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DESCRIPTION JPH06189389

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DESCRIPTION JPH06189389
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
bass compensation characteristic automatic control circuit suitable for application to, for
example, a small speaker system.
[0002]
2. Description of the Related Art In a small speaker system, there are many methods for
expanding the bass limit of the reproduction band by driving the speaker by increasing the
output voltage of the power amplifier, that is, the voice coil terminal voltage of the speaker in a
low frequency range. It is taken. Specifically, this is a method of inserting an equalizer having a
low-frequency rising characteristic into the input stage of the power amplifier, and compensating
for the shortage of the bass of the speaker by enhancing the electric drive power.
[0003]
In this method, when the input signal level to the equalizer is increased and the output sound
pressure of the speaker is increased, the input signal level of the power amplifier compensates
for low when the bass compensation characteristic of the equalizer is fixed. It becomes excessive
in the frequency range and causes clipping distortion in the power amplifier. That is, the output
sound pressure that distortion can tolerate, ie, the listening volume, is limited by the rated output
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power of the power amplifier.
[0004]
Therefore, when the rated output power of the power amplifier is small, even if it can be
reproduced without distortion to low tones when listening at low volume, there is a problem that
distortion is generated by low tones and the listening volume can not be raised.
[0005]
As a means to solve such problems, conventionally, a method of changing the compensation
amount of the low band compensation characteristic of the equalizer according to the input
signal level, that is, a method of performing level control such that the compensation amount
decreases as the input signal level increases. It is taken.
According to this method, when the input signal level is increased, the bass reproduction limit is
narrowed, but on the other hand, the output sound pressure which can tolerate distortion can be
increased.
[0006]
FIG. 7 shows an example of a conventional speaker system to which a bass compensation
characteristic automatic control circuit is connected. In the figure, 1a and 1b are input terminals
to which audio signals are supplied, 2 is a variable resistor for volume control, 3 is a bass
compensation characteristic automatic control circuit, 4 is a power amplifier, and 5 is a speaker.
[0007]
In this case, since the impedance values of the transistors constituting the control circuit 3 for
determining the compensation amount in the low frequency range are controlled by the output
level of the power amplifier 4 to change, as shown in FIG. The amount of compensation
decreases as the output voltage of the power amplifier 4 increases.
[0008]
In the configuration of the control circuit 3 shown in FIG. 7, the control from the low range rising
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characteristic to the flat characteristic is the limit of control.
[0009]
SUMMARY OF THE INVENTION The present invention provides a bass compensation
characteristic automatic control circuit that automatically changes control from a low band rising
characteristic to a flat characteristic or a low band lowering characteristic as the input signal
level increases.
[0010]
SUMMARY OF THE INVENTION The present invention comprises a low pass filter having a first
order Butterworth characteristic for dividing an audio band into two and a high pass filter, and
compresses and controls an output level within a predetermined level range of an input signal. A
low pass filter is configured to have level compression characteristics and the cutoff frequency
changes according to the amount of compression, and a high pass filter is configured to have an
output signal that has a predetermined cutoff frequency and a predetermined attenuation. And
input the same audio signal to the low pass filter and the high pass filter, and electrically
combine the output signals of the low pass filter and the high pass filter into an output signal,
thereby providing an amplitude frequency characteristic of the transfer function. However, as the
input signal level increases, the control changes automatically from the low range rising
characteristics to the flat characteristics or the low range lowering characteristics.
[0011]
According to the present invention, the output signal of the low pass filter is obtained by
electrically combining the output signals of the low pass filter and the high pass filter to which
the same audio signal is input. By adjusting the attenuation factor and cut-off frequency of the
high-pass filter in relation to the above, it is possible to optionally set the amplitude frequency
characteristics when the input signal level is high to flat characteristics or low-pass
characteristics (multiple steps) Become.
[0012]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present
invention will be described below with reference to FIG.
[0013]
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In the figure, 11a and 11b are input terminals to which an input audio signal is supplied, and 12a
and 12b are output terminals from which a bass-compensated output audio signal is obtained.
The input terminal 11 b and the output terminal 12 b are each a ground side terminal.
[0014]
Reference numeral 13 denotes a low pass filter (LPF). An input audio signal is supplied to the low
pass filter 13 from input terminals 11a and 11b.
That is, the input terminal 11 a is grounded via a series circuit of the resistor 14 and the
capacitor 15.
Further, a series circuit of a resistor 16 and an NPN transistor 17 is connected in parallel to the
capacitor 15.
The filter characteristic of the low pass filter 13 is a first order Butterworth characteristic in
which the attenuation factor of the attenuation region is -6 dB / oct.
[0015]
Reference numeral 18 denotes a high pass filter (HPF). An input audio signal is supplied to the
high pass filter 18 from input terminals 11a and 11b.
That is, the input terminal 11a is grounded via a series circuit of the capacitor 19 and the
resistors 20 and 21.
The filter characteristic of the high-pass filter 18 is a Butterworth characteristic whose
attenuation factor in the attenuation band is -6 dB / oct.
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[0016]
The output signal of low-pass filter 13, ie, the signal obtained at the connection point of resistor
14 and capacitor 15, and the output signal of high-pass filter 18, ie, the signal obtained at the
connection point of resistors 20 and 21 for voltage division are respectively synthesized The
signal is supplied to the unit 22 and synthesized, and is led out as an output audio signal at the
output terminal 12a.
[0017]
An input audio signal supplied to the input terminal 11a is supplied to a fixed terminal on the a
side of the changeover switch 23, and an output signal of the low pass filter 13 is supplied to the
fixed terminal on the b side.
The output signal of the changeover switch 23 is supplied to the control signal generation circuit
24. The control signal generation circuit 24 generates a control signal for controlling and
changing the resistance value between the emitter and the collector of the transistor 17 of the
low pass filter 13 according to the level of the input audio signal or the output signal of the low
pass filter 13. .
[0018]
That is, the output signal of the changeover switch 23 is amplified by the amplifier 25 and then
supplied to the voltage doubler peak rectification circuit including the capacitors 26 and 27 and
the diodes 28 and 29. The amplification degree of the amplifier 25 is set to an appropriate value
that can obtain an appropriate control characteristic. The output signal of the voltage doubler
peak rectification circuit is supplied via the resistor 30 to the base of the transistor 17 of the low
pass filter 13 as the control signal described above. The resistor 30 is for setting the base voltage
Vb of the transistor 17 to an appropriate value.
[0019]
Here, the control start time is determined by the capacitance of the capacitor 27, the output
resistance of the amplifier 25 and the forward resistance of the diode 29. The control return time
is determined by the capacitance of the capacitor 27 and the reverse resistance of the diode 29.
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[0020]
When the changeover switch 23 is connected to the a side, an input audio signal is supplied to
the control signal generation circuit 24, and a so-called forward control type control operation is
performed. On the other hand, when the changeover switch 23 is connected to the b side, the
output signal of the low pass filter 13 is supplied to the control signal generation circuit 24, and
the control operation of the backward movement type is performed.
[0021]
Next, the characteristics and synthesis conditions of the low pass filter 13 and the high pass filter
18 will be described in detail. (A) Let R1 and R0 be the resistances of the output level
compression control characteristics 14 and 16 of the low pass filter 13, C1 be the capacitance of
the capacitor 15, Rt be the resistance between the emitter and collector of the transistor 17, and
Assuming that the input signal voltage is E and the output signal voltage is VL, an equivalent
circuit of the low pass filter 13 is as shown in FIG.
[0022]
From FIG. 2, the output signal voltage VL is expressed by Equation 1.
[0023]
In Equation 1, Rx is a parallel combined resistance value of R1 and (R0 + Rt), and is represented
by Equation 2.
[0024]
In Equation 1, E.Rx / R1 is an output signal voltage at a low frequency satisfying .omega.C1Rx <1
and has compression characteristics.
Further, 1 / (1 + jωC1Rx) has low pass filter characteristics, and the cutoff frequency changes in
response to the change of Rx.
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Here, the cut-off frequency is a frequency at which the output voltage level of the filter drops 3
dB from the flat characteristic in the normal band, as is well known.
[0025]
First, the level compression characteristic of the output signal voltage in the low frequency range
of the pass band will be described.
[0026]
Generally, the resistance value Rt between the emitter and the collector of the silicon transistor
becomes almost infinite when the base voltage Vb is about 0.6 V or less.
However, when Vb is 0.6 V or more, Rt decreases rapidly, and when Vb is 1.5 to 2 V or more, Rt
maintains a low value of about several hundred ohms. Since the silicon transistor has such a
property, it has been conventionally used as a simple control element for level control.
[0027]
The base voltage Vb is generated by the control signal generation circuit 24 as described above,
and is a DC voltage corresponding to the level or the magnitude of the input signal or the output
signal of the low pass filter 13. Here, Vb is a DC voltage of a constant value corresponding to the
signal level when the input signal is a sine wave, but is a DC voltage whose magnitude changes
temporally in the case of a general audio signal. .
[0028]
In the case of the front type (using the switch 23 connected to the a side) that uses the input
signal as the control signal, Vb is proportional to the input signal level, but the back type that
uses the controlled output signal as the control signal In the case of connecting the switch 23 to
the b side), Vb is not proportional to the input signal level. However, in either case, Vb depends
on the input signal level.
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[0029]
Hereinafter, the level compression characteristic of E · Rx / R1 will be described for the case
where the input signal is a sine wave.
[0030]
Assuming that the input signal voltage at which Vb becomes the control start voltage (about 0.6
V) is E1, since Rt is a value close to infinity in this state, Rx = R1.
[0031]
Therefore, when the input signal voltage is E1, the output signal voltage VL1 'in the low
frequency range is expressed by Equation 3.
[0032]
Next, when the input signal level becomes high and Vb becomes 1.5 to 2 V or so, Rt becomes a
negligible value compared to R0.
Therefore, when Rt is omitted with respect to R0 in the equation 2, it is represented by the
equation 4.
[0033]
Therefore, assuming that the input signal voltage in this state is E2, the output signal voltage VL2
'in the low frequency range is expressed by equation 5.
[0034]
As apparent from the equation 5, in VL2 ', the voltage E2 is compressed to 1 / (1 + R1 / R0).
[0035]
Generally, in level compression, the input signal voltage is E, the compressed output signal
voltage is V0, the control start voltage of the input signal is E1, and the level compression ratio is
n.
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[0036]
By taking the logarithm of both sides of Eq. 6, n is represented by Eq. 7, and the level
compression ratio is represented as n: 1.
[0037]
Accordingly, the level compression ratio n2 when the input signal voltage is E2 is represented by
the equation 8, from the equations 3, 5, and 7.
[0038]
Ideally, the level compression ratio becomes a constant value (n2) over the range of the input
signal voltage from E1 to E2.
However, whether or not a constant compression ratio can be obtained depends on the control
characteristic of the resistance value Rt between the emitter and the collector of the transistor 17
by the base voltage Vb.
[0039]
FIG. 3 shows an example of level compression characteristics in the case where the input signal
level has a level compression ratio of a constant value of 4: 1 in the range from the control start
level to a level 12 dB higher.
(B) Output Voltage Frequency Characteristic of Low Pass Filter 13 The output level compression
characteristic in the low frequency range of the low pass filter 13, ie, the pass band, is as
described above.
[0040]
Therefore, how the output voltage frequency characteristic in the entire frequency range of the
low pass filter 13 changes with the input signal voltage E will be described.
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[0041]
First, at E1 where the input signal voltage E is the control start voltage, Rx = R1. Therefore, the
output signal voltage VL1 is represented by Eq. 9 from Eq.
[0042]
As apparent from the equation 9, VL1 has an output voltage characteristic of a constant value of
E1 in the low frequency range satisfying ωC1R1 << 1, and at the frequency of ωC1R1 = 1, | VL1
| is E1 / √2 In other words, the frequency is lowered by 3 dB, and at higher frequencies, there is
a first-order Butterworth characteristic which decreases in proportion to the frequency.
The frequency at which ω C 1 R 1 = 1, ie, the cutoff frequency f L 1 is expressed by equation 10.
[0043]
Next, when the input signal voltage E becomes E2, Rx is expressed by Equation 4. Therefore, the
output signal voltage VL2 when the input signal voltage E is E2 is represented by Equation 11.
[0044]
As apparent from Equation 11, VL2 is one in which E2 is compressed to 1 / (1 + R1 / R0).
Further, the cutoff frequency fL2 is expressed by Equation 12, and is a frequency that is (1 + R1 /
R0) times fL1.
[0045]
The above is the case where the input signal voltage E is E1 and E2, but when the level
compression ratio takes a constant value of n: 1 in the range of E1 to E2, the equation 1 E × Rx /
Assuming that R1 is VL ', Equation 13 is derived from Equation 6, and the relationship of
Equation 14 is established.
[0046]
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If equation 14 is modified, equation 15 is obtained.
However, E is a range of E1 to E2.
[0047]
Equation 16 shows that an input signal voltage E can obtain an n: 1 level compression ratio
within the range of E1 to E2 if Rx changes according to the relationship of Equation 16.
[0048]
Therefore, VL / E1 obtained by scaling the output signal voltage VL with E1 is represented by Eq.
16 from Eq. 1, Eq. 14, and Eq.
However, E is a range of E1 to E2.
[0049]
With regard to the output voltage frequency characteristic represented by Eq. 16, when the level
compression characteristic is the characteristic shown in FIG. 3 and fL1 is 120 Hz, the
characteristic shown by the solid line in FIG. 4 is obtained.
This characteristic is shown for the case where the level increase from the control start voltage
(E1) of the input signal voltage E is 0 dB, +4 dB, +8 dB, +12 dB.
(C) Assuming that the capacitance value of the output voltage-frequency characteristic capacitor
19 of the high-pass filter 18 is C4 and the resistance values of the resistors 20 and 21 are R3
and R4, respectively, the output voltage VH of the high-pass filter 18 in FIG. It is represented by
17.
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[0050]
In Equation 17, E / (1 + R3 / R4) gives the output signal voltage in the high frequency range, ie,
the pass band.
Further, the inside of {} indicates the high-pass filter characteristic of the first-order Butterworth
characteristic, and the cutoff frequency fH is expressed by Equation 18.
[0051]
Therefore, VH / E1 is represented by Eq. 19 from Eq.
[0052]
(D) Conditions for Combining Output Signals of Low-Pass Filter 13 and High-Pass Filter 18 For
conditions for combining both output signals of low-pass filter 13 and high-pass filter 18, the
level control range of low-pass filter 13 In the above-described input signal voltage E2 giving an
upper limit, it is either synthesized so as to have a flat characteristic over the entire band, or
synthesized so as to have a low-pass characteristic.
The composite output voltage frequency characteristic at E2 is determined by how much
attenuation is given to the output signal voltage of the high-pass filter 18 and how much the
cutoff frequency is set.
[0053]
At predetermined input signal voltage Ex within the range of E1 to E2 with n: 1 level
compression ratio, the low pass filter 13 and the high pass filter 18 are first passed in order to
combine the composite output voltage frequency characteristics into flat characteristics. From
the condition of equalizing both output signal voltages in the band, the relationship between the
attenuation factor [1 / (1 + R3 / R4)] of the output signal voltage of high-pass filter 18 and Ex /
E1 becomes several 20 from several 16 and several 19 , Given by the number 21.
[0054]
Next, from the condition of equalizing the cut-off frequencies of the low pass filter 13 and the
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high pass filter 18, the cut off frequency fH of the high pass filter is given by
[0055]
Here, a specific example of the combined output voltage frequency characteristic will be
described below.
[0056]
First, with respect to the input signal voltage E2 giving the upper limit of the level control range,
a specific example in which the combined output voltage frequency characteristic becomes flat in
the entire band is as follows.
[0057]
Assuming that the output voltage characteristic of the low-pass filter 13 is a characteristic shown
by a solid line in FIG. 4, E2 / E1 = 4, and since n = 4, if Ex = E2 in Eq. 21, then Eq. The
predetermined attenuation of the output signal level of the high-pass filter 18 is about -9 dB.
[0058]
Next, for the cut-off frequency fH of the high-pass filter 18, since the cut-off frequency fL1 of the
low-pass filter 13 is 120 Hz, fH = 120 × 40.75 = 339.4 ≒ 339 Hz
[0059]
The characteristic shown by the broken line in FIG. 4 is the output voltage frequency
characteristic of the high-pass filter 18 under the conditions described above.
[0060]
Therefore, the combined output voltage frequency characteristic is as shown in FIG.
As apparent from FIG. 5, the combined output voltage frequency characteristic maintains the low
band compensation characteristic with the low band rising characteristic as shown in the figure
until the input signal voltage E reaches the control start voltage E1, and exceeds E1. The amount
of compensation is reduced, and the entire band flat characteristic is obtained at the upper limit
voltage E2 of the control range, and the input signal voltage E higher than E2 is automatically
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controlled to maintain the flat characteristic as it is.
[0061]
Next, in E2, a specific example of the low-range lowering characteristic is as follows.
[0062]
In order to make the low-pass characteristic in E2, the output signal voltage in the passband of
the high-pass filter 18 in E2 is higher by a required value than the output signal voltage in the
passband of the low-pass filter 13 The attenuation amount of the output signal level of the highpass filter 18 may be determined.
[0063]
As an example, when the output signal level in the pass band of high pass filter 18 is set to be 2
dB higher than the output signal level in the pass band of low pass filter 13 at E2, the output
signal level of high pass filter 18 The required attenuation may be approximately −7 dB, which
is 2 dB less than approximately −9 dB.
[0064]
In this case, it is desirable that the combined output voltage frequency characteristic changes so
as to become a low band lowering characteristic from a low band rising characteristic to a flat
characteristic as the input signal level increases.
Since the input signal voltage with this flat characteristic is smaller than E2, the cutoff frequency
fH of the high-pass filter 18 needs to be a frequency different from that in the case of the flat
characteristic with the combined output voltage frequency characteristic E2. .
[0065]
With respect to this fH, Ex / E1 in which the output voltage in the pass band of the low pass filter
13 and the high pass filter 18 becomes the same under the condition that the required
attenuation of the output signal level of the high pass filter 18 is -7.03 dB. It is sufficient to
obtain fH from Eq.
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[0066]
Therefore, taking the characteristic of the low-pass filter 13 shown by the solid line in FIG. 4 as
an example, since the antilogarithm of −7.03 dB is 0.445, Expression 24 is obtained from
Expression 21 and Ex / E1 is It becomes.
[0067]
Therefore, from the equation 22, fH is fH = 120 × 2.940.75 = 269.4 ≒ 270 (Hz)
[0068]
FIG. 6 shows the combined output voltage frequency characteristics under the above conditions,
with the input signal voltage E in the range of E1 to E2.
[0069]
According to the present invention, the output signal of the low pass filter is obtained by
electrically combining the output signals of the low pass filter and the high pass filter to which
the same audio signal is input. By adjusting the attenuation factor and cut-off frequency of the
high-pass filter in relation to the frequency characteristic, the amplitude frequency characteristic
when the input signal level becomes high can be arbitrarily set as the flat characteristic or lowpass characteristic (multiple steps), It is suitable for application to a small speaker system.
[0070]
Brief description of the drawings
[0071]
1 is a block diagram showing an embodiment of the bass compensation characteristic automatic
control circuit according to the present invention.
[0072]
2 is a diagram showing an equivalent circuit of the low pass filter.
[0073]
3 is a diagram showing an example of the level compression characteristics of the low pass filter.
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[0074]
4 is a diagram showing the output voltage frequency characteristics (scaled by E1) of the low
pass filter and the high pass filter.
[0075]
5 is a diagram showing a combined output voltage frequency characteristics (scaled by E1) of the
low pass filter and the high pass filter.
[0076]
6 is a diagram showing a combined output voltage frequency characteristics (scaled by E1) of the
low pass filter and the high pass filter.
[0077]
7 is a diagram showing a configuration example of a conventional speaker system.
[0078]
8 is a diagram showing the output voltage frequency characteristics of the conventional bass
compensation characteristics automatic control circuit.
[0079]
Explanation of sign
[0080]
11a, 11b Input terminal 12a, 12b Output terminal 13 Low pass filter 18 High pass filter 22
Synthesizer 24 Control signal generation circuit
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