JPH05218903A - Echo canceller - Google Patents

Abstract

PURPOSE:To reduce the scale of the hardware by detecting a level of a residual echo so as to apply variable control to a delay time of transmission data thereby reducing number of taps of an adaptive filter. CONSTITUTION:An echo detection circuit 10 detects a level of a residual echo of a scramble transmission signal at training. Then a variable delay circuit 11 delays the scramble transmission signal and the circuit 10 detects the residual echo and the variable delay time is adjusted so as to minimize the residual echo level through the repetition as above. Since number of taps of an adaptive filter 9 is reduced, the scale of the hardware is reduced and the adjustment is facilitated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an echo canceller, and more particularly to an echo canceller used in a 2-wire / 4-wire conversion circuit for data communication.

[0002]

2. Description of the Related Art A conventional echo canceller includes a scrambler 1 for converting transmission data SD into scrambled transmission data SDS which is random data, as shown in FIG.
Coder 2 that encodes binary data and converts it into analog signals
And the analog signal from the coder 2 to the hybrid circuit 4
To the driver 3, a hybrid circuit 4 for 2-wire to 4-wire conversion, a filter 5 that removes noise from the received signal separated by the hybrid circuit 4, and a filter received signal RDF that is the received signal that has passed through the filter 5. Adder circuit 6 for subtracting the pseudo echo signal E from the filter reception signal R
A detection circuit 7 that determines and decodes "0" and "1" data from DF, and a descrambler 8 that removes randomness from the data decoded by the detection circuit 7 and converts it into reception data RD.
And an adaptive filter 9 for predicting and generating the pseudo echo signal E from the scrambled transmission data SDS.

Next, the operation of the conventional echo canceller will be described.

First, the hybrid circuit 4 performs 2-wire 4-wire conversion in which the analog output signal from the driver 3 is sent to the 2-wire side and the received signal from the 2-wire side is received by the filter 5 side. However, since a part of the output signal of the driver 3 returns to the hybrid circuit 4 as an echo, it becomes an interference signal with respect to the received signal, and the data is not correctly received.

Therefore, the scrambled transmission data SDS
Was input to generate a pseudo echo signal E by the adaptive filter 9 and subtracted from the filter reception signal RDF to cancel the echo so that correct data can be received.

The adaptive filter 9 is a kind of digital filter. As shown in FIG. 4, N-1 delay circuits DL are provided.
1 to DLN−1, N coefficient circuits C1 to CN, and an accumulator circuit AC1 with a known tap number N transversal filter.

[0007]

In the above-mentioned conventional echo canceller, the actual echo delay time changes according to the distance between the lines on the two-wire side, whereas the scrambled transmission data is transmitted through the hybrid circuit. Since the time to return to the receiving side as an echo through the adder is constant and the time to be input to the adder as a pseudo echo via the adaptive filter is also constant, these values are eliminated when the echo is canceled by the adder. It is necessary either to adjust the delay time of the echo to the delay time of the actual echo in advance, or to increase the number of taps of the adaptive filter sufficiently to cope with changes in the delay time of the actual echo. However, there are drawbacks in that it is difficult to make adjustments to cope with changes in the distance between the two lines, and the scale of the hardware increases.

[0008]

The echo canceller of the present invention is a hybrid circuit for connecting a two-wire type transmission line and a four-line type transmission line having a transmission line and a reception line, and a transmission input from the transmission line. An adaptive filter for predicting and generating a pseudo echo signal from data and a subtraction circuit for subtracting the output of the adaptive filter and the received signal separated by the hybrid circuit are provided, and the pseudo echo signal for the adaptive filter is generated. In an echo canceller that performs training for initial setting for, a level detection circuit that detects the level of a residual echo from the output of the subtraction circuit at the time of the training, and is inserted between the transmission line and the hybrid circuit, When the transmission data is delayed so that the level of the residual echo is minimized by delaying the transmission data. It is constituted by a variable delay circuit for varying the.

[0009]

Embodiments of the present invention will now be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the echo canceller of the present invention.

As shown in FIG. 1, the echo canceller of this embodiment has a scrambler 1, a coder 2, a driver 3, a hybrid circuit 4, a filter 5, and an adder circuit 6 similar to those of the above-mentioned conventional example. In addition to the detection circuit 7, the descrambler 8 and the adaptive filter 9, a bell detection circuit 10 for detecting the level of the residual echo from the output of the addition circuit 6
And a delay circuit 11 for delaying the time when the scrambled transmission data SDS which is the scrambled transmission data is input to the coder 2.

Next, the operation of this embodiment will be described.

FIG. 2A shows the circuit of the present embodiment shown in FIG. 1A when the delay time of the delay circuit 11 is 0, and FIG.
FIG. 4 is a waveform diagram showing an actual echo EA, a pseudo echo E, and a residual echo ER when the delay time of the delay circuit 11 is optimally set.

First, the training of the echo canceller is performed. When the delay time of the delay circuit 11 is 0,
As shown in (A), there are residual echoes. here,
The impulse response waveform of the pseudo echo E is the adaptive filter 9
The number of taps is N, which corresponds to the value of the coefficient of 1. Here, if there is a time difference τ between the actual echo EA and the delay time of the pseudo echo E to the adder circuit 6, the echo at the portion of this time difference τ is not erased and is filtered as the residual echo ER. It is superimposed on the filtered received signal RDF which is the received signal.

The level of the residual echo at this time is detected by the level detection circuit 10 and stored. next,
A small delay time t is given to the scrambled transmission data SDS by the delay circuit 11, the residual echo level at that time is detected again by the level detection circuit 10, and this is stored. Similarly, the delay circuit 11 adds a small delay time to the scrambled transmission data SDS to obtain the delay time tm that minimizes the level of the residual echo ER detected by the level detection circuit 10. Next, the delay time tm is added to the scramble transmission data SDS. This completes the training of the echo canceller and shifts to the actual data communication state.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications can be made. For example, instead of inserting the delay circuit on the coder side, it is of course possible to insert the delay circuit on the input side of the adaptive filter and adjust the delay amount by the output of the level detection circuit without departing from the gist of the present invention. is there.

[0017]

As described above, in the echo canceller of the present invention, the level detection circuit for detecting the level of the residual echo during training and the delay time of the transmission data so that the level of the residual echo is minimized. Since the number of taps of the adaptive filter can be reduced by providing the variable delay circuit that is variable, there is an effect that the scale of hardware can be reduced and the adjustment corresponding to the distance change of the line on the two-wire side can be facilitated. .

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an echo canceller of the present invention.

FIG. 2 is a waveform diagram showing an actual echo, a pseudo echo, and a residual echo when the delay time of the delay circuit in the echo canceller of this embodiment is 0 and when the delay time is optimally set.

FIG. 3 is a block diagram showing an example of a conventional echo canceller.

FIG. 4 is a block diagram showing an example of an adaptive filter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 scrambler 2 coder 3 driver 4 hybrid circuit 5 filter 6 addition circuit 7 detection circuit 8 descrambler 9 adaptive filter 10 level detection circuit 11 delay circuit AC1 accumulation circuit C1 to CN coefficient circuit DL1 to DLN-1 delay circuit

Claims (1)

[Claims] 1. A hybrid circuit connecting a 2-wire transmission line and a 4-wire transmission line having a transmission line and a reception line, and an adaptive circuit for predicting and generating a pseudo echo signal from transmission data input from the transmission line. An echo canceller that includes a filter and a subtraction circuit that subtracts the output of the adaptive filter and the received signal separated by the hybrid circuit, and performs an initial setting training for generating the pseudo echo signal for the adaptive filter. In the above, the level detection circuit for detecting the level of the residual echo from the output of the subtraction circuit during the training, and the level of the residual echo by delaying the transmission data inserted between the transmission line and the hybrid circuit are And a variable delay circuit that varies the delay time of the transmission data so as to minimize the delay time. Echo canceller to.