, the telephone connection is interfaced with an analog modem. 
     Both the digital and analog modems have a transmitter and a receiver. The digital modem has a transmitter that pulse modulates digital data in the sense that it generates and transmits pulse levels and a receiver that receives and demodulates signals in accordance with the standard V.34 communications protocol. Generally, the V.34 protocol employs a form of quadrature amplitude modulation/demodulation. The analog modem has a transmitter that transmits and modulates signals in accordance with the V.34 communications protocol and a receiver that demodulates the pulse levels into digital data. 
     Communications over the digital network are conducted in accordance with pulse code modulation (PCM). Moreover, communications over the analog connection occur via encoding of digital data on analog waveforms. 
     With the foregoing configuration, asymmetric data communications are realized. Specifically, the analog modem communicates to the digital modem using the V.34 communications protocol at a data rate of between 33,600 b/s and 2400 b/s, inclusive, while the digital modem communicates to the analog modem at a data rate of between 64,000 b/s and 2400 b/s, inclusive. 
     Worth noting is that the invention can also be broadly viewed as providing a method for bidirectionally communicating information between first and second communications devices along a connection. The method can be summarized as follows: concurrently communicating first and second signals in opposite directions along the connection between the first and second communications devices, and modulating the first and second signals with different modulation techniques. 
     The invention has numerous advantages, a few of which are delineated hereafter, as examples. 
     An advantage of the invention is that data transfers as high as 64 kb/s are achieved through the telephone network from the digital modem to the analog modem, while data is transferred in the reverse direction at up to 33,600 b/s using a conventional V.34 communications protocol. 
     Another advantage of the invention is that it provides for full duplex communication of signals along the analog telephone connection by using two different modulation techniques, one for each signal that is transferred in opposite directions. 
     Another advantage of the invention is that because of the two types of modulation that are utilized, echo distortion is minimized. Conventional V.34 modulation utilizes low symbol rates (approximately half) as compared to that described in U.S. Pat. No. 5,394,437 to E. Ayanoglu et al. Thus, each symbol is spread over more than 2 PCM samples so that quantized echo distortion is reduced. Further, V.34 modulation provides detection at the output of a trellis code Viterbi decoder after echo cancellation, receive equalization, precoding, and flexible symbol timing recovery. Finally, V.34 utilizes conventional transmit power levels with modest preemphasis, but no pre-equalization which causes high peak signal levels. 
     Another advantage of the invention is that it provides for higher speed transactions when a user is interacting with the internet or a computer controlled database. 
     Another advantage of the invention is that it is simple in design and reliable in operation. 
     Another advantage of the invention is that it can be implemented with only minor modifications to existing modem designs. 
     Other objects, features, and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional objects features, and advantages be included herein within the scope of the present invention, as defined by the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating principles of the present invention. In the drawings, like reference numerals designate corresponding parts throughout the several views. 
     FIG. 1 is an electronic block diagram of a possible implementation for coupling together digital and analog modems over a telephone network; and 
     FIGS. 2A and 2B show an electronic block diagram of a possible implementation of an asymmetric modem communications system and method in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 2A and 2B show an electronic block diagram illustrating the asymmetric modem communications system and method of the invention, which is generally denoted by reference numeral  130 . The asymmetric modem communications system  130  provides for asymmetric communications between the digital modem  101  and the analog modem  102 . Specifically, the asymmetric modem communications system  130  enables communications corresponding with the V.34 protocol from the analog modem  102  to the digital modem  101 , while concurrently permitting in a full duplex manner high speed communications from the digital modem  101  to the analog modem  102  using pulse level modulation. As a result, communications from the analog modem  102  to the digital modem  101  can take place at a data rate between 2400 b/s and 33,600 b/s, inclusive, while communications from the digital modem  101  to the analog modem  102  can take place between 2400 b/s and 64,000 b/s, inclusive. 
     In the discussion hereafter, the apparatus for enabling V.34 communications from the analog modem  102  to the digital modem  101  will be described first, and then the apparatus for permitting pulse level communications from the digital modem  101  to the analog modem  102  will be described thereafter. 
     Further note that the elements of the analog modem  102  and the elements of the digital modem  101 , as described hereafter, can be implemented with respective software, firmware, hardware, or a combination thereof. In the preferred embodiment, the elements of the modem  102  as well as the modem  101  are implemented in software that is stored in a memory and that configures and drives a suitable digital signal processor (DSP) situated in the respective modem. Furthermore, the foregoing software can be stored on any computer-readable medium for use by or in connection with a computer-related system or method. In the context of this document, a computer-readable medium is an electronic, magnetic, optical, or other physical device or means that can contain or store a computer program for use by or in connection with a computer-related system or method. 
     V.34 Communications 
     With reference first to FIG. 2B, a serial stream of digital transmit data  132  is communicated to the analog modem  102  from data terminal equipment (DTE), for example but not limited to, a home personal computer (PC). 
     The V.34 transmitter  134  is configured to receive the bit stream of transmit data  132  from the DTE. The V.34 transmitter  134  implements the standard and conventional modulation suggested by the V.34 recommendation, for example but not limited to, trellis encoding, mapping, scrambling, etc. The V.34 transmitter  134  outputs a data stream  136  that complies with the V.34 protocol. As is well known in the art, the V.34 protocol is a form of quadrature amplitude modulation. Moreover, the data stream  136  corresponds with one of the fourteen possible V.34 speeds between 2400 b/s, and 33,600 b/s, inclusive. 
     A sampling switch  138  is configured to receive a V.34 modulated signal  136  from the transmitter  134  and adapted to pulsate the signal to a Hilbert filter  141 . The sampling switch  138  operates at the symbol rate of the modem  102 . Thus, for a V.34 modem, the symbol rate is 3429, 3200, 3000, 2800, 2743, or 2400 hz. For purposes of simplicity, a sampling rate of 3200 hz will be utilized in the discussion hereafter so that downstream rate figures are whole numbers. Note that a sampling rate of 3200 hz corresponds to a maximum data transfer rate of 31.2 kb/s. 
     The Hilbert filter  141  receives the signal  139  from the sampling switch  138  and shifts the frequency of the signal constellation to the passband specified by the V.34 recommendation so that the carrier frequency is either 1829 hz or 1920 hz pursuant to the V.34 specification. Moreover, the Hilbert filter  141  implements a bandpass filter to limit the bandwidth (bw) to approximately 3200 hz centered around the carrier frequency. 
     A sampling switch  143  receives code words from the Hilbert filter  141  and switches the code words at an appropriate rate. In this example where the sampling rate of switch  138  was 3200 hz, then the rate of the switch  143  should be 9600 hz. 
     An interpolator  146  receives code words from the switch  143  at 9600 hz and outputs the signal  147  at 16,000 hz, as indicated by switch  148 . The interpolation process is performed so that pulse levels can be received in the reverse direction at 16,000 hz. A conventional analog modem would not need the interpolator  146 . 
     A DAC  153  receives the code words at 16,000 hz. Preferably, the DAC  153  has a resolution of 14 to 16 bits. The DAC  153  converts the digital signal  151  to an analog signal  154 . A sample-and-hold circuit  155  associated with the DAC  153  samples the analog signal  154  at 16,000 hz to produce a 16,000 hz analog signal  157  for a low pass filter (LPF). 
     The LPF  159  attenuates high frequencies in the signal  157 , while permitting passthrough of lower frequencies. The cut off frequency is preferably about 3700 hz in this exemplary implementation. 
     A hybrid  162  receives a filtered analog signal  164  from the LPF  159  and combines it with an incoming pulse level modulated signal. Specifically, the hybrid  162  includes an amplifier  166   a  configured to receive the signal  164  and to provide an amplified signal to a resistor (R)  168  and another amplifier  166   b . The resistor  168  is connected to a transformer  172  and the amplifier  166   b . Thus, as is apparent from the configuration, the transmit signal is passed from the LPF  159  through the amplifier  166   a , through the resistor  168 , and through the transformer  172  to the copper loop associated with the telephone connection  118 . Moreover, the incoming pulse level modulated signal is passed from the cooper loop  118  through the transformer  172 , and through the amplifier  166   b.    
     With reference to FIG. 2A, a hybrid  103  is connected to the copper loop  118 . This hybrid  103  is within the PSTN and is typically located at a telephone company central office. The hybrid  103  is configured just as the hybrid  162 . The hybrid  103  includes a transformer  174  connected to a resistor  176  and an amplifier  178 . Further, an amplifier  182  is connected to the amplifier  178  and the resistor  176 . 
     A LPF  186  receives an amplified analog signal  184  from the amplifier  178  of the hybrid  103 . The LPF  186  attenuates high frequencies, while permitting passthrough of lower frequencies. Preferably, the cutoff frequency is approximately 3700 hz. 
     A linear-analog-to-mu converter  107  receives a filtered linear analog signal  187  from the LPF  186  and converts it to a compressed mu-law digital signal  192  having a series of mu-law digital code words. To arrive at the mu-law digital signal  192 , the linear analog signal  187  is digitized to a companded digital signal that is quantized to a nearest 8-bit mu-law signal level. 
     The digital network  113  receives the mu-law code words  192  serially from the converter  107  and passes them to a digital modem  101  via connection  193 , which is typically a T1 carrier connection, a basic rate ISDN connection, or a primary rate ISDN connection. A sampling switch  194  communicates the mu-law code words to a mu-to-linear-digital converter  196 . The mu-to-linear-digital converter  196  is configured to perform a mu-law expansion operation on the mu-law digital signal. In this regard, the converter  196  converts each 8-bit mu-law code word to a 14-bit linear code word  198 . 
     A subtractor  202  receives the signal  198  and combines it with an echo canceler signal  204  in order to produce an enhanced signal  206  that is substantially free of echo from the local transmit signal. 
     An interpolator  208  receives the enhanced signal  206 ,  206   a  from the subtractor  202  and performs an interpolate function. After interpolation, the 8000 hz signal  206 ,  206   a  is converted to a 9600 hz signal  212 , as indicated by switch  214 . 
     A Hilbert filter  216  receives the 9600 hz signal  212  and restricts the frequency of the signal constellation to the passband specified by the V.34 recommendation. The Hilbert filter  216  implements a bandpass filter to limit the bandwidth to about 3200 hz. The output  218  from the filter  216  is switched, as indicated by sampling switch  221  at 9600 hz. 
     An equalizer  225  receives the signal  223  and equalizes the channel. Essentially, the equalizer  225  eliminates the effects of the LPF  186 , the copper loop  118 , and the LPF  159  (FIG. 2B) by boosting attenuated frequencies. The equalizer  225  essentially implements an inverse transfer function. 
     A V.34 receiver  228  receives the equalized signal  226  from the equalizer  225  and implements the standard and conventional demodulation suggested by the V.34 recommendation. The V.34 receiver  228  outputs receive data  232  in the form of a digital data stream to a DTE, for example, a computer with a large database. 
     B. Pulse Level Communications 
     A possible implementation of the apparatus for implementing pulse level communications is now described hereafter. As previously mentioned, the pulse level communications in accordance with the present invention enable data transfers at a high speed from the digital modem  101  to the analog modem  102 . One form of pulse modulation is described in detail in the previously mentioned U.S. Pat. No. 5,394,437. 
     Referring to FIG. 2A, transmit data  234  from the local DTE is communicated to a serial-to-parallel converter  236  of the digital modem  101 . The transmit data  234  may be scrambled by a scrambler (not shown for simplicity) so that the data is random. This minimizes direct current (DC) frequency content and helps the echo cancelers and equalizers of the system  130  to operate correctly. 
     The serial-to-parallel converter  236  converts the incoming serial data stream to a parallel data stream. In the preferred embodiment, the converter  236  outputs 5 to 8 bit code words, depending upon the number of desired levels. Use of 5, 6, 7, or 8 bit code words corresponds to 32, 64, 128, 256 pulse levels, respectively, and to data rates of 40, 48, 56, or 64 kb/s, respectively, as the code words are clocked to the digital network  113  at 8000 hz. 
     The map table  241  receives the code words  238  and converts them to an expanded linear digital format of, for example, 16 bits each. In the preferred embodiment, the map table  241  is essentially a lookup table. The mapping allows for pulse levels to be omitted where necessary to prevent ambiguity at the pulse decoder in the analog modem  102 . Channel noise or distortion on the copper loop  118  may reduce resolution in the pulse decode particularly for pulse levels corresponding to very low signal levels. In this case, many is of the low levels should not be used. Using the map table  241  provides a convenient method for omitting the unresolvable pulse levels. 
     A pulse transmitter  244  receives the linear digital code words (which represent pulse levels)  242  from the map table  241 . The pulse transmitter  244  can shape the signal  242 , if necessary, and can serve to digitally filter the signal  242 . The pulse transmitter  244  drives the linear digital signal  247 , via a sampling switch  246  that operates at 8000 hz, to a linear-digital-to-mu converter  252 . 
     The linear-digital-to-mu converter  252  converts the linear digital code words  247  to mu-law code words  248 . The words  248  are passed into the digital network  113  via sampling switch  249 , which operates at 8000 hz in synchronism with the 8000 hz switch  246 . The connection  251  from the switch  249  to the network  113  is typically a T1 or ISDN connection. 
     The 8000 hz signal  247  from the pulse transmitter  244  is also passed to an echo canceler  254 . The echo canceler  254  receives the linear code words from the pulse transmitter  244  as well as the linear digital signal  206  produced by the subtractor  202 . The echo canceler  254  shifts the phase and attenuates the amplitude of the signal  247   b  and correlates it with the signal  206   b  in order to produce an echo canceler signal  204  that can be combined with the incoming receive signal  198  to eliminate echo. The echo canceler  254  is well known in the art. 
     The codec  106 , which is located typically in a telephone company central office, receives the 8000 hz mu-law signal  256  from the digital network  113 . Specifically, the signal  256  is transferred to a mu-to-linear-analog converter  109 . The converter  109  produces a decompressed linear analog signal  258  by converting the mu-law code words to a linear analog signal  258 . 
     A LPF  264  receives the linear analog signal  258  from the converter  109  and filters the signal  258 . The LPF  264  attenuates higher frequencies while permitting pass through of lower frequencies. The cutoff frequency is a function of the configuration of the codec  106  and is typically about 3700 hz. The LPF  264  communicates a filtered analog signal to the hybrid  103 , which places the analog signal on the copper loop  118 . 
     Referring again to FIG. 2B, the hybrid  162  of the analog modem  102  receives the analog signal and passes it to an LPF  276 , which filters the signal. In the preferred embodiment, the cutoff frequency is around 8000 hz. The LPF  276  prevents distortion from being modulated into the system. A sampling switch  279  samples the filter signal  278  at 16,000 hz to produce a signal  282  for an ADC  284 . 
     The ADC  284  converts the analog signal  282  to a digital signal. Preferably, the ADC  284  has a precision of 14-16 bits. The ADC  284  outputs a digital signal  286  to a subtractor  285 . 
     The subtractor  285  combines the digital signal  286  with an echo canceler signal  289  from an echo canceler  292 . The subtractor  285  produces a signal  288  that is substantially free of echo from the local transmit signal. The enhanced signal  288  is passed back to the echo canceler  292  as well as to an equalizer  294 . 
     The equalizer  294  equalizes the channel by eliminating the effects of the LPF  276 , the copper loop  118 , and the LPF  264  (FIG.  2 A). It essentially boosts attenuated frequencies and performs an inverse transfer function. The equalizer  294  receives the linear digital signal  288  and provides an equalized linear digital signal  296  to a sampling switch  298 , which operates at 8000 hz in this example. The switch  298  provides 14-16 bit code words  302  to a pulse decoder  304 . 
     The pulse decoder  304  may be utilized to shape and/or filter the signal  302 , if desired or necessary. The pulse decoder  304  forwards a linear digital signal  306  having 14-16 bit code words to a map table  308 . 
     The map table  308  converts the linear binary code words to particular quantized amplitude levels (voltage levels). In the case when 5, 6, 7, or 8 bit code words are output by the serial-to-parallel converter  236  at the digital modem  101 , the code words are transformed into one of 32, 64, 128, or 256 different quantized levels. In essence, the map table  308  of the analog modem  102  performs the inverse of the map table  241  in the digital modem  101 . 
     A parallel-to-serial converter  312  receives the binary data code words  311  from the map table  308  and converts them to a serial digital data stream  314 , which is passed to the local DTE. As mentioned, use of 5, 6, 7, or 8 bit code words corresponds to 32, 64, 128, 256 pulse levels, respectively, and to data rates of 40, 48, 56, or 64 kb/s, respectively. 
     Many variations and modifications may be made to the preferred embodiment of the invention as described previously without departing from the spirit and scope of the invention. For example, the data rates and switching speeds may be varied. All such modifications and variations are intended to be included herein within the scope of the present invention, as is defined by the following claims. Other examples of possible modifications are set forth hereafter. 
     Fractional bit rates can be employed with the pulse level modulation technique to transfer a number of levels, other than precisely 32, 64, 128, or 256. For instance, a modulus converter as described in U.S. Pat. No. 5,103,227 to Betts, the disclosure of which is incorporated herein by reference, can be situated between the serial-to-parallel converter  236  and the map table  241  of the digital modem  101  and between the map table  308  and the parallel-to-serial converter  312  in the analog modem  102  so that 11 bits are converted into 2 code words (thus, effectively, 5.5 bit words) of 46 levels each. 
     The code levels that are utilized by the pulse level modulation scheme may be determined to optimize data communications. In essence, the system is tested to determine which levels are unresolvable by the analog modem  102 . Moreover, unresolvable levels are discarded and not utilized. In order to check levels, the digital modem  101  transfers levels to the analog modem  102 , which in turn records the received level at the output of the equalizer  294 . The analog modem  102  evaluates the received power levels, makes a determination as to which are resolvable and which are not, constructs a table of usable resolvable levels, and advises the digital modem  101  of the same. 
     Slave timing may also be employed to eliminate the need for the interpolator  146  (FIG.  2 B),  208  (FIG.  2 A). In this regard, the timing of the V.34 transmitter  134  in the analog modem  102  is locked to the timing of the pulse transmitter  244  of the digital modem  101 . In this configuration, the output sample rate of Hilbert filter  141  could be increased directly to the 16000 hz sampling rate. 
     In order to avoid transferring DC and to eliminate high frequencies, the digital modem  101  may be designed to send a vacant bit. This will force a spectral shape to the signal exiting the copper loop  118 . The foregoing concept is described in U.S. Pat. No. 5,394,437 to E. Ayanoglu et al. 
     Finally, in the claims hereafter, the structures, materials, acts, and equivalents of all “means” elements and steps are intended to include any structures, materials, or acts for performing the functions specified in connection with said elements.