Public address system

Abstract

To provide a loudspeaker device capable of preventing the occurrence of howling with a simple circuit configuration. [Solution] The loudspeaker includes a detection unit (70) that detects when the level of an amplified signal output from a microphone amplifier (20) having output characteristics including a linear region and a non-linear region following the linear region has reached the non-linear region, and a signal processing unit (40) that inverts the amplified signal when the detection unit (70) detects that the level of the amplified signal has reached the non-linear region. The detection unit (70) determines that the level of the amplified signal has reached the non-linear region when the level of the amplified signal is equal to or greater than a predetermined detection set value.

Description

[Technical field]

[0001] The present invention relates to a loudspeaker system, and is suitable for use in, for example, hearing aids and karaoke systems. [Background technology]

[0002] It is a well-known fact that feedback cannot be prevented. When speaker sound above a certain level enters a microphone, feedback occurs. Measures can be taken to prevent feedback, such as narrowing the microphone's directivity, to minimize the amount of sound returning to the microphone. Among public address systems, hearing aids have a relatively high gain. In hearing aids for people with moderate to severe hearing loss, it is difficult to suppress feedback with volume control alone. In hearing aids for people with severe hearing loss, there are cases in which the gap between the ear and the hearing aid is filled with silicone rubber or the like to isolate the earphone from the microphone. However, feedback can occur even with a slight amount of sound leakage, so this is not a complete solution.

[0003] Current methods for preventing feedback involve dealing with feedback that has already occurred. General public address systems, including hearing aids for mild hearing loss, are digitized, and feedback waveforms for rare loud sounds are altered to make them less noticeable. If the volume exceeds this limit, feedback cannot be suppressed and continues. Some hearing aid instruction manuals include a warning that "this does not mean that feedback will be eliminated." There are also reports of preventing feedback by feeding forward the signal based on assumed sound field conditions, but this is not perfect.

[0004] The technology described in Patent Document 1 suppresses howling by inverting the phase of the signal when the occurrence of howling is detected, but since howling has already occurred, it does not fundamentally prevent howling. The howling suppression technology described in Patent Document 1 is a noise canceller system itself.

[0005] Patent Document 2 describes a hearing aid that includes a sound input unit that converts sound into an electric signal, an amplifier unit that amplifies the electric signal converted by the sound input unit, a sound output unit that converts the amplified signal amplified by the amplifier unit into sound, a howling detection unit that detects howling when the level of the amplified signal exceeds a preset value that distinguishes between sound and howling, and a condition change unit that performs at least one of the following change processes when the howling detection unit detects howling: changing the phase of the howling signal passing through the amplifier unit and inputting it to the sound output unit, lowering the level of the howling signal, and inputting a dummy signal to the sound output unit instead of the howling signal, and the condition change unit detects howling and performs the change process in the howling detection unit, and then terminates the change process at a predetermined timing. However, this hearing aid cannot prevent howling from occurring in advance. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2000-341787 A [Patent Document 2] JP 2020-10296 A Summary of the Invention [Problem to be solved by the invention]

[0007] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide a loudspeaker system capable of preventing the occurrence of howling with a simple circuit configuration. [Means for solving the problem]

[0008] In order to solve the above problems, the present invention provides: a detection unit that detects when a level of an amplified signal output from a microphone amplifier having output characteristics including a linear region and a non-linear region following the linear region reaches the non-linear region; a signal processing unit that inverts the amplified signal when the detection unit detects that the level of the amplified signal has reached the non-linear region; It is a public address system having the following features.

[0009] In this loudspeaker, the microphone amplifier may be built in the loudspeaker or may be built in the microphone. The microphone may be integrated with the loudspeaker or may be externally attached to the loudspeaker. This loudspeaker typically has an operational amplifier that forms a positive-phase signal and a negative-phase signal from the amplified signal output from the microphone amplifier, and an output amplifier that amplifies the signal output from the operational amplifier. A speaker that is integrated with the loudspeaker or an external speaker is connected to the output amplifier.

[0010] The detection unit is not particularly limited as long as it can detect that the level of the amplified signal output from the microphone amplifier has reached the non-linear region of the output characteristics of the microphone amplifier, and is configured as necessary, for example, by a comparator, a flip-flop, etc. The signal processing unit is not particularly limited as long as it can invert the amplified signal when the detection unit detects that the level of the amplified signal has reached the non-linear region, and is configured as necessary, for example, by a toggle switch, etc. The detection unit typically determines that the level of the amplified signal has reached the non-linear region when the level of the amplified signal is equal to or greater than a predetermined detection setting value.

[0011] In this sound amplification system, feedback can be considered as a phenomenon of loop oscillation with the space, i.e., the sound field, as the feedback circuit. Feedback occurs in a loop from microphone → sound system → speaker → sound field → speaker. The feedback signal of the loop is a sine wave signal. Loop oscillation is expressed by the following equation (1). S=1+B+B 2 +B 3 +B 4 +B 5 +…+B N =[B (N+1) -1] / (B-1) …(1) Here, S represents the microphone output of the fed-back sine wave signal, and is the microphone input each time it goes around the loop. If the gain of the public address system is A and the gain of the feedback circuit is β, then B = Aβ (loop transfer function). If B>1, then the initial input (first term 1) becomes the sum of a geometric series multiplied by B each time it goes around the loop. Equation (1) indicates that oscillation will occur after N times going around the loop. B=1 is oscillation.

[0012] The frequency of the sine wave signal, which is the feedback signal of the loop, is thought to have a filter characteristic, as standing waves are generated by the reflection of the sound from objects present in the sound field at each time. For example, it is sometimes observed that the tone of the feedback changes when a person moves in a sound field.

[0013] We will now explain the first term 1 (normalized signal) in equation (1). When the power switch of a public address system is turned on, for example, suddenly changing signals or suddenly occurring sounds are always input to the public address system. When the pulse waveforms of these signals or sounds are viewed in the frequency domain using a Fourier transform, they have a wide bandwidth. Therefore, there is always a signal that matches the filter frequency of the sound field at that time. This signal is the first term 1 of S.

[0014] The condition for loop oscillation is (A) Gain B is 1 or more. (B) There is a feedback circuit (sound field) for filter characteristics. (C) The phase rotation is 360 degrees. There are three conditions. By not satisfying one of these conditions, the occurrence of feedback can be prevented. Because this sound amplification system is used as a sound amplification system, condition (A) cannot be used. Condition (B) cannot be used because it is determined by the environment in which the sound amplification system is placed. As a result, in order to prevent the occurrence of feedback due to loop oscillation, condition (C) must not be satisfied.

[0015] In this sound amplification system, the sine wave electrical signal portion obtained by inputting a loop filter feedback sine wave signal superimposed on the voice (current voice) to the microphone generates a phase shift in the non-linear region of the microphone amplifier's output characteristics. This (voice + sine wave) is detected and inverted to eliminate the phase shift of the sine wave and return it to the normalized signal (the first term 1 in equation (1)), preventing loop oscillation, i.e., howling. No matter how many times the feedback sine wave signal circulates, it has no phase shift and is the same phase as the normalized signal unless it enters the non-linear region of the microphone amplifier's output characteristics. No matter how many times it circulates, the signal that first entered the non-linear region of the microphone amplifier's output characteristics is always inverted and returned to the normalized signal. Effect of the Invention

[0016] According to the present invention, it is possible to prevent the occurrence of howling before it occurs, and moreover, a simple circuit configuration including a detection section and a signal processing section is sufficient. [Brief description of the drawings]

[0017] [Figure 1] 1 is a schematic diagram showing a loudspeaker apparatus according to an embodiment of the present invention; [Diagram 2] 3 is a schematic diagram showing output characteristics of a microphone amplifier of the loudspeaker audio device according to the embodiment of the present invention. [Diagram 3] 4 is a schematic diagram showing an example of an input signal to a microphone amplifier of the loudspeaker audio device according to the embodiment of the present invention. [Figure 4] 2 is a schematic diagram showing an example of a circuit configuration of a detection unit of the loudspeaker audio device according to the embodiment of the present invention. [Diagram 5] 5 is a schematic diagram for explaining the operation of the loudspeaker audio system according to the embodiment of the present invention; FIG. [Figure 6] 5 is a timing chart for explaining the operation of the loudspeaker sound system according to the embodiment of the present invention. [Figure 7] 3 is a schematic diagram showing a circuit configuration of a detection unit used in an example of the loudspeaker audio system according to the embodiment of the present invention. FIG. [Figure 8]11 is a photograph, substituted for a drawing, showing a signal observed when a detection unit is turned on in the example of the loudspeaker sound system according to the embodiment of the present invention. [Figure 9] 10 is a photograph substituted for a drawing showing a waveform of a signal in a precursor state to the occurrence of howling, measured with a detection unit turned off in an example of the loudspeaker sound system according to an embodiment of the present invention. [Figure 10] 11 is a photograph substituted for a drawing showing a waveform of a signal after howling occurs, measured with a detection unit turned off in an example of the loudspeaker sound system according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, modes for carrying out the invention (hereinafter referred to as "embodiments") will be described.

[0019] <One embodiment> [Sound amplifier] FIG. 1 shows a sound amplification device according to an embodiment. As shown in FIG. 1, the sound amplification device includes a microphone 10, a microphone amplifier 20, an operational amplifier 30, a signal processing unit 40, an output amplifier 50, a speaker 60, and a detection unit 70. The microphone amplifier 20 may be built into the microphone 10, or may be connected externally. The sound input to the microphone 10 is converted into an electric signal, and the electric signal is amplified by the microphone amplifier 20. The amplified signal amplified by the microphone amplifier 20 is input to the operational amplifier 30. The operational amplifier 30 has a linear output characteristic and a gain of 0 dB, and forms a positive-phase signal Φ0 or a negative-phase signal Φπ according to the amplified signal input. These positive-phase signal Φ0 and negative-phase signal Φπ are supplied to the signal processing unit 40, and a toggle switch 41 can be used to switch between the positive-phase signal Φ0 and the negative-phase signal Φπ according to the signal output from the detection unit 70. The output of the signal processing unit 40 is supplied to the output amplifier 40 via an audio volume 42. The output amplifier 50 is connected to a speaker 60 .

[0020] In FIG. 1, the parts enclosed by thin dashed lines are the electric processing part of the microphone 10, the microphone amplifier 20, the operational amplifier 30, the signal processing part 40, the output amplifier 50, the electric processing part of the speaker 60, and the detection part 70, which are the areas where the electromagnetic field is generated, and the parts enclosed by thicker dashed lines are the areas where the sound field is formed by the sound input to the microphone 10 and the sound generated from the speaker 60. The mechanism of howling is considered to be based on the interaction (information exchange) between the electromagnetic field and the sound field. The microphone 10 is a converter that converts the sound field into an electromagnetic field, and the speaker 60 is a converter that converts the electromagnetic field into a sound field. The feedback sound signal in FIG. 1 attenuates with the square of the distance between the speaker 60 and the microphone 10, and does not contribute to the oscillation.

[0021] Fig. 2 shows the output characteristics of the microphone amplifier 20. As shown in Fig. 2, the output characteristics of the microphone amplifier 20 are characterized in that they have a linear region and a non-linear region following this linear region.

[0022] The detection unit 70 is connected between the microphone amplifier 20 and the signal processing unit 40. The detection unit 70 detects a precursor state of feedback, that is, the level of the amplified signal output from the microphone amplifier 20 reaches the non-linear region of the output characteristics of the microphone amplifier 20. That is, as shown in FIG. 2, a signal as shown in FIG. 3 (current sound + sine wave signal which is a feedback signal of the loop) is input to the microphone 10, and a signal as shown in FIG. 2 is input to the microphone amplifier 20. The amplified signal as shown in FIG. 2 is output by the input signal to the microphone amplifier 20. In FIG. 2, the level of this amplified signal just reaches the non-linear region, and the detection unit 70 detects this. Here, the amplified signal is obtained by converting the current sound + sine wave signal which is a feedback signal of the loop into an electric signal, and the sine wave signal as an electric signal corresponding to the sine wave signal is superimposed on the electric signal corresponding to the current sound, so the level of the amplified signal is the level of the position of the sine wave signal.

[0023] An example of the circuit configuration of the detection unit 70 is shown in Fig. 4. As shown in Fig. 4, the detection unit 70 has a comparator 71 and a flip-flop (FF) 72. An amplified signal output from the microphone amplifier 20 is input to an inverting input terminal (-) of the comparator 71 via a resistor 73. A non-inverting input terminal (+) of the comparator 71 is connected to a ground line (GND) 75 via a volume 74. A power supply voltage V CC A resistor 77 and a variable resistor 78 are connected in series between a power supply line 76 that supplies V and a ground line 75. The volume 74 is made up of a variable resistor 78 and a capacitor 79. By adjusting the variable resistor 78 with the volume 74, it is possible to adjust a detection set value for determining whether or not the level of the amplified signal has reached the non-linear region. The output of the comparator 71 is supplied to a flip-flop 72. A resistor 80 is connected between the output terminal of the comparator 71 and the ground line 75. The flip-flop 72 is connected in series with a power supply voltage V CC The flip-flop 72 is connected between a power supply line 76 which supplies a voltage Vout and a ground line 75. The output of the flip-flop 72 makes it possible to switch between a positive phase signal Φ0 and a negative phase signal Φπ output from the operational amplifier 30.

[0024] [Sound amplifier operating] The operation of this loudspeaker will be described. Assume that the actual sound input to the microphone 10 and the sine wave signal, which is the feedback signal of the loop, have waveforms as shown in Fig. 3, and that this input causes the microphone amplifier 20 to output an amplified signal as shown in Fig. 2. In order to explain the operation of this loudspeaker, the operational amplifier 30, the signal processing unit 40, and the detection unit 70 of this loudspeaker are shown in Fig. 5. In Fig. 5, the timing charts of the amplified signal (1) output from the microphone amplifier 20, the signals (2) and (3) of the comparator 71 and the flip-flop circuit 72 of the detection unit 70, the positive-phase signal Φ0 (4) and the negative-phase signal Φπ (5) of the operational amplifier 30, and the output signal (6) of the signal processing unit 40 are shown in Figs. 6A to 6F.

[0025] As shown in FIG. 6A, the input amplified signal reaches a detection set value that is determined in advance to determine whether the level of the amplified signal reaches a non-linear region. As shown in FIG. 6B, the output of the comparator 71 maintains a high level when the input signal is lower than the detection set value, and outputs a low level when the input signal exceeds the detection set value. As shown in FIG. 6C, the flip-flop 72 starts operating when the input signal (2) goes from high level to low level, and the output signal of the flip-flop 72 goes from low level to high level, and goes from high level to low level when the input signal (2) next goes from high level to low level. The output signal of the flip-flop 72 toggles the toggle switch 41 of the signal processing unit 40. As shown in FIGS. 6D and E, every time the level of the amplified signal output from the microphone amplifier 20 reaches the detection set value, the toggle switch 41 toggles the signal passing through the signal processing unit 40 from the positive phase signal Φ0 to the negative phase signal Φπ, or from the negative phase signal Φπ to the positive phase signal Φ0. As shown in FIG. 6F, a discontinuity occurs in the output signal of the signal processor 40 due to an inversion operation from a positive-phase signal Φ0 to a negative-phase signal Φπ or from a negative-phase signal Φπ to a positive-phase signal Φ0.

[0026] [Example of a loudspeaker system] The omnidirectional back electret condenser microphone WM-61A manufactured by Panasonic Corporation was used as the microphone 10 with a built-in microphone amplifier 20 (preamplifier). The WM-61A has an impedance of 2.2 kΩ or less, a standard operating voltage of 2 V, and a sensitivity of -35±4 dB (0 dB=1 V / Pa, 1 kHz).

[0027] For an experiment on the occurrence of feedback, the loudspeaker system shown in Fig. 1 was actually assembled. In the detection section 70, a feedback prevention switch 81 consisting of a toggle switch was provided between the comparator 71 and the flip-flop 72, as shown in Fig. 7. When the feedback prevention switch 81 is turned off, no signal is output from the flip-flop 72, so the toggle switch 41 of the signal processing section 40 does not work, and it is not possible to switch between the positive phase signal Φ0 and the negative phase signal Φπ, and therefore it becomes impossible to invert the amplified signal.

[0028] An experiment on howling was carried out by inputting sound using the non-directional back electret condenser microphone WM-61A while observing the amplified signal output from the microphone amplifier 20 with an oscilloscope at a position between the microphone amplifier 20 and the detection unit 70 in Fig. 1. The oscilloscope screen and the sound input to the WM-61A were recorded in MP4. The inventor input the sound, and a person's voice on an FM broadcast was played in the background.

[0029] FIG. 8 shows the result of recording and recording in MP4 with the anti-feedback switch 80 turned on. In the screen of FIG. 8, the N-shaped waveform indicates the voice of the inventor repeatedly saying "Anti-feedback switch is on," and the group of waves on the right side of the screen indicates the voice of an FM broadcast playing in the background. In FIG. 8, a waveform corresponding to the output signal shown in FIG. 2 output from the microphone amplifier 20 and a waveform corresponding to the input signal shown in FIG. 2 input to the microphone amplifier 20 thereafter are observed, and it can be seen that the waveform corresponding to the latter input signal has a smaller amplitude than the waveform corresponding to the former output signal. In this state, it is clear that the loop oscillation shown by the above formula (1) is suppressed and no howling occurs. This is the result of inverting the amplified signal when it is detected that the level of the amplified signal output from the microphone amplifier 20 has reached the nonlinear region of the output characteristic of this microphone amplifier 20.

[0030] 9 and 10 show the results of video and audio recording in MP4 format with the anti-feedback switch 81 switched from on to off. The time axis and voltage axis of the oscilloscope screen are the same as those in FIG. 8. When the anti-feedback switch 81 is turned off, that is, when the anti-feedback functions of the detection unit 70 and the signal processing unit 40 are not working, loop oscillation occurs rapidly based on the above formula (1), and after passing through a state just before feedback occurs as shown in FIG. 9, feedback actually occurs as shown in FIG. 10.

[0031] As described above, according to this embodiment, the detection unit 70 is provided between the microphone amplifier 20 and the signal processing unit 40, and this detection unit 70 detects that the level of the amplified signal output from the microphone amplifier 20 has reached the non-linear region of the output characteristics of this microphone amplifier 20, and the amplified signal is inverted by the signal processing unit 40, thereby making it possible to prevent the occurrence of howling. Moreover, this loudspeaker sound device can be realized simply by providing the detection unit 70 and the signal processing unit 40 to an existing sound device, and a simple circuit configuration is sufficient.

[0032] Although one embodiment of the present invention has been specifically described above, the present invention is not limited to the above-described embodiment, and various modifications based on the technical concept of the present invention are possible.

[0033] For example, the numerical values, configurations, shapes, methods, etc. given in the above-described embodiments are merely examples, and numerical values, configurations, shapes, methods, etc. different from these may be used as necessary. [Industrial Applicability]

[0034] INDUSTRIAL APPLICABILITY The present invention can be applied to hearing aids, karaoke devices, and the like, and can also be applied to various applications in which a microphone and a speaker are used to amplify sound. [Explanation of symbols]

[0035] 10: microphone, 20: microphone amplifier, 30: operational amplifier, 40: signal processing unit, 50: output amplifier, 60: speaker, 70: detection unit

Claims

1. a detection unit that detects when a level of an amplified signal output from a microphone amplifier having output characteristics including a linear region and a non-linear region subsequent to the linear region reaches the non-linear region; a signal processing unit that inverts the amplified signal when the detection unit detects that the level of the amplified signal has reached the non-linear region; A public address system having the above construction.

2. 2. The sound amplifier according to claim 1, wherein said detection section determines that the level of said amplified signal has reached said non-linear region when the level of said amplified signal is equal to or greater than a predetermined detection set value.

3. 2. The loudspeaker system according to claim 1, wherein the microphone amplifier is built in the loudspeaker system or in a microphone.

4. 4. The loudspeaker system according to claim 3, wherein the microphone is integrated with the loudspeaker system or is externally attached to the loudspeaker system.

5. 2. The loudspeaker system according to claim 1, further comprising an operational amplifier for forming a positive phase signal and a negative phase signal from the amplified signal output from the microphone amplifier, and an output amplifier for amplifying the signal output from the operational amplifier.

6. 6. The sound amplification system according to claim 5, wherein a speaker integrated with the sound amplification system or an external speaker is connected to the output amplifier.

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