JPS59225626A - Echo canceller device for data transmitter - Google Patents

Abstract

PURPOSE:To decrease the required bit number of an operation processing circuit by combining a processing pre echo canceller and an adaptive type digital processing echo canceller and eliminating a part of an echo signal from an input signal of the latter in advance so as to narrow the dynamic range of an A/D converter. CONSTITUTION:A transmission signal from a transmitter 2 is given a specific frequency characteristic by a filter circuit 22 in a pre echo canceller 21 and attenuated properly by an attenuation circuit 23, then subtracted from a reception signal including an echo signal incoming through a terminal 6 and a hybrid transformer 5 by a subtraction circuit 24. When the frequency characteristic of the filter 22 and the attenuation of the attenuation circuit 23 are decided properly, the absolute amount of the echo signal included in the signal inputted to the digital processing adaptive type echo canceller 13 is decreased, the number of bits required for an A/D converter 7 is reduced and the number of bits of the operation processing circuit is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an echo canceller device for a data transmission device that uses a two-wire line to transmit one signal in both directions.

<Prior Art> As a method for mutually transmitting bidirectional data signals using a two-wire line, there is a method that uses an echo canceller device and uses the same frequency band at the same time.

Conventionally, this type of echo canceller device has been considered to create analog pseudo-echo signals and perform echo cancellation, but in order to perform echo cancellation adaptively when connected to various lines, it is necessary to Analog/digital (A/D)
) conversion and digital/analog CD/A) A method is used in which a pseudo echo is created by digital signal processing using conversion.

For example, a configuration shown in FIG. 1 has been proposed as an apparatus for eliminating echoes by digital processing. The data sequence applied to the transmission data input terminal 1 is applied to the transmitter 2 as well as to the pseudo echo signal generator 3 at the same time as in the normal case. The pseudo echo signal generator 3 includes, for example, a transversal filter realized by digital arithmetic processing and a control section that adaptively controls the tap coefficients of the transversal filter. The baseband N-like echoes generated by the pseudo-echo signal generator 3 are subjected to modulation similar to the modulation performed within the transmitter 2 by the modulator 4 . The output of the transmitter 2 is sent through a hybrid transformer 5 and further through a connection terminal 6 to a two-wire line (not shown).

The received signal including the echo signal received from the terminal 6 through the hybrid transformer 5 is digitized by the A/D converter 7, and the received signal including the echo signal is digitally processed by the subtracter 8 to the modulator. The output of 4 is subtracted. Ideally, the output r of the subtracter 8 is only the received signal, which is converted to analog a by the D/A converter 9 and given to the receiver 11, where the received data is restored and sent to the received data output terminal. 12.

- The output r of the force estimator 8 is a limiting error signal for the adjustment of the pseudo-echo signal generator 3 and is therefore applied to the pseudo-echo signal generator 3 and used for adaptively adjusting the pseudo-echo. The pseudo echo signal generator 3, modulation G 4 , A/D converter 7, subtractor 8, and D/A converter 9 within the broken line in the figure constitute a digital processing adaptive echo canceller 13. The echo canceller 13 of this configuration example must operate at a frequency higher than the Nyquist frequency of one transmission band, and has the drawback that high-speed arithmetic processing is required.

FIG. 2 is a circuit example showing a second conventional example of a digital processing echo canceller device, in which the echo canceler is integrated with the receiver. The operation of the second conventional example is as follows. The basic operation of this echo canceller 190 is the same as the first conventional example (Fig. 1), but in this second conventional example, the output of the A/D converter 7 is equalized by the equalizer 14, and then demodulated. The subtracter 16 subtracts the output of the pseudo echo signal generator 17, and echo cancellation is performed using the demodulated baseband component.

The output of the subtracter 16 is also supplied to the determination circuit 18, and the error signal adjusted and collected by the pseudo echo signal generator 17 is the difference between the result of determining the received signal in the determination circuit 18 and the received signal (represented by e in the figure). ). Since this second conventional example performs echo cancellation in the baseband band, the operation of the echo canceller is the same as the data symbol rate, and the arithmetic processing speed is also reduced compared to the first conventional example.

However, when the line loss is large, an echo signal that is larger than the received signal may be added to the received signal, and even in this case, it is necessary to In both the first and second conventional examples, it is necessary to widen the dynamic range of the A/D converter 7, which increases the required word length (number of bits) and has the disadvantage of complicating the hardware configuration. Furthermore, as the required word length increases, the number of bits for digital operations also increases, causing the problem that the scale of the arithmetic processing circuit also increases. Note that if the transmission method is one in which baseband transmission is performed between terminals, the modulator 4 in FIG. 1 and the demodulator 15 in FIG. 2 are unnecessary.

Summary of the Invention The purpose of the present invention is to narrow the dynamic range of the A/D converter in the adaptive echo canceller, reduce the number of bits required for the arithmetic processing circuit, and reduce the circuit size accordingly. However, it is an object of the present invention to provide an echo canceller device for a data transmission device that can be constructed at low cost.

According to this invention, an analog processing pre-echo canceller and an adaptive digital processing echo canceller are combined, and a part of the echo signal is removed from the input signal of the adaptive digital processing echo canceler in advance to reduce the relative level difference with the received signal. do.

<Embodiment> FIG. 3 shows an embodiment in which the present invention is applied to an apparatus of the type shown in FIG. An input data sequence applied to input terminal 1 is applied to adaptive echo canceller 13 and transmitter 2, and is modulated in a predetermined manner by transmitter 2 to become a transmission signal. The transmission signal passes through the hybrid transformer 5 and is sent out to the two-wire line from the terminal 6. In this invention, the transmitter 2
The transmitted signal is applied to the filter circuit 22 in the pre-echo canceller 21 and given a specific frequency characteristic,
After being suitably attenuated by the attenuation circuit 23, it is subtracted by the subtraction circuit 24 from the received signal including the echo signal arriving through the terminal 6 and the hybrid transformer 5.

Therefore, filter circuit 2.20 Frequency characteristics and attenuation circuit 2
If the amount of attenuation in step 3 is appropriately determined, the absolute amount of the echo signal included in the signal input to the digital processing adaptive echo canceller 13 will be small, and the number of bits required for the A/D converter 7 will be small. It is possible to reduce the number of bits of the arithmetic processing circuit used for digital signal processing.

In addition, since the level difference between the received signal and the echo signal becomes smaller, an analog automatic gain adjustment circuit is installed before the A/D converter 7, and an automatic gain adjustment circuit is installed in the preset stage of the A/D converter 7. Since it is also possible to set the signal level automatically, the calculation accuracy of digital signal processing can be improved even with the same number of bits.

A method for appropriately selecting the frequency characteristics of the filter circuit 220 and the amount of attenuation of the attenuation circuit 23 will be described in detail below. Note that the operation of the adaptive echo canceller 13 is the same as that of the conventional example shown in FIG. 1, so a description thereof will be omitted.

The cause of the generation of echoes to be canceled by the echo canceller is mainly due to impedance mismatch between the line and the balanced circuit network in the hybrid transformer 5, which causes the transmitted signal to sneak around to the received signal side.

For example, the echo signals on a telephone line include the echo generated by the hybrid transformer 5 in the own transmission equipment (hereinafter referred to as near-end echo), and the hybrid transformer used for 2-wire to 4-wire conversion within the line. echo yJ') caused by
, far-end echo). Although the far-end echo includes a time-varying element, it is subject to round-trip attenuation on the line, so it generally has a signal level that is comparable to or lower than the received signal. On the other hand, while the balanced circuit network of the hybrid transformer in one transmission device is fixed, it is necessary to connect lines with various impedances, so the near-end echo is about 10 to 20 dB below the transmitted signal. It can only be certified to the level of . Therefore, Mil device co-canceller 21
The desired effect can be expected by canceling only the near-end echo.

On the other hand, errors in received data depend on the received signal to echo level ratio, so when the received signal level is greatly attenuated, i.e. in the case of a telephone line, the near-end echo path is The frequency characteristics of the filter circuit 22 and the amount of attenuation of the attenuation circuit 23 may be determined by approximating the frequency characteristics.

In addition, since the pre-echo canceller 21 can obtain the desired effect by approximately reducing the level difference between the echo signal level and the received signal level, the input impedance seen from the hybrid transformer 5 to the line side when the loss is large. However, it is possible to determine the characteristics of the pre-echo canceller 21 by approximating the frequency characteristics of the near-end echo path when a circuit corresponding to the characteristic impedance is connected instead of the line by using the fact that it can be approximated to the characteristic impedance of the line. good.

Furthermore, it is also possible to use a frequency characteristic that approximates only the amplitude-frequency characteristic of the near-end echo path.

In the following explanation, the amount of attenuation of the attenuation circuit 23 is assumed to be constant regardless of the line to which it is connected, but in order to further improve the accuracy of canceling the echo signal, the attenuation circuit 23 may be configured as shown in FIG. Good. In the figure, the signal level holding circuit 25
is activated only during the period when the transmitter 2 is transmitting a signal in half-duplex mode, and detects and holds the peak value of the signal (ie, echo signal) from the received signal input terminal 26. The held peak value is input to the variable attenuator 27. The variable attenuator 27 attenuates the peak value of the output of the filter circuit 22 inputted from the input terminal 28 so as to match the peak value of the echo signal, outputs it from the output terminal 29, and adds it to the subtraction circuit 24. By doing this, the near-end echo signal in the output signal of the subtraction circuit 24 always becomes zero at its peak position, regardless of the type of line connected, and the echo signal level can be reduced. The operation of the signal level holding circuit 25 may be performed during a half-duplex training period that is performed prior to full-duplex data communication.

Although the embodiments described above are directed to a general telephone line that includes a carrier section, the present invention can also be applied as is since the far end echo signal is simply eliminated even when only a pair of lines are used. . In addition, although the adaptive echo canceller 13 used in the embodiment is that of the conventional example 1 (FIG. 1), the conventional example 2 (FIG. 2) also has the configurations of (7) and other REs. It is obvious that the present invention can also be applied to an adaptive echo canceller.

<Effects> As explained above, the present invention is composed of a pre-echo canceller that performs echo cancellation using analog processing and an adaptive echo canceler that performs echo cancellation using digital processing. Since the echo cancellation is performed and then the adaptive digital echo canceller performs precise echo cancellation, the pre-echo canceller consisting of an analog circuit can have a simple configuration, while the input signal of the adaptive digital echo canceller Since the included echo signal component is attenuated to the same extent as the received signal component, the number of bits required for the A/D converter and arithmetic processing circuit is small, simplifying the circuit scale, and using the same number of bits. In some cases, there is an advantage that echo cancellation can be performed with high accuracy. Therefore, if this invention is applied to a modem that performs full-duplex communication using a 4-wire line, it will be possible to reduce the cost without increasing the number of bits of the A/D converter and arithmetic processing circuit used in the modem. It is possible to configure a modem that performs full-duplex communication over a two-wire line.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of an example of a transmission device including a conventional echo canceller device, and FIG. 2 is a block diagram showing a transmission device including another conventional example of an echo canceller device.
FIG. 3 is a block diagram showing the configuration of an embodiment of the inventive device, and FIG. 4 is a configuration diagram of an example in which the amount of attenuation of the attenuation circuit 23 in FIG. 3 is automatically adjusted. 1: Transmission data input terminal, 2: Transmitter, 3: Pseudo echo signal generator, 4: Modulator, 5: Hybrid transformer, 6
: 2-wire line connection terminal, 7: A/D converter, 8: Subtractor, 9: D/A converter, 11: Receiver, 12: Received data output terminal, 13: Digital processing adaptive echo Canceller, 21: Pre-echo canceller, 22: Filter circuit, 23: Attenuation circuit, 24: Subtraction circuit.

Claims (1)

[Claims] (1) In an echo canceller device for a data transmission device that performs full-duplex communication using a two-wire line, there is a filter circuit that gives specific frequency characteristics to the output signal of the data transmission device, and the output of the filter circuit is attenuated. It has a pre-echo canceller comprising an attenuation circuit and a subtraction circuit that subtracts the output of the attenuation circuit from the received signal, and the output of the pre-echo canceller is used as the received signal for the adaptive echo canceller. Echo canceller device for data transmission equipment.