JP2862629B2 - Digital transmission equipment - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital transmission device, and more particularly, to a digital transmission device having an echo suppression function.

[Prior Art] Conventionally, there is a digital transmission apparatus as shown in FIG.

The digital transmission device shown in FIG. 3 has a transmission signal line 11 for transmitting a transmission signal 101 from the terminal device to a transmission line 10, and a reception signal line 12 for transmitting a reception signal from the transmission line 10 to the terminal device. The digital transmission device is located at a connection point between the transmission signal line 11, the reception signal line 12, and the transmission line 10,
A hybrid circuit 1 that performs two-wire to four-wire conversion, an echo canceller 2 that generates an echo replica 103 of the transmission signal 101, a filter 3 that removes noise of the signal from the hybrid circuit 1, and an echo replica 103 from the received signal 102 And a subtractor 4 for reducing the

In this digital transmission device, a transmission signal 101 is transmitted to the transmission line 10 through the hybrid circuit 1. on the other hand,
The signal arriving from the transmission path 10 passes through the hybrid circuit 1 and passes through the filter 3 to become a received signal 102. This allows
Transmission and reception can be performed simultaneously using one transmission path.

Incidentally, the received signal 102 includes not only the signal transmitted from the other party but also the transmission signal 101 in the hybrid circuit 1.
Leak through. This is called an echo and the received signal
Disturb 102. Therefore, in order to remove the echo from the received signal 102, an echo replica 103 having the same waveform as the echo is generated by the echo canceller 2, and the subtractor 4 cancels the echo included in the received signal 102 by the echo replica 103. The reception output 104 is obtained.

An example of this type of digital transmission device is disclosed in Japanese Patent Application Laid-Open No. 63-100828.

This transmission device includes a hybrid circuit, an echo canceller, an adder, and a line equalizer, and connects a filter that emphasizes and passes high-frequency components to a reception signal output terminal of the hybrid circuit. Be composed. This filter is inserted for the purpose of restoring the high frequency component, which is more strongly attenuated than the low frequency component, between the transmission path, the hybrid circuit, and the output point of the adder to the addition point of the adder.

As for the echo canceller type digital transmission device, for example, IEE, Transaction on Communication, COM30 (19
1982) pp. 2095-2108 (IEEE Trans.Comunication)
COM30 (1982) PP209-2108).

[Problems to be Solved by the Invention] Meanwhile, in order to improve the performance of removing the echo generated in the hybrid circuit, it is necessary to cancel the echo having a long response time waveform including a low frequency. For this purpose, it is necessary to set the tap length of the echo canceller to be large in accordance with a long response time waveform. But,
When the tap length is increased, there is a problem that the scale of hardware increases.

In this regard, the technique shown in FIG. 3 described above does not consider the miniaturization of the echo canceller without lowering the echo canceling performance.

On the other hand, the technique disclosed in Japanese Patent Application Laid-Open No. 63-100828 discloses that a filter for emphasizing high frequency components is provided in order to recover attenuation of high frequency components. However, there is no explanation as to why the recovery of the high frequency band component can reduce the size of the echo canceller.
The attenuation of the high frequency band component occurs mainly in the transmission path, and causes a problem not with the echo component but with the transmission signal from the other party. Therefore, it is considered that the recovery of the high frequency band component does not directly contribute to improving the elimination performance of the echo generated in the own device. In order to improve the performance of the echo canceller, it is necessary to remove low frequencies, and this point is not considered.

An object of the present invention is to provide a digital transmission device equipped with an echo canceller that is reduced in size without deteriorating the echo canceling performance.

[Means for Solving the Problems] The above object is achieved by inserting and connecting a filter having a characteristic of attenuating a low-frequency component including an echo component between a hybrid circuit and a subtractor.

As this filter, a filter having a transmission zero near DC is used. The transmission zero may be one or more. Preferably, one having two transmission zeros near DC is used. This type of filter has a frequency characteristic having a gain proportional to the square of the frequency near DC.

According to the present invention, a digital communication device is provided by connecting a terminal device to a digital transmission device. Further, a digital exchange connected to a transmission line via a digital transmission device is provided. Further, an information network is provided in which each of the terminal device and the exchange has a digital transmission device, and the terminal device and the exchange are connected to a transmission line via each digital transmission device.

[Operation] Since the filter inserted and connected between the hybrid circuit and the subtractor has a characteristic of attenuating low frequency components including echo components, a signal received from the hybrid circuit is a low frequency component. Decays. For this reason, the low frequency component contained in the echo is removed.

As a result, the echo canceller only needs to be able to remove only the high frequency range of the frequency components of the echo. That is, since the replica to be generated does not need to include the low frequency band component, the necessary tap length is reduced, and the hardware scale can be reduced.

In this case, since the low-frequency component of the echo is removed by the filter as described above, the echo suppression performance does not deteriorate.

Examples Examples of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of the configuration of an information network incorporating one embodiment of the digital transmission apparatus of the present invention.

The information network shown in FIG. 1 is configured by connecting a communication device on the subscriber side and an exchange using a transmission line 10. The communication device on the subscriber side includes a terminal device 30 and a digital transmission device 20. In addition, exchange 40
The digital transmission device 20 is also connected to the digital transmission device 20.

The digital transmission device 20 transmits a transmission signal from the terminal device 30.
It has a transmission signal line 11 for transmitting 101 to the transmission line 10 and a reception signal line 12 for transmitting a reception signal from the transmission line 10 to the terminal device. The digital transmission device 20 includes a hybrid circuit 21 that performs two-wire to four-wire conversion at a connection point between the transmission signal line 11, the reception signal line 12, and the transmission line 10, and an echo canceller 22 that generates an echo replica of the transmission signal. A subtractor 24 for subtracting an echo replica from a received signal; a filter 23 inserted and connected between the hybrid circuit 21 and the subtractor 24;
And a DFE (decision feedback equalizer) 25 for correcting distortion of the received signal waveform based on the output signal waveform of the output signal 24.

Except for the filter 23, each of the above-described components can be basically the same as that used in the conventional digital transmission device. However, for example, an echo canceller 22 or the like having a smaller hardware scale than the conventional hardware can be used due to the effects of the present embodiment described later.

The filter 23 has two transmission zeros near the direct current. As shown in FIG. 2, the filter 23 has a gain (amplification factor) proportional to the square of the frequency f, asymptotically to the curve shown by the broken line near the direct current. This is because the filter has two transmission zeros in direct current.

Further, when transmitting and receiving a signal of 80 KHz, for example, the filter 23 has a cutoff frequency of 1 to 10 KH, for example.
Selected in the range of z.

 Here, an example of the transfer function of the filter 23 will be described.

Here, Z = exp (S / 180 KHz) S is a complex frequency In the information network configured as described above, communication between the terminal device 30 and the exchange 40 is performed as follows. When a transmission signal is output from the terminal device 30, the transmission signal is transmitted from the hybrid circuit 21 to the transmission path 10 via the transmission signal line 11 of the digital transmission device 20. This signal is transmitted on transmission line 10
Is input to the hybrid circuit 21 of the digital transmission device 20 on the exchange 40 side, and further output to the reception signal line 12. The received signal passes through a filter 23 and is sent to a subtractor 24. Here, the echo replica generated by the echo canceller 12 is subtracted from the received signal, and a hybrid circuit
The echo generated at 21 is removed. The output of the subtractor 24 is DF
The state of the waveform is detected by E25, and the detection information is fed back to the subtractor 24 so as to cancel the distortion of the waveform. Then, it is sent to the exchange 40 as a reception output, and further sent to the other party's subscriber.

On the other hand, the transmission signal from the other party is transmitted from the exchange 40 to the transmission signal line 11 of the digital transmission device 20 and the hybrid circuit.
21, the transmission path 10, the hybrid circuit 21, the filter 23, the subtractor 24, and the DFE 25 are sent in this order, and input to the terminal device 30.

Here, the operation of the filter 23 will be described with reference to FIG.

FIG. 4 shows an example of an echo waveform generated when a single transmission signal wraps around a reception signal.

As is apparent from FIG. 5, when a conventional filter having no transmission zero point in direct current is used, the received signal output from the filter has a low frequency component as shown in FIG. To approach zero. in this case,
To remove echoes even in the low frequency range where the response time is long,
Since it is necessary to increase the tap length of the echo canceller, the circuit scale of the echo canceller increases.

On the other hand, when the filter of the present embodiment is used, the low-frequency component is removed, so that the filter output signal becomes zero in a short time and the response disappears in a short time as shown in FIG. Therefore, according to the present embodiment, it can be seen that the echo in the low frequency range is removed by the filter. As a result, it is sufficient for the echo canceller 22 to remove echoes in a high frequency range equal to or higher than the cutoff frequency of the filter 23. Therefore, the tap length can be small, and the echo canceller 22
Can be reduced in circuit scale.

As described above, in the digital transmission apparatus according to the present embodiment, the response of the echo can be shortened. Can be kept to the same extent. Of course, when designing to improve the accuracy of the hybrid circuit, it goes without saying that the performance can be higher than that of the conventional circuit.

Since the filter removes low frequency components of the received signal, the waveform of the received signal is distorted. This distortion is DF
It can be easily modified by using E25. Therefore, even if a problem such as waveform distortion occurs due to the use of a filter when reproducing the received signal, it is avoided.

Next, including the embodiments described above, the hybrid circuit 21, the echo canceller 22, the filter 23, and the subtractor 24, which are preferably used in the digital transmission device of the present invention, will be described.
One embodiment will be described with reference to FIGS. 5 to 9. FIG.

As shown in FIG. 5, the hybrid circuit 21
A transformer 211 for transmitting / receiving a signal to / from 10, a resistor 212 connected between the transmission signal line 11 and the transformer 211, and a coefficient for multiplying a transmission signal transmitted from the transmission signal line 11 by a constant coefficient and outputting the result And a subtractor 214 connected between the connection point between the resistor 212 and the transformer 211 and the reception signal line 12 and subtracting the signal output from the coefficient unit 213 from the reception signal. Also, the transmission line side terminal of the transformer 211 includes:
A DC blocking capacitor 215 is provided. This capacitor can be omitted when a direct current is not flowing through the transmission line 10.

The echo canceller 22, the filter 23 and the subtractor 24
This is realized by the signal processing device 60. The filter 23
An A / D (analog / digital) converter 50 for converting a signal from the hybrid circuit into digital data is provided on the input side of the circuit.

Specifically, the signal processing device 60 is, for example, as shown in FIG.
gital Signal Processor) DSP (hereinafter abbreviated as DSP) 61
And R for storing the operation program of DSP61, fixed data, etc.
An OM 62, a RAM 63 for storing data and the like required in the processing process by the DSP 61, an input device 64 for receiving signals from the transmission signal line 11, the reception signal line 12, and the like, and an output device 65 for outputting a calculation result of the DSP 61, And a bus line 66 for connecting these elements. These can be constituted by one chip as an LSI. Of course, it can also be constituted by a multi-chip IC.

The signal processing device 60 not only functions as the echo canceller 22, the filter 23, and the subtractor 24, but also performs a PLL function and a received signal processing function. The signal processing device 60 is provided independently of a signal processing device (not shown) that performs a PLL function, a reception signal processing function, and the like, and dedicated functions of the echo canceller 22, the filter 23, and the subtractor 24 are provided. May be executed.

The addresses of the ROM 62 and the RAM 63 are determined, for example, as shown in FIG. That is, in the storage area of the ROM 62, storage areas for filter, EC (echo canceller), PLL, and reception signal processing are set, and instructions, fixed data, and the like are stored in the respective storage areas. .
In the storage area of RAM63, PL for EC (echo canceller), PL
L, a storage area for receiving signal processing and a storage area for filtering are set, and data necessary for processing, processing results, and the like are stored.

In the filter storage area of the ROM 62, a filter processing program for realizing the filter 23 having the characteristic of attenuating low frequency range components, for example, a calculation program for a filter having a transfer function represented by the above-described first formula is stored. . In the storage area for the EC (echo canceller), an echo replica is generated and subtracted from the received signal.
An echo canceller processing program for executing an echo canceler operation by the echo canceller 22 and the subtractor 24 is stored.

DSP61, according to the instructions and data stored in ROM62,
The signal data input from the input device 64 is processed, and the result is output from the output device 65. In this embodiment, DSP6
1 executes a plurality of processes as described above. FIG. 8 shows an example of this signal processing procedure. In FIG. 8, filter processing 81, EC (echo canceller) processing 82, and
Instructions are read from the ROM 62 in the order of the PLL and the received signal processing 83, and are sequentially executed. These processes are executed cyclically.

In this embodiment, prior to the EC process 82, the filter process 81
Is performed, and the low frequency band component included in the received signal is removed. Therefore, in the EC processing 82, since the data processing for the low frequency band having a large data amount is not required when generating the echo replica, the storage capacity of the EC storage area of the RAM 63 is significantly reduced, for example, to about 50%. be able to. It should be noted that the storage capacity of the RAM 63 can be reduced to about 2/3 of that of the related art even if the storage area for filtering is increased.

In the echo canceller processing, the processing data amount is reduced, so that the DSP 61 can perform processing without using expensive high-speed processing. Note that a general-purpose processor may be used instead of the DSP as long as the processing speed can be handled.

FIG. 9 shows an example of a data flow in signal processing for realizing the function of the filter 23.

According to this data flow, filter processing is executed by combining data addition, coefficient multiplication, and data delay. That is, the input data obtained by multiplying the data delayed by Z- ¹ by the coefficient K4 and the data obtained by multiplying the data delayed by Z- ¹ by the coefficient K5 are added. . Furthermore, to what coefficient K1 is multiplied to the added data, and that the coefficient K2 is multiplied to data received delay Z ^-1, further, coefficient data subjected to delay Z ^-1 K
The value multiplied by 3 is added. by this,
The input data is filtered and output.

As described above, according to each of the above-described embodiments, the burden on the echo canceller can be reduced, and the hardware scale can be reduced. Further, in order to suppress the occurrence of echo, it is possible to reduce the size of a transformer of a hybrid circuit and a capacitor for blocking direct current, which have been difficult to reduce in size.

[Effect of the Invention] As described above, according to the present invention, the response of the echo can be shortened, and the echo canceling performance is not reduced.
This has the effect of reducing the size of the echo canceller.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an example of the configuration of an information network incorporating an embodiment of the digital transmission apparatus of the present invention, and FIG. 2 is a graph showing an example of the frequency characteristics of a filter used in the digital transmission apparatus of the present invention. FIG. 3 is a block diagram showing the configuration of a conventional digital transmission apparatus, and FIG.
FIG. 5 is a graph showing an example of an echo waveform generated when a single transmission signal wraps around a reception signal. FIG. 5 is a block diagram showing an example of a configuration of a hybrid circuit and a signal processing device used in the digital transmission device of the present invention. 6, FIG. 6 is a block diagram showing an example of the configuration of the signal processing device, FIG. 7 is an explanatory diagram showing ROM and RAM address mapping used in the configuration of the signal processing device, and FIG. And FIG. 9 is a flowchart showing an example of a data flow in signal processing for realizing the function of the filter. 10 transmission line, 11 transmission signal line, 12 reception signal line,
20 ... Digital transmission device, 21 ... Hybrid circuit,
22 ... Echo canceller, 23 ... Filter, 24 ... Subtractor, 25 ... DFE (decision feedback equalizer), 30 ... Terminal device, 4
0 ... switchboard, 60 ... signal processor, 61 ... DSP (digital signal processor), 62 ... ROM, 63 ... RA
M, 64: Input device, 65: Output device, 66: Bus line.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) H04B 3/20-3/23 H04B 7/005-7/01 H03H 21/00

Claims (5)

(57) [Claims] 1. A hybrid circuit for transmitting a transmission signal to a transmission path and receiving a signal arriving from the transmission path, an echo canceller for generating an echo replica by using the transmission signal as an input, and converting an echo replica from the reception signal. A subtractor that subtracts and removes an echo included in the received signal, and is inserted and connected between the hybrid circuit and the subtractor;
A filter having two transmission zeros near DC, wherein the filter has a transfer function: Here, Z = exp (S / 180kHz), where S is a complex frequency. 2. A digital communication device comprising a terminal device connected to the digital transmission device according to claim 1. 3. A digital exchange, which is connected to a transmission line via the digital transmission device according to claim 1. 4. A terminal device and an exchange each comprising the digital transmission device according to claim 1, wherein the terminal device and the exchange are connected to a transmission line via each digital transmission device. Information network. 5. A processor for executing signal processing, and an RO for storing an operation program of the processor, fixed data, and the like.
M, a RAM for storing data and the like necessary in the process of processing by the processor, a transmission signal, an input device for capturing a signal such as a reception signal, and an output device for outputting a calculation result of the processor, Is stored at least a filter processing program for realizing a filter having a characteristic of attenuating low frequency components, and an echo canceller processing program for generating an echo replica and performing an echo canceller operation of subtracting this from a received signal, The processor sequentially reads out these programs, executes a filtering process on the received signal, obtains a received signal in which the low-frequency component is attenuated, executes an echo canceller operation on the received signal, and executes the filtering process. Is assumed to have two transmission zeros near DC. Ri, is its transfer function, Here, Z = exp (S / 180kHz). S is a complex frequency.