JP2000138542A - Audio signal processor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the unnecessary high-frequency noise which is produced in amplifying audio signals by using a switching amplifier with a simple constitution. SOLUTION: This audio signal processor is provided with a correcting means 3 which corrects the frequency characteristic of inputted audio signals correspondingly to the attenuation at a low-pass filter connected to the succeeding stage, a switching amplifying means 4 which amplifies the audio signals corrected by means of the correcting means 3 and in which the turning on/off of a power element in a power amplifying stage is controlled with a high frequency, and a low-pass filter 5 which returns the square wave of the amplified output of the amplifying means 4 to a waveform similar to that of the audio signals and supplies the audio signals which are having the similar waveform to a loudspeaker device 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an audio signal processing device for amplifying an audio signal to drive a speaker device, and more particularly to a switching amplifying device in which a power element of a power amplifying stage is on / off controlled. The present invention relates to an audio signal processing device provided as a device.

[0002]

2. Description of the Related Art In recent years, a so-called low-loss amplifier has been developed as a type of audio amplifier for amplifying an audio signal supplied to a speaker device. This low-loss amplifier is a so-called switching amplifier whose efficiency is improved by turning on / off a power element of a power amplification stage at a high frequency.

In the switching amplifier, in the ideal state, when a voltage is applied to an element for amplifying power (that is, when the element is turned off), no current flows into the element, and current flows through the element. (That is, when the element is turned on), there is a characteristic that no voltage is applied between the corresponding terminals of the element, and therefore, compared to the class B amplifier generally used for the conventional audio, It has the characteristic that the efficiency is very high.

Since the output signal of the switching amplifier is generally a square wave, the speaker device cannot be driven as it is. Therefore, it is necessary to supply a square wave output from the switching amplifier to a low-pass filter to restore a waveform similar to the original audio signal, and then supply the analog signal to a speaker reproducing device.

[0005]

By the way, the switching amplifier has a problem that it emits extremely high frequency unnecessary radiation noise into the air because it switches high power at high frequency. Such high-frequency unnecessary radiation noise interferes with other devices or the device itself, resulting in malfunction of the device or deterioration of performance. Because of the problem of unnecessary high-frequency radiation noise, it is extremely difficult to design a switching amplifier.In addition, a higher-power amplifier emits higher-frequency unnecessary radiation noise. It is in a situation where the high efficiency, which is the biggest characteristic of, cannot be fully utilized.

Here, in the case of a switching amplifier for an audio signal, the above-mentioned low-pass filter that processes the output of the switching amplifier is related to one of the factors that cause unnecessary high-frequency noise. That is, a circuit called an LC second-order low-pass filter using a coil and a capacitor is generally used as a low-pass filter for an audio signal. This low-pass filter is a speaker device to which an audio signal as an output signal is supplied. When the cutoff frequency f _{0 in} the low-pass filter and the impedance of the speaker reproducing device are determined, the values of the coil and the capacitor constituting the low-pass filter are uniquely determined. . The cutoff frequency f ₀ is set to a frequency slightly higher than the highest audible frequency (about 20 kHz), and the impedance of the speaker device is about 4Ω to 8Ω.

On the other hand, when there is a reactive current regenerated by a power supply through a coil and a capacitor constituting a low-pass filter, and the output waveform of the amplifier is a square wave, the output current waveform is a triangular waveform obtained by integrating the square wave. Becomes Since this triangular wave current contains many high-frequency components, many high-frequency unnecessary radiation noises are emitted from a loop through which this current flows. Therefore, it is important to reduce the absolute value of the high-frequency triangular wave current in order to reduce unnecessary high-frequency radiation from the switching amplifier main body and the route through which the high-frequency triangular wave current flows. However,
In the case of an audio signal circuit, since the constants of the elements constituting the low-pass filter are uniquely determined as described above, it has been difficult to reduce the absolute value of the high-frequency triangular wave current.

It is an object of the present invention to reduce high frequency unnecessary radiation noise when an audio signal is amplified using a switching amplifier.

[0009]

In order to achieve this object, the present invention provides a frequency characteristic of an input audio signal.
Correction means for correcting in accordance with the amount of attenuation in a low-pass filter connected to the subsequent stage, and switching for amplifying the audio signal corrected by the correction means and for controlling the power element of the power amplification stage to be turned on / off at a high frequency Amplifying means, and a low-pass filter for returning an output square wave, which is an amplified output of the switching amplifying means, to a similar waveform of an audio signal and supplying the audio signal returned to the similar waveform to the speaker device.

According to the present invention, the characteristics of the low-pass filter connected downstream of the switching amplifying means can be changed by an amount corresponding to the amount of correction by the correcting means.
It is possible to select characteristics for reducing high frequency unnecessary radiation noise.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described.
This will be described with reference to FIGS.

In this embodiment, the present invention is applied to an audio signal processing device for amplifying an audio signal supplied to a speaker reproducing device, and FIG. 1 shows the overall configuration. The audio signal obtained at input terminal 1 is
A preamplification process is performed by the amplifier 2. The pre-amplification process here may include a process for adjusting a reproduced sound state such as sound quality adjustment and volume adjustment, in addition to the signal amplification process.

The audio signal processed by the preamplifier 2 is supplied to a switching amplifier 4 via a frequency characteristic correction circuit 3. Here, the frequency characteristic correction circuit 3 is configured in the form of a low-pass filter, and is a circuit that corrects the attenuation of the high-frequency characteristic generated by the low-pass filter 5 connected downstream of the switching amplifier 4. Further, the frequency characteristic correction circuit 3 also has a role of an anti-aliasing filter for removing an alias which is a spurious signal generated in the switching amplifier 4. Details of the characteristics of the frequency characteristic correction circuit 3 will be described later.

The switching amplifier 4 comprises:
Amplifying means for performing on / off control of the power element of the power amplification stage at a high frequency to perform amplification, and the output signal is a square wave. The output of the switching amplifier 4 is supplied to an output filter 5. This output filter 5
Is composed of a low-pass filter, and performs processing for returning a square wave to a waveform similar to the original audio signal waveform. The low-pass filter constituting the output filter 5 has the cutoff frequency f ₀ set to a value of about 30 kHz which is a frequency slightly higher than the highest audible frequency.

The audio signal having a waveform similar to the original audio signal waveform by the output filter 5 is supplied to a speaker reproducing device 6, and an audio signal is supplied to a voice coil of a speaker unit in the speaker reproducing device 6. Drive the speaker unit.

Here, the configuration of the low-pass filter constituting the output filter 5 is shown in FIG. The low-pass filter here is an LC2 composed of a coil L and a capacitor C.
This is a circuit called a next-order low-pass filter. That is, the input terminal a is connected to a load resistor R on the speaker reproducing device side via a coil L constituting a low-pass filter, and a connection point between the coil L and the load resistor R is grounded via a capacitor C. There is a configuration to do.

An example of the value of each element constituting the low-pass filter will be described. Here, the cut-off frequency f ₀ of the low-pass filter is 30 kHz.
Here, the load impedance value R of the speaker reproducing device 6 is set to 8Ω. In the case of a normal low-pass filter, when the impedance value R of the speaker device and the cutoff frequency f _{0 of the} filter are determined, the coils L,
The value of the capacitor C needs to be determined by the following equation.

[0018]

L / C = 2 × R ²

[0019]

F ₀ = 1 / [2 × π × √ (L × C)]

By using the equations (1) and (2), the coil L ≒ 60 μH and the capacitor C ≒ 0.47
The value of μF is obtained. In the case of the conventional circuit, a low-pass filter using a coil and a capacitor of this value was used. In the present embodiment, however, a low-pass filter using a coil and a capacitor of a different value is used. Is configured. That is, in this example, the values of the coil and the capacitor constituting the output filter 5 are selected so as to satisfy the following equation (3) instead of the equation (1).

[0021]

[Equation 3] [L / C]> [2 × R ² ]

[Equation 3] is obtained by transforming the above-described equation 1 into an inequality. That is, in the case of this example, when only the characteristics of the output filter 5 are viewed, the impedance with the speaker reproducing device 6 is not matched, and the inductance value of the coil L is smaller than the case where the impedance is matched. Is set to a large value.
Alternatively, the capacitance value of the capacitor C is set to a smaller value than when the impedance is matched. Preferably, the inductance value of the coil L is set to a larger value and the capacitance value of the capacitor C is set to a smaller value than in the case where the impedances are matched.

Here, when the cut-off frequency f ₀ of the low-pass filter is 30 kHz and the load impedance value R of the speaker reproducing device 6 is 8Ω, the coil L = 12
0 μH and capacitor C = 0.22 μF. When a low-pass filter is configured under such conditions, the frequency characteristic of the low-pass filter is such that the pass level of a frequency above a frequency considerably lower than the cutoff frequency f ₀ is attenuated.

FIG. 3 shows the frequency characteristics of the low-pass filter of this embodiment. The conventional low-pass filter has the characteristic of performing impedance matching with a speaker that satisfies the above-mentioned equations (1) and (2). (That is, L ≒ 60μ
The frequency characteristic LINE3 in the case of H, C ≒ 0.47 μF is a substantially flat characteristic up to the maximum audible frequency of 20 kHz, but the low-pass filter having the characteristics of this example (that is, L = 120 μH, C = 0.22 μF), the frequency characteristic LINE4 is 20 kHz.
The frequency characteristic is such that the pass level decreases from a frequency (about several kHz) which is considerably lower than z.

As described above, the characteristics of the output filter 5 are selected. In this embodiment, the frequency characteristic correction circuit 3 connected before the switching amplifier 4 is used.
Is selected so as to correct the frequency characteristic of the output filter 5 as the frequency characteristic of the low-pass filter constituting the above. Specifically, a frequency characteristic having a somewhat high kurtosis Q near the cutoff frequency f ₀ of the output filter 5 is set. For example, when the frequency characteristic of the output filter 5 is LINE 4 in FIG.
Of about 20 kHz as a total frequency characteristic of the signal passing through the frequency characteristic correction circuit 3, the switching amplifier 4, and the output filter 5.
The frequency characteristic LINE2 is almost flat up to that.

With this configuration, the deterioration of the frequency characteristics at high frequencies due to the impedance mismatch of the output filter with respect to the speaker is corrected, and the frequency response becomes flat as a whole circuit. Can be performed well. In this embodiment, the correction circuit 3 for setting the characteristics of the output filter and correcting the characteristics of the output filter is provided, so that the high frequency unnecessary radiation noise generated in the switching amplifier 4 and its surroundings is provided. Can be minimized.

The reduction of high frequency unnecessary radiation noise is performed according to the following principle. That is, when a low-pass filter is connected to the output stage of the switching amplifier 4, as described in the section of [Problems to be Solved by the Invention], the power supply is connected via the coil and the capacitor constituting the low-pass filter. When the output waveform of the amplifier is a square wave, the triangular waveform obtained by integrating the square wave becomes the reactive current. Since this triangular wave current contains many high-frequency components, many high-frequency unnecessary radiation noises are emitted from a loop through which this current flows. The value of the high-frequency triangular wave current is represented by the following equation.

[0028]

## EQU4 ## I _L = [(V _IN -V _OUT ) × T] / L

[0029]

## EQU5 ## I _C = (C × V _OUT ) / T

Here, I _L is the current flowing through the coil L, I _L
C is the current flowing through the capacitor C, V _IN and V _OUT are the input voltage and output voltage of the filter 5, respectively, and T is 1 / of the switching cycle of the switching amplifier 4.
The time of 2 is shown.

As derived from the equations (4) and (5), the values of the high-frequency triangular wave currents I _L and I _C are inversely proportional to the inductance of the coil L and proportional to the capacitance of the capacitor C. . Therefore, by increasing the inductance value of the coil L and decreasing the capacitance value of the capacitor C,
The high-frequency triangular wave currents I _L and I _C are reduced, and high-frequency unnecessary radiation noise is reduced.

Therefore, with the circuit configuration according to the present embodiment, the audio signal amplified and processed by the switching amplifier 4 and supplied to the speaker device can maintain the correct frequency characteristics while maintaining the correct frequency characteristics. High frequency unnecessary radiation noise from peripheral circuit components can be significantly reduced. For this reason, equipment incorporating the circuit of this example can easily clear the high-frequency noise regulation, and the degree of freedom of the installation location of the switching amplification means is increased. Can be easier.

FIG. 4 shows an example of a circuit diagram in the case where the processing circuit of the present embodiment shown as a block diagram in FIG. 1 is actually assembled as a circuit. The circuit configuration will be described below with reference to FIG. 4. An audio signal supplied from an audio signal source V1 (corresponding to the preamplifier 2 in FIG. 1) is transmitted through a series circuit of resistors R2 and R3. The signal is supplied to the + input terminal of the operational amplifier X1 included in the frequency characteristic correction circuit 3. This operational amplifier X
The connection point between the negative input terminal 1 and the resistor R3 is grounded via a capacitor C3. The operational amplifier X1 is supplied with power of a predetermined voltage from a positive power supply V2 and a negative power supply V3. The output terminal of the operational amplifier X1 is grounded via a series circuit of the resistors R4 and R5, and the negative input terminal of the operational amplifier X1 is connected to the connection point between the resistors R4 and R5. Further, the connection point between the resistors R2 and R3 is connected to the output terminal of the operational amplifier X1 via the capacitor C2.

The output terminal of the operational amplifier X1 is connected to a resistor R6
To the + input terminal of the amplifier circuit X2 constituting the switching amplifier 4. A resistor R is connected between the + input terminal of the amplifier circuit X2 and the resistor R6.
7 is grounded. Power of a predetermined voltage is supplied to the amplifier circuit X2 from the + power supply V4 and the − power supply V5. The output terminal of the amplifier circuit X2 is connected to the negative input terminal of the amplifier circuit X2, and the output terminal of the amplifier circuit X2 is
The output filter 5 (low-pass filter) is connected to one end of a coil L1. The other end of the coil L1
Similarly, one end of a capacitor C1 constituting the filter 5 is connected, and the other end of the capacitor C1 is grounded. The connection point between the coil L1 and the capacitor C1 is connected to the speaker reproducing device 6. In FIG. 4, the speaker reproducing device is shown as a load resistor R1.

In the case of such a configuration, the values of the coil L1 and the capacitor C1 constituting the output filter 5 are set to the values of the characteristics described above, instead of the values obtained by impedance matching with the impedance value of the load resistor R1. At the same time, the value of each element is selected so that the attenuation of the high-frequency characteristics in the low-pass filter is corrected by the operational amplifier X1. By adopting the circuit configuration shown in FIG. 4, it is possible to provide an audio signal amplifying device in which high frequency unnecessary radiation noise is reduced.

In the above-described embodiment, a specific device in which the switching amplifier and its peripheral circuits are incorporated has not been particularly described.
The circuit configuration of the present embodiment can be applied to various devices that perform amplification processing to drive a speaker device.
For example, the present invention is applied to a so-called high-fidelity audio reproducing amplifier separate from a speaker reproducing apparatus, or a relatively small audio apparatus in which an audio source such as a disk reproducing section or a tuner section and a speaker section are integrated. It can also be applied to an amplifier. Further, the present invention can be applied to an amplifying unit for driving a speaker device incorporated in a television receiver.

[0037]

According to the present invention, the characteristics of the low-pass filter connected downstream of the switching amplifying means can be changed by an amount corresponding to the amount of correction by the correcting means.
It is possible to select characteristics for reducing high frequency unnecessary radiation noise. Therefore, it is possible to supply power to the speaker device while minimizing the generation of unnecessary high-frequency radiation noise in the switching amplifier. In addition, it is easy to clear the high-frequency noise regulation, and the degree of freedom of the installation location of the switching amplifying means is increased, so that it is possible to easily design a device in which the switching amplifying means is incorporated.

In this case, the low-pass filter is configured such that the value of the triangular wave current obtained by integrating the output square wave of the switching amplifying means is smaller than the value obtained when impedance matching with the speaker device connected to this circuit is performed. By setting the characteristics, it is possible to effectively reduce the occurrence of high-frequency unnecessary radiation noise in the switching amplifier.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an example of an overall configuration according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing an example of a low-pass filter according to one embodiment of the present invention.

FIG. 3 is a waveform chart showing an example of frequency characteristics according to an embodiment of the present invention.

FIG. 4 is a circuit diagram showing a circuit example according to an embodiment of the present invention.

[Explanation of symbols]

3: Frequency characteristics correction circuit, 4: Switching amplifier, 5: Output filter, 6: Speaker reproduction device

Claims (2)

[Claims] 1. A correcting means for correcting the frequency characteristic of an input audio signal in accordance with the amount of attenuation in a low-pass filter connected to a subsequent stage, and amplifying the audio signal corrected by the correcting means, and A switching amplifying unit in which a power element of an amplifying stage is on / off controlled at a high frequency; and an output square wave which is an amplified output of the switching amplifying unit is returned to a similar waveform of the audio signal, and the audio signal is returned to the similar waveform. Audio signal processing device comprising: a low-pass filter that supplies a signal to a speaker device. 2. The audio signal processing device according to claim 1, wherein the low-pass filter has a triangular current value obtained by integrating the output square wave smaller than a value obtained when impedance matching with the speaker device is performed. An audio signal processing device whose characteristics are set to be