Audio signal processing apparatus, audio system, and audio signal processing method

Abstract

Provided is a technique capable of reducing noise components included in the output sound of a high-frequency-band speaker in a case where an audio signal output from an amplification device is supplied to the high-frequency-band speaker and a low-frequency-band speaker. An audio signal processing device that supplies an output audio signal to a high-frequency-band speaker, a low-frequency-band speaker, and an amplification device that supplies a first audio signal includes: a high-pass filter that generates a high-frequency-band audio signal by removing low-frequency-band frequency components from an input audio signal; an amplitude limiting section that generates a second audio signal by amplitude-limiting the input audio signal so as not to exceed a reference value corresponding to a clipping voltage at which the first audio signal output from the amplification device is clipped; a low-pass filter that generates a low-frequency-band audio signal by removing high-frequency-band frequency components from the second audio signal generated by the amplitude limiting section; and a combining section that generates the output audio signal by combining the high-frequency-band audio signal generated by the high-pass filter and the low-frequency-band audio signal generated by the low-pass filter.

Description

Technical Field

[0001] This invention relates to an audio signal processing device, an audio system, and an audio signal processing method. Background Technology

[0002] Patent document 1 discloses an audio system that supplies an audio signal output from an amplification device to both a high-frequency loudspeaker (high-frequency loudspeaker) and a low-frequency loudspeaker (low-frequency loudspeaker).

[0003] Patent Document 1: Japanese Patent Application Publication No. 2012-74780

[0004] In the case of the audio system described in Patent Document 1 being installed in a vehicle or similar vehicle, it is sometimes impossible to increase the power supply voltage of the amplification device. Therefore, if a large-amplitude audio signal is supplied to the amplification device, the amplification device easily outputs an audio signal with a waveform clamped by the power supply voltage (clamping voltage). The clamped waveform contains frequency components in the high-frequency band caused by the clamping.

[0005] If the high-frequency components caused by clamping are supplied from the amplification device to the high-frequency loudspeaker, the high-frequency loudspeaker will output noise caused by the high-frequency components. Summary of the Invention

[0006] One objective of the present invention is to provide a technique for reducing the noise component contained in the output sound from the high-frequency loudspeaker when an audio signal output from an amplification device is supplied to both a high-frequency loudspeaker and a low-frequency loudspeaker.

[0007] One aspect of the present invention relates to an audio signal processing apparatus for supplying an amplification device that supplies a first audio signal to a high-frequency loudspeaker and a low-frequency loudspeaker, and supplies an output audio signal. The audio signal processing apparatus includes: a high-pass filter that generates a high-frequency audio signal by removing low-frequency components from the input audio signal; an amplitude limiting unit that generates a second audio signal by limiting the amplitude of the input audio signal to a reference value that does not exceed a clamping voltage corresponding to the clamping voltage of the first audio signal output from the amplification device; a low-pass filter that generates a low-frequency audio signal by removing high-frequency components from the second audio signal generated by the amplitude limiting unit; and a synthesis unit that generates the output audio signal by synthesizing the high-frequency audio signal generated by the high-pass filter and the low-frequency audio signal generated by the low-pass filter.

[0008] Other aspects of the present invention relate to an audio signal processing apparatus for an amplification device that supplies a first audio signal to a high-frequency loudspeaker and a low-frequency loudspeaker, and supplies an output audio signal. The audio signal processing apparatus includes: a high-pass filter that generates a high-frequency audio signal by removing low-frequency components from the input audio signal; a first low-pass filter that generates a filter output signal by removing high-frequency components from the input audio signal; an amplitude limiting unit that generates a second audio signal by limiting the amplitude of the filter output signal to a reference value that does not exceed a clamping voltage corresponding to the clamping voltage of the first audio signal output from the amplification device; a second low-pass filter that generates a low-frequency audio signal by removing high-frequency components from the second audio signal generated by the amplitude limiting unit; and a synthesis unit that generates the output audio signal by synthesizing the high-frequency audio signal generated by the high-pass filter and the low-frequency audio signal generated by the second low-pass filter.

[0009] Another aspect of the present invention relates to an audio system comprising the aforementioned audio signal processing device, the high-frequency loudspeaker, the low-frequency loudspeaker, and the amplification device.

[0010] Another aspect of the present invention relates to an audio signal processing method implemented by a computer, which supplies an output audio signal to an amplification device that supplies a first audio signal to a high-frequency band speaker and a low-frequency band speaker. In this audio signal processing method, a high-frequency band audio signal is generated by removing low-frequency band components from the input audio signal; a second audio signal is generated by limiting the amplitude of the input audio signal in a manner that does not exceed a reference value corresponding to the clamping voltage of the first audio signal output from the amplification device; a low-frequency band audio signal is generated by removing high-frequency band components from the generated second audio signal; and the output audio signal is generated by combining the generated high-frequency band audio signal and the generated low-frequency band audio signal.

[0011] Another aspect of the present invention relates to an audio signal processing method implemented by a computer, which supplies an output audio signal to an amplification device that supplies a first audio signal to a high-frequency band speaker and a low-frequency band speaker. In this audio signal processing method, a high-frequency band audio signal is generated by removing low-frequency band components from the input audio signal; a filter output signal is generated by removing high-frequency band components from the input audio signal; a second audio signal is generated by limiting the amplitude of the filter output signal to a reference value that does not exceed the clamping voltage corresponding to the clamping voltage of the first audio signal output from the amplification device; a low-frequency band audio signal is generated by removing high-frequency band components from the generated second audio signal; and the output audio signal is generated by combining the generated high-frequency band audio signal and the generated low-frequency band audio signal. Attached Figure Description

[0012] Figure 1 This is a diagram illustrating an example of the audio system 1 according to the first embodiment.

[0013] Figure 2 This is a diagram representing an example of the first tone signal c1.

[0014] Figure 3 This is a diagram showing an example of an audio signal processing device 40.

[0015] Figure 4 This is a diagram illustrating an example of amplitude limiting performed by the amplitude limiting unit 103.

[0016] Figure 5 This is a diagram illustrating an example of the operation of audio system 1.

[0017] Figure 6 This is a diagram representing the first variation.

[0018] Figure 7 This is a diagram illustrating an example of the operation of the sound system 1 in the first modified example. Detailed Implementation

[0019] A: Implementation Method 1

[0020] A1: Audio System 1

[0021] Figure 1 This diagram illustrates an example of the audio system 1 according to the first embodiment. The audio system 1 is a system that supplies a first audio signal c1 to both a high-frequency speaker 10 and a low-frequency speaker 20. The first audio signal c1 is a signal that represents sound through a waveform. The audio system 1 is mounted on a vehicle 100, such as an automobile.

[0022] The vehicle 100 is driven by its passengers. The vehicle 100 can also perform automatic driving without being driven by its passengers. The vehicle 100 includes an audio system 1, wheels 2a to 2d, an operating unit 3, and an audio source 4.

[0023] Wheels 2a and 2b are the front wheels of vehicle 100. Wheels 2c and 2d are the rear wheels of vehicle 100. Vehicle 100 may have additional wheels besides wheels 2a to 2d.

[0024] The operation unit 3 is a touch panel. The operation unit 3 is not limited to a touch panel; it can be a control panel with various operation buttons. The operation unit 3 accepts operations performed by passengers of the vehicle 100. Hereinafter, "passengers of the vehicle 100" will be referred to as "users".

[0025] Sound source 4 generates sound signal a1. Sound signal a1 represents the sound. The larger the amplitude of sound signal a1, the louder the sound represented by sound signal a1.

[0026] The audio system 1 includes a high-frequency loudspeaker 10, a low-frequency loudspeaker 20, a storage device 30, an audio signal processing device 40, and an amplification device 50. The storage device 30 may also be an external element of the audio system 1.

[0027] High-frequency speaker 10 is an example of a speaker for the high-frequency band. The high-frequency band refers to the high-frequency range. Low-frequency speaker 20 is an example of a speaker for the low-frequency band. The low-frequency band refers to the low-frequency range. High-frequency speaker 10 and low-frequency speaker 20 are located in the passenger compartment of vehicle 100. The crossover frequency of high-frequency speaker 10 and low-frequency speaker 20 is 3kHz. The crossover frequency of high-frequency speaker 10 and low-frequency speaker 20 is not limited to 3kHz; for example, it can be higher than 3kHz or lower than 3kHz.

[0028] Storage device 30 is a recording medium readable by a computer (e.g., a non-transitory recording medium readable by a computer). Storage device 30 includes non-volatile memory and volatile memory. Non-volatile memory includes, for example, ROM (Read Only Memory), EPROM (Erasable Programmable Read Only Memory), and EEPROM (Electrically Erasable Programmable Read Only Memory). Volatile memory includes, for example, RAM (Random Access Memory).

[0029] Storage device 30 stores program p1. Program p1 specifies the operation of audio signal processing device 40. Storage device 30 can store program p1 that can be read from the storage device of a server (not shown). In this case, the server's storage device is an example of a recording medium that can be read by a computer.

[0030] The audio signal processing device 40 includes one or more CPUs (Central Processing Units). One or more CPUs are an example of one or more processors. The audio signal processing device, processor, and CPU are each an example of a computer.

[0031] The audio signal processing device 40 reads program p1 from the storage device 30. The audio signal processing device 40 performs various functions by executing program p1.

[0032] The audio signal processing device 40 generates an output audio signal b1 based on the audio signal a1. The output audio signal b1 is a signal that represents sound through a waveform. The output audio signal b1 is a signal supplied to the amplification device 50.

[0033] The amplification device 50 generates a first tone signal c1 by amplifying the output tone signal b1. The amplification device 50 supplies the first tone signal c1 to the high-frequency speaker 10 and the low-frequency speaker 20.

[0034] The amplification device 50 has a fixed gain (amplification ratio). When the amplified signal h1 obtained by amplifying the output tone signal b1 with the gain of the amplification device 50 exceeds the clamping voltage of the amplification device 50, the amplification device 50 generates a signal obtained by clamping the amplified signal h1 with the clamping voltage, which is used as the first tone signal c1.

[0035] Figure 2 This diagram illustrates an example of the clamped first tone signal c1. Voltages +VF and -VF are the clamping voltages. The condition where the amplified signal h1 exceeds the clamping voltage includes the condition where the amplified signal h1 is greater than voltage +VF and the condition where the amplified signal h1 is less than voltage -VF.

[0036] The clamped first tone signal c1 will contain the higher harmonics of the first tone signal c1. The higher harmonics of the first tone signal c1 constitute the high-frequency band components. If the higher harmonics of the first tone signal c1 are supplied to the high-frequency loudspeaker 10, the high-frequency loudspeaker 10 will reproduce the noise caused by the higher harmonics of the first tone signal c1.

[0037] The clamping voltage that affects the generation of noise depends on the power supply voltage of the amplifier 50. The power supply voltage of the amplifier 50 is supplied from the vehicle 100. Therefore, if the vehicle 100 cannot supply a high power supply voltage to the amplifier 50, clamping, which is a cause of noise, is likely to occur in the amplifier 50.

[0038] The audio signal processing device 40 generates an audio signal that is not clamped in the amplification device 50, and uses it as the output audio signal b1.

[0039] A2: Audio signal processing device 40

[0040] Figure 3This diagram illustrates an example of an audio signal processing apparatus 40. The audio signal processing apparatus 40 functions as an adjustment unit 101, an HPF (High Pass Filter) 102, an amplitude limiting unit 103, an LPF (Low Pass Filter) 104, and a synthesis unit 105 by executing program p1. At least one of the adjustment unit 101, HPF 102, amplitude limiting unit 103, LPF 104, and synthesis unit 105 can be constructed from circuits such as a DSP (Digital Signal Processor) and an ASIC (Application Specific Integrated Circuit). The adjustment unit 101 can be an external element of the audio signal processing apparatus 40. The adjustment unit 101 can be omitted.

[0041] The adjustment unit 101 generates the input audio signal d1 by adjusting the amplitude of the audio signal a1. For example, the adjustment unit 101 generates the input audio signal d1 by adjusting the amplitude of the audio signal a1 in accordance with the user's volume operation.

[0042] If the operation unit 3 receives an operation from the user to increase the volume, the adjustment unit 101 increases the amplitude of the audio signal a1. For example, if the operation to increase the volume is an operation that only amplifies the bass component of the audio signal a1, the adjustment unit 101 generates the input audio signal d1 by increasing the amplitude of the bass component of the audio signal a1 and maintaining the amplitude of the treble component of the audio signal a1.

[0043] If the operation unit 3 receives a volume reduction operation from the user, the adjustment unit 101 reduces the amplitude of the audio signal a1. For example, if the volume reduction operation is only to reduce the bass component of the audio signal a1, the adjustment unit 101 generates the input audio signal d1 by reducing the amplitude of the bass component of the audio signal a1 and maintaining the amplitude of the treble component of the audio signal a1.

[0044] The adjustment unit 101 can also generate an input audio signal d1 by adjusting the amplitude of the audio signal a1 in accordance with the state of the vehicle 100. The state of the vehicle 100 is, for example, the speed of the vehicle 100 or the acceleration of the vehicle 100. For example, the greater the acceleration of the vehicle 100, the greater the amplitude of the audio signal a1 set by the adjustment unit 101. The greater the speed of the vehicle 100, the greater the amplitude of the audio signal a1 set by the adjustment unit 101. Without adjusting the amplitude of the audio signal a1, the adjustment unit 101 outputs the audio signal a1 as the input audio signal d1.

[0045] The input tone signal d1 contains low-frequency and high-frequency components. Low-frequency components are those with a cutoff frequency less than 10² HPF. High-frequency components are those with a cutoff frequency greater than 10⁴ LPF. The amplitude of the signal composed of low-frequency components is greater than the amplitude of the signal composed of high-frequency components.

[0046] The HPF 102 is a second-order IIR (Infinite Impulse Response) filter. However, the HPF 102 is not limited to second-order IIR filters; it can also be a first-order or third-order IIR filter. The HPF 102 can also be a digital filter, such as a FIR (Finite Impulse Response) filter, which is different from an IIR filter.

[0047] The cutoff frequency of HPF 102 is the same as the crossover frequency of the high-frequency speaker 10 and the low-frequency speaker 20. The cutoff frequency of HPF 102 may also be different from the crossover frequency of the high-frequency speaker 10 and the low-frequency speaker 20. For example, the cutoff frequency of HPF 102 may be a frequency within a range of ±E% of the crossover frequency of the high-frequency speaker 10 and the low-frequency speaker 20. E% is, for example, 20%. E% is not limited to 20%; for example, it may be a value less than 20% or a value greater than 20%.

[0048] HPF 102 generates high-frequency band signal d2 by removing low-frequency band components from the input tone signal d1.

[0049] The amplitude limiting unit 103 generates the second tone signal d3 by limiting the amplitude of the input tone signal d1 to a reference value that does not exceed the clamping voltage corresponding to the clamping voltage of the first tone signal c1 output from the amplification device 50.

[0050] Figure 4 This diagram illustrates an example of amplitude limiting performed by the amplitude limiting unit 103. Signal amplitude +R and signal amplitude -R are reference values, respectively.

[0051] for Figure 3 The amplitude limiting unit 103 shown, if viewed differently, prevents the first tone signal c1 from being clamped in the amplification device 50 by limiting the amplitude of the input tone signal d1 in a manner that does not exceed a reference value. That is, by limiting the amplitude of the input tone signal d1 in a manner that does not exceed a reference value, the amplitude limiting unit 103 limits the voltage level of the output tone signal b1 to the voltage level at which the first tone signal c1, which is not clamped in the amplification device 50, is generated.

[0052] In the amplification device 50, the voltage level at which the first tone signal c1 is not clamped is a voltage level that does not exceed the clamping voltage of the amplification device 50. The voltage level at which the first tone signal c1 is not clamped in the amplification device 50 can also be a voltage level different from the voltage level at which the first tone signal c1 is clamped in the amplification device 50.

[0053] The reference value is predetermined in such a way that the first tone signal c1 is not clamped in the amplification device 50. Furthermore, corresponding to an increase in the reference value, the amplitude of the output tone signal b1 increases. Corresponding to a decrease in the reference value, the amplitude of the output tone signal b1 decreases.

[0054] The reference value is, for example, a value predetermined based on the clamping voltage and gain of the amplification device 50. Based on the clamping voltage and gain of the amplification device 50, the maximum amplitude of the signal that will not be clamped even when input to the amplification device 50 for the first tone signal c1 is defined. For example, the maximum amplitude of the signal that will not be clamped even when input to the amplification device 50 for the first tone signal c1 is defined by dividing the clamping voltage of the amplification device 50 by the gain of the amplification device 50. Hereinafter, the maximum amplitude of the signal that will not be clamped even when input to the amplification device 50 for the first tone signal c1 is referred to as the "first maximum amplitude".

[0055] A reference value is, for example, a value that limits the maximum amplitude of the output audio signal b1 to less than the "first maximum amplitude". As an example, the reference value is the amplitude value that limits the maximum amplitude of the output audio signal b1 to F% of the "first maximum amplitude". F% is, for example, 80%. F% is not limited to 80% and can be a value less than 80% (e.g., 70%) or a value greater than 80% (e.g., 85%). For example, the larger the value obtained by dividing the estimated amplitude of the high-frequency components of the input audio signal d1 by the estimated amplitude of the low-frequency components of the input audio signal d1, the smaller F% can be. Furthermore, the estimated amplitudes of the high-frequency components and the low-frequency components of the input audio signal d1 are predetermined based on a sample of the input audio signal d1.

[0056] Furthermore, as described later, the output tone signal b1 is generated by combining the high-frequency tone signal d2 generated by HPF 102 and the low-frequency tone signal d4 generated by LPF 104 in the synthesis unit 105. The low-frequency tone signal d4 is generated by removing the high-frequency components from the second tone signal d3 generated by amplitude limiting unit 103.

[0057] Therefore, for example, if the gain of LPF 104 and the gain of synthesis unit 105 are both "1x", the reference value can be the value of the amplitude obtained by limiting the maximum amplitude of the output tone signal b1 to the amplitude obtained by subtracting the estimated value of the amplitude of the high-frequency tone signal d2 from the "first maximum amplitude". The estimated value of the amplitude of the high-frequency tone signal d2 is predetermined based on a sample of the high-frequency tone signal d2.

[0058] The reference value is preset in program p1. The amplitude limiting unit 103 generates the second tone signal d3 by limiting the amplitude of the input tone signal d1 in a manner that does not exceed the preset reference value. If the amplitude limiting unit 103 limits the amplitude of the input tone signal d1, the first tone signal c1 will not be clamped by the clamping voltage in the amplification device 50.

[0059] If the amplitude of the input tone signal d1 is limited by the amplitude limiting unit 103 to generate the second tone signal d3, the second tone signal d3 will contain high-order harmonics associated with the amplitude limitation. The high-order harmonics associated with the amplitude limitation in the amplitude limiting unit 103 are the high-order harmonics of the second tone signal d3. If the high-order harmonics associated with the amplitude limitation in the amplitude limiting unit 103 are supplied to the high-frequency loudspeaker 10 via the amplification device 50, the high-frequency loudspeaker 10 will output noise.

[0060] LPF 104 generates a low-frequency band tone signal d4 by removing high-frequency band components from the second tone signal d3 generated by the amplitude limiting section 103. That is, LPF 104 removes the higher harmonics associated with amplitude limiting in the amplitude limiting section 103 from the second tone signal d3.

[0061] The LPF 104 is a second-order IIR filter. However, the LPF 104 is not limited to second-order IIR filters; it can also be a first-order or third-order IIR filter. The LPF 104 can also be a digital filter, such as an FIR filter, which is different from an IIR filter.

[0062] The cutoff frequency of LPF 104 is the same as that of HPF 102. The cutoff frequency of LPF 104 is also the same as the crossover frequency of the tweeter 10 and the woofer 20. The cutoff frequency of LPF 104 may also be different from the crossover frequency of the tweeter 10 and the woofer 20. For example, the cutoff frequency of LPF 104 may be a frequency within ±E% (e.g., 20%) of the crossover frequency of the tweeter 10 and the woofer 20.

[0063] LPF 104 removes the higher harmonics associated with amplitude limitation in amplitude limiting section 103 from the second tone signal d3, and further removes the high-frequency band tone signal d2 contained in the input tone signal d1. Therefore, the low-frequency band tone signal d4 does not have either the higher harmonics associated with amplitude limitation in amplitude limiting section 103 or the high-frequency band tone signal d2 contained in the input tone signal d1.

[0064] The synthesis unit 105 generates an output tone signal b1 by combining the high-frequency tone signal d2 generated by the HPF 102 and the low-frequency tone signal d4 generated by the LPF 104. That is, the synthesis unit 105 generates the output tone signal b1 by adding the high-frequency tone signal d2 (which is not present in the low-frequency tone signal d4) to the low-frequency tone signal d4. Therefore, the output tone signal b1 includes the high-frequency tone signal d2 removed by the LPF 104. Furthermore, the output tone signal b1 is a signal having a voltage level that generates a first tone signal c1 that is not clamped in the amplification device 50. The synthesis unit 105 supplies the output tone signal b1 to the amplification device 50.

[0065] The amplification device 50 generates a first tone signal c1 by amplifying the output tone signal b1. While the amplification device 50 amplifies the output tone signal b1, no clamping occurs within the amplification device 50. Therefore, no high-order harmonics associated with clamping occur in the amplification device 50. Consequently, the first tone signal c1 does not contain high-order harmonics associated with clamping. The amplification device 50 supplies the first tone signal c1 to both the high-frequency speaker 10 and the low-frequency speaker 20.

[0066] The high-frequency loudspeaker 10 reproduces the sound corresponding to the high-frequency components of the first tone signal c1. The first tone signal c1 does not contain high-order harmonics associated with clamping, therefore the high-frequency loudspeaker 10 does not reproduce noise caused by these high-order harmonics. Thus, the noise component in the output sound from the high-frequency loudspeaker 10 is reduced. The low-frequency loudspeaker 20 reproduces the sound corresponding to the low-frequency components of the first tone signal c1.

[0067] A3: Description of the actions

[0068] Figure 5 This is a diagram illustrating an example of the operation of audio system 1.

[0069] In step S101, the adjustment unit 101 generates an input sound signal d1 by adjusting the amplitude of the sound signal a1 in accordance with the user's operation.

[0070] Next, in step S102, HPF 102 generates a high-frequency band tone signal d2 by removing low-frequency band components from the input tone signal d1. Furthermore, step S102 can also be performed between steps S103 and S105, which will be described later.

[0071] Next, in step S103, the amplitude limiting unit 103 generates a second tone signal d3 by limiting the amplitude of the input tone signal d1 in a manner that does not exceed a reference value. The second tone signal d3 generated by the amplitude limiting unit 103 contains higher harmonics associated with the amplitude limitation in the amplitude limiting unit 103.

[0072] Next, in step S104, LPF 104 generates a low-frequency band tone signal d4 by removing high-frequency band components from the second tone signal d3. The low-frequency band tone signal d4 does not contain either the high-order harmonics associated with amplitude limitation in amplitude limitation section 103 or the high-frequency band tone signal d2 contained in the input tone signal d1.

[0073] Next, in step S105, the synthesis unit 105 generates an output audio signal b1 by combining the high-frequency bandgap signal d2 generated by the HPF 102 and the low-frequency bandgap signal d4 generated by the LPF 104. Therefore, the output audio signal b1 includes the high-frequency bandgap signal d2 removed by the LPF 104. In addition, the output audio signal b1 has a voltage level that will not cause clamping in the amplification device 50.

[0074] Next, in step S106, the synthesis unit 105 supplies the output audio signal b1 to the amplification device 50.

[0075] Next, in step S107, the amplification device 50 generates a first tone signal c1 by amplifying the output tone signal b1. The first tone signal c1 is not clamped in the amplification device 50. Therefore, the first tone signal c1 does not contain the higher harmonics associated with clamping.

[0076] Next, in step S108, the amplification device 50 supplies the first audio signal c1 to both the high-frequency speaker 10 and the low-frequency speaker 20.

[0077] Next, in step S109, the high-frequency speaker 10 and the low-frequency speaker 20 reproduce the sound based on the first tone signal c1. The first tone signal c1 does not contain high-order harmonics associated with clamping. Therefore, noise caused by high-order harmonics associated with clamping is not emitted from the high-frequency speaker 10. As a result, the noise component contained in the output sound from the high-frequency speaker 10 is reduced.

[0078] A4: Summary of the first implementation method

[0079] The amplitude limiting unit 103 generates a second tone signal d3 by limiting the amplitude of the input tone signal d1 to a value not exceeding a reference value. The LPF 104 removes high-order harmonics caused by the amplitude limiting in the amplitude limiting unit 103 and the high-frequency band tone signal d2 contained in the input tone signal d1 from the second tone signal d3 generated by the amplitude limiting unit 103. The synthesis unit 105 generates an output tone signal b1 by combining the high-frequency band tone signal d2 generated by the HPF 102 and the low-frequency band tone signal d4 generated by the LPF 104. The amplification device 50 generates a first tone signal c1 by amplifying the output tone signal b1. The amplification device 50 supplies the first tone signal c1 to both the high-frequency speaker 10 and the low-frequency speaker 20.

[0080] Therefore, the low-frequency speaker 20 is able to output a powerful sound and reduce the noise contained in the output sound from the high-frequency speaker 10.

[0081] The reference value used by the amplitude limiting unit 103 is a predetermined value based on the clamping voltage of the amplification device 50 and the gain of the amplification device 50.

[0082] Therefore, in accordance with the amplification characteristics of the amplification device 50, such as the clamping voltage and the gain of the amplification device 50, the low-frequency speaker 20 is able to output a powerful sound and limit the occurrence of noise (noise caused by clamping) contained in the output sound from the high-frequency speaker 10.

[0083] The cutoff frequency of HPF 102 is the same as that of LPF 104. Therefore, compared to a structure where the cutoff frequencies of HPF 102 and LPF 104 are different, it is possible to reflect the frequency components of the input tone signal d1 onto the frequency components of the output tone signal b1.

[0084] The cutoff frequency of HPF 102 and LPF 104 is not limited to 3kHz; for example, it can be higher or lower than 3kHz.

[0085] The cutoff frequencies of HPF 102 and LPF 104 can be different from each other. In this case, it is also possible to reduce the noise (caused by clamping) contained in the output sound from the high-frequency loudspeaker 10.

[0086] The order of HPF 102 is the same as that of LPF 104. Therefore, the phase characteristics of the high-frequency band signal d2 input to the synthesizer 105 are consistent with the phase characteristics of the low-frequency band signal d4 input to the synthesizer 105. As a result, compared with a structure where the order of HPF 102 is inconsistent with that of LPF 104, the quality of the sound represented by the output audio signal b1 is improved.

[0087] The order of HPF 102 and LPF 104 is not limited to 2nd order; for example, it can be higher or lower than 2nd order.

[0088] The order of HPF 102 and LPF 104 can be different from each other. In this case, it is possible to reduce the noise (caused by clamping) contained in the output sound from the high-frequency loudspeaker 10.

[0089] The adjustment unit 101 generates the input audio signal d1 by adjusting the amplitude of the audio signal a1 in accordance with the user's volume operation. Therefore, the user's volume operation is not reflected in the gain of the amplification device 50, but rather in the amplitude of the input audio signal d1. Thus, even if the amplitude of the input audio signal d1 increases in accordance with the user's volume operation, clamping in the amplification device 50 can be suppressed. Therefore, noise (caused by clamping) contained in the output sound from the high-frequency loudspeaker 10 can be reduced.

[0090] B: Variation Example

[0091] The following shows variations of the first embodiment. Two or more embodiments selected arbitrarily from the following embodiments may be appropriately combined to the extent that they do not contradict each other.

[0092] B1: First Variation

[0093] Figure 6 This is a diagram illustrating the first modification. Hereinafter, with respect to the first modification, we will mainly describe the points that differ from the first embodiment. For example, the first modification differs from the first embodiment in the presence of LPF 106 and HPF 107.

[0094] In the first variation, the storage device 30 stores program p2 instead of program p1. The audio signal processing device 40 reads program p2 from the storage device 30. The audio signal processing device 40 functions as the adjustment unit 101, amplitude limiting unit 103, LPF 104, synthesis unit 105, LPF 106, and HPF 108 by executing program p2. HPF 108 includes HPF 102 and HPF 107. At least one of the adjustment unit 101, amplitude limiting unit 103, LPF 104, synthesis unit 105, LPF 106, and HPF 108 may also be constructed from circuits such as DSP and ASIC.

[0095] The LPF 106 is an example of a first-order low-pass filter. The LPF 106 is a second-order IIR filter. The LPF 106 is not limited to a second-order IIR filter; it can also be a first-order or third-order IIR filter. The LPF 106 can also be a digital filter, different from an IIR filter, such as an FIR filter.

[0096] The cutoff frequency of LPF 106 is the same as that of HPF 102 and LPF 104. The cutoff frequency of LPF 106 is the same as the crossover frequency of the tweeter 10 and the woofer 20. The cutoff frequency of LPF 106 may also be different from the crossover frequency of the tweeter 10 and the woofer 20. For example, the cutoff frequency of LPF 106 may also be a frequency within ±E% (e.g., 20%) of the crossover frequency of the tweeter 10 and the woofer 20.

[0097] The LPF 106 generates the filter output signal g1 by removing high-frequency components from the input tone signal d1.

[0098] In the first modified example, the amplitude limiting unit 103 generates a second tone signal g2 by limiting the amplitude of the filter output signal g1 to a reference value that does not exceed the clamping voltage corresponding to the clamping voltage of the first tone signal c1 output from the amplification device 50. The second tone signal g2 contains higher harmonics caused by the amplitude limiting in the amplitude limiting unit 103. These higher harmonics are the higher harmonics of the second tone signal g2.

[0099] The LPF 104 in the first modification is an example of a second low-pass filter. The LPF 104 in the first modification generates a low-frequency band tone signal g3 by removing high-frequency components from the second tone signal g2. The low-frequency band tone signal g3 is the signal from the second tone signal g2 after removing higher harmonics caused by amplitude limitation in the amplitude limiting section 103. Furthermore, the LPF 106 removes high-frequency components from the input tone signal d1, therefore the low-frequency band tone signal g3 does not contain the high-frequency band tone signal d2 contained in the input tone signal d1.

[0100] The HPF 107 is a second-order IIR filter. However, the HPF 107 is not limited to second-order IIR filters; it can also be a first-order or third-order IIR filter. The HPF 107 can also be a digital filter, such as an FIR filter, which is different from an IIR filter.

[0101] The cutoff frequency of HPF 107 is the same as that of HPF 102, LPF 104, and LPF 106. The cutoff frequency of HPF 107 is the same as the crossover frequency of the tweeter 10 and the woofer 20. The cutoff frequency of HPF 107 may differ from the crossover frequency of the tweeter 10 and the woofer 20. For example, the cutoff frequency of HPF 107 may also be a frequency within ±E% (e.g., 20%) of the crossover frequency of the tweeter 10 and the woofer 20.

[0102] HPF 107 generates a high-frequency band sound signal g4 by removing low-frequency components from the high-frequency band sound signal d2 generated by HPF 102.

[0103] HPF 108 comprises HPF 102 and HPF 107. Therefore, the cutoff frequency of HPF 108 is the same as that of LPF 104 and LPF 106. HPF 102 and HPF 107 are both second-order IIR filters, thus the phase order of HPF 108 is fourth. HPF 108 generates the high-frequency band signal g4 by removing low-frequency components from the input tone signal d1.

[0104] In the first modification, the synthesis unit 105 generates an output audio signal b1 by combining the high-frequency band tone signal g4 generated by the HPF 108 and the low-frequency band tone signal g3 generated by the LPF 104. Therefore, the output audio signal b1 includes the high-frequency band tone signal g4 removed by the LPF 106. Furthermore, the output audio signal b1 is a signal with a voltage level that does not clamp in the amplification device 50. The synthesis unit 105 supplies the output audio signal b1 to the amplification device 50.

[0105] Figure 7 This is a diagram illustrating an example of the action in the first variation. In Figure 7 In the middle, to and Figure 5 The processes shown are labeled with the same numbers. The following mainly focuses on those that are identical to... Figure 5 The different processing methods shown are explained.

[0106] In step S201, following step S101, HPF 108 generates a high-frequency band tone signal g4 by removing low-frequency band components from the input tone signal d1. Furthermore, step S201 can be performed between steps S202 and S205, which will be described later.

[0107] Next, in step S202, LPF 106 generates filter output signal g1 by removing high-frequency components from the input tone signal d1.

[0108] Next, in step S203, the amplitude limiting unit 103 generates a second tone signal g2 by limiting the amplitude of the filter output signal g1 in a manner that does not exceed a reference value. The second tone signal g2 contains higher harmonics associated with the amplitude limiting in the amplitude limiting unit 103.

[0109] Next, in step S204, LPF 104 generates a low-frequency band tone signal g3 by removing high-frequency band components from the second tone signal g2. The low-frequency band tone signal g3 does not contain either the high-order harmonics associated with amplitude limitation in amplitude limitation section 103 or the high-frequency band tone signal g4 contained in the input tone signal d1.

[0110] Next, in step S205, the synthesis unit 105 generates an output tone signal b1 by combining the high-frequency tone signal g4 generated by HPF 108 and the low-frequency tone signal g3 generated by LPF 104. Steps S106 to S109 are then executed.

[0111] According to the first modification, similar to the first embodiment, the low-frequency speaker 20 is capable of outputting a powerful sound and reducing the noise contained in the output sound from the high-frequency speaker 10. Furthermore, the first modification includes an LPF 106, thus improving the quality of the output sound signal b1 compared to a structure without an LPF 106.

[0112] The cutoff frequencies of HPF 108, LPF 104, and LPF 106 are all equal. Therefore, compared to a structure where the cutoff frequencies of HPF 108, LPF 104, and LPF 106 are different, this structure can reflect the frequency components of the input tone signal d1 onto the frequency components of the output tone signal b1.

[0113] The cutoff frequencies of HPF 108, LPF 104, and LPF 106 are not limited to 3kHz; for example, they can be higher or lower than 3kHz.

[0114] The cutoff frequencies of HPF 108, LPF 104, and LPF 106 can be different from each other. In this case, it is also possible to reduce the noise (caused by clamping) contained in the output sound from the high-frequency speaker 10.

[0115] The order of HPF 108 is equal to the sum of the orders of LPF 104 and LPF 106. Therefore, the phase characteristic of the high-frequency band signal g4 input to the synthesizer 105 is consistent with the phase characteristic of the low-frequency band signal g3 input to the synthesizer 105. Consequently, compared to a structure where the order of HPF 108 is inconsistent with the sum of the orders of LPF 104 and LPF 106, the quality of the sound represented by the output audio signal b1 is improved.

[0116] The combined order of HPF 108, LPF 104, and LPF 106 is not limited to 4th order; for example, it can be higher or lower than 4th order.

[0117] The combined order of HPF 108, LPF 104, and LPF 106 can be different from each other. For example, HPF 102 or HPF 107 can be omitted. In this case, it is also possible to reduce the noise (caused by clamping) contained in the output sound from the high-frequency loudspeaker 10. In addition, omitting HPF 102 or HPF 107 simplifies the structure.

[0118] B2: Second Variation

[0119] In the first embodiment and the first variation, the gain of the amplification device 50 can be variable. In this case, the reference value can be changed, for example, in accordance with a change in the gain of the amplification device 50. For example, it can also be increased in accordance with a decrease in the gain of the amplification device 50. Figure 4 The baseline value +R shown indicates a decrease. Figure 4 The reference value shown is R.

[0120] The reference value can be predetermined based on the clamping voltage of the amplification device 50 and the maximum gain of the amplification device 50. For example, firstly, the maximum amplitude of the signal that will not be clamped even if the clamping voltage of the amplification device 50 is divided by the maximum gain of the amplification device 50 is determined. The reference value is a voltage whose maximum amplitude limits the output tone signal b1 to F% of the maximum amplitude of the signal that will not be clamped even if the output tone signal b1 is input to the amplification device 50.

[0121] According to the second variation, even if the gain of the amplification device 50 is variable, the low-frequency speaker 20 is able to output a powerful sound and reduce the noise contained in the output sound from the high-frequency speaker 10.

[0122] B3: Third Variation

[0123] In the first embodiment and the first to second modifications, the audio system 1 is not limited to being mounted on a vehicle 100, but can also be mounted on vehicles such as the vehicle 100 where it is difficult to supply a high power voltage to the amplification device 50. Vehicles where it is difficult to supply a high power voltage to the amplification device 50 are, for example, small airplanes or small boats.

[0124] According to the third variation, for example in a small aircraft or small ship, the low-frequency speaker 20 is able to output a powerful sound and reduce the noise contained in the output sound from the high-frequency speaker 10.

[0125] C: Based on the above methods and variations, the methods can be mastered.

[0126] Based on the above method and at least one of the variations, the following method can be mastered.

[0127] C1: Method 1

[0128] The audio signal processing apparatus according to the first aspect of the present invention is for an amplification device that supplies a first audio signal to a high-frequency loudspeaker and a low-frequency loudspeaker, and supplies an output audio signal. The audio signal processing apparatus includes: a high-pass filter that generates a high-frequency audio signal by removing low-frequency components from the input audio signal; an amplitude limiting unit that generates a second audio signal by limiting the amplitude of the input audio signal to a reference value that does not exceed a clamping voltage corresponding to the clamping voltage of the first audio signal output from the amplification device; a low-pass filter that generates a low-frequency audio signal by removing high-frequency components from the second audio signal generated by the amplitude limiting unit; and a synthesis unit that generates the output audio signal by synthesizing the high-frequency audio signal generated by the high-pass filter and the low-frequency audio signal generated by the low-pass filter.

[0129] According to this method, when the first tone signal output from the amplification device is supplied to both the high-frequency band loudspeaker and the low-frequency band loudspeaker, the noise component contained in the output tone from the high-frequency band loudspeaker can be reduced.

[0130] C2: Method 2

[0131] In the example of the first method (the second method), the cutoff frequency of the high-pass filter is equal to the cutoff frequency of the low-pass filter. According to this method, compared with a structure where the cutoff frequencies of the high-pass filter and the low-pass filter are different, it is possible to reflect the frequency components of the input audio signal to the frequency components of the output audio signal.

[0132] C3: The third method

[0133] In the example of the first or second method (the third method), the order of the high-pass filter is equal to the order of the low-pass filter. According to this method, the phase characteristics of the high-frequency band signal input to the synthesizer are consistent with the phase characteristics of the low-frequency band signal input to the synthesizer. Therefore, compared to a structure where the orders of the high-pass filter and the low-pass filter are inconsistent, the quality of the sound represented by the output audio signal is improved.

[0134] C4: Method 4

[0135] The audio signal processing apparatus according to the fourth aspect of the present invention is for an amplification device that supplies a first audio signal to a high-frequency loudspeaker and a low-frequency loudspeaker, and supplies an output audio signal. The audio signal processing apparatus includes: a high-pass filter that generates a high-frequency audio signal by removing low-frequency components from the input audio signal; a first low-pass filter that generates a filter output signal by removing high-frequency components from the input audio signal; an amplitude limiting unit that generates a second audio signal by clamping the filter output signal at a reference value not exceeding a clamping voltage corresponding to the clamping voltage of the first audio signal output from the amplification device; a second low-pass filter that generates a low-frequency audio signal by removing high-frequency components from the second audio signal generated by the amplitude limiting unit; and a synthesis unit that generates the output audio signal by synthesizing the high-frequency audio signal generated by the high-pass filter and the low-frequency audio signal generated by the second low-pass filter.

[0136] According to this method, when the first audio signal output from the amplification device is supplied to both the high-frequency band loudspeaker and the low-frequency band loudspeaker, the noise component contained in the output audio from the high-frequency band loudspeaker can be reduced. Furthermore, this method includes a first low-pass filter, thus improving the quality of the output audio signal compared to a structure without a first low-pass filter.

[0137] C5: Method 5

[0138] In the example of the fourth method (the fifth method), the cutoff frequencies of the high-pass filter, the first low-pass filter, and the second low-pass filter are equal to each other. According to this method, compared to a structure where the cutoff frequencies of the high-pass filter, the first low-pass filter, and the second low-pass filter are different, it is possible to reflect the frequency components of the input audio signal to the frequency components of the output audio signal.

[0139] C6: Method 6

[0140] In examples of the fourth or fifth method (the sixth method), the order of the high-pass filter is equal to the sum of the orders of the first and second low-pass filters. According to this method, the phase characteristics of the high-frequency band signal input to the synthesizer are consistent with the phase characteristics of the low-frequency band signal input to the synthesizer. Therefore, compared to a structure where the order of the high-pass filter is inconsistent with the sum of the orders of the first and second low-pass filters, the quality of the sound represented by the output audio signal is improved.

[0141] C7: The 7th method

[0142] In any example of any of the methods 1 to 6 (method 7), the reference value is a predetermined value based on the clamping voltage and gain of the amplification device. According to this method, corresponding to the amplification characteristics of the amplification device such as the clamping voltage and gain of the amplification device, the low-frequency loudspeaker can output a powerful sound and limit the occurrence of noise contained in the output sound from the high-frequency loudspeaker.

[0143] C8: Method 8

[0144] In an example of any of the first to seventh methods (the eighth method), an adjustment unit is further included. This adjustment unit generates the input audio signal by adjusting the amplitude of the audio signal in accordance with the user's volume operation. The high-frequency speaker and the low-frequency speaker are located in the vehicle. According to this method, the user's volume operation is not reflected in the gain of the amplification device, but in the amplitude of the input audio signal. Therefore, even if the amplitude of the input audio signal increases in accordance with the user's volume operation, clamping in the amplification device can be suppressed. Thus, in the vehicle, the noise contained in the output sound from the high-frequency speaker can be reduced.

[0145] C9: The 9th Method

[0146] The audio system according to the ninth aspect of the present invention includes an audio signal processing device of any one of the first to eighth aspects, the high-frequency loudspeaker, the low-frequency loudspeaker, and the amplification device. According to this aspect, when a first audio signal output from the amplification device is supplied to both the high-frequency loudspeaker and the low-frequency loudspeaker, the noise component contained in the output sound from the high-frequency loudspeaker can be reduced.

[0147] C10: Method 10

[0148] The audio signal processing method of the present invention (the 10th aspect) is implemented by a computer. It supplies an output audio signal to an amplification device that supplies a first audio signal to a high-frequency loudspeaker and a low-frequency loudspeaker. In this audio signal processing method, a high-frequency audio signal is generated by removing low-frequency components from the input audio signal; a second audio signal is generated by limiting the amplitude of the input audio signal with a reference value corresponding to the clamping voltage of the first audio signal output from the amplification device; a low-frequency audio signal is generated by removing high-frequency components from the generated second audio signal; and the output audio signal is generated by combining the generated high-frequency audio signal and the generated low-frequency audio signal.

[0149] According to this method, when the first tone signal output from the amplification device is supplied to both the high-frequency band loudspeaker and the low-frequency band loudspeaker, the noise component contained in the output tone from the high-frequency band loudspeaker can be reduced.

[0150] C11: Method 11

[0151] The audio signal processing method of the present invention (the 11th aspect) is implemented by a computer. It supplies an output audio signal to an amplification device that supplies a first audio signal to a high-frequency loudspeaker and a low-frequency loudspeaker. In this audio signal processing method, a high-frequency audio signal is generated by removing low-frequency components from the input audio signal; a filter output signal is generated by removing high-frequency components from the input audio signal; a second audio signal is generated by limiting the amplitude of the filter output signal to a reference value that does not exceed the clamping voltage corresponding to the clamping voltage of the first audio signal output from the amplification device; a low-frequency audio signal is generated by removing high-frequency components from the generated second audio signal; and the output audio signal is generated by combining the generated high-frequency audio signal and the generated low-frequency audio signal.

[0152] According to this method, when the first tone signal output from the amplification device is supplied to both the high-frequency band loudspeaker and the low-frequency band loudspeaker, the noise component contained in the output tone from the high-frequency band loudspeaker can be reduced.

[0153] Explanation of the label

[0154] 1…Audio system, 3…Operating unit, 4…Sound source, 10…High frequency loudspeaker, 20…Low frequency loudspeaker, 30…Storage device, 40…Audio signal processing device, 50…Amplification device, 100…Vehicle, 101…Adjustment unit, 102…HPF, 103…Amplitude limiting unit, 104…LPF, 105…Synthesis unit, 106…LPF, 107…HPF, 108…HPF.

Claims

1. An audio signal processing apparatus, which provides an output audio signal to an amplification device that supplies a first audio signal to a high-frequency loudspeaker and a low-frequency loudspeaker. The audio signal processing device includes: A high-pass filter generates a high-frequency signal by removing low-frequency components from the input audio signal. An amplitude limiting unit generates a second tone signal by limiting the amplitude of the input tone signal in a manner that does not exceed a reference value corresponding to the clamping voltage that clamps the first tone signal output from the amplification device. A low-pass filter that generates a low-frequency band tone signal by removing high-frequency components from the second tone signal generated by the amplitude limiting portion; and The synthesis unit generates the output sound signal by combining the high-frequency band sound signal generated by the high-pass filter and the low-frequency band sound signal generated by the low-pass filter.

2. The audio signal processing apparatus according to claim 1, wherein, The cutoff frequency of the high-pass filter is the same as the cutoff frequency of the low-pass filter.

3. The audio signal processing apparatus according to claim 1 or 2, wherein, The order of the high-pass filter is the same as the order of the low-pass filter.

4. The audio signal processing apparatus according to claim 1 or 2, wherein, The reference value is a pre-defined value based on the clamping voltage of the amplification device and the gain of the amplification device.

5. The audio signal processing apparatus according to claim 1 or 2, wherein, It also includes an adjustment unit that generates the input audio signal by adjusting the amplitude of the audio signal in accordance with the user's volume operation. The high-frequency loudspeaker and the low-frequency loudspeaker are located in the vehicle.

6. An audio signal processing apparatus, which provides an output audio signal to an amplification device that supplies a first audio signal to a high-frequency loudspeaker and a low-frequency loudspeaker. The audio signal processing device includes: A high-pass filter generates a high-frequency signal by removing low-frequency components from the input audio signal. The first low-pass filter generates the filter output signal by removing high-frequency components from the input tone signal; An amplitude limiting unit generates a second tone signal by limiting the amplitude of the filter output signal in a manner that does not exceed a reference value corresponding to the clamping voltage that clamps the first tone signal output from the amplification device. A second low-pass filter generates a low-frequency tone signal by removing high-frequency components from the second tone signal generated by the amplitude limiting portion; and The synthesis unit generates the output sound signal by combining the high-frequency band sound signal generated by the high-pass filter and the low-frequency band sound signal generated by the second low-pass filter.

7. The audio signal processing apparatus according to claim 6, wherein, The cutoff frequencies of the high-pass filter, the first low-pass filter, and the second low-pass filter are all equal.

8. The audio signal processing apparatus according to claim 6 or 7, wherein, The order of the high-pass filter is equal to the sum of the orders of the first low-pass filter and the second low-pass filter.

9. The audio signal processing apparatus according to claim 6 or 7, wherein, The reference value is a pre-defined value based on the clamping voltage of the amplification device and the gain of the amplification device.

10. The audio signal processing apparatus according to claim 6 or 7, wherein, It also includes an adjustment unit that generates the input audio signal by adjusting the amplitude of the audio signal in accordance with the user's volume operation. The high-frequency loudspeaker and the low-frequency loudspeaker are located in the vehicle.

11. A sound system comprising: The audio signal processing apparatus according to any one of claims 1 to 10; The high-frequency band loudspeaker; The low-frequency loudspeaker; and The amplification device.

12. A sound signal processing method, implemented by a computer, for supplying an output sound signal to an amplification device that supplies a first sound signal to a high-frequency loudspeaker and a low-frequency loudspeaker. In the aforementioned audio signal processing method, A high-frequency band signal is generated by removing low-frequency band components from the input audio signal. The second tone signal is generated by limiting the amplitude of the input tone signal in a manner that does not exceed a reference value corresponding to the clamping voltage that clamps the first tone signal output from the amplification device. A low-frequency band tone signal is generated by removing high-frequency components from the generated second tone signal. The output sound signal is generated by combining the generated high-frequency sound signal and the generated low-frequency sound signal.

13. A sound signal processing method, implemented by a computer, for supplying an output sound signal to an amplification device that supplies a first sound signal to a high-frequency loudspeaker and a low-frequency loudspeaker. In the aforementioned audio signal processing method, A high-frequency band signal is generated by removing low-frequency band components from the input audio signal. The filter output signal is generated by removing high-frequency components from the input audio signal. The second tone signal is generated by limiting the amplitude of the filter output signal in a manner that does not exceed a reference value corresponding to the clamping voltage that clamps the first tone signal output from the amplification device. A low-frequency band tone signal is generated by removing high-frequency components from the generated second tone signal. The output sound signal is generated by combining the generated high-frequency sound signal and the generated low-frequency sound signal.

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