JPH0779290B2 - Eco-removal device - Google Patents

Description

The present invention relates to an echo canceller for realizing two-wire bidirectional digital transmission, and more specifically, to an echo canceller.
The present invention relates to an improvement of an echo removing device that removes echo leaking from a transmitting circuit to a receiving circuit on the 4-line side of a line / 4-line conversion circuit.

[Conventional technology]

An echo canceller is known as a well-known technique for realizing two-wire bidirectional digital transmission using a pair wire. IEEE TRANSACTIONS ON ACOUSTICS, SPEECH,
AND SIGNAL PROCESSING), Volume 27, Issue 6, 1979, 768-
On page 781, such an echo canceller is shown. The echo canceller generates a pseudo echo (echo replica) corresponding to a transmitted data sequence by using an adaptive filter having a tap coefficient corresponding to the length of the impulse response of the echo. As for the adaptive filter, the transversal type is described in the above-mentioned document, and the transversal type has a coefficient memory for each tap. By generating an echo replica with such an adaptive filter,
The echo canceller operates so as to suppress the echo leaking from the transmission circuit to the reception circuit in the 2-line / 4-line conversion circuit. At this time, each tap coefficient of the adaptive filter is sequentially corrected by taking the correlation between the transmission signal and the difference signal obtained by subtracting the echo replica from the mixed signal in which the echo and the received signal are mixed.

FIG. 5 shows a principle diagram of the structure of the echo canceller shown in the above-mentioned document. Binary data to be transmitted is supplied to the input terminal 1 and input to the encoder 2. The encoder 2 uses the input binary data,
A zero level or positive or negative pulse is generated according to the coding rule of the transmission line code adopted. The generated pulse is sent to the 2-wire transmission line 12 via the hybrid (HYB) (2-wire / 4-wire conversion circuit) 3, and at the same time, it is supplied to the multi-level code adaptive filter 13 corresponding to this pulse. . On the other hand, the received signal is supplied to the low pass filter 4 via the transmission line 12 and the hybrid 3. After the unnecessary high-pass component is removed by the low-pass filter 4, the filter output is input to the demodulator 6 via the subtractor 5. Demodulator 6
Has a function of line equalization, timing extraction, identification, etc., and a reception signal demodulated as binary data appears at the output terminal 7. Here, due to impedance mismatch in the hybrid 3, the output of the encoder 2 leaks into the receiving circuit as an echo and is input to the low pass filter 4. This echo causes a problem when demodulating a received signal, which is a problem. The adaptive filter 13 and the subtractor 5 are for canceling echoes that interfere with the demodulation of the received signal.
By adaptively generating an echo replica with the adaptive filter 13, the echo contained in the output of the low pass filter 4 is removed.

In this way, echoes leaking from the transmitting circuit to the receiving circuit are removed by using one adaptive filter 13.

[Problems to be solved by the invention]

However, in an actual circuit, it is very difficult to satisfy the condition that the positive pulse and the negative pulse generated by the encoder 2 in FIG.
Considering the LSI, it is usually necessary to allow about 5% of asymmetric components. At this time, the number of staying echoes increases due to the asymmetrical components of the positive and negative pulses, making it difficult to suppress the echo to a desired level. In order to solve this, conventionally, two kinds of memories corresponding to positive and negative transmission pulses have to be prepared as tap coefficients, and therefore one adaptive filter is used as shown in FIG. The conventional configuration has a problem in that the required memory capacity is doubled and therefore the hardware scale is increased.

An object of the present invention is to provide an echo canceling device having a small hardware scale.

[Means for solving problems]

According to the present invention, an echo removing device for removing an echo leaking from a transmitting circuit to a receiving circuit in a 2-line / 4-line converting circuit receives binary data to be transmitted and receives zero level or An encoder that generates positive or negative pulses and at the same time generates corresponding three-valued data, and receives the three-valued data supplied from the encoder, and considers that the positive or negative pulses are symmetrical. A first adaptive filter that generates a first echo replica by controlling the sign for one type of tap coefficient, and receives the ternary data only when the positive pulse occurs or the negative pulse The data converter that generates "1" only when occurs and the output of this data converter operates to enable the tap coefficient output only when this output is "1". A second adaptive filter that generates an echo replica, an adder that obtains the sum of the first echo replica and the second echo replica, and subtraction of the output of this adder from a mixed signal in which echo and received signals are mixed And using the output of the subtractor, the input of the first adaptive filter updates the tap coefficient generating a positive or negative pulse, and at the same time the input of the second adaptive filter is It is characterized in that the first and second adaptive filters are adaptively operated by updating only the tap coefficient which is "1".

[Action]

The echo canceling apparatus of the present invention cancels echoes caused by asymmetrical generation of positive and negative pulses by using two adaptive filters. When using two adaptive filters, the first adaptive filter considers the positive and negative pulses as symmetric and removes the echo, while the second adaptive filter removes the echo due to the asymmetry of the positive and negative pulses. It becomes possible to generate the echo replica required for echo cancellation by sharing these two adaptive filters so that the desired echo replica adds the outputs of the first and second adaptive filters. Obtained. Since the amplitude of the echo due to the asymmetry is smaller than the amplitude of the symmetric echo, the number of taps in the adaptive filter determined by the length of the impulse response of the echo path is small, and it is desirable to consider two adaptive filters The coefficient memory is also smaller than in the conventional method having two types of positive and negative.

〔Example〕

 Next, the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. In FIG. 1, the functional blocks given the same reference numerals as in FIG. 5 have the same functions as in FIG.
That is, the encoder 2 is for converting binary data to be transmitted into a transmission line code, and outputs a pulse according to a predetermined coding rule. Some examples of coding rules are the second
This is shown in Figures and 4. This will be described later. The hybrid 3 sends out the pulse generated by the encoder 2 to the two-wire transmission line 12, while supplying the received signal from the other side to the low pass filter 4 and the low pass filter 4
Then remove unnecessary high frequency components.

Further, the demodulator 6 demodulates the received signal after echo removal into binary data and sends it to the output terminal 7. Like the case of FIG. 5, it has functions of line equalization, timing extraction, identification and the like. Have.

In such a transmission / reception system, in the case of FIG. 1, two adaptive filters 9 and 10 are used as adaptive filters for echo removal, and further a data conversion circuit 8 and an adder 11 are used. The output of the adder 11 is supplied to the subtractor 5 '.

The first adaptive filter 9 is an adaptive filter that receives a signal supplied from the encoder 2 and generates a first echo replica, and the second adaptive filter 10 is supplied from the encoder 2. It is an adaptive filter that receives an output of a data conversion circuit 8 that converts a signal and generates a second echo replica.

The adder 11 obtains the sum of the first echo replica and the second echo replica, and the subtractor 5 ′ subtracts the output of the adder 11 from the mixed signal in which the echo and the received signal are mixed. To

In this way, in the device for removing the echo leaking from the transmission circuit to the reception circuit on the 4-wire side of the 2-wire / 4-wire conversion circuit,
An encoder 2 for converting binary data to be transmitted into a transmission path code, and a signal supplied from the encoder 2
First adaptive filter 9 for generating the echo replica, a data conversion circuit 8 for converting the signal supplied from the encoder 2, and a second echo replica for receiving the output of the data conversion circuit 8. Second adaptive filter 10 for
At least a subtractor 5'for subtracting the output of the adder 11 from the mixed signal in which the echo and the received signal are mixed, and the output of the subtractor 5 ' It is adapted to eliminate echoes by adapting the first and second adaptive filters 9, 10 using them.

The difference between FIG. 1 and FIG. 5 is that the adaptive filter 13 of FIG. 5 for generating an echo replica is the first adaptive filter 9 and the second adaptive filter 10 in FIG. Is added, the data conversion circuit 8 for supplying data to the adaptive filter 10 is added, and the echo replicas from these two adaptive filters 9 and 10 are added. The point is that the adaptive filters 9 and 10 are adapted by using the output thereof after addition by the device 11 and application to the subtracter 5 '.

Next, AMI (Alternate Mark Inve), which is a typical ternary code,
rsion) code as an example, the operation of FIG. 1 will be described with reference to the waveform examples of the respective parts shown in FIG.

FIG. 2A shows the coding rule of the AMI code. In this encoding rule, 0 is output for input data "0" as shown in (a) of FIG. 2 (a). Further, as shown in (b), +1 and -1 are alternately output for the input data "1". FIG. 2 (b) (A) shows binary data to be transmitted supplied to the input terminal 1 of FIG. The horizontal axis is the time axis, and the data cycle is T seconds. The output waveform of the encoder 2 shown in FIG. 1 for converting the binary data to be transmitted into the AMI code is shown in FIG. 2 (b) (D). As is clear by comparing (A) and (D) in FIG. 2 (b), when the input binary data is "0", the zero level is set, and when it is "1", the positive pulse or Negative pulses are output alternately. However, in a real circuit, it is very difficult to satisfy the condition that the positive pulse and the negative pulse are completely symmetrical,
The waveform shown in (D) contains some asymmetrical components as shown by the solid line. The waveform shown in (D) represents a case where the amplitude of the negative pulse is slightly smaller than that of the positive pulse, and such a case will be described for the time being. At this time, the residual echo increases due to the asymmetry of the positive and negative pulses, and it becomes difficult to suppress the residual echo to a desired level. In this embodiment, this problem is solved as follows.

First, the AMI-encoded output waveform in which the positive and negative pulses are asymmetrical shown in FIG. 2B, the waveform in which the positive and negative pulses are symmetric in FIG. 2E and the asymmetry of the positive and negative pulses in FIG. Divide into the waveform to be considered. It is clear that the waveform shown in (D) can be obtained by adding the waveforms shown in (E) and (F). Therefore, an adaptive filter for removing the echo caused by the symmetrical waveform of the positive and negative pulses shown in (E) is used.
An adaptive filter 10 for eliminating the echo caused by the waveform for correcting the asymmetry of the positive and negative pulses shown in the filters 9 and (F) is introduced. The adaptive filter 9 is supplied with a ternary code of 0, +1 or -1 generated by the encoder 2 as shown in FIG. 2 (b) (B). On the other hand, the data conversion circuit 8 which has received the signal shown in FIG. 2B has a value of -1, that is, +1 only when it detects that a negative pulse has been input, and 0 otherwise. The output signal shown in (C) of (b) is
Supply to the filter 10. Therefore, the adaptive filter 10 generates the echo replica corresponding to the correction amount only when the -1 pulse is transmitted. On the other hand, the adaptive filter 9 correlates the ternary data string supplied from the encoder 2 with each tap coefficient, adds them, and outputs them. Therefore, only one type is actually prepared as a coefficient memory. The adder 11 adds a positive / negative symmetrical echo replica by the adaptive filter 9 and an echo replica for asymmetry correction by the adaptive filter 10 to generate a desired echo replica and supplies it to the subtracter 5 '.
In this way, it is possible to obtain a corresponding echo replica even if the transmitted pulse is positive and negative asymmetrical without having a coefficient memory for positive and negative.

The data conversion circuit 8 in FIG. 2 has a function that satisfies the input / output relationship described below. Input ternary signal is +1 or 0
In case of, 0 is output, and in case of -1, +1 is output. That is, the output becomes +1 only for the negative pulse, which corresponds to (C) in FIG. 2 (b). The ternary signal supplied to the data conversion circuit 8 is actually represented by 2 bits,
If 0, +1 and -1 are defined as "00", "01" and "11" respectively, the detection of -1 is the same as the detection of "11". Therefore, use the OR circuit 33 shown in FIG. Can be done. The input signals 31 and 32 in FIG. 3 correspond to the respective 2 bits supplied from the encoder 2 to the data conversion circuit 8, and the output signal 34 corresponds to the output of the data conversion circuit 8.

The adaptive filters 9 and 10 are the transversal type described in the above-mentioned reference, or IEEE TRANSACTIONS ON COMMUNICATIONS, 29, 11
No. 1981, pp. 1573-1581, and can be realized by using the memory type. As described above, the transversal type has a coefficient memory for each tap, while the memory type has a RAM (Random Access) for each tap output.
Memory) address. Therefore, if the number of taps is N, the former basically requires N memories, whereas the latter requires 2 ^N memories. Here, considering the adaptive filter 13 of FIG. 5 showing the conventional example, when the positive and negative pulses to be transmitted are asymmetric, the transversal type corresponds to the memory corresponding to the positive pulse and the negative pulse as a coefficient. Two types of memory are required, which doubles the memory capacity (2N). Also,
In the memory type, the capacity is required to be doubled (2 ^{N + 1} pieces) to distinguish positive and negative pulses. On the other hand, the adaptive filters 9 and 10 of FIG. 1 showing an embodiment of the present invention.
Does not need to double the memory capacity when the positive and negative pulses to be transmitted are asymmetrical, regardless of whether they are realized by the transversal type or the memory type. This is for the following reason. First, the adaptive filter 9 which treats pulses as symmetry and corresponds to both positive and negative pulses with one type of coefficient is
Since it is not necessary to have tap coefficients separately for positive and negative, half the memory capacity of the conventional adaptive filter of FIG. 5 is sufficient. Further, the length of the impulse response of the echo due to the asymmetry of the pulse is significantly shorter than that of the positive / negative symmetrical pulse. This means that the number of taps of the adaptive filter 10 that corrects the pulse asymmetry is significantly reduced. Therefore, adaptive filters 9 and 10
The total memory capacity of the conventional adaptive filter
It becomes less than 13, and it is possible to reduce the hardware scale. This is effective regardless of whether the adaptive filter is a transversal type or a memory type.

Next, as another embodiment of the present invention, a case of binary code will be described with reference to FIG.

(A) and (b) of FIG. 4 (a) represent binary codes for transmitting +1 and -1 pulses for data "0" and "1", respectively. Also in this case, the explanation has already been made with reference to FIG.
It can be considered exactly as in the case of the AMI code. However, the adaptive filter 9 is supplied with a binary signal of +1 and -1 from the encoder 2 instead of the ternary signal of +1, 0, -1. Further, the data conversion circuit 8 to which the binary signal is supplied at the same time has a function that satisfies the input / output relationship described below. That is, input 2
0 is output when the value signal is +1 and +1 is output when the value signal is -1. If the +1 and -1 binary signals supplied to the data conversion circuit 8 are defined by "01" and "11" like the AMI code, the data conversion circuit 8 is also realized by an OR circuit. be able to. The signal from the data conversion circuit 8 is supplied to the adaptive filter 10 to generate an echo replica for asymmetry correction corresponding to the negative pulse.

FIG. 4 (b) is a bi-phase code, and changes from +1 to -1 for data "0" and "1", respectively, as shown in (a) and (b) of FIG. 4 (b). The pulse to be turned on and the pulse of the opposite phase are sent out. At this time, the adaptive filter 9 is also supplied with binary signals of +1 and -1 from the encoder 2. Further, the data conversion circuit 8 to which the binary signal is supplied at the same time has a function that satisfies the input / output relationship described below. That is, 0 is output when the input binary signal is +1 and +1 is output when the input binary signal is -1. The signal from the data conversion circuit is supplied to the adaptive filter 10 to generate an asymmetry-corrected echo replica for the pulse corresponding to "1". That is, the output of the adaptive filter 10 operates as a correction component for the non-reference pulse. Also at this time, the data conversion circuit 8 can be realized by an OR circuit.

In the embodiments described so far by using the waveform converting circuit of FIG. 3 and the waveform examples of the respective parts of FIG. 2, the echo replica corresponding to the negative pulse is generated with the positive pulse as the reference, but the negative replica is generated. It is also possible to use the level of the pulse of. At this time, the input signal 32 shown in FIG. 3 is supplied to the logical sum circuit 33 through a newly added negation circuit. With such slight modification of the data conversion circuit, the present invention can be implemented with reference to the level of the negative pulse. Further, it is apparent that the present invention can be applied to binary or ternary transmission line codes other than the AMI code described as the embodiment and the binary code shown in FIG. 4 by slightly modifying the data conversion circuit 8. .

〔The invention's effect〕

As described above, according to the present invention, the number of taps of the adaptive filter can be reduced as compared with the conventional case when the echo caused by the positive / negative asymmetric transmission pulse is removed, and the required memory capacity is small. Contribute to the reduction of the hardware scale of the entire device.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a waveform example of each part for explaining the coding rule of an AMI code which is a coding example by an encoder and the circuit operation of FIG. FIG. 3, FIG. 3 is a diagram showing an example of a data conversion circuit, FIG. 4 is a diagram showing an example of encoding used for explanation of another embodiment of the present invention in the case of a binary code, and FIG. It is a block diagram which shows a prior art example. 1 ... input terminal 2 ... encoder 3 ... hybrid 4 ... low pass filter 5 '... subtractor 6 ... demodulator 7 ... output terminal 8 ... data conversion circuit 9,10 ... adaptive Filter 11 …… Adder 12 …… 2-wire transmission line

Claims (1)

[Claims] 1. An echo removing device for removing an echo leaking from a transmitting circuit to a receiving circuit in a 2-line / 4-line converting circuit, receives binary data to be transmitted, and receives a zero level according to a coding rule of a transmission line code. Alternatively, an encoder that generates positive or negative pulses and at the same time generates corresponding three-valued data, and receives the three-valued data supplied from the encoder, and the positive or negative pulses are symmetric. A first adaptive filter that generates a first echo replica by controlling the sign for one type of considered tap coefficient, and the three-valued data is received, and only when the positive pulse is generated or the negative pulse is generated. A data converter that generates "1" only when a pulse is generated, and the output of this data converter is received, and the tap coefficient output operates only when this output is "1". Second adaptive filter for generating the echo replica of the above, an adder for obtaining the sum of the first echo replica and the second echo replica, and the output of this adder is subtracted from the mixed signal in which the echo and the received signal are mixed. And using the output of the subtracter to update the tap coefficient at which the input of the first adaptive filter is generating a positive or negative pulse, and at the same time, the input of the second adaptive filter. An echo canceller characterized in that the first and second adaptive filters are adaptively operated by updating only the tap coefficient having "1".