JPS61192133A - Echo canceller - Google Patents

Abstract

PURPOSE:To suppress an echo component at the discriminating point in a reception signal by providing the 1st correcting means for correcting a signal with a higher Baud rate than that of a reception signal and for inputting the result to a line equalizer and the 2nd correcting means for correcting an output signal of the line equalizer in the same Baud rate as that of the reception signal to suppress the uncorrected echo signal at the discriminating point of the reception signal. CONSTITUTION:An echo signal from a transmission digital signal sent to a reception side via a hybrid transformer 1 is subtracted from an echo replica 1 signal at a subtractor 6 and the result is given to a line equalizer 7 after the uncorrected residual peak at the output of the line equalizer 7 is suppressed to a prescribed value or below. An echo replica 2 signal for coarse remaining echo correction on a time axis generated including the discriminating point of the reception signal from the 2nd stage echo canceller 8 is supplied to a subtractor 11 and the echo component is suppressed substantially at the discriminating point of the reception signal by subtracting the echo replica 2 signal from the remaining echo signal.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an echo canceller for a bidirectional digital transmission system using a hybrid circuit.

In digital subscriber line transmission systems and the like, bidirectional digital transmission is made possible via a two-wire transmission line by using a hybrid circuit.

In such a transmission system, the transmitted signal goes around to the receiving side via the hybrid circuit and becomes an echo signal, which causes problems in receiving digital data. Although technical means have been developed to avoid such inconveniences, there is still room for improvement in these means.

[Conventional technology]

Conventionally developed echo cancellers are equipped with an echo canceller that cancels the echo (i) that comes around via the hybrid circuit at a baud rate faster than the baud rate of the received signal, making it possible to eliminate the influence of the echo signal on the line equalizer output. We try to eliminate it as much as possible.

[Problem that the invention seeks to solve]

Since this echo canceller cannot completely cancel the echo, echo correction remains at the decision point, and the inconvenience caused by the presence of the echo signal remains unsolved.

Furthermore, in order to suppress the peak value of the residual echo correction to a predetermined value or less, it becomes necessary to increase the number of sampling points used in the correction. This is not a good idea as it will drastically increase the amount of hardware.

Furthermore, since it is required that the dynamic range required for echo correction be relatively large, the correction circuit must be highly accurate, in other words, it must have a large number of bits.

[Means for solving problems]

The present invention provides an echo canceller that solves the above-mentioned problems, and includes means for transmitting digital signals to and from the hybrid circuit in a bidirectional digital transmission system using a hybrid circuit. a first correction means for receiving a received signal from the hybrid circuit, correcting an echo signal from the hybrid circuit at a baud rate higher than the baud rate of the received signal, and inputting the received signal to a line equalizer; and upon receiving the output signal of the line equalizer, correct the output signal of the line equalizer at the same baud rate as the baud rate of the received signal so as to suppress the correction residual of the echo signal at the decision point of the received signal. and a second correction means for carrying out the correction.

[Effect]

According to the echo canceller of the present invention, in the first correction means that receives the transmitted digital signal input to the hybrid circuit and the received signal of the hybrid circuit, the echo signal included in the received signal is transmitted to the line etc. at a baud rate higher than the baud rate of the received signal. The received signal is corrected so as to suppress the corrected residual peak value at the output of the converter to a predetermined value or less, and is further corrected in the second correcting means so as to suppress the corrected residual of the echo component at the decision point of the received signal. Echo components at decision points in the signal are substantially suppressed.

〔Example〕

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

FIG. 1 shows an embodiment of the invention. 1 is a hybrid transformer of a bidirectional digital transmission system. 2 is a first stage echo canceller which receives the transmitted digital signal on line 3 and the echo cancel-1 signal on line 4 and sends out an echo replica-1 signal on line 5. Reference numeral 6 denotes a multiplier which receives the echo replica-1 signal and the received signal from the hybrid transformer l, calculates the difference between them, and generates an echo cancel-1 signal on line 4.

7 is a line equalizer which receives the signal on line 4 and applies a required equalization effect to the signal.

8 is a second stage echo canceller which receives the transmitted digital signal on line 3 and the echo cancel-2 signal on line 9 and sends out an echo replica-2 signal on line 10. A subtracter 11 receives the echo replica-2 signal and the output signal of the line equalizer 7, takes the difference between them, and generates an echo cancel-2 signal on the line 9.

FIG. 2 shows an example of an echo canceller used in the first stage echo canceller 2 and the second stage echo canceller 8. This exemplary echo canceller is a table reference type echo canceller using a random access memory (RAM) 20, and echo replica data is stored in the RAM 20. The data is updated during the echo canceller training period as follows. That is, echo replica data sequentially generated via an error detection circuit 21, an adder circuit 22, and a flip-flop (FF) 23 is sequentially stored at an address generated via a shift register 24 and a control circuit 25. An update will be made. Then, the echo replica data in the RAM 20 is sent to an adder circuit 22 via a flip-flop (FF) 26.
and is also supplied to a digital-to-analog (D/A) converter 27. The output signal of the D/A converter 27 is sent to the hybrid transformer 1 as an echo replica-1 signal.
The subtracter 6 is supplied with the received signal from the subtracter 6 to generate an echo cancel-1 signal therefrom.

Further, in this embodiment, the second stage echo canceller also has the same configuration as the first stage echo canceller, as described above. The difference lies in the sampling clock. That is,
Whereas the second stage echo canceller operates with a required sampling clock, such as a baud rate sampling clock, the first stage echo canceller operates with a high speed sampling clock that is a predetermined value higher than the baud rate. This high-speed sampling clock can also have a period considerably lower than the sampling clock required by conventional echo cancellers.

The operation of the echo canceller of the present invention configured as described above will be explained using FIG.

Echo signal sent from the transmitting digital signal to the receiving side via the hybrid transformer 1 [(3-1) in Figure 3]
At the time that
)), and from there the echo replica-1 signal for fine echo correction on the time axis ((3-3 in Figure 3)
) is generated and supplied to the subtracter 6. In the subtracter 6, the echo replica-1 signal is subtracted from the echo signal. Thereby, correction is made to the extent that the corrected residual peak value at the output of the line equalizer 7 can be suppressed to a predetermined value or less. Thus, the echo cancellation-1 signal [(3
-4)) is fed to the input of the line equalizer 7.

Since the line equalizer 7 has a high-gain filtering characteristic, the signal that is equalized in the line equalizer 7 and output from it has a signal as shown in (3-5) in Fig. 3. A residual echo signal (echo correction residual) is generated and supplied to the subtracter 11.

This subtracter 11 also includes the judgment point of the received signal generated from the second stage echo canceller 8 operating at the same baud rate as the received signal ((3-6) in Figure 3). The echo replica-2 signal for coarse residual echo correction on the time axis [(3-7 in Figure 3)
) is supplied.

By subtracting the echo replica-2 signal from the residual echo signal in the subtracter 11 that receives the above two signals, an echo cancel-2 signal [third (
3-8)) is obtained.

In the above embodiment, the first-stage echo canceller and the second-stage echo canceller both obtain echo replica signals by training, and after that period, use the obtained echo replica signals. An adaptive version of the echo canceller may be used to dynamically deal with the situation.

Further, the gist of the present invention can also be applied to a case where a transversal type echo canceller is used.

[Effects of the Invention] As explained above, according to the present invention, it is possible to sufficiently correct the echo at the decision point of the signal, and the accuracy required for each echo canceller can be relaxed without increasing the circuit scale. These advantages can be achieved, such as the ability to advantageously develop an echo canceller into an LSI.

[Brief explanation of drawings]

FIG. 1 is a diagram showing one embodiment of the present invention, FIG. 2 is a diagram showing a specific configuration example of the echo canceller used in the embodiment of FIG. 1, and FIG. 3 is a diagram showing signal waveforms at various parts of the embodiment of FIG. 1. FIG. In the figure, 1 is the hybrid nodotrans, 2 is the first
1. 6.11 is a subtracter, 7 is a line equalizer, and 8 is a second stage echo canceller. Figure 1 Figure 2 Figure 3 Judgment point

Claims (1)

[Claims] In a two-way digital transmission system using a hybrid circuit, a transmission digital signal to the hybrid circuit and a reception signal from the hybrid circuit are received, and an echo signal from the hybrid circuit is corrected at a baud rate higher than the baud rate of the reception signal. a first correction means for inputting the received signal to a line equalizer; and a first correction means for receiving a digital signal transmitted to the hybrid circuit and an output signal of the line equalizer to calculate a correction residual of an echo signal at a decision point of the received signal. an echo canceller comprising: second correction means for correcting the output signal of the line equalizer at the same baud rate as the baud rate of the received signal so as to suppress the received signal.