KR20010019438A - Adaptive echo cancel apparatus with programmable analog filter - Google Patents

Abstract

PURPOSE: An adaptive echo cancel apparatus using a characteristic programmable low pass filter is provided to appropriately cope with the change of the characteristic of a line and promote the efficiency of a radio communication system, thereby reducing the manufacturing cost of a digital circuit. CONSTITUTION: The first operation part(131) synthesizes a frequency characteristic signal of an echo path to a transmitting signal and the transmitting signal of the other side sensed with characteristic of a line. A filter part(81) filters the transmitting signal by the transmitting signal and a variable filter control signal for outputting an echo cancel signal. The second operation part(132) synthesized the synthesized signal of the first operation part(131) and the echo cancel signal of the filter part(81). A filter control part(82) filters the transmitting signal and the filter signal with high frequency according to the output signal of the second operation part(132) for controlling the filter part(81).

Description

Adaptive echo cancel apparatus with programmable analog filter

The present invention relates to an echo cancel device of a wired communication system in which transmission and reception in both directions are simultaneously performed through a pair of lines. More particularly, the present invention relates to minimizing the amount of echo in a received signal. The present invention relates to an adaptive echo canceller using a characteristic variable analog filter capable of actively responding to changes in a transmission / reception line.

In a system in which two-way communication of signals is performed simultaneously, such as an asymmetric digital subscriber line (ADSL), in the case of frequency overlap multiplexing (FOM), an echo signal generated by a transmission signal is processed by an echo canceller. In case of ADSL DMT (Descrete Multi-Tone), the maximum signal loss to recognize the received signal is 45dB. In order to accurately receive the relative signal, there must be a difference of at least 20dB between the echo signal and the received signal. Therefore, in the case of ADSL, a echo canceller having an echo return loss of 65dB or more is required to prevent the transmission / reception efficiency degradation due to the echo. Digital echo cancellers are commonly used due to the nonlinearity of A / D and D / A converters.

FIG. 1 illustrates an input impedance Z _i to the transformer 10 as Z _B 11-1 to generate an echo signal as a signal amplified by the transmission signal amplifier 12 and to subtract the received signal to receive signal amplifier 13. The echo is amplified by) and the echo is removed. The output signal of the transformer 11 is transmitted to the termination resistor (R _t- 100 kV) 15 via the telephone line 14.

2 is a characteristic transfer function C (s) 11-2 using an active filter instead of the impedance Z _B 11-1 of FIG. 1 to remove the echo by implementing the characteristic transfer function H (s) of the echo generation path. Structure.

Unlike FIG. 1 and FIG. 2, FIG. 3 is an echo controller using a digital filter and a Least Mean Sqaure (LMS) algorithm. The transmission signal is converted into an analog signal through the D / A converter 30, which is transmitted to the receiving end of the other side through the hybrid transformer 31 and the telephone line. The transmission signal on the other side and the echo signal through the echo generation path are synthesized, introduced into the reception path, and converted into a digital signal via the A / D converter 32. The digital filter controller 33 controls the characteristics of the digital filter so as to minimize reflection, and the output of the adaptive digital filter 34 as the optimized digital filter removes the echo signal from the received signal at the calculator 35 and outputs it. .

1 and 2, the conventional analog hybrid circuit implements the input impedance Z _i or the characteristic transfer function H (s) as a passive or active element for echo cancellation. In general, due to the diversity of distance and installation structure, the telephone line, which is a signal transmission medium, has a problem in that the input impedance Z _i or the characteristic transfer function H (s) is not constant and the characteristic change is severe.

4 is an example of a telephone line (ANSI line) generally used to measure the performance of a device in a digital subscriber line (Digital Subscriber Line). Here, the line from the central office (CO) to the subscriber remote terminal (RT) is indicated in feet / line (AWG).

FIG. 5 shows that the input impedance Z _i and the characteristic transfer function H (s) in FIGS. 1 and 2 are obtained with respect to various lines of FIG.

As described above, the analog filter having the fixed characteristics of FIGS. 1 and 2 has a problem in that it is difficult to cope with various characteristic changes according to the line change and in each case, an integer design of a different circuit is required. In the case of using the digital adaptive filter of Fig. 3, the echo can be optimally removed by appropriately and effectively responding to the above line environment. However, in order to implement a high performance digital eliminator, it is necessary to increase the number of bits of a D / A converter and the number of taps of a digital filter, thereby increasing the cost of implementation. In addition, due to nonlinearities in the D / A converter and the analog amplifier stage, it is difficult to implement echo cancellation performance (65 dB in the case of ADSL) which is required only by the digital echo canceller.

The technical problem to be achieved by the present invention is a variable frequency characteristic variable analog filter and a signal error-least mean square (SE-LMS) algorithm implementation circuit in a digital wired communication system by configuring an adaptive analog echo canceller to effectively remove the characteristic variable echo An adaptive echo canceller using an analog filter is provided.

1 and 2 are circuit diagrams of an echo canceller using a conventional analog hybrid circuit.

3 is a block diagram showing the configuration of an adaptive echo canceller for canceling echo in a conventional digital signal processing scheme.

4 is a diagram illustrating an example of a telephone line that is generally used to measure the performance of a device in a digital subscriber network line.

5 is a frequency characteristic diagram of the input impedance Z _i and the characteristic transfer function H (s) of FIGS. 1 and 2 for various telephone lines of FIG. 4.

6 is a diagram for explaining a general adaptive algorithm.

FIG. 7 illustrates four types of the adaptive algorithm of FIG. 6.

8 is a block diagram showing the configuration of an adaptive echo cancellation device using a characteristic variable analog filter according to the present invention.

9 to 11 are diagrams for describing the variable characteristic analog filter of FIG. 8.

FIG. 12 is a detailed diagram of an analog filter controller using a sine error least squares algorithm of FIG. 7.

FIG. 13 is a detailed view of FIG. 8.

An adaptive echo canceller using a characteristic variable analog filter for solving the technical problem to be solved by the present invention comprises a first calculation unit for synthesizing a frequency characteristic signal of a reverberation path with respect to a transmission signal and a relative transmission signal in which a characteristic of a line is detected. ; A filter for outputting an echo cancellation signal by filtering the transmission signal according to the transmission signal and the variable filter control signal; A second calculator configured to synthesize the synthesized signal of the first calculator and the echo cancellation signal of the filter; And a filter controller for outputting a variable filter control signal for controlling the filter by high frequency filtering the transmission signal and the filter signal according to the output signal of the second calculator.

Hereinafter, with reference to the accompanying drawings will be described in detail the present invention.

6 is a diagram for explaining a general adaptive algorithm.

FIG. 7 illustrates four types of the adaptive algorithm of FIG. 6.

FIG. 7 (a) shows an analog filter controller using an analog least square method (hereinafter referred to as LMS), FIG. 7 (b) shows an analog filter controller using a sign data LMS, and FIG. 7 (c). Is an analog filter controller using a sign error LMS, and FIG. 7 (d) is a diagram showing an analog filter controller using a sign sign LMS.

8 is a block diagram showing the configuration of an adaptive echo cancellation device using a characteristic variable analog filter according to the present invention.

The apparatus shown in FIG. 8 is a variable characteristic analog filter 81 capable of varying frequency characteristics with a transmission signal amplifier 80 for amplifying a transmission signal V _T (t), an output of the transmission signal amplifier 80, and a filter control signal. From the analog filter controller 82, the transformer 83, the telephone line 84, and the terminating resistor 85, which outputs a filter control signal for optimizing the characteristics of the variable characteristic analog filter 81 by sensing the characteristic of the line. The echo component of the transmitted transmission signal V _L (t) and the echo signal e _H (t) from the transmit signal amplifier 80 and the echo correction signal e _c (t) of the variable characteristic analog filter 81 are synthesized to remove the echo component. And a received signal amplifier 87 for amplifying by amplification.

9 to 11 are diagrams for describing the variable characteristic analog filter of FIG. 8.

FIG. 12 is a detailed diagram of an analog filter controller using a sine error least squares algorithm of FIG. 7.

The apparatus shown in FIG. 12 includes a frequency characteristic variable analog filter C (s) 120, an echo path frequency characteristic function H (s) 121, an adder 122, a comparator 123, a switching unit 124, and a high frequency. Attenuation filter 125.

FIG. 13 is a detailed view of FIG. 8.

The apparatus shown in FIG. 13 outputs the output signal e _H (t) of the H (s) 130 and the H (s) 130 and the counterpart transmission signal V _L (t) representing the frequency characteristics of the echo path of the transmission signal. A first adder 131 to be added, a second adder 132 to add the output signal of the first adder 131 and the echo correction signal e _c (t), a plurality of analog filters 81-1, 2, 4, 6) a variable characteristic analog filter 81 and a filter 82-1 composed of a plurality of multipliers 81-3, 5, 7 and an adder 81-8 to output an echo correction signal e _c (t); , Analog comparator 82-2, switching unit 82-3, and high frequency attenuation filters 82-4, 5, 6 to output a filter control signal for optimizing the characteristics of the variable characteristic analog filter 81 It consists of a filter controller 82.

Next, the present invention will be described in detail with reference to FIGS. 6 to 13.

The actual transmission line is composed of various structures, and accordingly, the frequency characteristics of the input impedance or echo return path of the telephone line also change. Therefore, in order to maximize echo, it is necessary to optimize the circuit parameters for each telephone line. However, the conventional echo cancellation apparatus using a fixed integer value is difficult to cope with this.

FIG. 6 is a structure of a general adaptive system, and when applied to this, it is possible to optimize an efficient echo attenuation for a characteristic change of a telephone line. 6 adds 61 weighting parameters w _i (t) for each input signal and adds the target response signal 62 again to output an error signal e (t). Here, the weight integer w _i (t) can be defined as follows by the basic continuous-time LMS algorithm.

Where ρ is the convergence constant.

Considering the simplicity of the implementation of the actual adaptive system, analog adaptive algorithms are commonly used LMS, SD-LMS, SE-LMS, SS-LMS.

FIG. 7A illustrates a multiplication operation after comparing the levels of the first comparator 70-1 and the second comparator 70-2 using an error signal of Equation 1 as a filter controller using an LMS algorithm. -3), integral 70-4 and amplification 70-5 output w _i (t) which is a weighted constant.

FIG. 7 (b) is a filter controller based on the SD-LMS algorithm, and multiplies (71-2) the integral output value of the comparator 71-1 and the error signal of the error signal using the error signal of Equation 1, and integrates (71). After amplification (71-4), w _i (t), which is a weight integer, is output.

FIG. 7 (c) is a filter controller based on the SE-LMS algorithm, and multiplies (72-2) the output of the comparator 72-1 and the error signal using the error signal of Equation 1, and integrates 72-3. After amplification 72-4, a weight integer w _i (t) is output.

FIG. 7 (d) is a filter controller based on the SS-LMS algorithm, and multiplies (73-1), integrates (73-2), and amplifies (73-3) the error signal of Equation 1 with the partial derivative of the error signal. When the weight integer w _i (t) is output.

Among the SD-LMS and SS-LMS algorithms, there is a risk of divergence, and the SE-LMS algorithm has a certain amount of error in convergence. The weight constant w _i (t) by the adaptive algorithm is implemented as follows.

By using the adaptive echo canceller having the structure as shown in FIG. 8, optimization according to the telephone line can be efficiently performed.

The transmission signal amplifier 80 amplifies the transmission signal V _T (t). The variable characteristic analog filter 81 varies the frequency characteristic with the output of the transmission signal amplifier 80 and the filter control signal. The analog filter controller 82 detects the characteristics of the output and the line of the reception signal amplifier 87 and outputs a filter control signal for optimizing the characteristics of the variable characteristic analog filter 81. The relative transmission signal V _L (t) transmitted from the transformer 83, the telephone line 84, and the termination resistor 85 is an echo signal e _H (t) which is an output signal of the transmission signal amplifier 80 passing through the resistor R _pt . Mixed with). The echo correction signal e _c (t) which is an output signal of the variable characteristic analog filter 81 is input to the reception signal amplifier 87 together with the mixed V _L (t) and e _H (t) to remove the echo component. Output V _R (t).

In the electronic hybrid structure shown in FIG. 8, an echo canceller (analog filter controller 82 and a variable characteristic analog filter) are used to convert a mixed signal of a transmission signal V _L (t) and an echo signal e _H (t) into an echo hybrid. The echo component in the received signal V _R (t) is removed by correcting with e _c (t) through 81).

If the two transmission signals V _T (t) and V _R (t) are Uncorrelated Stationary Random Processes, the mean square value Q may be defined as follows.

Minimizing echo in Equation 4 is the mean square value of V _R (t) Same as minimizing (DC power supply). In order to implement this adaptive analog system, a design of the frequency characteristic variable analog filter 81 and a filter controller 82 for optimizing the same is required.

9 to 11 illustrate the variable characteristic analog filter of FIG. 8, and FIG. 9 includes an adder 90, a characteristic function A (s) 91, and a characteristic function B (s) 92. In the same basic feedback system, the transfer function for input / output signals V _I and V _O is defined as follows.

10 is applied to the adder 100, the integrator -k / s 101, the multipliers (102, 103) using the OP amplifier, each signal and the transfer function has the following relationship.

In the above equation, the external control signals k _r and k _{p allow} the amount of gain, zero, and pole to be varied so that the characteristics of the filter can be adjusted.

FIG. 11 may be implemented using a variable gain amplifier 111 instead of the multiplier 113 for pole varying as an example of a commonly used programmable low pass filter. In this case, divergence due to the position of the right half plane of the pole can be prevented.

In the present invention, the adaptive echo canceller is implemented using the SE-LMS algorithm having the least influence of the offset among the analog adaptive algorithms.

Implementing the frequency characteristic equation of Figure 8 is as follows.

If both sides of the equation (7) are partial to the weighted constant w _{i which} is a programmable parameter,

Assuming that C (s) is a linear equation, the weighted constant w _{i, which} is a variable integer, becomes r ₀ , r _k , and p _k .

Obtaining the value of Equation 8 using the above equation is as follows.

From above, various LMS algorithms defined in Equation 2 can be easily implemented.

12 is a block diagram of an analog filter controller using the SE-LMS algorithm. The signal V _R (t) from which the echo component is removed is a characteristic of the transmission signal V _L (t) by the frequency characteristic variable analog filter 120. The function C (s), the echo path frequency characteristic function H (s) 121, and the counterpart transmission signal V _L (t) are added by the adder 122 and output. The V _R (t) signal compares the received signal level in the comparator 123 and controls the switching of the signal obtained by smoothing the echo component removal signal of the switching unit 124 by a weighted integer. The switching output echo component removal signal is output after passing through the high frequency attenuation filter 125.

An analog adaptive echo cancellation device using the SE-LMS algorithm filter controller is shown in FIG. The echo signal e _H (t) generated along the H (s) 130 in which the transmission signal V _T (t) represents the frequency characteristic of the echo path is generated by the counterpart transmission signal V _L (t) and the first adder 131. Are synthesized to generate the received signal V _R (t).

The first analog filter 81-1 of the characteristic variable analog filter 81 passes through the transmission signal V _T (t) to be input to the adder 81-8, and the second analog filter 81-2 transmits the transmission signal. The multiplier 81-3 is multiplied with the filter control signal r ₁ through V _T (t) and the filter control signal p ₁ and input to the adder 81-8. The k-th analog filter 81-4 is multiplied by the filter control signal r _k through the transmission signal V _T (t) and the filter control signal p _k , and input to the adder 81-8. The n-th analog filter 81-6 passes through the transmission signal V _T (t) and the filter control signal p _n , multiplies the filter control signal r _n by the multiplication operation 81-7, and inputs it to the adder 81-8. In the synthesized signal from the adder 81-8, the echo signal is removed from the received signal V _R (t) to obtain a received signal e _c (t) that is close to the counterpart transmission signal V _L (t). Accordingly, the second adder 132 outputs a signal from which the echo component is removed.

In order to minimize the echo signal, the analog filter controller 82 using the SE-LMS algorithm includes an analog filter 82-1, a comparator 82-2, a switching unit 82-3, and a high frequency attenuation filter 82-4. 5,6). The analog filter 82-1 filters the output signal of the second multiplier 81-5 and outputs the filtered signal to the first switching unit 82-31. The comparator 82-2 compares the level of the received signal V _R (t) and outputs 1 when it is larger than 0 and -1 when it is smaller than 0 to perform a switching control operation of the switching unit 82-3. As a signal input to each of the switching units 82-31, 32, and 33, a signal obtained by simplifying the transmission signal V _T (t) is input by the equation (10). The signal output from each of the switching units 82-31, 32, and 33 is input to each of the high frequency attenuation filters 82-4, 5, and 6, and then filtered. The final filter control signals -r ₀ , -r _k ,- will print p _k .

The present invention is not limited to the above-described embodiments and can be modified by those skilled in the art within the spirit of the invention.

As described above, according to the present invention, it is possible to solve the disadvantage that the existing analog hybrid circuit having fixed characteristics does not adequately respond to the characteristic change of the line. In this way, it is possible to reduce the cost of digital circuits by making the wired communication system more efficient.

Claims (3)

A first calculating unit for synthesizing a frequency characteristic signal of a reverberation path with respect to a transmission signal and a transmission signal of a counterpart on which a characteristic of a line is detected; A filter for outputting an echo cancellation signal by filtering the transmission signal according to the transmission signal and the variable filter control signal; A second calculator configured to synthesize the synthesized signal of the first calculator and the echo cancellation signal of the filter; And And a filter controller for outputting a variable filter control signal for controlling the filter by high frequency filtering the transmission signal and the filter signal according to the output signal of the second calculator. The method of claim 1, wherein the variable characteristic analog filter First to n-th filters for combining and filtering a predetermined signal among the variable filter control signals and the transmission signal; First to n-th multipliers for multiplying outputs of the second to n-th filters with a predetermined signal among the variable filter control signals; And And an adder configured to add an output signal of the first filter and an output signal of the first to n-th multipliers to output a signal from which echo is removed. The method of claim 1, wherein the filter control unit A filter for filtering the output of the k-th multiplier among the first to n-th multipliers; A comparator comparing the output signal level of the second calculator and outputting a switching control signal; A switching unit for switching the transmission signal or the output of the first to k-th filters or the output of the filter by a switching control signal of the comparator; And Adaptive echo canceller using a characteristic variable analog filter, characterized in that for outputting the filter control signal by attenuating the high frequency signal for the output of the switching unit.