JPS58114554A - Howling preventing device - Google Patents

Abstract

PURPOSE:To prevent the howling in response to the fluctuation of delay time of an acoustic path, by measuring the delay time of sound waves between a speaker and a microphone, generating a pseudo audio signal based on the measurement and subtracting the signal from an input signal to the microphone. CONSTITUTION:A reception signal 22 inputted to a reception system input terminal 21 is branched partly and applied to a variable delay line 212 and given to a pseudo sound path 213 with a delay. This output is given to a subtractor 210, where a pseudo sound signal is subtracted from the signal inputted from a transmission system input terminal 28. The delay time at a variable delay line 212 is controlled with an output of a delay time measuring device 215. The measurement of the delay time is realized by providing an ultrasonic wave generator near a speaker 24, locating an ultrasonic wave receiver near a transmitter 27 and measuring the propagation time. Thus, since echo can be cancelled while correcting the delay time one after another, the howling can be prevented effectively.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a howling prevention device used for conference telephones and the like.

Conventionally, a general cobodus has been used in this type of device. Bordus uses a voice switch, which compares the levels of the transmitting signal and the receiving signal, selectively passes the signal with the higher level, and allows the signal with the lower level to pass without causing howling. It is necessary to provide attenuation of When such a voice switch is used, there is a drawback that when a simultaneous two-way call is made, the 1c'1m signal is inevitably cut off (or attenuated), resulting in a disconnection of the call. Even in the case of a one-way call using only the received signal, there is a drawback that the signal that has passed through the acoustic path to the transmitting path cuts off the end of the received signal, impairing the quality of the call.

Adaptive echo cancellers are applied to solve the above-mentioned drawbacks. The adaptive echo canceller consists of a pseudo signal outputted from the receiving circuit to the transmitting circuit through a transversal filter with a tap coefficient of T'[tap coefficient, and a receiving signal I'! lJ
The taps of the transversal filter are subtracted from the loop signal appearing from the acoustic path to the transmitting circuit 1111 via all the subtractors, and the tap adjustment of the transversal filter is performed so as to minimize the residual signal appearing at the output of the subtractor. This is what I have done to correct it. When the correction of the tap coefficients is completed, the tap coefficients represent the impulse response of the acoustic path, the residual signal is minimized, and howling can be prevented. However, at the start of operation, it is naturally difficult to prevent howling, and controls are required to prevent it.
A+ is required separately. Furthermore, since the tap coefficients are corrected by considering the acoustic path as linear, if the listener speaks while the tap coefficients are being corrected, the tap coefficients will be incorrectly corrected. Even after the tap coefficient has been corrected,
It is necessary to correct the tap coefficient according to changes in the acoustic path, but in this case, it is necessary to determine by monitoring the transmitting and receiving signal levels, etc. whether there is a two-way conversation or whether the listener is speaking.
1ljl control is required in which all tap coefficient corrections are performed only when a received signal arrives. Making these corrections in an environment where normal room noise is present requires precision, especially in the high frequency range. Also,
A correction amount calculation circuit is required to correct the tap coefficients using the residual signal. Therefore, the disadvantages are that the control is complicated and the scale of the hardware is large.

Furthermore, the power in the high frequency range of audio is generally small, and in the high frequency range, the level difference between the room noise and the interference signal becomes small, making it extremely difficult to modify the tap coefficient. This means that the spectrum of the audio signal is the curve S in Figure 1.
Or 1QdB per octave in the high frequency range as shown in R.
This is because, on the other hand, the decline in the high range of the room original sound is only about 5 dB per octave, as shown by curve N in the figure. Note that when the indoor noise level is about i60 phon, it intersects the listening level curve R in the high frequency range. Therefore, the suppression of the JA interference signal in the tube region is limited by this S/N ratio.

In order to increase this S/N ratio, the caller must place his or her lips close to the microphone, and each conference participant must prepare a microphone close to their lips.

Further, FIG. 1 is a diagram showing an acoustic spectrum at a normal microphone position where a conference phone is used, where a curve S is a transmission level and a curve N is a room noise level. In addition, curve R is 2 dB higher than the listening level and 1 dB higher than the transmitting level.
, that is, the vocalization level is 55 at a point 1m in front of the lips.
dB, the listening level is about 7 Q dB. Therefore, if the received sound waves enter the microphone through the bend, it will naturally cause howling, so even if the characteristics of the microphone are made directional, and the relative positional relationship between the speaker and the microphone is taken into consideration, howling can be prevented sufficiently. It is difficult.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned conventional drawbacks and to provide an effective howling prevention device with a simple configuration.

The howling prevention device of the present invention includes a variable delay line that delays a received signal, a pseudo acoustic path that outputs a pseudo feedback signal by multiplying the output signal of the variable delay line by a fixed tap coefficient, and a transmitting system transmission line. a subtractor inserted therein to subtract and output an output signal of the pseudo acoustic path from a wrap-around signal; and a delay time measuring circuit to output an output signal corresponding to a delay time of the acoustic path. The impulse response characteristic of is set in advance using measured values, etc., and the delay time measuring circuit 2 and the variable delay line correct the temporal change between the delay time of the acoustic path at the time of setting and the delay time of the acoustic path during use. It is characterized by

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

FIG. 2 is a block diagram showing one embodiment of the present invention.

In other words, the received signal 22 input to the receiver input terminal 21
is transmitted to the speaker 24 from the output terminal 23 of the receiving system and converted into acoustic energy, which is then heard by the listener 26. Further, the received signal 22 is branched and delayed by a variable delay line 212, and the pseudo acoustic path 2iai=subtractor 2
given to 10. The delay time of the one-way variable delay line 212 is controlled by the output of the open-side limiter 215 at the time of delay.

The pseudo acoustic path 213 is constituted by, for example, a transversal filter, and includes an electroacoustic conversion characteristic and an air propagation characteristic from the speaker 24 to the transmission system input terminal 28 via the microphone 27 (hereinafter referred to as a round signal transmission path). It simply matches the impulse response of the acoustic path. However, most of the delay time is due to the delay line 21.
Make it compatible with 2. The impulse response of the acoustic path is not a simple one, and several loudspeakers 24. Sound field 25. Characteristics around the microphone 27.

This includes, of course, amplitude characteristics, delay characteristics, etc. The wrap-around signal manually input to the transmitting system input terminal 28 is transmitted by the subtractor 210.
The difference between the signal and the output signal of the pseudo acoustic path 213 is calculated and outputted from the transmission system output terminal 211.

Therefore, if the impulse responses of the delay line 212 and the pseudo transmission path 213 match those of the acoustic path, the residual signal will be small and howling will be prevented.

FIG. 3 shows an example of an impulse response of an acoustic path. That is, when an impulse as shown in FIG. A response waveform appears. Most of the delay time Td is based on the propagation time of the sound wave in the sound field 25.

FIG. 4 shows an example of the configuration of the pseudo acoustic path 213.

The register 42 is a register that stores the output signal 41 of the variable delay line 212 (shown in FIG. 2), and the register 43 is a register that stores the impulse response '1sst' of the acoustic path described above. The contents of registers 42 and 43 are multiplied by a multiplier circuit 44, and the multiplication results are accumulated by an accumulator 45. The output signal 46 of the accumulation circuit 45 is the output signal of the pseudo acoustic path 213 and is sent to the subtractor 210. Since the above configuration constitutes a general transversal filter with fixed tap coefficients, the output signal 46 is
This becomes a pseudo wrap-around signal that is the same as the wrap-around signal in the acoustic path. The impulse response can be stored in the register 43 by applying an impulse to the receiving output terminal 23 and accumulating the impulse response waveform appearing at the transmitting input terminal 28, as shown in FIG. 2, for example. However, if the impulse response is measured separately and the register 41
If you preset it to 3, you can achieve it with simple hardware. In this case, even if the contents of register 43 are erased due to a failure such as a power outage (as long as the data remains)
There is no need to remeasure. Furthermore, since it is possible to perform measurements with the influence of room noise removed, more precise settings can be made.

Now, when the envelope of the audio waveform at the receiving output terminal 23 is as shown in FIG. 5(a), the transmitting input terminal 28 has a delay time T, as shown in FIG. 4, an envelope slightly deformed depending on the characteristics of the acoustic path appears.The envelope of the output waveform of the delay line 212 appears after the delay time side.The difference between this delay time Td and TJ is determined by the pseudo acoustic path 213. This can be solved by increasing the number of taps in the transversal filter reference ↓.In other words, the setting of the register 43 is the period of the impulse response length Tr shown in Fig. 3 plus the difference between the above delay times. Delay time T
Making the difference between J and − small is useful for constructing a more economical pseudo acoustic path.

Now, the upper limit of the frequency band of the howling prevention device is set to 3400.
In Hertz, the impulse response waveform of the transmitting system input terminal 28 is sampled at 8 kilohertz (period: 125 microseconds).

Then, the sampled value every 125 microseconds is preset in the register 4-3 so as to correct the difference between the delay time 'rd and TJ. That is, the tap coefficients are fixed. If you make a call in this state,
Since the wraparound signal of the received signal is canceled by the output signal of the pseudo acoustic path, the residual signal appearing at the transmission system output terminal 211 is small. In other words, howling is prevented.

However, if the propagation time of the acoustic path when measuring the impulse response differs from the propagation time during the actual meeting,
Since the delay time Td of the acoustic path changes, the taps of the transversal filter are shifted isometrically. That is, the residual signal output to the transmitting system output terminal 211 becomes a dog, and the howling prevention effect is reduced. In this embodiment, the delay time measurement circuit 215 is used, and the output signal 214 is used to calculate the delay time measurement circuit 215. By controlling the delay time of the 1-cho-variable delay line 212, the howling prevention effect is prevented from decreasing due to a change in the delay time. Delay time measurement circuit 215
It is not necessary to measure the physical delay time, and it may be a circuit that measures only the change in the delay time Td. It is easy to set the delay time of the variable delay line to be controlled by the above change. For example, the variable delay line 212 is configured with a voltage controllable variable delay line, and the delay time measurement circuit 21
The above control is possible by selecting an output gain of 5. The delay time can be measured, for example, by measuring the time until a built-in signal similar to (slightly deformed) the received signal at the receiving output terminal 23 appears at the transmitting input terminal 28. Since the signal changes from moment to moment and does not have exactly the same waveform as the wrap-around signal appearing at the transmission system input terminal 28, a somewhat complicated circuit is required to determine the identity. For example, you can apply a 1000 Hz sine wave signal to the receiving system output terminal 23 and measure the time until the above signal appears at the transmitting system input terminal 28VC. It's not right. Howling can be sufficiently prevented if it is a short meeting by measuring before the meeting. However, if the delay time is measured using ultrasonic waves, it can be measured even during a conference without disturbing the conference. In this embodiment, the speaker 24
Place an ultrasonic transmitter near the transmitter 270, place an ultrasonic receiver near the transmitter 270, measure the ultrasonic propagation time between the transmitter and receiver, and determine the ultrasonic propagation time when measuring the MMe impulse response. The system is configured to control the delay time of the 0I variable delay line by generating a DC voltage corresponding to the difference in ultrasonic propagation time during a conference. Therefore, throughout the meeting,
It is possible to measure the delay time and control the variable delay line, and it is possible to prevent howling in response to changes in the delay time of the acoustic path within the venue.

According to this embodiment, there is no need for a circuit that controls switches by detecting the level of the receiving signal and the sending signal, etc., as in the past, and there is no risk of the end of the conversation being cut off, ensuring excellent call quality. There is an effect that can be done.

In addition, a circuit for detecting a residual signal due to only the received signal, a tap coefficient correction value calculation circuit for minimizing the residual signal, a tap coefficient correction circuit, and a tap coefficient correction circuit in the initial state. There is no need for any control circuits to prevent howling, and the amount of hardware can be significantly reduced. In addition, since it is possible to increase the impulse amplitude for impulse response measurement within the range where non-linear distortion from speakers, microphones, etc. occurs, it is possible to eliminate the influence of room noise and to measure impulse responses with high precision. This is possible, and therefore, the howling prevention effect is also great. Accuracy can be further improved by measuring impulse responses during times when indoor noise is low.
Sufficient margin for howling prevention can be ensured even in the high-frequency range of audio. Furthermore, since the characteristics of the pseudo acoustic path are not disturbed by simultaneous two-way communication, there is no unnatural noise in the pseudo loop signal, which has the effect of completely improving the speech quality.

As described above, the present invention achieves great effects with a simple configuration.

In the above-mentioned embodiment, the speaker, microphone, and delay time measuring circuit were each explained as one each for convenience, but in an actual conference, a plurality of speakers may be used.

When microphones are used, it is possible to provide a pseudo line and a variable delay line corresponding to each microphone. In this case, the delay time measuring circuit may be used in common, or a plurality of them may be used to more precisely correct the delay time. Of course, these modifications are included in the present invention.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of the acoustic power spectrum at the microphone position, FIG. 2 is a block diagram showing an embodiment of the present invention,
Fig. 3 is a diagram showing an example of the impulse response of the acoustic path, Fig. 4 is a block diagram showing an example of the configuration of the pseudo acoustic path of the embodiment shown in Fig. 2, and Fig. 5 is a diagram showing an example of the configuration of the pseudo acoustic path of the embodiment shown in Fig. 2. FIG. 3 is a diagram showing an example of the envelope of the audio waveform of each main part of the embodiment. In the figure, 21... Receiving system input terminal, 22... Received signal, 23... Receiving system output terminal, 24... Speaker, 25... Sound scar, 26... Listener, 27... ... Microphone, 28 ... Speech system input terminal, 41 ... Output signal of variable delay line 212, 42.43 ... Register, 44
... Multiplication circuit, 45 ... Accumulation circuit, 46 ... Output signal of pseudo acoustic path, 210 ... Subtractor, 211
... Sending system output terminal, 212 ... Variable delay line, 21
5...Delay time measurement circuit. Agent Patent Attorney Resident 1) Toshi So (15) Broom 1 Diagram - fI3g Fu - Diagram 2

Claims (1)

[Claims] (1) A variable delay line that delays a received signal, a pseudo acoustic path that outputs a pseudo loop signal by multiplying the output signal of the variable delay line by a fixed tap coefficient, and a pseudo acoustic path that is inserted into the transmitting system transmission line. an impulse response of the pseudo acoustic path, comprising a subtractor that subtracts and outputs the output signal of the pseudo acoustic path from the sag signal, and a delay time measuring circuit that outputs an output signal corresponding to the delay time of the acoustic path. The characteristics are set in advance using measured values, etc., and the time change between the delay time of the acoustic path at the time of setting and the delay time of the acoustic path during use is corrected by the delay time 8i++ constant circuit and the variable delay line. Howling prevention device with % characteristics. (2. The howling prevention device according to claim 1, wherein the delay time measuring circuit measures the propagation time of the ultrasonic wave.