JP2000138542

Patent Translate
Powered by EPO and Google
Notice
This translation is machine-generated. It cannot be guaranteed that it is intelligible, accurate,
complete, reliable or fit for specific purposes. Critical decisions, such as commercially relevant or
financial decisions, should not be based on machine-translation output.
DESCRIPTION JP2000138542
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
audio signal processing apparatus for amplifying an audio signal to drive a speaker apparatus,
and in particular to amplifying switching amplification means in which power elements of a
power amplification stage are controlled on / off The present invention relates to an audio signal
processing apparatus provided as
[0002]
2. Description of the Related Art In recent years, as one type of audio amplifier for amplifying
and processing an audio signal supplied to a speaker device, a so-called low loss amplifier has
been developed. This low loss amplifier is called a so-called switching amplifier in which the
efficiency is improved by turning on and off the power element of the power amplification stage
at high frequency.
[0003]
In the ideal state, this switching amplifier does not allow current to flow into the element when
voltage is applied to the element that amplifies power (that is, when the element is off), and when
current flows through the element. Since the voltage is not applied between the terminals of the
element (when the element is turned on), the efficiency is extremely high compared to a class B
amplifier generally used conventionally for audio. It has the feature of being high.
08-05-2019
1
[0004]
Since the output signal of this switching amplifier is generally a square wave, it is not possible to
drive the speaker device as it is.
Therefore, after the square wave output from the switching amplifier is supplied to the low pass
filter to return to the similar waveform of the original audio signal, it is necessary to supply it to
the speaker reproducing apparatus.
[0005]
By the way, since the switching amplifier switches high power at high frequency, there is a
problem that very large high frequency unwanted radiation noise is radiated to the air. Such high
frequency unwanted radiation noise interferes with other devices or the devices themselves,
resulting in malfunction or deterioration of the devices. Due to the problem of the high frequency
unwanted radiation noise, not only is the design of the switching amplifier very difficult, but also
the large power amplifier emits larger high frequency unwanted radiation noise, so this amplifier
It is in a situation where it is not possible to make full use of the high efficiency that is the
biggest feature of
[0006]
Here, in the case of a switching amplifier for an audio signal, the above-mentioned low pass filter
for processing the output of the switching amplifier is concerned as one of the generation factors
of high frequency unnecessary noise. That is, as a low pass filter for an audio signal, a circuit
called an LC second-order low pass filter using a coil and a capacitor is generally used, but this
low pass filter is a speaker device to which an audio signal as an output signal is supplied. When
the cutoff frequency f0 of the low pass filter and the impedance of the speaker reproduction
apparatus are determined, the values of the coil and capacitor constituting the low pass filter are
uniquely determined. The cutoff frequency f0 is set to a frequency slightly higher than the
highest audio frequency (about 20 kHz), and the impedance of the speaker device is about 4 Ω
to 8 Ω.
08-05-2019
2
[0007]
On the other hand, when there is a reactive current regenerated to the power supply through the
coil and capacitor forming the low pass filter, and the output waveform of the amplifier is a
square wave, the output current waveform is a triangular wave obtained by integrating the
square wave. Since this triangular wave current contains many high frequency components,
many high frequency unwanted radiation noises are emitted from the loop through which this
current flows. Therefore, in order to reduce high frequency unwanted radiation from the
switching amplifier body and the route through which the high frequency triangular current
flows, it is important to reduce the absolute value of the high frequency triangular current.
However, in the case of a circuit for an audio signal, 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 current.
[0008]
An object of the present invention is to reduce high-frequency unwanted radiation noise in the
case of amplifying an audio signal using a switching amplifier.
[0009]
SUMMARY OF THE INVENTION In order to achieve the above object, the present invention
corrects the frequency characteristic of an input audio signal according to the amount of
attenuation in a low pass filter connected in the subsequent stage; Switching amplification means
for amplifying the audio signal corrected by this correction means and for which the power
element of the power amplification stage is on / off controlled at high frequency, and an output
square wave as an amplification output of the switching amplification means And a low pass
filter for supplying the audio signal restored to the similar waveform to the speaker device.
[0010]
According to the present invention, it becomes possible to change the characteristics of the low
pass filter connected in the latter stage of the switching amplification means by the amount
corresponding to the correction amount in the correction means, and to select the characteristics
that reduce high frequency unwanted radiation noise. Becomes possible.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will
be described below with reference to FIGS.
08-05-2019
3
[0012]
In this embodiment, the present invention is applied to an audio signal processing apparatus for
amplifying and processing an audio signal supplied to a speaker reproducing apparatus, and the
whole configuration is shown in FIG.
The audio signal obtained at the input terminal 1 is pre-amplified by the pre-amplifier 2.
The preamplification processing in this case may include processing for adjusting the
reproduction sound state such as sound quality adjustment and volume adjustment, in addition to
signal amplification processing.
[0013]
The audio signal processed by the pre-amplifier 2 is supplied to the switching amplifier 4
through the 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 high frequency characteristics generated by
the low pass filter 5 connected to the subsequent stage of the switching amplifier 4.
The frequency characteristic correction circuit 3 also serves as 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.
[0014]
The switching amplifier 4 is amplification means for performing on / off control of the power
element of the power amplification stage at high frequency by on / off control, and the output
signal thereof is a square wave. The output of the switching amplifier 4 is supplied to an output
08-05-2019
4
filter 5. The output filter 5 is constituted by a low pass filter, and performs processing to restore
the square wave to a waveform similar to the original audio signal waveform. The low pass filter
constituting the output filter 5 sets the cutoff frequency f0 to a value of about 30 kHz which is a
frequency slightly higher than the highest audio frequency.
[0015]
The audio signal converted to a waveform similar to the original audio signal waveform by the
output filter 5 is supplied to the speaker reproduction device 6, and the audio signal is supplied
to the voice coil of the speaker unit in the speaker reproduction device 6. Drive.
[0016]
Here, the configuration of the low pass filter constituting the output filter 5 is shown in FIG.
The low-pass filter here is a circuit called an LC second-order low-pass filter configured of a coil
L and a capacitor C. That is, the input terminal a is connected to the load resistance R on the
speaker reproducing device side via the coil L constituting a low pass filter, and the connection
midpoint between the coil L and the load resistance R is grounded via the capacitor C As a
configuration.
[0017]
The cutoff frequency f0 of the low pass filter is 30 kHz, and the load impedance value R of the
speaker reproduction apparatus 6 is 8 Ω. In the case of an ordinary low-pass filter, when the
impedance value R of the speaker device and the cut-off frequency f0 of the filter are determined,
the values of the coil L and the capacitor C are set as follows to obtain impedance matching with
the speaker device. It needs to be determined by the formula.
[0018]
L / C = 2 × R2
[0019]
[Equation 2] f 0 = 1 / [2 × π × √ (L × C)]
08-05-2019
5
[0020]
The values of the coil L ≒ 60 μH and the capacitor C ≒ 0.47 μF can be obtained by using the
equations (1) and (2).
In the case of the conventional circuit, a low pass filter using a coil and a capacitor of this value
was used, but in this example, the output filter 5 is a low pass filter using a coil and a capacitor of
different values. Configure
That is, in this example, the values of the coil and the capacitor that constitute the output filter 5
are selected so as to satisfy the condition of the following equation 3 instead of the above
equation 1.
[0021]
[L / C]> [2 × R2]
[0022]
This [Equation 3] is an inequality of the equation of [Equation 1] described above.
That is, in the case of this example, when only the characteristic of the output filter 5 is seen, the
impedance of the coil L is set to a state in which the impedance with the speaker reproduction
device 6 is not matched and the impedance is matched. Is a large value. Alternatively, the
capacitance value of the capacitor C is set to a smaller value than when impedance matching is
performed. Preferably, the inductance value of the coil L is made larger and the capacitance
value of the capacitor C is made smaller than when impedance matching is performed.
[0023]
Here, assuming that the cutoff frequency f0 of the low pass filter is 30 kHz and the load
08-05-2019
6
impedance value R of the speaker reproduction device 6 is 8 Ω, coil L = 120 μH and capacitor C
= 0.22 μF. When the low pass filter is configured under such conditions, the frequency
characteristic of this low pass filter is such that the passing level of the frequency above the
frequency considerably lower than the cutoff frequency f0 is attenuated.
[0024]
The frequency characteristics of the low-pass filter of this example are shown in FIG. 3, which is a
conventional low-pass filter having impedance matching with the speaker satisfying the abovementioned [Equation 1] and [Equation 2] The frequency characteristic LINE3 in the case of a
filter (≒ 60 μH, CC0.47 μF) has a substantially flat characteristic up to 20 kHz which is the
highest audio frequency, but the low-pass filter of the characteristic of this example (ie L = 120
μH, The frequency characteristic LINE 4 in the case of the filter (C: 0.22 μF) is a frequency
characteristic in which the passing level is reduced from a frequency (approximately several kHz)
much lower than 20 kHz.
[0025]
Thus, the characteristic of the output filter 5 is selected. In this example, the frequency
characteristic of the output filter 5 is taken as the frequency characteristic of the low pass filter
constituting the frequency characteristic correction circuit 3 connected to the front stage of the
switching amplifier 4. Choose to correct the characteristics.
Specifically, the frequency characteristic has a kurtosis Q that is somewhat high in the vicinity of
the cutoff frequency f0 of the output filter 5. For example, when the frequency characteristic of
the output filter 5 is LINE 4 of FIG. 3, the frequency characteristic of the frequency characteristic
correction circuit 3 of this example is made to be LINE 1 of FIG. 3, and the frequency
characteristic correction circuit 3, switching amplifier 4, output The overall frequency
characteristic of the signal passed through the filter 5 is made to be a substantially flat frequency
characteristic LINE2 up to about 20 kHz.
[0026]
With this configuration, the decrease in the frequency characteristic at high frequency due to the
unmatching of the impedance of the output filter to the speaker is corrected, and the entire
circuit becomes a flat frequency characteristic, and the drive of the speaker reproduction device
08-05-2019
7
6 is favorable. You will be able to do it. In this example, since the characteristics of the output
filter are set as described above and the correction circuit 3 for correcting the characteristics of
the output filter is provided, high frequency unwanted radiation noise generated in the switching
amplifier 4 and its periphery Can be minimized.
[0027]
The reduction of the high frequency unwanted radiation noise is performed according to the
following principle. That is, when the low-pass filter is connected to the output stage of the
switching amplifier 4, as already described in the section of the [Problems to be solved by the
invention], the power supply is made via the coil and capacitor constituting the low-pass filter.
When the output waveform of the amplifier is a square wave, the waveform of the triangular
wave obtained by integrating the square wave becomes the invalid current. Since this triangular
wave current contains many high frequency components, many high frequency unwanted
radiation noises are emitted from the loop through which this current flows. The value of this
high frequency triangular wave current is shown by the following equation.
[0028]
## EQU4 ## IL = [(VIN-VOUT) .times.T] / L
[0029]
## EQU5 ## IC = (C × VOUT) / T
[0030]
Here, IL is the current flowing through the coil L, IC is the current flowing through the capacitor
C, VIN and VOUT are the input voltage and output voltage of the filter 5, respectively, and T is
1/2 of the switching period of the switching amplifier 4 Show.
[0031]
As derived from the equations (4) and (5), the values of the high frequency triangular current IL
and IC are inversely proportional to the inductance of the coil L and proportional to the
capacitance of the capacitor C.
08-05-2019
8
Therefore, by increasing the inductance value of the coil L and decreasing the capacitance value
of the capacitor C, the high frequency triangular current IL and IC are reduced, and high
frequency unwanted radiation noise is reduced.
[0032]
Therefore, with the circuit configuration according to the present embodiment, the audio signal
amplified by the switching amplifier 4 and supplied to the speaker device is supplied to the
speaker device while maintaining the correct frequency characteristics. Can be significantly
reduced.
Therefore, in the device incorporating the circuit of this example, it becomes easy to clear the
high frequency noise regulation, and the degree of freedom such as the installation place of the
switching amplification means is increased, and the design of the device incorporating the
switching amplification means It can be easy.
[0033]
Here, FIG. 4 shows an example of a circuit diagram in the case where the processing circuit
according to 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. The audio signal
supplied from the audio signal source V1 (here, corresponding to the pre-amplifier 2 in FIG. 1) is
transmitted through a series circuit of resistors R2 and R3. The signal is supplied to the + side
input terminal of the operational amplifier X1 constituting the frequency characteristic
correction circuit 3. The connection point between the negative input terminal of the operational
amplifier X1 and the resistor R3 is grounded via a capacitor C3. A power supply of a
predetermined voltage is supplied to the operational amplifier X1 from the positive power supply
V2 and the negative power supply V3. The output terminal of the operational amplifier X1 is
grounded via a series circuit of resistors R4 and R5, and the negative side input terminal of the
operational amplifier X1 is connected to the connection midpoint of the resistors R4 and R5.
Furthermore, the connection midpoint of the resistors R2 and R3 is connected to the output
terminal of the operational amplifier X1 via the capacitor C2.
08-05-2019
9
[0034]
The output terminal of the operational amplifier X1 is connected via the resistor R6 to the + side
input terminal of the amplifier circuit X2 which constitutes the switching amplifier 4. The
positive side input terminal of the amplifier circuit X2 and the resistor R6 are grounded via a
resistor R7. Power of a predetermined voltage is supplied to the amplification circuit X2 from the
positive power supply V4 and the negative power supply V5. The output terminal of the amplifier
circuit X2 is connected to the negative side input terminal of the amplifier circuit X2, and the
output terminal of the amplifier circuit X2 is connected to one end of the coil L1 constituting the
output filter 5 (low pass filter). The other end of the coil L1 is connected to one end of a
capacitor C1 which also constitutes the filter 5, 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 reproduction device 6. In FIG. 4, the speaker reproduction device is shown as a load
resistor R1.
[0035]
In such a configuration, the values of the coil L1 and the capacitor C1 constituting the output
filter 5 are not the values obtained by impedance matching with the impedance value of the load
resistor R1, but are made to have the above-mentioned characteristics and The value of each
element is selected so that the amount of attenuation of high frequency characteristics in the low
pass filter is corrected by the operational amplifier X1. With the circuit configuration shown in
FIG. 4, an amplification device for an audio signal in which high frequency unnecessary radiation
noise is reduced can be obtained.
[0036]
In the embodiment described above, the specific device in which the switching amplifier and its
peripheral circuits are incorporated has not been particularly described, but the circuit
configuration of this embodiment is for driving the speaker device. It is applicable to various
devices that perform amplification processing. For example, it is applied to an amplification
device for so-called high-fidelity audio reproduction that is separate from the speaker
reproduction device, or a relatively small-sized audio device in which an audio source such as a
disk reproduction unit or a tuner unit and a speaker unit are integrated. It can also be applied to
08-05-2019
10
an amplification unit. Furthermore, the present invention can also be applied to an amplification
unit for driving a speaker device incorporated in a television receiver.
[0037]
According to the present invention, it becomes possible to change the characteristics of the lowpass filter connected downstream of the switching amplification means by the amount
corresponding to the correction amount by the correction means, thereby reducing high
frequency unwanted radiation noise. It becomes possible to select characteristics. Therefore, it is
possible to supply power to the speaker device while minimizing the occurrence of high
frequency unwanted radiation noise in the switching amplification means. In addition, it becomes
easy to clear the high frequency noise regulation, and the degree of freedom such as the
installation place of the switching amplification means is increased, and the design of the device
in which the switching amplification means is incorporated can be facilitated.
[0038]
In this case, the low-pass filter is set so that the value of the triangular wave current obtained by
integrating the output square wave of the switching amplification means is smaller than the value
obtained by impedance matching with the speaker device connected to this circuit. By doing this,
the generation of high frequency unwanted radiation noise in the switching amplification means
can be effectively reduced.
08-05-2019
11