Abstract:
An improved data access arrangement device for use in full-duplex two-wire communication systems having a transmission impedance characteristic. The DAA includes a transmit amplifier and a receive amplifier adapted for connection to a two-wire line, a codec connected to the transmit amplifier for generating a transmit signal, a summation device connected to the receive amplifier, an echo cancellation path including a variable gain amplifier, where the path extends between the codec and the summation device, and wherein the variable gain amplifier is adjusted in accordance with the transmission impedance characteristic. The variable gain amplifier may include a gain register that is programmed by a microprocessor, or may have the gain set by user-configurable switches. The summation device is preferably an operational amplifier.

Description:
FIELD OF INVENTION 
     The present invention relates to telecommunications networks. More specifically, it relates to echo cancellation devices used in two-wire line full duplex communication systems, and more particularly to two-wire line modem devices. 
     BACKGROUND OF THE INVENTION 
     Past implementations of hybrids used to accomplish four-wire to two-wire conversion employ a transmission signal cancellation path to duplicate and cancel the portion of the outgoing transmission signal that is erroneously picked up by the receive amplifier. A hybrid used in a typical solid-state Data Access Arrangement (DAA) device will employ a gain stage that serves as the transmission signal cancellation path. 
     Telephone lines present a wide range of characteristic impedances. The characteristic impedance varies with frequency on a particular telephone line, and each two-wire subscriber line, or telephone line, will present a different characteristic. 
     Hybrids in present DAA devices use a fixed gain stage for cancellation that is not responsive to changes in the telephone line impedance. Such a fixed gain stage does not provide frequency dependent impedance matching. The DAA is designed to perform a coarse echo cancellation by providing a hybrid that is not precisely balanced. The prior art DAA devices do not provide for the connection of external impedance matching devices. 
     In view of recent telecommunication techniques that utilize a larger portion of the available bandwidth such as that specified by ITU-T recommendation V.90, it would be advantageous to provide a transmission signal cancellation path in a DAA device that is more flexible and responsive to varying telephone line conditions, and which is able to better model the telephone line impedance to provide better echo cancellation, especially at the band edges. 
     SUMMARY OF THE INVENTION 
     In accordance with preferred embodiments of the present invention, an improved data access arrangement is provided. The DAA provides an external pin for connecting an impedance matching circuit, and includes a variable gain stage to accommodate a variety of line impedance characterisitics. An improved data access arrangement device for use in full-duplex two-wire communication systems having a transmission impedance characteristic. The DAA includes a transmit amplifier and a receive amplifier adapted for connection to a two-wire line, a codec connected to the transmit amplifier for generating a transmit signal, a summation device connected to the receive amplifier, an echo cancellation path including a variable gain amplifier, where the path extends between the codec and the summation device, and wherein the variable gain amplifier is adjusted in accordance with the transmission impedance characteristic. The variable gain amplifier may include a gain register that is programmed by a microprocessor, or may have the gain set by user-configurable switches. The echo cancellation path also may include a frequency dependent impedance circuit, and more specifically, a bandpass filter. The impedance circuit may also be an active filter element, such as an operational amplifier. The filter is preferably external to the DAA integrated circuit so as to allow flexibility in the choice of filter parameters. The summation device is preferably an operational amplifier. 
    
    
     The foregoing and other features and advantages of a preferred embodiment of the present invention will be more readily apparent from the following detailed description, which proceeds with references to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention are described with reference to the following drawings, wherein: 
     FIG. 1 is a block diagram illustrating a preferred embodiment of the improved hybrid; 
     FIG. 2 is a circuit level diagram illustrating a preferred embodiment of the hybrid of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a block diagram illustrating an exemplary DAA and hybrid conversion system  10 , including a Host  12 , a Controller  14 , a Digital Signal Processor (DSP)  16 , a Codec  18 , and a Hybrid  20  interfacing to a telephone line  22  via Regulatory Circuitry  24 . The Host  12  transmits data to and receives data from the DSP  16  via the Controller  14 . The Codec  18  includes a Digital to Analog Converter (DAC)  30  used to convert digital samples, transmitted by the Host  12  and processed by the DSP  16 , to an analog transmission signal (TX) for transmission to the telephone line  22  via the Hybrid  20 . The Codec  18  further includes an Analog to Digital Converter (ADC)  32  used to convert an analog receive signal (RX), received from the telephone line  22  via the Hybrid  20 , to digital samples to be processed by the DSP  16 . The DSP  16  is also used to set the gain level of the codec  18  by controlling the contents of registers within the codec. The registers then determine the amplifier gain using techniques well known in the art. 
     The Hybrid  20  includes a Transmit Amplifier  40 , a Termination Impedance  50 , a Low Pass Filter (LPF)  60 , a DC Blocking Filter (DCBF)  70 , a Receive Amplifier  80 , a Summer  90 , a variable Gain Stage  100 , and a Band Pass Filter (BPF)  110 . The Hybrid  20  receives the analog transmission signal TX at node  34  from the DAC  30 . The transmission signal TX at node  34  is routed along a cancellation path and a transmission path to the telephone line  22 . The transmission signal along the transmission path is amplified by Transmit Amplifier  40  and passes through a Termination Impedance  50 . Preferably, the Termination Impedance  50  is preferably chosen to “match” the expected value of the characteristic line impedance (not shown) of the telephone line  22 . The transmission signal faces a “voltage divider” of sorts in the Termination Impedance  50  and the telephone line  22  impedance. 
     The outgoing transmission signal is filtered by the LPF  60  and is passed out of the Hybrid  20  to the telephone line  22  via the Regulatory Circuitry  24 . As is known by those of skill in the art, the Regulatory Circuitry  24  will typically include diode bridge rectifier circuit, ferrite beads, sidactor, and capacitor arrangements to provide isolation and overvoltage protection. 
     Conversely, the Hybrid  20  of FIG. 1 receives the incoming signal from the telephone line  22  via the Regulatory Circuitry  24 . A portion of the outgoing transmission signal is undesirably fed back along the receive path, forming and echo signal, and combines with the receive signal that comes from the LPF  60 . The echo signal, which is that portion of the outgoing transmission signal that is fed back, will vary in accordance with the voltage divided formed by the termination impedance  50  and the line impedance (not shown) of the telephone line  22 . The line characterisitics that shape the echo signal is generally referred to as the echo path or echo channel. The echo channel includes effects of the line impedance and the low pass filter  60 . The desired receive signal from the distant end together with the undesired echo pass through the DCBF  70  and next are amplified by the Receive Amplifier  80 . 
     As noted above, the analog transmission signal TX as received by the Hybrid  20  is directed at node  34  along the cancellation path and the transmission path to the telephone line  22 . The transmission signal cancellation path provides an echo cancellation signal at the Summer  90  in order to cancel the contribution to the receive signal that is due to the undesired feedback portion of the outgoing transmission signal. In the exemplary preferred embodiment shown in FIG. 1, The BPF  110  is cascaded with the variable Gain Stage  100  to form the transmission signal cancellation path. Preferably, the echo cancellation signal is shaped by BPF  110  and the Gain Stage  100  having gain G. The echo cancellation path also may include another type of frequency dependent impedance circuit such as a multiple pole-zero filter with the poles and zeros located at the desired frequencies. The impedance circuit may also include active filter elements, such as operational amplifiers. The filter is preferably external to the DAA integrated circuit so as to allow flexibility in the choice of filter parameters and circuit components. 
     Preferably, the value of the gain G is capable of being varied or changed and is not fixed. In certain preferred embodiments of the invention, the value of the feed back gain G is varied according to telephone line  22  conditions. The characteristic impedance of subscriber lines varies from country to country, thus a variable gain G allows the DAA to adapt its echo cancellation for the given environment. The range of gains may be set from −0.3 to −0.9, and more preferably, is set to one of four values: −0.50, −0.55, −0.60 or −0.65. It should be understood that in alternative embodiments of the DAA the feed back gain G can be fixed and non-variable. A typical fixed value for the gain G is −0.5, or −0.8. The gain may be set by a number of available techniques well known to designers in the electrical arts. For example, the gain may be set by user-controlled switches, such as DIP switches. The user, having knowledge of the typical line impedance in his telephone system, may refer to a chart and set the switches accordingly. Alternatively, the gain may be set by the user through software in a similar manner, resulting in the DSP setting amplifier gain registers, in a manner similar to the gain registers used in standard codec devices. The gain setting may also be done automatically by performing voltage measurements of a test signal passed through the DAA. As stated above, the DAA impedances together with the line impedance for a voltage divider. Hence, the line impedance may be determined by measuring a test signal placed on the line by the DAA. The DSP can then set the gain switches or registers as required. 
     Preferably, as described above, the Gain Stage  100  operates in conjunction with the BPF  110  to form the transmission signal cancellation path. In a preferred embodiment, the feed back gain G is −0.6. It should be understood that numerous implementations of the transmission signal cancellation path are contemplated within this disclosure including a Gain Stage  100  with a fixed feed back gain G cascaded with a BPF  110 ; a Gain Stage  100  with a variable feed back gain G cascaded with a BPF  110 ; a Gain Stage  100  with a variable feed back gain G with no cascaded filter; etc. 
     Finally, after the combined receive and transmission reflection signal is amplified by the Receive Amplifier  80 , the signal is summed at the Summer  90  with the transmission signal TX that has been filtered by the BPF  110  and multiplied by the feed back gain G of Gain Stage  100 , to yield the analog receive signal RX. The Gain Stage  100  with feed back gain G of FIG. 1 spans from node N 2  to node N 3,  as shown in FIG.  2 . The gain stage is preferably implemented as an operational amplifier. To adjust the gain, a programmable register is used to switch in different gain settings. The transmission cancellation path of FIG. 1 terminates in the Summer  90 . Summer  90  is also preferably implemented as an operational amplifier having the inputs summed prior to amplification. 
     An exemplary embodiment of the present invention is presented in FIG.  2 . FIG. 2 is a circuit diagram illustrating a preferred implementation,of the Hybrid  20  of FIG.  1 . The functional blocks of FIG. 1 are represented by components in FIG.  2 . 
     An exemplary Data Access Arrangement (DAA) integrated circuit chip  120  is represented in FIG. 2 by dotted lines. The DAA chip  120  includes four pins TX (transmission signal input at node N 1 )  91 , TXFB (transmission signal feed back at node N 2 )  92 , RXP (receive channel input from telephone line at node N 4 )  94 , DCT (DC termination at node N 5 )  95 , as well as a pin connected to signal ground (GND) at node N GND    97 . The DAA  120  includes codec  18  (not shown) within a single integrated circuit package. The TXFB pin allows for the connection of an external impedance circuit, specifically, a bandpass filter in the example of the preferred embodiment. Prior art devices do not provide external access to the feedback path, and hence are not flexible in their approach to echo cancellation. 
     The transmit data enters the DAA  120  by way of a digital data bus. The Codec  18  transforms the digital data to an analog signal, which is shown as voltage source V 1    125 . The transmit signal leaves the DAA  120  at pin TX of the DAA chip  120 . The resistor R 3    83  is shown to model the input resistance of the codec, and is equivalent to 100 MΩ. The resistor R 5    85  is used to model the input resistance to the variable gain amplifier, and is preferrably equal to 70 kΩ. 
     After amplification by the Transmit Amplifier  40  of FIG. 1, the transmission signal passes through the Termination Impedance  50 . The Termination Impedance  50  of FIG. 1 spans from node N 1    91  to node N 6    96  in FIG.  2  and includes resistor R 6    86 . Preferably, the resistor R 6  is chosen to match the characteristic impedance of the telephone line. In a preferred embodiment of the present invention, the resistor R 6  is equivalent to 600Ω. The Low Pass Filter (LPF)  60  of FIG. 1 is connected at node N 6    96  in FIG.  2  and includes capacitor C 2    72 . In a preferred embodiment, capacitor C 2    72  is equivalent to 0.033 μF. Capacitor C 3    73  provides filtering of high frequency noise components. The DCBF  70  of FIG. 1 spans from node N 6    96  to node N 4    94  in FIG.  2  and includes capacitor C 1    71 . Preferably capacitor C 1    71  is equivalent to 1 μF. Inductor L 1   61  is a gyrator that provides the necessary current voltage characteristic of the transmission line. It may be implemented with operational amplifiers and capacitive elements (not shown). Resistor R 2    82  represents the input impedance of the summer  90  and is equivalent to 70 kΩ. Resistor R 1    81  is an external power resistor that provides a resistive component of the DC termination. 
     The telephone line  22 , shown in FIGS. 1 and 2, has an associated characteristic line impedance. This line impedance is modeled in FIG. 2 as Z LINE    23 . As is known to those in the art, the line impedance associated with a telephone line can vary as a function of frequency and the size and geometry of the line. It should be understood that although the line impedance Z LINE    23  can be represented by a resistor of 600Ω, this representation may serve as an estimate so that attempts can be made to match the load of the telephone line. 
     As described above, the transmission cancellation path preferably includes the Band Pass Filter (BPF)  110  and the Gain Stage  100  with feed back gain G. The Band Pass Filter (BPF)  110  of FIG. 1 spans from node N 1    91  to node N 2    92  in FIG.  2  and includes capacitors C 4    74  and C 5    75  and resistor R 4    84 . In a preferred embodiment, the capacitors C 4    74  and C 5    75  are equivalent to 0.12 uF and 0.018 uF, respectively, while the resistor R 4    84  is equivalent to 649Ω. The BPF  110  serves to shape the echo cancellation signal to conform more closely to the frequency characteristics of the echo channel. This is done in part to counteract the effects of the low pass filter  60  and the DCBF  70  within the echo path. Without the BPF  110 , the echo would have incomplete cancellation, particularly near the band edges. 
     An operating environment for the improved DAA disclosed herein includes a processing system with at least one high speed Central Processing Unit (“CPU”), such as the DSP  16  and the host  12 , and a memory system. In accordance with the practices of persons skilled in the art of computer programming, the DAA is described with reference to acts and symbolic representations of operations or instructions that are performed by the processing system, unless indicated otherwise. Such acts and operations or instructions are sometimes referred to as being “computer-executed”, or “CPU executed.” 
     It will be appreciated that the acts and symbolically represented operations or instructions include the manipulation of electrical signals by the CPU. An electrical system with data bits causes a resulting transformation or reduction of the electrical signal representation, and the maintenance of data bits at memory locations in the memory system to thereby reconfigure or otherwise alter the CPU&#39;s operation, as well as other processing of signals. The memory locations where data bits are maintained are physical locations that have particular electrical, magnetic, optical, or organic properties corresponding to the data bits. 
     The data bits may also be maintained on a computer readable medium including magnetic disks, optical disks, organic disks, and any other volatile or non-volatile mass storage system readable by the CPU. The computer readable medium includes cooperating or interconnected computer readable media, which exist exclusively on the processing system or is distributed among multiple interconnected processing systems that may be local or remote to the processing system. 
     It should be understood that the programs, processes, methods, systems and apparatus described herein are not related or limited to any particular type of computer apparatus (hardware or software), unless indicated otherwise. Various types of general purpose or specialized computer apparatus may be used with or perform operations in accordance with the teachings described herein. 
     In view of the wide variety of embodiments to which the principles of the invention can be applied, it should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the present invention. For example, the Steps of the flow diagrams may be taken in sequences other than those described, and more or fewer elements or components may be used in the block diagrams. In addition, the present invention can be practiced with software, hardware, or a combination thereof. 
     The claims should not be read as limited to the described order of elements unless stated to that effect. In addition, use of the term “means” in any claim is intended to invoke 35 U.S.C. §112, paragraph 6, and any claim without the word “means” is not so intended. Therefore, all embodiments that come within the scope and spirit of the following claims and equivalents thereto are claimed as the invention.