JPS6173431A - Removing method of echo - Google Patents

Abstract

PURPOSE:To simplify control, to reduce the scale of a circuit and to shorten a converging time by adding a circuit consisting of a subtractor and a delay element applying the delay of Tsec, an interpolation filter and a circuit consisting of polarity detectors, a correlator and a multiplier to a conventional circuit. CONSTITUTION:A value obtained by subtracting a value obtained before Tsec from a current value appears in the output of a subtractor 16 by adding the subtractor 16 and the delay element 17 applying a delay time T (one bit width). Consequently, a condition that the probability to obtain the polarity of a residual echo component in a difference signal to be the output of a subtractor 10 precisely is not zero is obtained by a polarity detector 12 and the adaptability of an adaptive digital filter 8 is guaranteed, so that control is simplified. The correlated value of the outputs of the polarity detectors 12, 19 is calculated by a correlator 20 and multiplied by 2alpha (alpha is a constant) in a multiplier 21. The polarity of the output of the subtractor 16 appears in the output of the detector 12 and the polarity of an echo replica appears in the output of the detector 19. Therefore, the output of the correlator 20 is changed in accordance with the size of the residual echo. Thereby, the output concerned is scaled by multipying it by 2alpha in the multiplier and used as step size, the polarity of the output of the detector 12 is applied to said step size and the added value is fed back to the filter 8, so that the converging time is sharply shortened.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method of echo cancellation for realizing two-wire bidirectional digital transmission.

(Problems with the prior art) An echo canceller is known as a well-known technology for realizing two-wire bidirectional digital and digital transmission using bare wires. Speech and Signal Processing (IEEBTRANSAC'J'l0NSO
N ACOUS'rIC8,5PBE-CH,AND
5IGNAL PROCESSING) Vol. 27, No. 6, 1979, pp. 768-781). The echo canceller uses an adaptive filter with a tap coefficient equal to the length of the echo impulse response to generate a pseudo echo (echo replica) corresponding to the transmitted data series.
By generating tl-, the 2-wire/4-wire conversion circuit operates to suppress echoes leaking from the transmitting circuit to the receiving circuit. At this time, each tap coefficient of the adaptive filter is successively corrected by correlating the difference signal obtained by subtracting the echo replica from the mixed signal in which the echo and the received signal are mixed and the transmitted data. The correction of the coefficients of such an adaptive filter, that is, the convergence algorithm of the echo canceller, is described in the above-mentioned reference.

A typical example is the Stochastic Iteration Algorithm.
tion algorithm) and sine algorithm are known.

2.11 K that realizes bidirectional digital transmission is LS
A system that can apply digital device technology, which has recently made significant technological progress, is desirable.

At this time, if you try to configure a digital filter as the adaptive filter mentioned above, the analog/digital (
A/D) Control I part and digital/analog CD/
A) A converter is required. The number of bits required for the D/A converter is determined by the system requirements, and for example, approximately 12 bits are required for application to subscriber lines of public notification networks. On the other hand, the number of bits required for the A/D converter depends not only on the system conditions but also on the convergence algorithm of the echo canceller described above.

For example, when applied to a subscriber line of a public communication network, the Stochastic E-Iteration algorithm requires 8 bits and 8 degrees, whereas the sign and algorithm require only 1 bit. . However, in the sine algorithm, the tap coefficients of the adaptive filter are corrected depending on the polarity of the difference signal mentioned above.
If the polarity of the residual echo contained in the difference signal does not match the polarity of the difference signal, a problem arises in that adaptive operation becomes impossible. For example, when a binary code such as a biphase code is used as a transmission line code, the presence of a received signal causes a residual echo (difference between an echo and an echo replica).
When the level becomes comparable to the received signal level, the above-mentioned problem occurs. So during this time ffl? A conventional technique for solving the problem will be described next.

FIG. 5 shows a conventional example of an echo canceller employing the sine reference algorithm. Here the fifth
The circuit in the figure is 2#! It is assumed that they are connected oppositely via a transmission line 4. If the target cable is a subscriber cable, one is installed at the station IIC and the other is installed at the subscriber side. To simplify the explanation, we will assume baseband transmission and explain FIG. 5 as a subscriber side circuit.

In FIG. 5, a binary data series is supplied to an input terminal 1 and input to a transmitter 3 and an adaptive digital filter 8. In the transmitter 3, the binary data series is converted into a transmission line code, and then transmitted through a hybrid transformer (HY
B) 3 to the two-wire transmission line 4. On the other hand, a part of the transmission signal generated by the transmitter 2 appears as an echo component at the output of the no-brid transformer 3, resulting in a low-pass signal.
It is supplied to the filter (LP1'') 5.The received signal sent from the opposite side (in the current explanation, the station side) facing the circuit in FIG. - It is supplied to the low-pass filter 5 via the transformer 3. Therefore, the output of the low-pass filter 5 is
This results in a mixed signal consisting of a received signal and an echo. The role of the low-pass filter 5 is to suppress frequency components outside the desired signal band. The output of the low-pass filter 5 is fed to a subtracter 10. Here, the adaptive
Digital filter 8, D/A converter (I) AC)
A closed loop circuit consisting of 9, a subtracter 10, an adder 11, a polarity determination circuit 12, and a multiplier 13 operates to remove the echo of the mixed signal A that is the output of the low-pass filter 5. This is achieved by the adaptive digital filter 8 generating echo replicas. Therefore, the adaptive digital filter 8 will be explained in detail.

FIG. 6 shows a detailed block diagram of the adaptive digital filter 8 shown in FIG.

Input signals 105 and 106 in FIG. 6 are the binary data series (+1
or takes a value of -1) and the output of the multiplier 13. Furthermore, the output signal IQ7 in FIG.
This corresponds to the output signal of the adaptive digital filter 8 shown in the figure. The binary data series 105 is transmitted through the delay element 10
0m1 multiplier 101-1IOIs,...Ding, 101R
-, and coefficient generator A, , A, , . . .
A 11-1 is supplied. Delay element 100s, 100 seconds, ..., 10ONyR□ giving a delay of T seconds
are connected in this order, and each can be realized by a clip-flop. Here, N and R are positive concatenated numbers, and R is a divisor of N.

Further, the data rate of the binary data series 105 is 1/T bit/sec. Delay element 1ooi (1.2°...,
N/R-1) are respectively output to multipliers 101 . .

101, 10, ..., 101J1R-1 and coefficient generator Aj, Aj++, ......"'J+R-
s¥ζ is supplied.

However, J=summer×R. Multiplier 101, 101 +
R1..., 101 to 10s-R (K=0.1.
......, R-1), the coefficient generator A
+8 to K and A. .......

After each coefficient, which is the output of 8 to 10N-A, is multiplied by the input data, all of the multiplication results are input to an adder 102KK and added. R adders 102. .

The outputs of 102, ..., 102a-r are switch 1.
03 input contact. The switch 103 is a multi-contact switch with T gold dust cycles, and R adders 102. .

102s,...+102 The output of R-1 is selected and output in this order every T/R seconds, and the output signal 107
becomes.

The output signal 107 is an echo replica, and an echo replica is generated every T/R seconds. R is called an interpolation constant (interpolation factor) and is usually an integer greater than Ri2 in order to remove echo within a required signal band. On the other hand, switch 1 operates in synchronization with switch 103.
04 has the input and output reversed from the switch 103. That is, the switch 104 performs the function of sequentially distributing the input signal 106 to R contacts every T/R seconds. switch 10
The outputs of each of the four contacts are supplied to a coefficient generator that is present in the signal path that is input to the contact that corresponds to the switch 105 that operates synchronously.

Next, the coefficient generation circuit will be explained in detail.

Figure 7 shows the coefficient generator At (l = 0,...) in Figure 6.
. . , N-1). The input signal 200 in FIG. 7 is the binary data series 105 or the delay elements 100z and 100 in FIG.

...corresponds to the output signal of +100HyR-4.

Furthermore, the input signal 201 in FIG. 7 corresponds to the contact output of the switch 104 in FIG.

Furthermore, the output signal 203 in FIG. 7 corresponds to the output of the coefficient generator At in FIG. In FIG. 7, input signals 200 and 201 are supplied to a multiplier 204, and the multiplication result becomes one input of an adder 205. Adder 2
The output of 05 is fed back through all the delay elements 206 for T seconds, and the coefficients are updated every 7 seconds based on the correlation value of the input signals 200 and 201 supplied to the multiplier 204.
This is achieved by adding it to the pre-sampled coefficient value. Output signal 203 is the coefficient.

The echo replica generated by the adaptive digital filter 8 in FIG. 5, which has been explained above with reference to FIGS. 6 and 7, is supplied to the D/'A converter 9 and converted from a digital signal to an analog signal. and becomes one input of the subtracter 10. The subtracter 10 uses a difference signal (=[residual echo]+[received signal]) obtained by subtracting the echo replica from the mixed signal (=[echo]+[received signal]) which is the output signal of the low-pass filter 5. ]=[echo]−[echo replica]) is obtained and supplied to the receiving section 6, adder 11, and amplitude control circuit 14.

In the receiving section 6, clock extraction, demodulation of the received signal, etc. are performed, and the identified data appears at the output terminal 7.

The imaging width control circuit 4 has the function of controlling the maximum amplitude value of the random signal generated by the random signal generator 15 with reference to the amplitude or power of the difference signal output from the subtracter 10. The random signal having the maximum amplitude controlled by the amplitude control circuit 14 becomes one input of the adder 11. The difference signal which is the output of the subtracter lO and the amplitude control circuit 14
The amplitude-limited random signal output from adder I
After being added by NC, only the polarity thereof is detected by the polarity detector 12. Furthermore, the output of the polarity detector 12 is output to the multiplier 1
After being multiplied by 2α (α is a positive number) in Step 3, the signal is supplied to the adaptive digital filter 8 as an error signal. 6th
The input signal 106 in the figure corresponds to the error signal. In order for the aforementioned adaptive digital filter 8 to perform an adaptive operation, it is necessary for the polarity detector 12 to correctly detect the polarity of the residual echo. However, since the received signal is included in the difference signal that is the output of the subtractor IO, if we assume that the output of the subtractor 10 is directly input to the polarity detector 12 in FIG. When the signal level reaches the same level as the received signal level, the polarity detector 12
With the output of , the polarity of the residual echo cannot be accurately obtained. Therefore, the adaptive digital filter 8
will lose its ability to adapt.

Therefore, conventionally, as shown in FIG. 5, an adder 11.
The adaptive operation of the adaptive digital filter 8 is guaranteed by adding an amplitude control circuit 14 and a random signal generator 15 to add a random signal of the same level as the received signal level to the difference signal that is the output signal of the subtracter 10. The method was used. This method adds a random signal with the same level as the received signal to the difference signal.
Generates a probability of canceling the received signal. Since this probability is the probability that the polarity of the residual echo is correctly obtained by the polarity detector 12, the adaptive operation of the adaptive digital filter 8 is guaranteed;

However, in the conventional method shown in FIG. 5, it is necessary to generate a random signal, and in order to obtain the desired degree of echo suppression, the maximum value of the difference signal P plus the random signal to be added must be the received signal level. Is it complicated control to maintain the same level?
This has the drawback of requiring a large hardware scale. Furthermore, since the tag coefficients are updated using the polarity of the error signal, the conventional method that employs the sine algorithm has the drawback of a long convergence time.

(Object of the Invention) Therefore, an object of the present invention is to provide an echo removal method that is easy to control and requires small hardware.

Another object of the present invention is to provide an echo removal method with short convergence time.

(Structure of the Invention) According to the present invention, on the 4-wire side of the 2-wire 74-wire conversion circuit, an adaptive filter is used to remove echo money leaking from the transmitting circuit to the receiving circuit by using the generated pseudo echo. This is an echo removal method in which the echo is subtracted from a mixed signal in which the echo and the received signal are mixed to produce a difference signal.
1. Then, add the culm extension signal obtained by delaying the difference signal from the difference signal. The first error signal value is obtained by subtracting the value, the correlation between the polarity of the pseudo echo and the polarity of the 16th error signal is calculated as 9, and the signal obtained by multiplying the correlation signal by a constant is added to the signal obtained by multiplying the 16th error signal by a constant. 1
generate a second error signal by adding the negativity of the ψ difference signal of
An echo cancellation method is obtained, characterized in that the second error signal is fed back to the adaptive filter.

(Principle of the Invention) The first point of the present invention is to pay attention to the characteristics of the receiving eye pattern and to configure the following so that the probability that the received signal will be canceled does not become zero.

In other words, according to the characteristics of received eye data of transmission line codes including binary encoding systems, the current value and the value before LT (l is a positive integer) are almost the same value, or each absolute value with opposite polarity. The minimum probability that the values are almost the same takes a certain positive value that is not zero. Therefore, regarding the difference signal (= residual echo + received signal),
II to take the difference or phase between the current value and the value 6T seconds ago
: , it will be canceled with a certain positive probability that the receiving and receiving component is not zero. Therefore, if the polarity of the difference or sum is fully detected, the sign of the residual echo will be stolen with a probability of a certain positive value other than zero, so the adaptive operation of the adaptive filter is guaranteed.1. Ru.

The point of M2 of the present invention is that the step size is adaptively changed when updating the tap coefficients of the adaptive filter. In the present invention, when the residual echo is large, there is a strong correlation between the polarity of the residual echo and the polarity of the residual echo, whereas when the residual echo is small, there is no correlation between the two. The step size is adaptively changed depending on the correlation value.

Therefore, the convergence time can be significantly shortened compared to the conventional method.

(Example) Next, the present invention will be described in detail with reference to all the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention.

In the figure, functional blocks all assigned the same reference numbers as in FIG. 5 are numbered 4, which have the same functions as in FIG.

The difference between FIG. 1 and FIG.
2 and the circuit consisting of the polarity detector 19, correlator 20, and multiplier 21! ν, other configurations are 5th
It is exactly the same as the figure. Before explaining these differences, the overall configuration will be briefly described. The binary data series supplied to the input terminal l is supplied to the transmitter 2 and the adaptive digital filter 8. 2 at transmitter 2
After the value data series is converted into a transmission line code, it is sent to a two-wire transmission line 4 via a hybrid transformer 3.

Here, due to the impedance mismatch of the hybrid transformer 3, the output of the transmitter 2 leaks into the receiver circuit as an echo and is supplied to the low-pass filter 5. On the other hand, the received signal is also supplied to the low-pass filter 5 via the transmission line 4 and the hybrid transformer 3. Low pass
The mixed signal (=[echo]+[received signal]) whose unnecessary high frequency components have been suppressed by the filter 5 is supplied to the subtracter 10.

Therefore, the pseudo echo (echo replica) generated by the adaptive digital filter 8 is converted into an analog signal by the D/A converter 9, and then input to the subtracter IOK via the interpolation filter 22. Therefore, out of the component O of the difference signal (= [mixed signal] - [echo replica] = (echo] + [received signal] - [echo replica]) which is the output of the subtractor 10, the residual echo [= [echo] - (
When the echo replica]) becomes sufficiently smaller than the received signal, the received signal is accurately demodulated in the receiving section 6, and the received binary data sequence appears at the output terminal 7. Note that the interpolation filter 22 functions to suppress the high-order e component contained in the output of the D/A converter 9.

Here, the adaptive digital filter 8°D/A
A closed loop circuit consisting of a converter 9, an interpolation filter 22, a subtracter 10, a subtracter 16, a polarity detector 12, and a multiplier 13 realizes the adaptive operation of the adaptive digital filter 8.

The configuration of the adaptive digital filter 8 may be the same as that shown in FIGS. 6 and 7, similar to that described in the conventional example of FIG. 5.

The output of the dominant detector 12 is multiplied by the output of the multiplier 21 in a multiplier 13 and supplied to the adaptive digital filter 8 as an error signal.

Next, we will explain in detail the relationship between the output of the polarity detector 12 and the polarity of the residual echo component of the difference signal that is the output of the subtracter 10.Before that, we will discuss the transmission line code. This figure shows typical examples of value symbols (
al is the bias x-s code! &, (b)FiMSK
(Minimum Shift Keying) The pulse waveforms of the codes are shown.

As shown in FIG. 2(a) K, in the Bike 2-1 code, pulse waveforms with inverted polarities are assigned to data of degrees 0 and 1. Both pulses have the characteristic that the polarity is reversed at the center of the 1-bit width T seconds, and the positive and negative values are balanced within 1 bit. Against this AK, the second
As shown in Figure (b), four types of pulse waveforms are prepared in the MSK code.

That is, 2i4ff pulse waveforms of the mode and the mode with the polarities inverted are prepared for the data of "O" and "1", respectively.

These two types of mode transitions are determined by the previous mode KJ: in FIG. 2 (indicated by thick arrows bl; the current mode is one peer). This N15K code has a feature that the polarity is always reversed at the pit boundary.

Note that in the MSK code, the positive and negative values are balanced within 1 bit for 111, but for 7
If so, the positive and negative are not balanced. However, as is clear from the direction of the thick arrow indicating the mode transition in Figure 2 (1)), if there is an even number of "0"s in a continuous pit series, there is a positive/negative balance order, and the DC component can be said to be almost negligible. The transmission path code shown in FIG. 2 is outputted from the transmitter 2VC in FIG. 1.

correspond to FIG. 2, respectively.
This is a receiving eye pattern of the SK code.

As shown in the figure, the receiving eye pattern has high frequency components cut off and becomes rounded. Now, pay attention to Figure 3 (al).The combinations of four sample points separated by T seconds are each (t・, t0', (11, 11/).

Os, 1. /) and (ts, ts'). At this time, if Am is the value obtained by subtracting the sample value of t = tm from the sample value of 1 = 1□' (m = On 11 2.3), ・A, n can be given as shown in Table 1. Recognize.

・Assuming that the probability of occurrence of Oo and "l" 4 is equal to 10, the probabilities of A・=O1As=O1A−"O, A, and 20 are 1, 1, and 1, respectively, from Table 1. becomes. In this example, we considered four sets of sample points separated by T seconds shown in Figure 3, but as is clear from the figure, no matter what the phase is, %V will not change other than those shown in Table 1, apart from a negative reversal. It turns out that this pattern is impossible. Therefore, the minimum probability that the value obtained by subtracting the sample value T seconds ago from the current sample value is zero is ten. Next, considering the receiving eye pattern of the MSK symbol in FIG. 3 (bl diagram), A1.l is given as shown in Table 2 with reference to all the mode transitions in FIG.

Assuming that the probabilities of appearance of "0" and "1" are equal and each is 10, then A= =Q, AI =0. Am = Q and A
, = 0 is 1, 11, and 10, respectively, as shown in Table 2. In this example, 4
As is clear from the figure, no pattern other than those shown in Table 1'VC is possible, apart from positive/negative reversals, regardless of the overall phase. . Therefore, even when M8 is a sign, the minimum probability that the value obtained by subtracting the sample value T seconds ago from the current sample acquisition is zero is . As mentioned above, using the By7 engineering code and the M8 code as examples, the minimum probability that the value obtained by subtracting the sample value T seconds ago from the current sample value is zero is both 10. I understand that. When considering transmission line codes other than these codes in the same way, it is clear that the minimum value of the probability has a value other than zero. Furthermore, up until now, the target value has been the value obtained by subtracting the sample value T seconds ago (the data rate is I/T pits/second) from the current sample value, but the current sample value is l.
It can be seen that the minimum probability that the value obtained by subtracting the sample value T seconds ago (l is a positive integer) becomes zero is also . Next, the meaning of this probability in the adaptive operation of the echo canceller will be explained with reference to FIG.

In one implementation of the invention shown in FIG. 1, reference numeral 17 is a delay element providing a delay of T seconds, reference numeral 161 or subtractor reference numeral 12 is a polarity detector. Here, in order for the adaptive digital filter 8 to perform an adaptive operation, the polarity detector 12 must generate a difference signal (=[echo]+[received signal]-[echo replica]) which is the output of the $c calculator 1o.
As mentioned above, it is necessary to have a condition that the probability of accurately obtaining the polarity of the residual echo [echo] - [echo replica]) component contained in ) is not zero. In FIG. 1, a subtracter 16 and a delay element 17 are added for the purpose of satisfying this condition, and the output of the subtracter 17 is as follows.
The value obtained by subtracting the value T seconds ago from the current value appears.

As mentioned in the explanation of Tables 1 and 2, it is clear that the received signal component in the difference signal that is the output of the subtractor 1o becomes zero with a probability of 10 or more at the output of the subtractor 16. It is. On the other hand, considering the residual echo component included in the output of the subtractor 16, from the current residual echo value T
A value obtained by subtracting the value of the residual echo seconds before is output from the subtracter 16 as a residual echo component. Since the current residual echo value and the residual echo value T seconds ago are uncorrelated, the residual echo value T seconds ago can be regarded as random noise. The amplitude distribution of the residual echo value T seconds ago is symmetrical between positive and negative, and the probability that the amplitude d is ldl≦δ (however, δ is insufficient) is:
Takes a positive value other than zero. Therefore, it can be seen that the probability that the polarity of the residual echo of the black cloth is accurately outputted by the polarity detector 12 which receives the output signal of the subtractor 16 is a certain positive value that is not zero. Therefore, the adaptive operation of the adaptive digital filter 8 is guaranteed. In addition, in FIG. 1, the delay element 17 was explained as providing a delay of T seconds, but as mentioned in the explanation of Tables 1 and 2, the amount of delay is l - T seconds (l is a positive integer). The same effect can be obtained as well.

Next, the operation of the correlator 20 shown in FIG. 1 will be explained.

The correlation value between the output of the polarity detector 12 and the output of the polarity detector 19 is calculated by the correlator 20 and is calculated by the multiplier 21 as 2α(α
is a constant) and is multiplied and supplied to the multiplier 13. Here, the output of the polarity detector 12 includes a difference signal (= [residual echo] + [received signal]) K, which is the output of the subtractor 10, and the polarity of the value obtained by subtracting the value T seconds ago from the current value. appears. On the other hand, the polarity of the echo replica appears in the output of the susceptibility detector 19. Therefore, if we pay attention to the fact that when the residual echo is large, there is a correlation between the habitability of the residual echo and the polarity of the echo replica, whereas when the residual echo is small, there is no correlation between the two. The output of the correlator 20 takes a large value when the residual echo is large, and a small value when the residual echo is small. Therefore, the multiplier 21 scales the output of the correlator 20 by a factor of 2α to obtain the step size and [7
The polarity of the output of the polarity detector 12 is added to this step size, and the adaptive digital filter 8
By returning to K, it becomes possible to significantly shorten the convergence time.

FIG. 4 is a block diagram showing a second embodiment of the invention. In this figure, functional blocks given the same reference numbers as in FIG. 1 have the same functions as in FIG. 1.

The difference between FIG. 4 and FIG. 1 is that the subtracter 16 in FIG. 1 is replaced with an adder 18 in FIG. 4, and the other parts are exactly the same. Therefore, in FIG. 4, regarding the difference signal that is the output of the subtractor lO, the current value of the difference signal and T
The sum with the value of the difference signal seconds before appears at the output of the adder 18,
The polarity of this sum value is detected by the polarity detector 12. Therefore, tables corresponding to Tables 2 and 3 will be obtained using FIG. 2 which shows an example of a transmission path code and FIG. 3 which shows an example of its reception eye pattern. First, paying attention to Figure 3 (al), the combinations of four sample points separated by T seconds are (t,, t,') and (t, respectively).

t,'), (tm, tm') and (t,,ta')
Assume that At this time, t=tffi′ (m=0,1
, 2°3) and the sample value of 1=1ffl as B1. , it can be seen that B- is given as shown in Table 3.

Similarly, Table 4 is obtained for FIG. 3H.

Table 3 Values of 8□ in case of bi7 engineering code Table 4 MS
Assuming that the probabilities of appearance of the BmO values lO" and "I" in the case of K code are equal and each is 10, then B,, l=0.Bm =0.Bm =o and B
, ==o, in the case of the biphase code shown in Table 3, are +1, ÷, and l, respectively, and in the case of the MSK code, shown in Table 4, they are 1, +, t, 10, and 1, respectively. Become. Therefore, the minimum probability that the sum of the current sample value and the sample value T seconds ago becomes zero is 2, and this holds true for the sampling phase of the current sample. Also, Tables 3 and 4
In the following, the cases of Bi7-Z code and MSK code are all shown, but if you consider other transmission path codes in the same way, 1. It is clear that the minimum probability that the sum of the current sample value and the sample value T seconds ago becomes zero is a short note that is not zero.

Furthermore, it goes without saying that the minimum probability that the sum of the current sample value and the sample value T seconds (ltri positive integer) ago is zero also has a non-zero value. Returning to the explanation of the figure, the difference signal that is the output of the subtracter 10 is supplied to the receiving section 6, as well as to the adder 18 and the delay element I7 that provides a delay of T seconds. Further, the output of the delay element 17 serves as one input of the adder 18. Therefore, the sum of the current value and the value T seconds ago appears at the output of the adder 18. It is clear that the received signal component in the difference signal which is the output of the subtracter 10 becomes zero at the output of the adder 16 with a probability factor or higher. On the other hand, considering the residual echo component included in the output of the adder 18, the sum of the current residual echo value and the residual echo from T seconds ago is output from the adder 18 as the residual echo component.

Since the current residual echo value and the residual echo value T seconds ago are uncorrelated, the residual echo value T seconds ago can be regarded as random noise. The amplitude distribution of the residual echo values T seconds ago is symmetrical, and the probability that the amplitude d satisfies ldl≦δ (however, Ozδ) is not zero but takes a certain positive value. Therefore, the polarity detector 12 receives the output signal of the adder 18.
It can be seen that the probability that the current polarity of the residual echo is correctly output takes a certain positive value that is not zero. Therefore, the adaptive operation of the adaptive digital filter 8 is guaranteed. In addition, in FIG. 4, the delay element 17 was explained as providing a delay of T seconds, but as stated in the explanation of Tables 3 and 4, the delay amount is expressed as l - T seconds (l is a positive integer). A similar effect can be obtained. In addition, correlator 2
The operation of 0 is the same as in FIG. 1, but the signal being supplied to the polarity detector 12 is
The difference is that the difference signal, which is the output of , is the sum of the current value and the value T seconds ago. Considering the residual echo component of the difference signal, the output of the correlator 20 changes depending on the size of the residual echo, as in FIG. 1, so it is possible to significantly shorten the convergence time. it is obvious.

The present invention has been described above in detail based on the embodiments, and the line stopper for compensating the line loss of the two-wire transmission line is included in the damaged part 6 in FIGS. 1 and 4. It may be considered, or it may be inserted between the low-pass filter 5 and the subtracter IO. Also, if MSK code is used, “0”
The structure of the adaptive digital filter 8 is due to two reasons: the pulse waveforms for ° and °1 are different, and each has both e mode and θ mode.
This is slightly different from the case of Z codes. That is, corresponding to the different pulse waveforms of "0" and °1, the tap coefficient 'f
It is necessary to prepare two types of t and update them individually, and it is also necessary for the transmitter 2 to distinguish between the tag coefficients of the mode signal. Furthermore, in the explanation so far, the delay amount of the delay element 17 is T seconds or 1-T seconds (lFi positive integer).
However, in practice, it goes without saying that a value around 1-T seconds is sufficient. Further, the interpolation filter 22 is not necessary if the purpose is to remove echoes only at sampling points where echo replicas are generated.

(Effects of the Invention) As described in detail above, according to the present invention, the difference signal (=
[Residual echo] + [Received signal]) For K, calculate the difference or sum of the current value and the previous value for l-T seconds (where I! is a positive integer and 1/T is the data rate). Since K, the received signal component is canceled with a probability of a certain positive value that is not zero. Therefore, by detecting the polarity of the difference or sum, the adaptive operation of the adaptive digital filter is guaranteed.

Further, according to the first and second inventions, although a delay of J-T seconds is given, the above-mentioned adaptive operation can be completely guaranteed by combining the delay element and the subtractor or adder 8, so that no complicated control is required. It is possible to provide a complete echo cancellation method that is simple and requires small hardware. In other words, according to the present invention, since the step size can be adaptively changed depending on the size of the residual echo, it is possible to significantly shorten the convergence time.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a first embodiment of the present invention, FIG. 2 is a diagram showing a sequence of pulse waveforms of a transmission path code, and FIG. 3 is a diagram showing an example of a reception eye pattern. FIG. 4 is a block diagram showing the second embodiment of the present invention, FIG. 5 is a block diagram showing a conventional example, FIG. 6 is a diagram showing the configuration of the adaptive digital filter, and FIG. 7 is a diagram showing the configuration of the coefficient generator. FIG. 3 is a diagram showing the configuration. In the figure, 2 is a transmitter, 3 is a hybrid transformer, 4H
2M transmission line, 5#i low-pass filter, 6 is a receiving section,
7 is an output terminal, 8 is an adaptive digital filter, 9 is a D/A converter, 10 and 16 are subtracters, 11 and 18 are adders, Minoru and 19 are polarity detectors, 13 and 21 are multipliers, 14 is amplitude control circuit,
15 is a random signal generator, 17 is a delay element, 20 is a correlator, 22 is an interpolation filter, -100 dew on 100, ..., 100N/R-t is a delay element, 1
01., IQ2+-...+ 101N-, is a multiplier. 102・-102m, -..., 102R-s
is an adder, 103 and 104 are multi-contact switches,
204 is a multiplier, 205 is an adder, and 206 is a delay element. Figure 2: o" (a) -1...(b) Figure 3: (a) (b)

Claims (1)

[Claims] An echo removal method for removing echoes leaking from a transmitting circuit to a receiving circuit using a pseudo echo generated by an adaptive filter on the 4-wire side of a 2-wire/4-wire conversion circuit, After subtracting the pseudo echo from a mixed signal in which signals are mixed to obtain a difference signal, the difference signal and a delayed signal obtained by delaying the difference signal are added or subtracted to obtain a first error signal, and the pseudo echo is The polarity of the echo and the first
The polarity of the first error signal is calculated by calculating the correlation with the polarity of the difference signal, and the polarity of the first error signal is added to the signal obtained by multiplying the correlation signal by a constant.
An echo cancellation method characterized in that the second error signal is generated and the second error signal is fed back to the adaptive filter.