JPS62176338A - Adaptive echo canceller - Google Patents

Abstract

PURPOSE:To eliminate the deterioration of talking quality due to the double talk detection by providing the 1st delay circuit retarding an echo signal by a prescribed delay time and giving the result to a subtractor and the 2nd delay circuit having nearly the same delay time, retarding a reception signal by the delay time and giving the result to the adaptive digital filter. CONSTITUTION:The 1st delay circuit 51 outputs an echo signal y1 retarding an echo signal (y) from an A/D converter 6 by a prescribed delay time mT and gives the signal y1 to a subtractor 10. Further, the 2nd delay circuit 52 outputs a reception signal x1 retarding a reception signal (x) from an A/D converter 7 by a prescribed delay time mT and gives the signal x1 to an adaptive digital filter 11. Then the time from the generation of double talk to the stop of the adaptive control operation is delayed by the delay times by the 1st and 2nd delay circuits. Thus, the effect of the detection delay of double talk in the double talk detection circuit is not given onto the adaptive digital filter and excellent talking quality is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an adaptive echo canceller (adaptive echo canceller) having a double talk detection circuit (two-way conversation detection circuit).

(Prior art) Conventionally, as a technology in this field, Japanese Patent Application Laid-Open No. 58-1
There were those described in JP-A No. 23240 and JP-A-51-11313. The configuration will be explained below using figures.

FIG. 2 is a block diagram showing an example of the configuration of a conventional adaptive echo canceler.

In FIG. 2, l is a handset, and this handset l
A hybrid coupler 2 for branching transmitted and received signals is connected to the transmitter, and the handset 1 is connected to a transmission line 3 on the transmission side and a transmission line 4 on the reception side through the hybrid coupler 2.

An adaptive echo canceller 5 is connected between the transmission line 3 on the transmission side and the transmission line 4 on the reception side in order to eliminate loopback of signals from the reception side to the transmission side in the hybrid coupler 2 . Since the adaptive echo canceller 5 has a circuit configuration that processes digital signals, the input/output side of the echo canceller 5 includes an A/D converter 6.7 for converting an analog signal into a digital signal, and an A/D converter 6. D/A converters 8 and 9 are connected to convert the signals into analog signals.

The adaptive echo canceller 5 includes a subtracter 10, an adaptive digital filter (ADF) 11, and a double talk detection circuit 12. The subtraction 310 takes the difference between the echo signal y digitally converted by the A/D converter 6 and the pseudo echo signal y that is the output of the adaptive digital filter 11, and generates an estimated error signal (hereinafter simply referred to as an error signal). ) is a circuit that outputs e. The adaptive digital filter 11 is a circuit that inputs the received signal For example, it is composed of a delay element, a multiplier, and an adder.

The double talk detection circuit 12 includes a level detection circuit 13 that detects the signal levels t and y of the echo signal y, and a level detection circuit 13 that detects the signal levels t and y of the echo signal y, and
The level detection circuit 14 detects the signal level Lx of the adaptive digital filter 11, and the comparison circuit 15 compares the signal levels Ly and Lx and outputs a detection signal C for temporarily stopping the adaptive control of the adaptive digital filter 11. has been done.

In the above configuration, when the near-end speaker N is not generating a signal from the handset 1, the adaptive digital filter 11
operates to make the error signal e zero by performing adaptive control to eliminate the echo signal y.

When the near-end speaker N generates a signal from the handset l (that is, enters a double talk state), the signal is digitally converted by the A/D converter 6 and then superimposed on the echo signal y. Then, since the level detection circuits 13 and 14 of the double talk detection circuit 12 detect the signal levels Ly and Lx on the transmitting side and the receiving side, respectively, the comparison circuit 15 detects the signal level L! It is detected that C has become larger than Lx by a predetermined value or more, and the detection signal C is provided to the adaptive digital filter 11. As a result, the adaptive control operation of the adaptive digital filter 11 is temporarily stopped.

Here, in the technique of Japanese Patent Application Laid-open No. 123240/1983,
The level detection circuits 13 and 14 are composed of circuits that output the sum of squares of input signals over a certain period of time.

In the level detection circuits 13 and 14, for example, the input received signal X and the output signal level L! The relationship is as shown in the following equation.

However, nT, (n-i)T: signal sampling time.There is one relational expression similar to this, for example, as follows.

LX(nr)=α(X(TIT)) +(1-a几x(
(n-uT)...(2) However, 0 × α × 1 LX (nT) = ,'X, l X ((n-+)
T) l −(3)I 菖O Lx<nT>= a l X(nr)l +
(1-a)Lx((n-+)T)...(4) However, in JP-A-51-11313, the input side of each level detection circuit 13. A plurality of unit double talk detection circuits each comprising a bandpass filter, a level detection circuit 13, 14, and a comparison circuit 15 are provided, and the signal band is divided into a plurality of subbands. A technique is disclosed in which each unit double talk detection circuit is used for a subband signal.

(Problems to be Solved by the Invention) However, the adaptive echo canceller having the above configuration has the following problems.

The technology of JP-A No. 58-123240 (1) 2~(
As is clear from equation 4), due to the integral effect, the larger N or the closer α is to O, the gentler the changes in the signal levels Lx and Ly will be compared to the changes in the signals x and y.

Therefore, even if double talk occurs and the echo signal y increases rapidly, it takes time for the signal level LY to increase, causing a problem in that detection of double talk is delayed and speech quality deteriorates.

Furthermore, with the technique of JP-A-51-11313, there is a problem in that the detection of double talk is delayed due to the signal delay caused by the bandpass filter installed on the input side of the level detection circuit 13 and 14, and the call quality deteriorates. There was a point.

The present invention provides an adaptive echo canceller that solves the problems of the prior art, such as deterioration in speech quality due to delay in double talk detection.

(Means for Solving the Problems) In order to solve the above problems, the present invention provides a subtracter that takes the difference between an echo signal and a pseudo echo signal approximated thereto and outputs an error signal; an adaptive digital filter that inputs the error signal, performs adaptive control to bring the error signal close to zero, and outputs the pseudo echo signal; and an adaptive digital filter that detects double talk based on the received signal and the echo signal and temporarily performs the adaptive control. a first delay circuit that delays the echo signal by a predetermined delay time and supplies it to the subtracter; and a delay time that is substantially the same as the delay time. and a second delay circuit that delays the received signal by the delay time and applies the delayed signal to the adaptive digital filter.

(Function) According to the present invention, since the adaptive echo canceller is configured as described above, the first. The second delay circuit works to delay the time from when double talk occurs until when the adaptive control operation stops. Therefore, the adaptive digital filter is not affected by the double talk detection delay caused by the double talk detection circuit. Therefore, the above-mentioned problem can be eliminated.

(Embodiment) FIG. 1 is a configuration block diagram of an adaptive echo canceller showing an embodiment of the present invention. Note that the same elements as the conventional elements in FIG. 2 are given the same reference numerals.

This adaptive echo canceller 50 differs from conventional echo cancellers in that it has a first delay circuit 51 on the (+) input side of the subtracter 10.
In addition, a second delay circuit 52 is connected to the reception input side of the adaptive digital filter 11.

Here, the first delay circuit 51 is a circuit that outputs an echo signal Te1 obtained by delaying the echo signal y from the A/II converter 6 by a predetermined delay time mT, and supplies it to the subtracter 10. The second delay circuit 52 also outputs a received signal X from the A/D converter 7 delayed by a predetermined delay time mT! 1 and sends it to adaptive digital filter 1.
This is a circuit that supplies to 1.

The operation of the adaptive echo canceller configured as described above will be explained.

If near-end talker N is not emitting a signal from handset l, then
Since the transfer function from the received signal After a delay of 10 minutes, adaptive control is performed to eliminate the echo signal yt, and the error signal e is brought to zero.

In addition, in the double talk detection circuit 12, as in the conventional case,
The level detection circuits 13 and 14 each output an echo signal y.
When the signal levels Ly and Lx of the received signal X are detected, the level comparison circuit 15 compares whether the signal level LY is greater than a predetermined value compared to Lx. If the signal level t, y is not in the bull talk state, the signal level t, y is Lx
Since the magnitude is not greater than the predetermined value compared to , the detection signal C is not output from the level comparison circuit 15, and the adaptive digital filter 11 continues the above-described adaptive control operation.

Now, when near-end talker N starts emitting a signal from handset l,
The signal is superimposed on the echo signal y and input to the delay circuit 51 and the level detection circuit 13. Then, since the signal level LY detected by the level detection circuit 13 becomes larger than the signal level Lx detected by one of the level detection circuits 14 by a predetermined value or more, the detection signal C is outputted from the level comparison circuit 15. The signal is applied to the adaptive digital filter 11.

Here, if the double talk detection circuit 12 detects double talk within the delay time m7 from the start of double talk, the echo signal y including the signal emitted by the near-end talker N is still in the delay circuit at the time of double talk detection. 51 and does not reach the adaptive digital filter 11. Therefore, the influence of the delay in double talk detection, that is, the disturbance of the tap coefficients of the adaptive digital filter 11 does not occur, and good speech quality can be obtained.

Note that the present invention is not limited to the illustrated embodiment, and various modifications may be made, such as using a different circuit configuration for the double talk detection circuit 12.

(Effects of the Invention) As explained in detail above, according to the present invention, the first. Since the second delay circuit is provided, the time from the occurrence of double talk to the time when the adaptive control operation is stopped is the same as that of the first delay circuit. It is delayed by the delay time of the second delay circuit. Therefore, the adaptive digital filter is not affected by the double talk detection delay in the double talk detection circuit, and good speech quality can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram of the configuration of an adaptive echo canceller showing an embodiment of the present invention, and FIG. 2 is a block diagram of the configuration of a conventional adaptive echo canceller. lO...Subtractor, 11...Adaptive digital filter, 12...Double talk detection circuit, 13, 14...Level detection circuit, 15...
. . . Level comparison circuit, 50 . . . Adaptive echo canceller, 51. 52 . . . 1st. Second delay circuit.

Claims (1)

[Claims] A subtracter that calculates the difference between an echo signal and a pseudo echo signal approximated thereto and outputs an error signal, and an adaptive control that inputs the received signal and the error signal to bring the error signal close to zero. an adaptive digital filter that detects double talk based on the received signal and the echo signal and temporarily stops the adaptive control; a first delay circuit that delays the echo signal by a predetermined delay time and supplies it to the subtracter; An adaptive echo canceller characterized in that it is provided with a second delay circuit that applies to a digital filter.