JP2006197580A - Sound signal amplitude limiter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sound signal amplitude limiter whereby a reproduced sound with a less sense of distortion can be obtained even when the amplitude of the sound signal is considerably limited so as to enhance the sound quality of hearing-aids and recording units or the like. <P>SOLUTION: The sound signal amplitude limit unit includes: an amplitude limit circuit 1 for placing a limit onto a received sound signal; a pre-stage subtractor 2 provided to an output side of the amplitude limiter circuit; a low pass filter 3 for cutting off high frequency components in an output signal from the subtractor; a delay circuit 4 connected in parallel with the low pass filter 3; and a subtractor 5 connected to the post-stage of the low pass filter 3 for providing a processing finished signal. The pre-stage subtractor 2 receives the sound signal and executes processing of subtracting an output signal of the amplitude limiter circuit from the received sound signal. The delay circuit 4 receives the sound signal and carries out delay processing to synchronize the output signal from the low pass filter with the received sound signal. The post-stage subtractor 5 receives the output signal from the delay circuit, subtracts the output of the low pass filter from the received output signal from the delay circuit, and provides an output of the subtraction result. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an audio signal amplitude limiter capable of obtaining reproduced sound with very little deterioration in sound quality.

  Conventionally, when an amplitude signal that greatly exceeds the capacity of an audio signal amplifier or the like is processed, the reproduced sound quality of the output signal is significantly lowered. Therefore, a limiter and an amplitude suppression circuit have been proposed in which the reproduction sound quality is hardly degraded even when a large amplitude signal is input.

  Patent Document 1 relates to the proposal of the present inventor. When the input signal waveform level rises and exceeds the upper limit of the limit amplitude, the output signal waveform stops at the upper limit of the limit amplitude, and the input signal waveform increases. When starting from descent, the descent start point or its vicinity becomes the upper limit value of the amplitude limit, and the subsequent waveform is output.

  However, in this conventional technique, when the input signal level is large, a large harmonic distortion is generated by the amplitude limiting operation, and a relatively high frequency component among the harmonics gives a considerably strong distortion feeling to the reproduced sound.

  In particular, when this type of technology is applied to a hearing aid, etc., even if the amplitude is very strong and accurate, even if it is used for a hearing impaired person with severe hearing loss, the sound quality will not deteriorate much. Although necessary, the technology of Patent Document 1 cannot meet this requirement.

  The inventor has also proposed the invention shown in Patent Document 2. In this prior art, when a signal exceeding the amplitude limit is input, the amplitude portion exceeding the specified level is more strongly suppressed by the low frequency signal than the high frequency signal. .

  In other words, when a person hears a natural sound, the greatest difference between a distant sound and a nearby sound is due to the level of the frequency, and the low sound is more attenuated by the distance than the high sound. Since the amplitude of the bass is large and the amplitude of the bass of a distant sound is small, it is considered that a person listens with a high level bass corrected to a small level.

  Therefore, according to the prior art of Patent Document 2, it is possible to suppress the amplitude of the large amplitude signal without changing the small amplitude signal even in the case of a bass signal. There is an advantage that a sound with a small level difference between a distant sound and a nearby sound can be reproduced.

  However, since the amplitude limit value cannot be set accurately in the prior art of Patent Document 2, for example, when used in a power amplifier for audio signals, there has been a drawback that the amplifier capacity cannot be fully exploited. Furthermore, the prior art of Patent Document 2 requires that the power supply voltage of the circuit be high in order to limit the amplitude of a signal having a large amplitude. In this case, it is easy to think of using negative feedback to process a signal having a relatively large amplitude with a circuit having a small dynamic range, but it is difficult to accurately set the limit amplitude with negative feedback.

JP 2001-320255 A Application for Japanese Patent Application No. 2004-226172

  The present invention has been proposed in order to solve the above-mentioned problems of the prior art. Even when a sound signal having a large amplitude is inputted, the determined amplitude limit value is hardly exceeded and reproduction is performed. An object of the present invention is to realize an audio signal amplitude limiter that outputs a sound signal in which the sound does not feel much distortion.

  In order to achieve the above object, the invention of claim 1 is directed to an amplitude limiting circuit for limiting the amplitude of an input audio signal, a subtracter provided on the output side of the amplitude limiting circuit, and an output from the subtractor. A low-pass filter that cuts high-frequency components in the signal; a delay circuit connected in parallel with the low-pass filter; and a subtractor that is arranged downstream of the low-pass filter 3 and outputs a processed audio signal. The audio signal similar to that input to the amplitude limiting circuit is input and the output signal of the amplitude limiting circuit is subtracted from the audio signal. The delay circuit is input to the amplitude limiting circuit. The audio signal similar to the above is input, and a delay process is performed to synchronize the input signal with the output signal from the low-pass filter. The subtractor in the subsequent stage includes the delay circuit. Enter a et of the output signal, characterized in that the input signal from the delay circuit and outputs by subtracting the output from the low pass filter.

  According to a second aspect of the present invention, the amplitude limiter circuit and a subtracter for inputting a voice signal similar to that input to the amplitude limiter circuit and subtracting the output signal of the amplitude limiter circuit from the voice signal. Is constituted by a small change amplitude signal deletion circuit.

  According to a third aspect of the present invention, an input audio signal is divided into a plurality of bands, and the amplitude limiting circuit, a subtractor, a low-pass filter, a delay circuit, and a subtractor at the subsequent stage are provided for each band. To do.

The invention of claim 4 is characterized in that the frequency characteristics of the low-pass filter and the delay circuit are the same or approximate.
The invention of claim 5 is characterized by comprising an equalizer that emphasizes high frequency components of the input signal or output signal.

  According to the present invention, although the amplitude is limited, the high-frequency component lost by the amplitude limitation or a part of the high frequency component lost by the amplitude limitation is added to the signal subjected to the amplitude limitation. Thus, it is possible to obtain a reproduced sound with little deterioration in sound quality. Also, by providing a delay circuit, it becomes possible to synchronize the signal whose amplitude has been limited and the input unprocessed signal, and it is possible to obtain an easy-to-hear reproduction that is close to the original sound from the difference between the two. . Further, when the frequency characteristics of the low-pass filter and the delay circuit are the same or approximate, more strict amplitude limitation can be performed. In addition, when the audio frequency band is divided into a plurality of parts and the amplitude is limited for each band, the distortion of the reproduced sound can be further reduced.

1. First Embodiment Hereinafter, a first embodiment of the present invention will be described in detail with reference to FIG.

(A) Configuration of First Embodiment In FIG. 1, reference numeral 1 denotes an amplitude limiting circuit that limits the amplitude of an input unprocessed audio signal, 2 denotes a subtractor connected to the subsequent stage, and 3 denotes a subtracter 2. 4 is a delay circuit connected in parallel with the low-pass filter 3, and 5 is a subtractor disposed downstream of the low-pass filter 3. The output is output to a speaker or the like.

  The subtracter 2 receives an unprocessed audio signal similar to that input to the amplitude limiting circuit 1 and performs a process of subtracting the output signal of the amplitude limiting circuit 1 from the audio signal. An unprocessed audio signal is input to the delay circuit 4, an output from the delay circuit 4 is input to the subtractor 4, and an output from the delay circuit 4 is output from the low-pass filter 3 in the subtractor 4. The output is subtracted.

(B) Operation of the First Embodiment The operation order of the amplitude limiter of FIG. 1 having such a configuration is represented by the components (a) to (f) of the amplitude limiter and the corresponding (a) to (a) of FIG. It will be described as follows according to the signal waveform diagram shown in (f).
(1) A specific configuration of the amplitude limiting circuit 1 is shown in FIG. 2 as an example. The amplitude limiting circuit 1 generates a signal such as (b) in which the input signal (a) is amplitude limited. Is output. The circuit configuration and operation of FIG. 2 will be described in detail later.

(2) The input signal (a) and the output signal (b) of the amplitude limiting circuit 1 are subtracted from each other by the subtracter 2, and the difference (c) is output from the subtractor 2. In this case, the input signal (a) is a signal that is sufficiently large to limit the amplitude. When the amplitude is not limited, the two input signals (a) and (b) of the subtracter 2 are the same, so that no signal appears in (c) and (d) of FIG. Therefore, the signal (e) appears as it is in the output.

(3) The high frequency component of the output signal (c) of the subtracter 2 is cut by the low-pass filter 3. The low-pass filter 3 is characterized in that the cut-off frequency is relatively high and the frequency characteristics are flat from a low frequency to near the cut-off frequency. Therefore, the waveform (d) of the output signal from the low-pass filter 3 is substantially the same as the output signal of the computing unit 2 from the low frequency to near the cutoff frequency. On the other hand, a signal in a high frequency band equal to or higher than the cutoff frequency is not attenuated by the low-pass filter 3.

(4) One of the input signals (a) is delayed by the delay circuit 4 so as to be synchronized with the output signal (d) of the low-pass filter 3. Then, the delayed signal (e) is input to the subtracter 5. Since the delay circuit 4 delays the input signal (a) for synchronization in this embodiment, the output signal (e) from the delay circuit 4 has the same waveform although the phase is different from that of the input signal. is there.

(5) The output (d) of the low-pass filter 3 and the output (e) of the delay circuit 4 are input to the subtracter 5 and subtracted from each other, and the difference signal (f) appears in the output circuit of the subtractor 5. When amplitude limiting, subtraction processing, or frequency cut-off processing is performed, signal delay occurs during the processing. Therefore, in the present invention, the delay circuit 4 delays the input signal input to the subtracter 5 so as to establish synchronization.

(6) In the first embodiment, the low-pass filter 3 and the delay circuit 4 have different frequency characteristics. That is, the low-pass filter 3 uses a flat characteristic from the low frequency band to the middle frequency band, and a curvilinear characteristic from which the cutoff processing is performed as the frequency increases from the high frequency band. . On the other hand, the delay circuit 4 does not have such frequency characteristics but uses a circuit that exhibits flat characteristics in the entire frequency band.

  Therefore, the signal (e) having the same level as the output signal (a) of the amplitude limiting circuit 1 is output to the output circuit of the subtractor 5 in the frequency range where the characteristics of the low-pass filter 3 are flat. However, in the frequency range where the characteristics of the low-pass filter 3 are not flat (that is, the high-frequency range), the output signal from the low-pass filter 3 does not appear, whereas the output signal (e) of the delay circuit 4 is at the center of FIG. It appears in the output circuit (f) of the subtractor 5 like the described waveform. As a result, as the output signal (f) from the subtracter 5, a signal exceeding the amplitude limit included in the input signal (a) is also output, and the amplitude limit is not so much performed.

(7) In the circuit according to the first embodiment, even if the amplitude is limited, the low-pass filter 3 does not cause much distortion in the reproduced sound. However, due to this high cut, the amplitude is not so limited in the frequency band where the characteristics of the low-pass filter 3 are not flat. However, since the amplitude of a high frequency audio signal is usually small and is not so audible, even if this amplitude limiting operation is incomplete, it is not a big problem in practice.

(C) Specific Example of Amplitude Limiting Circuit 2 Next, an example of the amplitude limiting circuit 2 in the first embodiment will be specifically described with reference to FIG. The amplitude limiting circuit 1 detects a signal in a section where a constant voltage changes in the same direction from the point where the voltage change direction of the input signal is reversed, and outputs the detected component signal. The amplitude limiting circuit used in the first embodiment is not limited to that shown in FIG. 3, but the circuit shown in FIG. 3 can process a signal having a relatively large amplitude with a circuit having a small dynamic range.

  That is, in the amplitude limiting circuits of Patent Document 1 and Patent Document 2, a circuit having a larger dynamic range than the output signal amplitude is required. Therefore, a relatively high voltage power source is required. In order to process a signal having a relatively large amplitude in a circuit having a small dynamic range, it is easy to think of using negative feedback, but it is difficult to set the limit amplitude accurately in negative feedback. However, by combining the amplitude limitation by negative feedback and the amplitude limitation circuit used in Patent Document 1 or Patent Document 2, a signal having a relatively large amplitude can be processed with a small dynamic range circuit.

  FIG. 2 is a circuit example in which amplitude suppression by negative feedback is combined with amplitude limitation used in Patent Documents 1 and 2. In FIG. 2, the portion surrounded by a dotted line is a portion for performing amplitude suppression processing by negative feedback. That is, the circuit composed of the operational amplifier OP1, the operational amplifier OP2, the capacitor C1, the capacitors C2, R1, R2, R3, R4, the diode D1, and the diode D2 in FIG. 2 is an amplitude suppression circuit using negative feedback.

  When the waveform signal shown in FIG. 3A is input to the INPUT of this circuit, the waveform signal shown in FIG. 3C appears at the output terminal of the operational amplifier OP1. The waveform in FIG. 3B is a signal waveform that appears at the output terminal of the operational amplifier OP1 when the waveform in FIG. 3A is input to INPUT in the circuit without the diode D1 and the diode D2. Therefore, in order to obtain the same result as that of FIG. 3 (d) by the amplitude limiting circuit of Patent Document 1 or Patent Document 2, a circuit that can handle the amplitude of FIG. 3 (b) is required.

The operation of outputting the waveform of FIG. 3C will be described as follows.
(1) The horizontal axis of FIG. 3 is a time axis. 1, 1 ′, 1 ″, 1 ′ ″, 2, 2 ′, 2 ″, 2 ′ ″ indicate operating voltages at which the amplitude limiting circuit starts to operate, and 1 and 2 indicate initial values thereof. It is. The voltage between 1 and 2 is determined by the forward voltage when a current flows through the diode D1 and the diode D2 in the circuit of FIG.

(2) The output terminal potential of the operational amplifier OP1 changes in the same manner as in FIG. 2B from t1 to t2. This is because the current does not flow through the diode D1 and the diode D2 during this period, so that the output potential of the operational amplifier OP2 does not move.

(3) From t2 to t3, the waveform in FIG. 3 (c) changes in the same manner as in FIG. 3 (a). The reason is that a current flows through the diode D1, and the output potential of the operational amplifier OP2 moves together with the output potential of the operational amplifier OP1. At this time, the potential at which the waveform suppression circuit starts to operate is shifted to 1 ′ and 2 ′ by the output potential of the operational amplifier OP1.

(4) Since the output potential of the operational amplifier OP2 does not move until the next t3 to t4, the output potential of the operational amplifier OP1 changes in the same manner as in FIG.
(5) Since the output potential of the operational amplifier OP2 moves together with the output potential of the operational amplifier OP1 from t4 to t5, the output potential of the operational amplifier OP1 changes as in FIG. At this time, the potential at which the waveform limiting circuit starts operating shifts to 1 ″ and 2 ″.

(6) Similarly, the output voltage of the operational amplifier OP1 varies with FIG. 3B from t5 to t6.
(7) Furthermore, the output voltage of the operational amplifier OP1 varies with FIG. 3 (a) from t6 to t7. At this time, the potential at which the waveform limiting circuit starts to operate is shifted to a voltage of 1 ′ ″, 2 ′ ″.

(8) After t7, the same operation as described above is performed and the waveform signal shown in FIG. 3C appears at the output of the operational amplifier OP1.
(9) The circuit constituted by the operational amplifier OP1, the capacitor C3, the diode D3, the diodes D4 and R5 is equivalent to the circuit used in Patent Document 1 or Patent Document 2, and the waveform signal in FIG. The waveform is shaped as shown in FIG.

(10) In other words, in order to obtain the signal shown in FIG. 3 (d) with the circuits used in Patent Document 1 and Patent Document 2, a circuit capable of handling a dynamic range larger than the signal shown in FIG. 3 (b) is required. In the case of the circuit of FIG. 2, it is sufficient to handle a dynamic range larger than the waveform of FIG.

(D) Other Specific Example of Amplitude Limiting Circuit 1 FIG. 4 shows another specific example of the amplitude limiting circuit 1 according to the first embodiment, and a circuit in which the amplitude limiting value changes in conjunction with fluctuations in the power supply voltage. It is an example. That is, in order to produce as large a volume as possible with an audio power amplifier or the like of a small device, it is desirable that the waveform limiting circuit of the present invention operates at a peak value immediately before the power amplifier is saturated and distortion increases. For this purpose, the amplitude limit value needs to be changed in conjunction with fluctuations in the power supply voltage. FIG. 4 shows an example of the circuit (partial block diagram).

  When the power supply voltage changes in the high direction in the circuit of FIG. 4, the voltage across R8 increases and the output voltage of the operational amplifier OP3 decreases. As a result, the emitter potential at which the emitter current starts to flow through the transistor Q1 decreases. Further, since the voltage across R12 also increases, the emitter potential at which the emitter current starts to flow through the transistor Q2 increases. Therefore, the amplitude limit value of this circuit becomes large. When the power supply voltage is lowered, the limit amplitude value is also reduced by the operation opposite to the above.

  Note that how much the voltage gain and the amplitude limit value of the operational amplifier OP3 and the operational amplifier OP4 should be determined should be determined in relation to the voltage gain and the maximum amplitude of the power amplifier provided in the subsequent stage.

2. Second Embodiment (A) Configuration of Second Embodiment Next, a second embodiment of the present invention will be described with reference to FIG. In the second embodiment, since the amplitude is satisfactorily limited to a high frequency, the low-pass filter 3 and the delay circuit 4 in the first embodiment have the same or close frequency characteristics. Further, an equalizer 6 that emphasizes a high frequency is provided in the input circuit or the output circuit.

(B) Action of Second Embodiment The action of the second embodiment is as follows.
(1) The audio signal input to the circuit of FIG. The purpose of this is to correct in advance the high-frequency attenuation caused by the low-pass filter 3 and the delay circuit 4 in the subsequent stage.

(2) One of the output signals of the equalizer 6 is input to the amplitude limiting circuit 1, and only the signal from the point where the change direction of the sound signal waveform is inverted to the change of a certain level is extracted and output (in the second embodiment). A specific example of the amplitude limiting circuit 1 is shown in FIG. 6, and its circuit operation will be described later).

(3) The amplitude limiting circuit 1 outputs a signal as shown in FIG. This is a waveform significantly different from the input signal FIG. 7A, but the reproduced sound is not as bad as expected. However, it is difficult to obtain a sound quality that can be practically used when the amplitude is strongly limited. Most of the distortion of the reproduced sound is harmonics accompanying the deformation of the waveform. For this reason, the harmonics are removed from the subsequent circuits.

(4) The output signal of the amplitude limiting circuit 1 is subtracted from the output signal of the equalizer 6 by the next subtracter 2. Therefore, the output of the subtracter 2 is a signal obtained by subtracting the output signal of the amplitude limiting circuit 1 from the output signal of the equalizer 6.

(5) The output signal of the subtracter 2 is attenuated in high frequency by the next low pass filter 3, and the output signal of the equalizer 6 is subtracted from the signal shifted by the delay circuit 4 in the final stage subtracter 5. This is because the shift by the delay circuit 4 is synchronized with the low-pass filter 3 because a delay occurs in the low-pass filter 3.

(6) In this way, a signal having much less harmonic components than the output of the amplitude limiting circuit 1 is output from the subtractor 5 at the final stage. This signal is significantly deformed compared to the input signal, but no significant distortion is felt in the reproduced sound.

(7) In the second embodiment, the frequency characteristics of the low-pass filter 3 and the frequency characteristics of the delay circuit 4 are the same or approximate. That is, in the first embodiment, since the frequency characteristic of the delay circuit is flat even in the high frequency band, there is little attenuation of the signal in the high frequency band, and as shown in the waveform in the center portion of FIG. Even in the high frequency band, the delay circuit 4 outputs a signal (e) equivalent to the input signal (a). However, in the second embodiment, since the frequency characteristics of the delay circuit are the same as or approximate to those of the low-pass filter 3, the high-frequency component in the signal also receives the same attenuation as the low-pass filter in the delay circuit 4, and the characteristics are flat. As with very low frequency bands, strict amplitude limiting is performed.

  In this case, the higher the frequency, the lower the gain of the entire circuit, and the higher frequency signal is attenuated. Therefore, in the second embodiment, this is corrected by providing the equalizer 6, but the equalizer 6 may be provided on the output side. However, in the case of an audio signal, the amplitude of a high frequency signal is often small, so that it is often possible to obtain a preferable result when it is provided in the previous stage (input side).

(C) Specific Example of Amplitude Limiting Circuit 1 As the amplitude limiting circuit, the same one as that used in the first embodiment can be used. In the second embodiment, an example is shown in FIG. Use a configuration. The circuit of FIG. 6 and its operation will be described with reference to the waveform diagram of FIG.

(1) The distance between the upper and lower thin lines in FIG. 7B indicates the forward voltage of D1 + the forward voltage of D2.
(2) When a signal as shown in FIG. 7A is input to the INPUT of FIG. 6, a signal having the same waveform is output to the output until t1-t2.
(3) However, since the diode D2 is turned on at t3-t4, the output voltage does not change. At this time, the capacitor C1 is charged with the current i1.

(4) Since the direction of waveform change is reversed at t3, the diode D2 is turned off and the waveform of FIG. 7A appears at OUTPUT until t3-t4.
(5) At t4, D1 is turned ON and the OUTPUT waveform does not change until t4-t5. At this time, the capacitor C1 is charged with a current i2 and a reverse current as in the case of (3).

(6) The same operation is performed after t5, and a signal having a waveform as shown in FIG. 7B is output to OUTPUT.
(7) In the signal of FIG. 7B, the large amplitude signal in the signal of FIG. 7A is severely lost, but there is little loss of small signals. Therefore, even if the audio signal is passed through this circuit, the intelligibility of the reproduced sound is not so bad.

3. Third Embodiment (A) Configuration of Third Embodiment The third embodiment shown in FIG. 9 has a simpler circuit configuration than the second embodiment shown in FIG. In the third embodiment, the amplitude limiting circuit 1 and the subtracter 2 in FIG. This small change amplitude signal deletion circuit 7 outputs a signal in which a signal in a section that changes by a certain voltage in the same direction from the point where the voltage change direction of the input signal is reversed is deleted from the input signal. The configuration is shown in FIG. 10, but the circuit operation will be described later.

(B) Operation of the third embodiment The operation of the third embodiment having such a configuration is as follows.

(1) The audio signal input to the circuit shown in FIG. The purpose of this is to correct in advance the high-frequency attenuation caused by the low-pass filter 3 and the delay circuit 4 in the subsequent stage.

(2) One of the output signals of the equalizer 6 is input to the small change amplitude signal deletion circuit 7, and the same signal as the signal obtained by subtracting the output signal from the amplitude limiting circuit 1 from the output signal from the equalizer 6 in FIG. It is. That is, the small change amplitude signal deletion circuit outputs a signal equivalent to the output of the subtracter 2 in FIG. 5 of the second embodiment.

(3) The output of the small change amplitude signal deletion circuit 7 is subtracted from the output signal of the equalizer 6 shifted in the delay circuit 4 again by the subtracter 5 after the high frequency is attenuated by the next low pass filter 3. The shift by the delay circuit 4 is to synchronize the amount of delay with the delay occurring in the low-pass filter 3.

(4) In this way, the subtracter 5 outputs a signal equivalent to the output of the subtracter 5 in the second embodiment.

(C) Configuration of Small Change Amplitude Signal Deletion Circuit 7 The small change amplitude signal deletion circuit 7 used in the third embodiment will be described with reference to FIGS.

(1) The circuit of FIG. 10 outputs the voltage across the capacitor, with the positions of the capacitor and diode of FIG. 6 being interchanged.

(2) The voltage across the capacitor C2 is obtained by subtracting the voltage across the diodes D3 and D4 from the INPUT voltage. Since the voltage across the diodes D3 and D4 is equivalent to the voltage across the diodes D1 and D2 in FIG. 6, the voltage across the capacitor C2 is the same as subtracting the output voltage in FIG. 6 from the input voltage. That is, the output signal of the small change amplitude signal deletion circuit 7 of FIG. 9 is equivalent to the output signal of the first stage subtracter 2 of FIG.

  FIG. 11 is a practical circuit of FIG. The forward voltages of the diodes D3 and D4 in FIG. 10 are determined by the characteristics of the elements, and it is difficult to make them arbitrary values. Therefore, it is often inconvenient to handle small signals with the circuit of FIG. However, the practical circuit of FIG. 11 operates as if the forward voltage of the diode is R4 / (R3 + R4). The reason is that when the both-ends pressure of the diode D3 or D4 is equal to or lower than the forward voltage, the gain of the operational amplifier is (R3 + R4) / R4.

  The circuit shown in FIG. 11 can also be used for noise elimination of an audio signal as shown in (9) of an embodiment described later. In other words, noise often has a small amplitude, and if a small amplitude signal is deleted, a signal with good S / N can be produced.

4). Fourth Embodiment The amplitude limiting circuit performs amplitude limitation collectively from a low frequency to a high frequency of an audio signal. However, in this method, when the amplitude of a large low frequency signal is limited, a harmonic signal in the middle frequency band is generated to the extent that it can be heard. In order to reduce the sense of distortion, it is effective to divide the voice band into a plurality of parts.

  FIG. 12 shows a fourth embodiment of the present invention, and is a block diagram of a circuit for processing an audio signal band divided into two. This operation will be described as follows.

(1) The input signal is high-frequency emphasized by the equalizer 6 having the same characteristics as in FIG.
(2) One of the output signals of the equalizer 6 is input to the low-pass filter 3a via the small change amplitude signal deletion circuit 7a having the same characteristics as the small change amplitude signal deletion circuit 7 of FIG. The cut-off frequency of the low-pass filter 3a is lower than that of the low-pass filter 3 of FIGS. 5 and 9 (for example, 1 kHz or less).

(3) The output signal of the equalizer 6 is also input to the delay circuit 4a, where the delay of the same time as in the case of the low-pass filter 3a is performed. The frequency characteristic of the delay circuit 4a is preferably as flat as possible.

(4) The output signals of the low-pass filter 3a and the delay circuit 4a are input to the subtractor 5a, and their difference signal appears in the output circuit. At this time, as the output of the subtracter 5a, a signal having a smaller limiting effect is output as the frequency signal is higher although the amplitude is correctly limited at a lower frequency.

(5) The output signal of the subtracter 5a is input to the second small change amplitude signal deletion circuit 7b and the delay circuit 4b. The subsequent circuit operation is the same as the part excluding the equalizer 6 in FIG. 9, and the output of the low-pass filter 3b is subtracted from the output of the delay circuit 4b by the subtractor 5b, and the difference is output from the subtractor 5b. It is often the case that the delay circuit 4a is omitted and the output of the equalizer 6 is directly input to the subtracter 5a without harm.

  Next, an embodiment in which the audio signal amplitude limiter of the present invention and the amplitude limiter circuit used in the audio signal amplitude limiter are applied to specific products will be described with reference to FIGS. The advantage when the amplitude limiter of the present invention is used in these devices is that a sound with less sense of distortion or loudness than that of Patent Document 2 can be obtained.

  In each figure, reference numeral 13 is a microphone, 14 is a microphone amplifier, 15 is an amplitude limiting circuit of the present invention, 16 is a power amplifier, 17 is a preamplifier, 18 is an attenuator, 19 is an amplitude limiting circuit of the present invention, 20 Is a power amplifier, 22 is a microphone amplifier, 21 is a microphone, 23 is an earphone, 24 is a speech network circuit, 25 is a hands-free circuit, 26 is a dial circuit, 27 is a handset, 28 is a speaker, 28 is a recording / playback head, Reference numeral 30 is a recording amplifier, 31 is a reproduction amplifier, 32 is a sharp directional microphone, and 33 is an analysis circuit.

(1) Soundproof protector FIG. 13 is a perspective view showing an example of a soundproof protector incorporating the audio signal amplitude limiter of the present invention. The soundproof protector includes a soundproof protector body 10 and its ear pads. And a circuit block 12 and a microphone 13 attached to the outside of the ear pad. As the circuit block, as shown in FIG. 14, the output of the microphone 13 is amplified by the microphone amplifier 14, processed by the amplitude limiter 15 of the present invention, and then from the speaker 11 via the power amplifier 16. It is the composition to output.

  This type of soundproofing protector has a drawback that it cannot talk while worn because it cannot hear small sounds when worn. However, according to the configuration shown in FIG. 13, when external sound is collected by the microphone 13 and transmitted to the user's ear through the speaker 11, only the loud sound is restricted by the amplitude limiter 15 and the small sound is allowed to pass. Because of this, you can talk while wearing soundproofing protective equipment.

(2) Power Amplifier FIG. 15 is a block diagram showing an example of a power amplifier incorporating the audio signal amplitude limiter of the present invention. An input signal is input to the amplitude limiter 19 of the present invention via the preamplifier 17 and the volume 18. Then, it is output via the power amplifier 20.

(3) Hearing Aid or Hearing Aid FIG. 16 is a block diagram showing an example of a hearing aid or a hearing aid incorporating the audio signal amplitude limiter of the present invention, and is input from the microphone 21 via the microphone amplifier 22 and the input volume 18. The audio signal is subjected to amplitude limitation 19 and then output from the earphone 23 via the output volume 18 and the power amplifier 20 to obtain a playback sound that is easy to hear.

  In addition, if the amplitude limiter of this invention is used, the hearing aids which can hear very well also to a highly deaf person can be manufactured. The reason is that the amplitude limiting operation of the present invention that can reduce the dynamic range of sound can convert a human voice into a sound that can be heard even by a severely deaf person with a very small audible range.

(4) Telephone FIG. 17 is a block diagram showing an example of a telephone incorporating the audio signal amplitude limiter of the present invention with respect to a telephone having the hands-free circuit 25. The hands-free provided in parallel with the handset 27 is shown in FIG. By transmitting the input of the microphone 21 on the circuit 25 side through the amplitude limiter 19, the transmission voice can be clarified.

(5) Loudspeaker FIG. 18 is a block diagram showing an example of a loudspeaker incorporating the audio signal amplitude limiter of the present invention. Basically, like the hearing aid of FIG. 16, the amplitude of the input sound is shown. By performing the restriction, the loud sound of the small sound volume that is difficult to hear by eliminating the large sound volume input is effectively implemented. In addition, there is a feature that can produce a large volume with relatively small power.

(6) Tape Recorder FIG. 19 is a block diagram showing an example of a tape recorder incorporating the audio signal amplitude limiter of the present invention. Like the telephone, the amplitude limit is applied to the input signal from the microphone 21 on the recording side. 19 is given. The audio signal whose amplitude is limited in this way is recorded on the magnetic tape via the recording / reproducing head 29 and is output from the speaker 28 via the reproducing amplifier 31 and the like. In this case, recording with high intelligibility and easy listening is possible.

(7) Telephoto microphone FIG. 20 is a block diagram showing an example of a telephoto microphone incorporating the audio signal amplitude limiter of the present invention, and uses a highly directional microphone 32 to record distant audio. Is. In this case, since the microphone 32 with strong directivity records a small sound of a far sound source, it is necessary to increase its sensitivity. On the other hand, an audio signal having a predetermined volume or more may be input. However, in this embodiment, since the input of a loud sound signal is restricted by the amplitude limiter 19, a small sound far away can be clearly recorded.

(8) Speech Recognition Device FIG. 21 is a block diagram showing an example of a speech recognition device incorporating the speech signal amplitude limiter of the present invention. By applying the speech signal subjected to amplitude limitation to the analysis device 33, Perform voice recognition. The recognized data is output from a communication interface or the like. In this speech recognition apparatus, it is possible to improve the recognition rate by limiting the amplitude of the input signal.

(9) Noise Eliminating Circuit The amplitude limiter of the present invention is characterized by a combination of the amplitude limiting circuit 1, the subtractors 2 and 5, the low-pass filter 3, and the delay circuit 4. However, the amplitude limiting circuit used for the amplitude limiter of the present invention alone can be used in the devices (1) to (8). FIG. 22 is a block diagram showing a configuration when the small amplitude signal deletion circuit of FIG. 11 is used for a hearing aid for noise cancellation. Further, the amplitude limiting circuit shown in FIG. 2, FIG. 4 or FIG. 6 can be used in place of the circuit of FIG.

  INDUSTRIAL APPLICABILITY The present invention can be widely used to improve the characteristics of devices that handle sound, such as hearing aids that can be heard with low noise and audio devices that can produce a large volume with low power.

The block diagram which shows 1st Embodiment of the audio | voice signal amplitude limiter of this invention. FIG. 3 is a circuit diagram showing an example of an amplitude limiting circuit used in the first embodiment of the present invention. FIG. 3 is a waveform diagram for explaining the operation of the amplitude limiting circuit of FIG. 2. The circuit diagram which shows the other example of the amplitude limiting circuit used for 1st Embodiment of this invention. The block diagram which shows 2nd Embodiment of the audio | voice signal amplitude limiter of this invention. The circuit diagram which shows an example of the small change amplitude signal extraction circuit used for 2nd Embodiment of this invention. FIG. 7 is a waveform diagram for explaining the operation of the small change amplitude signal extraction circuit of FIG. 6. The block diagram which shows the state of the output signal from each block in the 1st and 2nd embodiment of this invention. The block diagram which shows 3rd Embodiment of the audio | voice signal amplitude limiter of this invention. The circuit diagram which shows an example of the small change amplitude signal erasure circuit used for 4th Embodiment of this invention. FIG. 11 is a circuit diagram showing a practical example of the small change amplitude signal erasing circuit of FIG. 10. The block diagram which shows 4th Embodiment of the audio | voice signal amplitude limiter of this invention. The perspective view which shows an example of the soundproof protector incorporating the audio | voice signal amplitude limiter of this invention. The block diagram which shows the structure of the soundproof protector of FIG. The block diagram which shows an example of the power amplifier incorporating the audio | voice signal amplitude limiter of this invention. The block diagram which shows an example of a hearing aid or a hearing aid incorporating the audio | voice signal amplitude limiter of this invention. The block diagram which shows an example of the telephone incorporating the audio | voice signal amplitude limiter of this invention. The block diagram which shows an example of the loudspeaker incorporating the audio | voice signal amplitude limiter of this invention. The block diagram which shows an example of the tape recorder incorporating the audio | voice signal amplitude limiter of this invention. The block diagram which shows an example of the telephoto microphone incorporating the audio | voice signal amplitude limiter of this invention. The block diagram which shows an example of the speech recognition apparatus incorporating the audio | voice signal amplitude limiter of this invention. The block diagram which shows the structure at the time of using the small amplitude signal deletion circuit of FIG. 11 for a hearing aid for noise cancellation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Amplitude limiting circuit 2 ... Subtractor 3 ... Low pass filter 4 ... Delay circuit 5 ... Subtractor 10 ... Soundproof protector main body 11 ... Speaker 12 ... Circuit block 13 ... Microphone 14 ... Microphone amplifier 15 ... Amplitude limiting circuit 16 of this invention ... Power amplifier 17 ... Preamplifier 18 ... Attenuator 19 ... Amplitude limiting circuit 20 of the present invention ... Power amplifier 22 ... Microphone amplifier 21 ... Microphone 23 ... Earphone 24 ... Speech network circuit 25 ... Hands-free circuit 26 ... Dial circuit 27 ... Handset 28 ... Speaker 29 ... Recording / playback head 30 ... Recording amplifier 31 ... Playback amplifier 32 ... Sharp directional microphone 33 ... Analytical circuit R1-R14 ... Resistors C1-C4 ... Capacitors D1-D4 ... Diodes Q1, Q2 ... Transistors OP1-OP4 ... Operational amplifier OUTPUT ... Output circuit INPUT ... Input times GND ... ground circuit + Vcc ... plus power supply circuit -Vcc ... minus the power supply circuit

Claims (5)

An amplitude limiting circuit that limits the amplitude of the input audio signal, a subtracter provided on the output side of the amplitude limiting circuit, a low-pass filter that cuts high-frequency components in the output signal from the subtractor, and a low-pass filter A delay circuit connected in parallel; and a subtractor arranged after the low-pass filter 3 for outputting a processed audio signal;
The subtracter inputs a sound signal similar to that input to the amplitude limiting circuit and performs a process of subtracting the output signal of the amplitude limiting circuit from the sound signal.
The delay circuit inputs a sound signal similar to that input to the amplitude limiting circuit, and performs a delay process for synchronizing the input signal with the output signal from the low-pass filter,
The subtractor at the subsequent stage inputs an output signal from the delay circuit, and subtracts an output from the low-pass filter from an input signal from the delay circuit, and outputs the result. vessel.   A small change amplitude signal is deleted between the amplitude limiter circuit and a subtractor that inputs the same audio signal as that input to the amplitude limiter circuit and subtracts the output signal of the amplitude limiter circuit from the audio signal. The audio signal amplitude limiter according to claim 1, comprising a circuit.   The input audio signal is divided into a plurality of bands, and the amplitude limiting circuit, the subtractor, the low-pass filter, the delay circuit, and the subsequent stage subtractor are provided for each band. 2. The audio signal amplitude limiter according to 2.   2. The audio signal amplitude limiter according to claim 1, wherein frequency characteristics of the low-pass filter and the delay circuit are the same or approximate.   3. The audio signal amplitude limiter according to claim 1, further comprising an equalizer that emphasizes a high frequency component of the input signal or the output signal.

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