Abstract:
The present invention discloses communications devices which improves communications of pulse code modulation modems which use communications lines with multiple digital-to-analog conversions. One embodiment of the present invention includes a plurality of codecs ( 312 ) which receive analog signals through multiple communications channels ( 310 ). Digital data streams generated by the codecs ( 312 ) from the analog signals is transmitted to an associated one of a plurality of digital signal processors ( 214 ). The digital signal processors ( 214 ) process the decoded data and then transfers the data to a digital modem ( 216 ) for further transmission.

Description:
This application claims priority under 35 USC § 119(c)(1) of provisional application Ser. No. 60/123,881 filed Mar. 11, 1999. 

   TECHNICAL FIELD OF THE INVENTION 
   This invention relates generally to telecommunication and more particularly to pulse code modulation (“PCM”) modems using communication lines with multiple digital-to-analog converters. 
   BACKGROUND OF THE INVENTION 
   An exemplary prior art telephone system is shown in FIG.  1 . The telephone system  100  includes a V.PCM digital modem  110  which is connected to a digital channel  112  that terminates at central office  114 . The V.PCM digital modem, also referred to as a V.90 modem, is described in the International Telecommunications Union, Telecommunication Standardization Sector (ITU-T), Recommendation V.90 (1998), herein incorporated by reference in it&#39;s entirety. The V.PCM digital modem  110  transmits PCM octets through the digital channel  112  to the central office  114 . At the central office  114  the bytes are processed by a convert-and-filter device  116  which includes a μ-law (or A-law) digital-to-analog converter (“DAC”) followed by an analog PCM filter. The output of the convert-and-filter device  116  is then transmitted to a subscriber via an analog channel  118  which consists, for example, of two copper wires which typically are referred to as a twisted pair. 
   In some telephone systems the analog channel  118  consists of multiple ones of these twisted pairs, each twisted pair referred to as a “line”, which are then in turn connected to a unit (“EU”)  120  which is located outside the central office  114 . The unit  120  includes a conversion device  122  which includes an analog front end (“AFE”) and a μ law (or A-law ) analog-to-digital converter (“ADC”). In the conversion device  122 , once each line is processed by the μ law (or A-law) ADC, an output bit stream of distorted PCM octets  124  is sent to a second unit  126  (called “RU”) via a digital modem (e.g., a digital subscriber line (“DSL”) modem), not shown. At the RU unit  126  the bits are processed by conversion device  128  which includes another μ-law (or A-law) DAC and a PCM filter. The resultant analog signal is sent to a subscriber via analog channel  130  and a V.PCM analog modem  132 . 
   For many years the public digital telephone network (DTN) has been used for data transmission between modems. Typically, a modulated carrier is sent over a local loop to a service provider (e.g., a Regional Bell Operating Company), whereupon the service provider quantizes the signal for transmission through the DTN. A service provider that is located near the receiving location converts the digital signal back to an analog signal for transmission over a local loop to the receiving modem. This system is limited in the maximum achievable data rate at least in part by the sampling rate of the quantizers, which is typically 8 kHz (which rate is also the corresponding channel transmission rate, or clock rate, of the DTN). 
   Furthermore, the analog-to-digital (A/D) and digital-to-analog (D/A) conversions are typically performed in accordance with a non-linear quantizing rule. In North America, this conversion rule is known as mu.-law. A similar non-linear sampling technique known as A-law is used in certain areas of the world such as Europe. The non-linear A/D and D/A conversion is generally performed by a coder/decoder (“codec”) device located at the interfaces between the DTN and local loops. Alternatively, these devices are referred to herein as a DAC and an ADC. 
   It has been recognized that a data distribution system using the public telephone network can overcome certain aspects of the aforesaid limitations by providing a digital data source connected directly to the DTN, without an intervening codec. In such a system, the telephone network routes digital signals from the data source to a client&#39;s local subscriber loop without any intermediary analog facilities, such that the only analog portion of the link from the data source to the client is the client&#39;s local loop (plus the associated analog electronics at both ends of the loop). The only codec in the transmission path is the one at the DTN end of the client&#39;s subscriber loop. 
   Because of the existence of two DACs in the exemplary telephone system  100 , those lines effectively cannot carry 56K signals. Thus, subscribers who use 56K modems are generally unable to use these lines. 
   SUMMARY OF THE INVENTION 
   The present invention discloses improved operation of pulse code modulation modems using communications channels with multiple digital-to-analog converters. 
   These and other features of the invention that will be apparent to those skilled in the art from the following detailed description of the invention, taken together with the accompanying drawings. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram illustrating an exemplary prior art telephone system; 
       FIG. 2  shows a first embodiment of an EU unit in accordance with the present invention; 
       FIG. 3  is a block diagram of a second embodiment of an EU unit in accordance the present invention; and 
       FIG. 4  illustrates a third embodiment of an EU unit in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  shows a block diagram illustrating a first embodiment of an EU unit  200  in accordance with the present invention. The EU unit  200  receives as input signals from two or more analog lines  210 . The input signals are processed by a coder/decoder (“codec”)  212  associated with each of the analog lines  210 . The outputs from multiple ones of the codecs  212  are processed by one of multiple digital signal processors (“DSPs”)  214  included in the EU unit  200 . Each of the codecs  212  in the EU unit  200  shown in  FIG. 2  also include a linear ADC, not shown, which digitizes the signal before being output from the codec  212 . The output of the ADC is then transmitted to one of the multiple DSPs  214  for further processing, discussed in more detail hereinbelow. After processing by the DSP  214 , the resultant signal is transferred to a digital subscriber line (“DSL”) modem  216  which transmits its output signal to an RU unit, not shown, via line  220 . The embodiment of the EU unit  200  shown in  FIG. 2  requires no change in the RU unit such as that shown in the prior art telephone system  100  in FIG.  1 . 
   The DSP  214  shown in  FIG. 2  executes in accordance with software instructions to implement a feed-forward equalizer. In a typical PCM modem call, only a subset of the available PCM levels are used. Therefore a conventional modem can detect with high confidence which PCM level was sent and is thus capable of implementing a decision-directed-equalizer (“DFE”). The DSP  214  shown in  FIG. 2 , however, is unable to detect which levels are used, and thus the probability of symbol error is too high to implement a DFE and thus a DFE should not be used. Usually there is a lot of attenuation at DC and at 4 Khz. When the DFE is not in use the FFE should be very long, i.e., 200 filter taps. During data-mode operation of the modem, a least mean square (“LMS”) algorithm is used to adapt the equalizer&#39;s taps only when the detected symbol has a magnitude equal to or greater than a predetermined threshold. This is because in μ law (or A law) the difference between adjacent levels increases as the magnitude increase. Therefore, the probability of error with relatively large symbols is much smaller than the probability of error in relatively small symbols. 
   Equalizer training options in the EU unit  200  shown in  FIG. 2  are as follows. The central office sends a pre-defined training signal to the EU unit  200  over the analog line  210  a short time before the beginning of the V.PCM modem start-up procedure. Since there is no practical method of determining in advance whether the call is a V.PCM call, in a telephone system which implements the embodiment of the EU unit  200  shown in  FIG. 2 , the central office sends the pre-defined training signal before every call. The length of the training signal should range from 0.5 to 1 seconds. The training signal can be sent at the beginning of the call (with the disadvantage of being heard by the user) or a short time prior to the call (i.e., 1 minute). Alternatively, signals that would otherwise be used for other functions, such as a Caller ID signal, may be used. 
   When installing the EU unit  200 , a special training signal is sent to the EU unit  200 . This can be done, for example, by dialing up the EU unit  200  from a remote modem, not shown, which is digitally connected to the telephone network. The EU unit  200  then waits for the training signal of phase  3  and trains according to that training signal. This requires (1) making sure it is a 56K call and not a V.34 call or voice call; and (2) re-generating the line-probing signal (the frequencies comb). 
   A second embodiment of the EU unit  300  in accordance with the present invention is shown in FIG.  3 . The embodiment of the EU unit  300  shown in  FIG. 3  is a higher cost solution relative to the embodiment of the EU unit  200  shown in  FIG. 2  which also enables a subscriber to use a PCM modem at high speed. The EU unit  300  shown in  FIG. 3  receives as input analog signals on analog lines  310 . A codec  312  is associated with each of the analog lines  310  and decodes the received analog signal. The decoded signal from each of the codecs  312  is then sent to an associated analog PCM modem  314 . Each of the analog PCM modems  314  are bidirectionally connected to an associated digital PCM modem  320 . 
   In the EU unit  300  shown in  FIG. 3 , each of the digital PCM modems  320  communicates with a subscriber modem, not shown, as if it was the far end digital modem and each of the analog PCM modems  314  communicates with the far end digital modem as if he was the subscriber. The two PCM modems  314  and  320  operate “back to back” and bidirectionally transfer bit streams to each other. Additional control is required between the analog PCM modem  314  and the digital PCM modem  320  in that the two modems may connect at a different data rate. For example, the PCM modem receiver in the EU unit  300  may connect at 48 Kbps while the subscriber&#39;s PCM modem receiver connects at 49 Kbps. In this case, a controller unit, not shown, generates additional, non-informative bits, such as a long stream of ones in the case of V.34 connection. The V.34 modem is described in the International Telecommunications Union, Telecommunications Standardization Sector (ITU) Recommendation V.34 (1998) herein incorporated by reference in it&#39;s entirety. 
     FIG. 4  shows a third embodiment of an EU unit  400  in accordance with the present invention. The EU unit  400  is typically a medium cost (relative to the cost of implementing the EU unit  200  shown in  FIG. 2  or the EU unit  300  shown in  FIG. 3 ) solution that enables a subscriber to use a PCM modem at high speed. The EU unit  400  shown in  FIG. 4  includes a codec  412  which accepts as input analog signals received through analog line  410 . The decoded signal from the codec  412  is then input to DSP  418  which is bidirectionally connected to a DSL modem  446 . The DSP  418  implements an analog PCM receiver  416 . The output signal from the analog PCM receiver  416  is connected to switch  434  for output to the DSL modem  446 . The analog PCM receiver  416  is also operable to generate line condition information which is output on line  426  to a signal generator  430 . The output of the signal generator  430  is also connected to the DSL modem  446  through switch  434 . The signal generator  430 , is also bidirectionally connected to a state machine  422  through line  428 . The state machine  422  is in turn bidirectionally connected to the analog PCM receiver through line  424 . In addition, the state machine  422  is bidirectionally connected to a v.34 start-up receiver  436 . The v.34 start-up receiver  436  accepts as input signals from the DSL modem  446  through line  442  and generates parameters which are transmitted to the analog PCM receiver  416  through line  440 . The received signals from the DSL modem  446  are also transmitted to the codec  412  and to the analog PCM receiver  416  through line  442 . 
   The analog PCM receiver  416  (which is contemplated to operate at between 8 and 12 million instructions per second) and the V.34 start-up receiver  436  are contemplated to operate only during start up. The idea generally is that the V.34 start-up receiver  436  listens to the information passed from the analog modem to the digital modem, and using that information determines the parameters characterizing the signal transmission. These parameters are later used in downstream transmission (from the digital modem to the analog modem). These parameters enhance the performance of the analog PCM receiver  416  and enable the analog PCM receiver  416  to better handle a null at DC. The reason for that is that the EU unit  400  is operable to determine which PCM levels are used and which are not and thus is operable to decode the symbols with a relatively low symbol error rate and therefore a DFE could be used. 
   The above mentioned parameters generated by the v.34 start-up receiver  442  are used in phase  3  and phase  4  of modem initialization and in data mode. Thus, the performance of the analog PCM receiver  416  is enhanced when operating during phase  3  and phase  4  of modem initialization and during data mode. The determination, however, of whether to proceed with a V.PCM call or to fall back to V.34 modem operation is made by the analog modem, during phase  2 . Thus, there is a risk that the analog modem will decide to fall back to V.34 because the analog PCM receiver  416  is not operating at peak performance levels. To overcome this problem the signal generator  430  in included to generate clean signals during modem initialization phase  2  and in particular the line probing signal. 
   Other Embodiments 
   Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.