JPS6041849A - Loudspeaker telephone set - Google Patents

Abstract

PURPOSE:To prevent howling due to a large change in an echo path by inserting a temporary loss to a howling group when the echo path changes largely and the estimating operation of an echo canceller is not in time. CONSTITUTION:An echo path fluctuation detecting circuit 310 transmits always a supervisory signal without a listening range indoors at a prescribed time interval via an amplifier 309 and a speaker 307. This signal is reflected by, e.g. a caller and inputted to a detection circuit 310. The detection circuit 310 detects only the supervisory signal by using a filter, detects the delay from a transmission timing and the change in the attenuation, and discriminates it as the presence of a large change in the echo path when the amount of change exceeds a prescribed value and transmits a control signal to a transmission/reception decision circuit 303 so as to command the operation of voice switches 301, 305. When it is discriminated that the cancellation capability of th echo canceller is recovered afterward, the detection circuit 310 outputs a voice switch release signal.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention is directed to preventing ringing caused by acoustic coupling in a loudspeaker telephone when the sound emitted from a speaker is reflected by a human-powered wall and enters a microphone. Concerning the configuration of the echo canceller.

[Technical background of the invention]

Amplified telephones that allow you to talk without using a handset using the amplified sound from a speaker are extremely convenient because you can use both hands freely during a call, and we believe that they will become indispensable, especially for conference calls, which have been attracting attention in recent years. It will be done.

This loudspeaker telephone uses an amplifier to amplify the incoming sound and sends it to the speaker, so the sound emitted from the speaker is picked up by the microphone.
This signal is amplified by the transmitting amplifier, and this signal is input again to the receiving amplifier with sidetone generated by the hybrid coil, thereby creating a signal loop. Therefore, if a public address telephone is configured simply by a microphone, a speaker, and an amplifier, this signal loop may cause howling, making it impossible to make a call.

Conventionally, in order to prevent this howling, a voice switch system has been widely used for loudspeaker telephones to break this loop by inserting a loss in the receiving side when transmitting a voice and in the transmitting side when receiving a voice. However, this system has the disadvantages that simultaneous communication is impossible due to a loss occurring in one of the communication paths, and that it gives the speaker an unnatural switching sensation when the loss is switched.

On the other hand, with advances in digital signal technology, echo cancellers have become relatively easy to implement using LSI and the like, and have attracted attention as a technology to replace the voice switch method.

This echo canceller is s d! As shown in Figure 1, in a loudspeaker telephone, the signal n (t) input to the microphone is
+ Of the y(t), only the signal y(t) that comes out from the speaker and is reflected by walls etc. is canceled, thus preventing the shearing that occurs when the various signal loops mentioned above are cut off between the speaker and the microphone. can. Therefore, if this echo canceller is used, there is no need to introduce loss into the communication path, so better speech quality can be obtained compared to the voice switch system.

This echo canceller can be configured as shown in FIG. 2, for example.

In the same figure, a transversal filter 201 is a filter (this filter (referred to as a pseudo-echo path), and is generally constituted by a transversal filter having tap coefficients approximating the impulse response of the transfer function.

This circuit generates a pseudo echo signal y (t) which is a close approximation of the signal So 9 (
By subtracting (key y(t)), the echo signal y(t
) to extract the true transmission signal n(t).

Here, the tap coefficients of the transversal filter 201 are determined sequentially by a widely known learning algorithm such as the learning identification method using the signal x (t) from the speaker and the output signal e (t) of the subtracter 202. I get glared at. In the figure, an estimation circuit 203 inputs the tap coefficients from the filter 201, modifies them according to the learning identification method, and returns them to the filter 201 again. By sequentially performing this process, the tap coefficients of the transversal filter are finally The coefficients are made to approximate the impulse response of the transfer function of the reflected signal.

Note that this tap coefficient estimation needs to be performed only when the microphone input is the reflected signal y (t). Otherwise, the reflected signal y (t) will be masked by the transmitted speech signal n (tJ, making estimation of the tap coefficient inaccurate.
Compare the power of l+y(11) and received signal x(tJ,
When the power of n(t) and y(t) is larger than the value at which the power of x(t) starts to decrease by a certain value, it is determined that the transmission signal n(t) exists, and the tap coefficient of the estimation circuit 203 is Forbid updates.

[Problems with background technology]

As mentioned above, the echo canceller is an effective technology for preventing howling in loudspeaker telephones, but it still has the following problems, which are obstacles to its practical use.

The problem is that the echo canceller must always have a pseudo echo path that accurately approximates the transfer function of the echo path. If the pseudo-echo path is inaccurate, the reflected signal y (t) will not be sufficiently attenuated, and as described above, the margin of the signal loop will be reduced and howling will occur.

This problem seems to be solved because the echo canceller constantly performs estimation operations based on voice, but in reality, depending on the movement of the speaker in front of the loudspeaker, for example, the echo path may change. If the transfer function changes significantly, there is a possibility that the echo canceller cannot be estimated in enough time to cause powling. A more important problem is the change in the echo path when the double talk dechefter operates. At this time,
Since the echo canceller does not perform estimation operation,
If the echo path changes due to the speaker moving, the error between the pseudo echo path and the actual echo path increases, causing howling.

[Purpose of the invention]

The present invention has been devised in view of the above-mentioned problems, and its purpose is to eliminate the need for double-root detector operation even when there is a large change in the echo path without impairing the advantages of the echo canceller. The objective is to provide a loudspeaker telephone that does not generate howling even when the time is high.

[Summary of the invention]

The gist of the present invention is to send new sound waves (for example, ultrasonic waves) or radio waves outside the audio band to the echo path, detect changes in the echo path based on the reflected signal of this signal, and detect changes in the echo path if the echo path changes significantly. If the estimated operation of the echo canceller is not in time or the double talk detector does not estimate the operation of the echo canceller, a temporary error is inserted into the howling loop to prevent howling from occurring. It is something.

Note that this loss can be removed when it is determined that the echo canceller has been correctly estimated and the echo path has become a pseudo echo path, for example, based on the l/Hel ratio of the received signal and the transmitted signal.

[Embodiments of the invention]

An embodiment of the invention is shown in FIG. 3 and will be described in detail below.

In the figure, a microphone 101, a speaker 104, an echo canceller 105, a subtracter 106, an amplifier 102
, ], 03, The high-grid coil 107 is part of a public address telephone using a conventional echo canceller, and its operation has already been explained in FIG. In the present invention, a voice switch method is also used to supplement the operation of the echo canceller. This part includes the transmission/reception determination circuit 303, switches 301, 305, and insertion losses 302 and 304. When talking, press switch 305 and 301 to B.
By turning the switches 305 and 301 to the A side, a loss is inserted into the receiving side, and when receiving a call, by turning the switches 305 and 301 to the A side, a loss is inserted into the sending side. This voice switch operation is performed only immediately after the echo path has changed significantly according to the output of the echo path change detection circuit, and is otherwise prohibited when the switch 305 selects the B side and the switch 301 selects the A side.

Echo path fluctuation detection circuit 310, ゛, amplifier 308, 30
7. Microphone (or oscillator) 306, speaker (or resonator, in the case of a resonator, the filter 403 in Fig. 4 is unnecessary)
307 and the cancellation amount detection circuit 311 are the central part of the present invention.

Here, the echo path change detection circuit 310 always sends a monitoring signal (for example, ultrasonic wave) outside the audible range into the room via the amplifier 309 and the speaker 307 at regular time intervals. This signal is reflected by, for example, a speaker, is detected by a microphone 306 and an amplifier 308, and is input to an echo path variation detection circuit 310. This circuit uses a filter to detect only the monitoring signal (-1
Delays with the transmission timing and changes in attenuation are detected, and when these changes exceed a certain value, it is determined that there has been a significant change in the echo path, and a control signal is sent to the transmission/reception determination circuit.
Instructs voice switch operation.

Thereafter, the echo path fluctuation detection circuit 310 instructs the transmission/reception determination circuit 303 to operate the voice b switch until the cancellation amount determination circuit determines that the echo canceller's cancellation ability has been recovered and issues a voice switch release signal. continue.

As described above, if the echo path changes significantly due to the movement of the speaker, etc., it can be detected by the echo path change detection circuit 310, and by using the voice switch method in combination, it can be detected by the echo path change detection circuit 310. Normal calls can be made without causing howling even when If the cancellation amount detection circuit detects that the amount of cancellation of the echo canceller has recovered, the voice switch is removed and the quality of the call using the echo canceller is restored again.

Note that the echo path variation detection circuit 310 and the cancellation amount detection circuit 311 shown in FIG. 3 can be realized with the configurations shown in FIGS. 4 and 6. First, the operation of the echo path fluctuation detection circuit will be explained with reference to FIG. 4 and the fifth time chart.

FIG. 5(a) is a diagram showing the output of the oscillator 401,
bJ is a diagram showing the output of the oscillator 402, (C) is a diagram showing the output of the amplifier 306, and (dl is the integration circuit (40
5), the figure (e) shows the output of the threshold circuit 406.
The figure if) shows the output of interval counter 4.
(gl is a diagram showing the output of the delay circuit 408, (11) is a diagram showing the output of the subtracter 409, and (1) is a diagram showing the output of the threshold circuit 409. diagram showing,
The same figure ('') shows the output of the delay circuit 411, the same figure (k
1 is a diagram showing the output of the subtracter 412, (1) is a diagram showing the output of the threshold circuit 413, and 1 of the diagram shows the output of the threshold circuit 41.
4, (n) is a diagram showing the output of the latch circuit 415, and (0) is the output of the R/S flip-flop 41.
FIG. 6 is a diagram showing the output of No. 6.

The oscillator 401 generates pulses at partial intervals τ as shown in FIG. 5 ta+. Then, the oscillator 402 generates an out-of-audible signal (for example, an ultrasonic wave) in synchronization with this pulse, and sends it into the room via an amplifier and a speaker.

The out-of-band signal sent into the room is reflected by the speaker, walls, etc., is picked up by a microphone and an amplifier after a delay of time τd, is extracted by a bandpass filter 403, and then leveled by a detection circuit 404 and an integration circuit 405. The detected waveform of fcl) is obtained.

This detection circuit measures the amount of delay and attenuation based on this waveform, and detects a wide change in the echo path. Examples of such detection means are shown below.

First, the change in delay amount is determined by the waveform (e) obtained by inputting the output of the integrating circuit 405 to the threshold circuit 406 and the waveform (e) obtained by inputting the output of the integrating circuit 405 to the threshold circuit 406.
It is detected based on the time difference between the rising edge and the waveform (1) of waveform (1).

As shown in FIG. 5, the amount of delay before the echo path changes is τd, and after the change it is assumed to be τd2. The interval counter 407 counts these time intervals and outputs the counter output as shown in (δ). In this example, τd1 is 10, τ,2 is 5
It has become. Further, the output of this counter is led to a delay circuit 408, and as shown in (g), one period τ of the oscillator 401 is
The output is delayed by a minute. These values are then subtracted by the subtractor 40
The difference is taken at 7 and input to threshold circuit 410. When the absolute value of the difference is greater than a certain value, the threshold circuit 410 considers that there has been a large change in the amount of delay, and outputs a logical value of "1" as shown in (i).

On the other hand, the change in attenuation amount is determined by using a delay circuit 411, taking the difference (lO) from the past level for a certain period of time τ, and
If the absolute value of this value is greater than a certain value, it is determined that there has been a large level fluctuation, and "1" is output as shown in 1).

Then, these outputs of the threshold circuits 410 and 413 pass through the output circuit 414, and at least one of them becomes "1".
When ``1'', R, /S flip-flop 41
Set 6 and get an output of (01.

The output "1" of this R/S flip-flop is held until the cancellation amount detection circuit detects that the amount of cancellation of the echo canceller has recovered, and is reset when the amount of cancellation of the echo canceller has been recovered.

Next, a configuration example of the cancellation amount detection circuit will be described.

The amount of cancellation can be detected based on the level, as shown in FIG. 6, for example.

In the figure, the speaker output x (t) and the echo canceller trial difference signal e (t) are detected and integrated by level detection circuits 601 and 602, and are detected and integrated by a logarithm circuit 603.
-LOG conversion is performed at 604, and the difference is taken at subtracter 605. The output of this subtracter is detected by a threshold circuit 606 to see if it is above a certain value, and if it is above a certain value, it is determined that the echo canceller has performed sufficient cancellation, and the voice switch operation is performed. To inhibit this, flip-flop 416 of FIG. 4 is reset.

The following is an example of the present invention, and each circuit in this example can be easily realized using current technology.

In this embodiment, ultrasonic waves are used as an example of signals outside the audible range, but this is not limited to ultrasonic waves. For example, electromagnetic waves can also be used. In this case, the microphone 30 in FIG.
6 and the speaker 307, the antenna functions as a light emitter and a sensor when using infrared light or the like.

Further, in this embodiment, the hybrid coil 107 is used to connect the loudspeaker motor to a two-wire line, but this coil is unnecessary if the loudspeaker motor is connected to a four-wire line.

Finally, expansion of the present invention, 1? Let's talk about I'i pathization.

In FIG. 3, when the gains of amplifiers 102 and 103 are large, even a slight decrease in the canceling performance of the echo canceller may cause howling. In this case, in order to prevent this howling using the present invention, it is necessary to increase the detection accuracy of the echo path fluctuation detection circuit, but if the out-of-band signal has directivity, No fluctuations can be detected. In this case, in order to prevent this, a plurality of speakers 307 and microphones 306 are arranged in different directions, and each has an echo path fluctuation detection circuit 310, thereby increasing the detection accuracy.

On the other hand, when the gain of the amplifiers 102 and 103 is small and the cancellation performance of the echo canceller is not required to be that great, for example, it is possible to detect echo path fluctuations by detecting changes only in the amount of delay or only in the level. Can be divided into departments.

This uses a constant echo canceller and a voice switch (however, the amount of loss is not that large).
The same applies to systems that reduce the burden on echo cancellers. In addition, in such a system, the loss of the voice switch becomes large when the echo path fluctuates.

〔Effect of the invention〕

As described above, by using the present invention, it is possible to prevent howling during double talk, which was difficult with conventional loudspeaker telephones using echo cancellers, and to prevent howling due to large changes in the echo path. Increases the practicality of loudspeaker telephones using

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of a conventional example, FIG. 2 is a configuration diagram of an echo canceller, FIG. 3 is a configuration diagram showing an embodiment of the present invention, and FIG. 4 is a diagram showing the configuration of an echo path fluctuation detection circuit. FIG. 5 is a timing Chickert diagram of the echo path fluctuation detection circuit, and FIG. 6 is a diagram showing the configuration of the cancellation amount detection circuit. 101.306...Microphone 104.307...Speaker 105...Echo-1 canceller 301.305...Switch 304...Transmission/reception judgment circuit 310...Echo path variation detection circuit 311...Cancellation Quantity detection circuit representative Patent attorney Nori Chika Kensuke (and 1 other person) Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6

Claims (6)

[Claims] (1) In a public address telephone that prevents acoustic coupling between a microphone, a speaker and a car using an echo canceller and suppresses the occurrence of howling, a means for transmitting a signal outside the audible band to the echo path where the acoustic coupling occurs; , means for detecting a change in the echo path based on the echo signal of the out-of-audible signal. (2) The loudspeaker telephone according to claim 1, wherein the signal outside the audible band is an ultrasonic wave. (3) The loudspeaker telephone according to claim 1, wherein the signal outside the audible band is an electromagnetic wave. (4) The loudspeaker telephone according to any one of claims 1 to 3, characterized in that when a change in the echo path is detected, a loss is inserted into at least one of the transmitting side and the receiving side. (5) The loudspeaker telephone according to claim 4, wherein the insertion of the loss is based on a tint pattern that is at least approximate to the power of the transmitted signal and the received signal. (6) Claim 1, characterized in that the means for detecting a change in the echo path is at least one of a change in the amount of delay and a change in the amount of attenuation of the echo signal of the signal outside the audible band. - A loudspeaker telephone as described in paragraph 4.