JPH02288734A - Echo canceller - Google Patents

Abstract

PURPOSE:To realize an echo canceller operated stably even in the case of environmental change by detecting a double talk depending the timewise correlation between a reception output and a transmission output. CONSTITUTION:A received input signal is subjected to nearly envelope detection in a power comparator circuit 7, a residual echo and a ratio ka are inputted to a correlation detection circuit 10, in which a ratio kb of a preceding and a new value of ka is obtained. When the ratio kb is kept within a prescribed fluctuation consecutively, the tap coefficient of the filter is revised. When the consecutive ratio change is monitored in this way, double talk is easily detected.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an echo canceller used in a video conference system or the like.

[Conventional technology]

Generally, echo cancellers are used to cancel transmission/reception crosstalk on bidirectional telephone lines, and to cancel acoustic echoes that enter microphone inputs from loudspeakers in audio conferences and video conferences using communication lines.

This echo canceller uses a transversal filter to generate an estimated pseudo-echo from the received input, and cancels the echo by subtracting it from the transmitted signal.It also corrects each tap coefficient of the transversal filter based on the cancellation error. Adaptive control is performed by repeating this process.

In general, adaptive control is performed on the assumption that the transmitted signal contains only echo or crosstalk components, so if the original transmitted signal is present (during a two-way call or a state called double talk), If updating of the tap coefficients is not stopped, corrections will be made in the wrong direction. Therefore, double talk detection and tap update/non-update control are essential functions for an adaptive echo canceller.

FIG. 4 is a block diagram of a conventional echo canceller.

Here, the double talk detection system is extracted from the echo canceller.

In the figure, 1 is an echo canceller, 2 is a double talk detector, 3 is a power comparison circuit, 4 is a power comparison judgment circuit, and 5
is a reference value circuit.

Next, the operation of the conventional echo canceller will be explained. First, in the case of non-double talk, the reception output Ro
ut goes around to the transmitting power Sin as crosstalk or acoustic echo. The amount of rotation varies depending on the conditions, but
This is the amount by which the normal reception output Rout is attenuated by a predetermined amount.

On the other hand, if there is double talk, the sender's power Sin
Since the original transmission signal is added to the signal and the level increases, the presence or absence of double talk can be detected from the comparison result of the power comparison circuit 3.

Another method is to determine double talk when the residual echo exceeds a predetermined reference value. This is because when there is no double talk, the residual echo falls within a predetermined tolerance range, and when double talk occurs, the original transmission signal is added and exceeds the predetermined reference value. It can be detected by

Conventionally, detection accuracy has been improved by using a combination of the above two types of detection circuits.

FIG. 5 is a block diagram of an echo canceller disclosed in Japanese Patent Laid-Open No. 62-299120.

In the figure, ■ is an echo canceller similar to that in Figure 4;
is a power comparison circuit that compares the sending human power Sin and the receiving human power R4n, 7 is a power comparing circuit that compares the receiving human power R4n and the transmitting output Rout, 8 is a selection circuit that selects a dark value, and 9 is a dark value that generates a dark value. It is a circuit.

Now, in the initial state of the echo canceller, since the amount of echo residual is large, a large value is selected as the darkness value. In a steady state where the amount of echo cancellation reaches a predetermined value or more, the darkness value is set to a sufficiently small value to enhance the detection ability. Here, the power comparator 7 calculates the receiving power Rin and the transmitting power Rou.
t, and the selection circuit 8 is controlled based on the comparison result. Then, the dark value supplied to the double talk detector 6 is switched according to this control state.

In this way, double talk is detected in accordance with the convergence state of the echo canceller.

[Problem to be solved by the invention]

Conventional double talk detection circuits are configured as described above, so when a sudden change in the environment occurs, such as the movement of people within a conference room, as in an audio conference or video conference,
There was a drawback that the residual echo suddenly increased in size, and as a result, double talk was determined and subsequent adaptive operations were not carried out normally.

The present invention has been made in order to eliminate the above-mentioned drawbacks, and aims to provide an echo canceller that can perform normal double talk detection and perform adaptive operation even when residual echo increases due to environmental changes. purpose.

[Means to solve the problem]

The echo canceller according to the present invention includes a power comparison circuit that detects data on the ratio between reception input and transmission output every predetermined period, and a power comparison circuit that detects ratio data between the reception input and transmission output at predetermined intervals, and a ratio data sent from the power comparison circuit and the previously detected ratio data. and a correlation detection circuit that detects the ratio of and sends a signal indicating double talk to the double talk detector only when the ratio is within a predetermined range and continues for a predetermined period.

[For production]

The correlation detection circuit performs double talk detection by monitoring the fluctuation state of the ratio data between the reception input and the transmission output.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram of an echo canceller showing one embodiment of the present invention. Here, as in FIG. 4, the double talk detection system is extracted from the echo canceller and shown.

In the figure, 1 is an echo canceller, and 2 is a double talk detector, which stops updating the coefficient taps of the echo canceller 1 when double talk is detected. 7 is a power comparator circuit that detects the ratio between the receiving input and the transmitting power; 10 is a power comparing circuit that obtains the ratio from the power comparing circuit 7 and monitors the temporal fluctuation of this value, and detects the temporal correlation between the receiving input and the transmitting output; This is a correlation detection circuit that determines the

Further, FIGS. 2(a) to 2(f) are time charts showing operation waveforms when double talk is not present.

Here, (a) in the same figure is the received input waveform, (b) is the received input level, (c) is the residual echo waveform, and (d) in the same figure.
is the transmission output level, (e) is the ratio change, and (f) is the transmission output level.
> indicates the detection output.

Now, in the power comparator circuit 7, the received human power signal (FIG. 2(a)) is subjected to substantially enrobe detection, and a received input level curve as shown in FIG. 2(b) is obtained. Similarly, a transmission output level curve as shown in FIG. 2(d) is obtained for the residual echo waveform (FIG. 2(C)).

Then, the ratio ka between the two is input to the correlation detection circuit 10, and the ratio kb between the prefix and the new value of ka is further determined (see Fig. 2).
e)) For a given input level, the kbO value does not change significantly. Here, the dotted line in the figure indicates that this ratio kb is unstable when the input level and residual level are small. When this ratio kb continuously falls within a predetermined variation, the tap coefficients of the filter are updated as shown in FIG. 2(f).

Further, when the above kb is in the unstable range, update/non-update is determined first based on the following conditions.

First, if the receiving human power level (Figure 2 (b)) is sufficiently low, the residual echo will be small and the system will become unstable due to residual noise, etc., so if the receiving human power level is less than a predetermined value, the It will not be updated. Furthermore, when the transmission output level is sufficiently low, the residual echo is sufficiently small and there is no need for updating, so that it is unconditionally non-updated. If the ratio ka between the received input level and the transmitted output level is greater than or equal to a predetermined value, it can be determined that there is double talk as in the conventional system, so no updating is performed.

Next, FIGS. 3(a) to 3(g) are time charts showing waveforms when there is double talk. Here, the same figure (a
) is the received input waveform, (b) is the received input level, (c) is the residual echo component, (d) is the original transmitted signal, (e) is the transmitted output level, (f) shows the ratio change, and (g) the same figure shows the detection output.

Now, if there is a residual echo component (Fig. 3 (C)) and the original transmission signal (Fig. 3 (d)) is added to it, the transmission output level (Fig. 3 (e)) is approximately a combination of both. It becomes what it is. As shown in FIG. 3(f), the change in the ratio kb is greatly disturbed in a certain section of the original transmission signal. Therefore, double talk can be easily detected by monitoring the continuity of this ratio change.

In other words, in the conventional method shown in Fig. 4, when an environmental change occurs and the echo component rapidly increases and exceeds the threshold for detecting double talk, double talk is falsely detected and adaptive action is not possible. I won't be able to do it.

In addition, in the conventional method shown in Figure 5, the dark value for double talk detection is changed depending on the level of the residual echo, but conversely, the original transmission output is also judged as an echo component and the filter is directed in the wrong direction. There is a risk that the tap coefficient of

In contrast, in this embodiment, even if the residual echo increases due to environmental changes, double talk is detected based on the presence or absence of temporal correlation with the input level, so stable adaptive operation can be performed.

In the above embodiment, a method is shown in which the signal level is obtained by envelope detection in the power comparison circuit 7, but the signal power may be used as the signal level as in the conventional method.

Next, power comparison circuit 7. Correlation detection circuit 10. A case will be described in which the double talk detector 2 is configured by software.

FIG. 6 is a flowchart showing level detection. First, when level detection starts, find the absolute value of the signal (
Step 61). Next, it is determined whether this absolute value (new value) is greater than or equal to the stored value (step 62→).

Here, if the saved value is larger than the new value, a predetermined amount of attenuation is given to the saved value to make it a new saved value (step 63).
. On the other hand, if the new value is greater than or equal to the stored value, the stored value is replaced with the new value and the process proceeds to the next step (step 64).

Further, FIG. 7 is a flowchart showing double talk control. First, when double talk control is started, the ratio ka = R/S between the input level R and the transmission level S is determined (step 71). Next, ka (t-1) and k
Ratio to a (t) kb=ka (tl)/ka(t
) is determined (step 72).

Then, whether the input level R is less than or equal to a predetermined value (step 73).
, it is determined whether the transmission level S is below a predetermined value (step 74) and whether the ratio ka is above a predetermined value (step 75). Here, from step 73 to step 75,
If either is rYESJ, non-update of coefficients is set (step 79). On the other hand, if all steps from step 73 to step 75 are rNOJ, kb (t-1)
It is determined whether /kb (t) is outside a predetermined range (step 76). Next, if it is outside the predetermined range in step 76, it is determined whether that state continues (step 78). Here, if they are consecutive, step 79 above
If they are not consecutive, proceed to the next step. On the other hand, if it is not outside the predetermined range in step 76, it is determined whether the state continues (step 77). Here, if they are continuous, set the coefficient update (step 80); if they are not continuous, proceed to the next step.

Note that (1) in the figure indicates time t, and (t-1) indicates time one sample before. Further, there is no problem in practical use even if the confirmation of the continuity of kb is omitted for the sake of simplification.

〔Effect of the invention〕

As described above, in the present invention, double talk is detected based on the temporal correlation between the reception input and the transmission output, so it is possible to obtain an echo canceller that operates stably even under environmental changes.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an echo canceller showing an embodiment of the present invention, FIGS. 2(a) to (f) are timing charts showing operating waveforms when double talk is not performed, and FIG. 3(a) is a block diagram of an echo canceller showing an embodiment of the present invention.
- (g) are time charts showing waveforms when there is double talk, Figure 4 is a block diagram of a conventional echo canceller, Figure 5 is a block diagram of another conventional echo canceller, and Figure 6 is a diagram showing level detection. FIG. 7 is a flowchart showing double talk control. DESCRIPTION OF SYMBOLS 1...Echo canceller, 2...Double talk detector, 7...Power comparison circuit, 10...Correlation detection circuit.

Claims (1)

[Claims] An echo canceller equipped with a double talk detector that detects double talk in a two-way telephone line, etc., includes a power comparison circuit that detects data on the ratio between reception input and transmission output every predetermined period. , Detects the ratio between the ratio data sent from this power comparison circuit and the previously detected ratio data, and only when the ratio is within a predetermined range and continues for a predetermined period, a signal indicating double talk is detected. an echo canceller comprising: a correlation detection circuit that sends a correlation detection circuit to the double talk detector.