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DESCRIPTION JP2011035661

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DESCRIPTION JP2011035661
The present invention relates to switching of an audio signal switching means in an audio
reproduction system having a configuration in which reproduction is performed by an
electroacoustic transducer via a low cut filter for blocking the low frequency range of the audio
signal after passing through audio signal switching means. Reduce the inherent noise generated.
A variable low cut filter (22) is disposed upstream of an audio signal switching means (12). When
audio signal switching is not performed, the cutoff frequency of the variable low cut filter 22 is
set to a low value, and the entire band of the audio signal output from the audio source 10 is
allowed to pass. When audio signal switching is performed, the cutoff frequency of the variable
low cut filter 22 is temporarily changed to a high value before the switching is performed, and is
returned to the original low value after the switching is completed. [Selected figure] Figure 1
Audio signal switching noise reduction circuit
[0001]
The present invention has a configuration in which an audio signal such as music is reproduced
by an electroacoustic transducer via a low cut filter (low frequency removal filter) for blocking
the low frequency range of the audio signal after passing through the audio signal switching
means. The present invention relates to an audio signal switching noise reduction circuit applied
to an audio reproduction system, in which noise generated uniquely with switching of audio
signal switching means is reduced.
[0002]
10-04-2019
1
In an audio reproduction system that reproduces audio such as music with a small speaker, if an
audio signal including a low frequency band is supplied to the speaker as it is, the low frequency
band does not contribute to reproduction, and the diaphragm of the speaker operates with a
large amplitude For this reason, the movable range of the diaphragm may be narrowed with
respect to the middle range or more contributing to reproduction, or the excessive amplitude of
the diaphragm may cause damage to the diaphragm and the drive signal line.
Therefore, in such an audio reproduction system, the amplification band of the amplifier is
limited so that a signal having a frequency lower than the low frequency reproduction capability
of the speaker is not supplied to the speaker. FIG. 2 shows a conventional audio reproduction
system in which the low band is band limited. The audio signal supplied from the audio source
10 such as a storage / recording medium reproducing apparatus, a television or radio tuner
passes through the audio signal switching means 12 and the low cut filter 14 removes
components not contributing to the reproduced sound, for example, components lower than 1
kHz. The signal is amplified by the amplifier 16 and reproduced by the speaker 18. The audio
signal switching unit 12 is, for example, an audio source switching switch, an audio signal cutoff
switch (such as a mute switch), a volume that gradually switches the volume of the audio signal,
etc. Switching of the audio signal (switching of the source type) by the switching instruction
means 20 such as switching instruction signal output means for automatically outputting the
switching instruction signal regardless of the switching operation of the Switching, switching the
sound on / off, etc. The low cut filter 14 is composed of a capacitor C1 and a resistor R1.
[0003]
JP 2007-259324 A JP JP 2001-203581 A JP JP 7-30497 A JP JP 4-78208 A
[0004]
In the audio reproduction system of the configuration of FIG. 2, the voltage Vc across the
capacitor C1 of the low cut filter 14 fluctuates according to the frequency and the amplitude of
the audio signal supplied from the audio source 10.
That is, for an audio signal below the cutoff frequency of the low cut filter 14, the voltage Vc
increases as the frequency of the audio signal decreases. FIG. 3 shows the voltage when the audio
signal switching means 12 is switched when the audio signal of the low cut filter 14 or less is
supplied from the audio source 10 and the audio signal is cut off by the audio signal switching
means 12. Indicates a waveform. (A) is an audio signal supplied from the audio source 10, and (b)
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2
is a potential at the connection point P of the capacitor C1 and the resistor R1 (voltage across the
resistor R1). Since the voltage Vc across the capacitor C1 is applied to the resistor R1 when the
audio signal is cut off by the audio signal switching means 12, the potential at the connection
point P of the capacitor C1 and the resistor R1 drops to -Vc. Thereafter, the charge of the
capacitor C1 is discharged through the resistor R1, so the potential at the connection point P of
the capacitor C1 and the resistor R1 returns to 0 V with a time constant of R1 · C1.
[0005]
As described above, in the audio reproduction system of FIG. 2, when the audio signal switching
means 12 is switched, the charge stored in the low cut filter 14 is discharged to the potential of
the connection point P of the capacitor C1 and the resistor R1. Undershoot occurs. Since the
frequency component of the undershoot portion is higher than the audio signal supplied from
the audio source 10, the abnormal sound is easily detected by the ear that sounds "petite" which
is not originally included in the audio signal. (Pety noise) is uttered from the speaker 18.
[0006]
The above has described the case where the audio signal switching means 12 cuts off the audio
signal, but the same phenomenon occurs when the audio signal switching means 12 is a volume
that switches the volume of the audio signal stepwise (stepwise), While changing the volume,
abnormal noises occur continuously as "puchipuchi ...". For example, in a mobile phone in which
the cut-off frequency of the low cut filter 14 is set to 1 kHz so that 1 kHz or less can not be
reproduced, when the audio signal of 100 Hz is supplied from the audio source 10, the audio
signal is removed by the low cut filter 14 When the volume is adjusted by operating the audio
signal switching means (volume) 12 in which the volume is switched stepwise in this case, the
stepwise switching of the volume is the connection of the capacitor C1 and the resistor R1. The
potential at the point P is gradually changed, and the connection point P is undershooted each
time the change is made. Therefore, while adjusting the volume, a petit noise of 1 kHz or more is
heard. In addition, there existed a technique of patent document 1-4 as a prior art which
prevented generation | occurrence | production of the noise accompanying switching of an audio
| voice signal.
[0007]
The present invention has been made in view of the above-described point, and an audio
reproduction having a configuration in which reproduction is performed by an electroacoustic
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transducer via a low cut filter for blocking the low frequency range of the audio signal after
passing through audio signal switching means. It is an object of the present invention to provide
an audio signal switching noise reduction circuit in which noise generated inherently with
switching of the audio signal switching means is reduced in a system.
[0008]
The present invention supplies an audio signal to the electroacoustic transducer via the audio
signal switching means which causes a step in the output signal waveform according to the
signal switching operation and further via the low cut filter which cuts off the low frequency
range of the audio signal. And an audio signal switching noise reduction circuit applied to an
audio reproduction system including switching instruction means for instructing switching of the
audio signal switching means, the noise reduction circuit being disposed on the front side of the
audio signal switching means A variable low-cut filter having a variable cutoff frequency, and the
low-cut filter having a cutoff frequency of the variable low-cut filter disposed downstream of the
variable low-cut filter when no switching instruction is issued from the switching instruction
means Lower than the cut-off frequency of the filter (which may be abbreviated as “following
side low cut filter”) (The cutoff frequency is set to 0 Hz (in other words, it is not a low-cut filter)
but all bands are allowed to pass), and when the switching instruction is issued, the switching
instruction is issued for the cutoff frequency of the variable low-cut filter. Then, the switching
instruction of the audio signal switching means is performed according to the switching
instruction, and the switching frequency of the variable low cut filter is changed after the
switching of the audio signal switching means is completed. And a control circuit that executes
control to return to the state before being output.
According to this, when a switching instruction is issued from the switching instruction means,
the low frequency component of the input signal of the audio signal switching means is
attenuated by the variable low cut filter prior to switching the audio signal switching means. The
voltage across the low cut filter capacitor is low. Therefore, when the audio signal switching
means is switched, undershoot or overshoot is less likely to occur, and the generation of
abnormal noise is suppressed. The cutoff frequency of the variable low cut filter is, for example, a
value that allows the entire band of the audio signal to pass when no switching instruction is
issued from the switching instruction means, and when the switching instruction is issued, the
rear end side It can be a value close to the cutoff frequency of the low cut filter.
[0009]
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In the present invention, the cutoff frequency of the variable low cut filter can be changed, for
example, by changing the time constant of the variable low cut filter while inserting a capacitor
in the audio signal path of the variable low cut filter. According to this, it is possible to prevent
the occurrence of a steep level difference in the output signal waveform of the variable low cut
filter, and to suppress the generation of new noise in the low cut filter on the rear stage side with
the change of the passband of the variable low cut filter. Can.
[0010]
In the present invention, the audio signal switching means may be, for example, any of an audio
source switching switch, an audio signal cutoff switch, and a volume for stepwise switching the
volume of the audio signal. The switching instruction means may be, for example, an operator
that is switched by the operator. The control circuit may control, for example, the cutoff
frequency of the variable low cut filter when the switching instruction is issued, in a band
substantially equal to the cutoff frequency of the low cut filter on the rear stage side. The audio
reproduction system is mounted on a mobile phone, and the low-cut filter of the rear-stage side
limits the low pass band of the audio signal to a band of approximately 1 kHz or more and the
electro-acoustic conversion mounted on the mobile phone When the voice is to be uttered from a
small speaker that constitutes a speaker, the control circuit sets the cut-off frequency of the
variable low cut filter to approximately 1 kHz when the switching instruction is issued. be able to.
[0011]
FIG. 1 is a block diagram showing an embodiment of the present invention. It is a block diagram
which shows the conventional audio reproduction system which band-limited the low region. FIG.
5 is a waveform chart showing an operation when the audio signal switching means 12 is
switched in the audio reproduction system of FIG. 2; It is a circuit diagram which shows the
structural example of the audio | voice signal switching means 12 of FIG. It is a circuit diagram
which shows the structural example of the variable low cut filter 22 of FIG. It is a characteristic
view which shows the example of band setting of the variable low cut filter 22 of FIG. It is a
timing chart which shows operation | movement when switching operation of the audio | voice
signal switching means 12 of FIG. 1 is performed. It is a block diagram which shows the
application example of the audio reproduction system | strain of FIG. It is a block diagram which
shows the other application example of the audio reproduction system | strain of FIG.
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[0012]
An embodiment of the present invention is shown in FIG. The same reference numerals as in FIG.
2 denote the same parts in FIG. An audio signal supplied from an audio source 10 such as a
storage / recording medium reproducing apparatus, a television or radio tuner passes through a
variable low cut filter (variable high pass filter) 22 and an audio signal switching means 12 and
does not contribute to reproduced sound by the low cut filter 14 A component lower than 1 kHz
is removed, amplified by the power amplifier 16 and reproduced by the speaker 18. The variable
low cut filter 22 is a low cut filter whose cut-off frequency is variably controlled, and allows the
audio signal supplied from the audio source 10 to pass the low pass band variably. The audio
signal switching unit 12 is, for example, an audio source switching switch, an audio signal cutoff
switch (such as a mute switch), a volume that gradually switches the volume of the audio signal,
etc. Switching, etc.). The low cut filter 14 is composed of a capacitor C1 and a resistor R1. The
switching instruction means 20 is a switching instruction for automatically outputting a
switching instruction signal regardless of a switching operation (a source switching operation, a
muting operation, a volume switching operation, etc.) operated by the operator or a switching
operation by a person. It is comprised as a signal output means etc. The control circuit 24
switches and controls the audio signal switching unit 12 based on the switching instruction of
the switching instruction unit 20 to switch the audio signal. The control circuit 24 variably
controls the cutoff frequency of the variable low cut filter 22 based on the switching instruction
of the switching instruction means 20.
[0013]
A configuration example of the audio signal switching means 12 is shown in FIG. Each of the
audio signal switching means 12 in FIGS. 4A to 4D switches the audio signal by switching the
audio signal path. That is, the audio signal switching means 12 of (a) constitutes an audio source
switching switch. Audio signals from a plurality of audio sources are input to the audio source
switching switch 12, and audio signals from a single audio source selected by the source
switching operation device or the like are selectively output by switching control of the control
circuit 24. A variable low cut filter (not shown) according to the present invention is disposed at
a position before the audio signal switching means 12 in the signal path of each audio source.
The variable low cut filter of the signal path of each audio source temporarily reduces the cutoff
frequency all at once (or for variable low cut filters of two audio sources before and after
switching) before and after the switching of the audio signal switching means 12 Be done. The
audio signal switching means 12 of (b) constitutes an audio signal cutoff switch. The audio signal
from the audio source 10 is input to the audio signal blocking switch 12 and is passed / blocked
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by the switching control of the control circuit 24 in response to the on / off operation of the
mute operator. The audio signal switching means 12 of (c) constitutes an electronic volume that
switches the volume of the audio signal in stages. The audio signal from the audio source 10 is
input to the resistors of the plurality of stages constituting the electronic volume 12 and switches
the resistors of the plurality of stages by the switching control of the control circuit 24 in
accordance with the volume switching operation of the volume switching operator. By switching
by the analog switch, the volume of the audio signal from the audio source 10 is switched
stepwise.
[0014]
A configuration example of the variable low cut filter 22 is shown in FIG. The variable low cut
filter 22 is composed of a capacitor C2 and series connected resistors R2 and R3. The resistance
value is R2 << R3 (e.g., R2: R3 = 1: 100). Connected to both ends of the resistor R3 is a switch S1
which shorts the both ends. The switch S1 is turned on / off by the control circuit 24. When the
switch S1 is turned off, the variable low cut filter 22 becomes a low cut filter by the capacitor C2
and the resistors R2 + R3, and the cut-off frequency becomes low. The cutoff frequency can be
set to, for example, a value that allows the entire band to pass as it is without band limiting the
band (20 Hz to 20 kHz for CD-DA sound source) of the audio signal supplied from the audio
source 10. When the switching of the audio signal switching means 12 is not performed, this
state is maintained. On the other hand, when the switch S1 is turned on, a low cut filter is formed
by the capacitor C2 and the resistor R2, and the cutoff frequency becomes high. When switching
the audio signal switching means 12, the state is temporarily shifted to this state. When shifting
this cutoff frequency, the capacitor C2 remains in the audio signal path, so that a large
undershoot does not occur with the switching of the switch S1, and no new petit noise as
perceived by the ear occurs.
[0015]
An example of band setting of the variable low cut filter 22 is shown in FIG. (A) is a reproduction
band of the speaker 18. Here, it is assumed that the entire audio reproduction system of FIG. 1 is
installed in a mobile phone, and the speaker 18 is compact and can reproduce a band of 1 kHz or
more. (B) is a band of an audio signal supplied from the audio source 10, and is, for example, 20
Hz to 20 kHz. (C) shows the pass band of the low cut filter 14 on the post stage side. It is set to
pass a band of 1 kHz or more in accordance with the reproduction band of the speaker 18. (D)
shows the pass band of the variable low cut filter 22 when the switch S1 is turned off. In order to
pass the band of the audio signal supplied from the audio source 10 as it is, the band of 20 Hz
10-04-2019
7
(or lower frequency) or more is set to pass. (E) shows the pass band of the variable low cut filter
22 when the switch S1 is turned on. It is set to pass a band of 1 kHz or more. In this setting, the
low band of the audio signal supplied from the audio source 10 is removed. The cutoff frequency
of the variable low cut filter 22 when the switch S1 is turned on does not have to be strictly
matched to the cutoff frequency of the low cut filter 14 on the rear stage side, and the noise
reduction effect and the change in timbre when switching The balance can be set. For example, if
both cutoff frequencies are set to be approximately equal, it is possible to reduce changes in
timbre at the time of switching while securing an abnormal noise reduction effect.
[0016]
Next, the variable control of the cutoff frequency of the variable low cut filter 22 and the
switching control of the audio signal switching means 12 by the control circuit 24 will be
described. FIG. 7 shows the operation when switching of the audio signal switching means 12 is
instructed by the switching operation of the switching instruction means 20. Before the
switching instruction means 20 is operated, the switch S1 is turned off, and the cutoff frequency
of the variable low cut filter 22 is set to a low frequency. Therefore, the audio signal supplied
from the audio source 10 passes through the audio signal switching means 12 in the same band
(without band limitation). At this time, the switching state of the audio signal switching means 12
is referred to as state A. When the switching instruction means 20 is switched and the switching
instruction signal is outputted from the switching instruction means 20, the switch S1 is turned
on immediately, and the cutoff frequency of the variable low cut filter 22 is shifted up to a high
frequency. As a result, the voltage Vc at both ends of the capacitor C1 of the low cut filter 14 on
the rear stage side decreases to near 0V. Thereafter, after a predetermined time T1 has passed
since the switching operation, the audio signal switching means 12 is switched from the state A
up to now to the state B according to the operation content of the switching instruction means
20. At this time, since the voltage Vc at both ends of the capacitor C1 drops close to 0 V, there is
no significant change in the potential at the connection point P of the capacitor C1 of the low cut
filter 14 and the resistor R1 even if the audio signal switching means 12 is switched. Therefore,
the large undershoot does not occur at the connection point P. Therefore, no petit noise as
perceived by the ear occurs. The time T1 can be set to, for example, a time longer than the time
constant of the low-cut filter 14 on the downstream side. After that, when the switch S1 is turned
on and when a predetermined time T2 (a timing after the switching of the audio signal switching
means 12 is finished) elapses, the switch S1 is turned off and the cutoff frequency of the variable
low cut filter 22 is the original. Shifted down to lower frequencies. As a result, the switched audio
signal passes through the audio signal switching means 12 while maintaining the band supplied
from the audio source 10.
10-04-2019
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[0017]
An application example of the audio reproduction system of FIG. 1 is shown in FIG. In this audio
reproduction system, the switch S2 is disposed between the audio signal switching means 12 and
the low cut filter 14 on the rear stage side in the audio reproduction system of FIG. 1 so that
reproduction by the speaker 18 and reproduction by the headphone 26 can be switched. It is a
thing. The switch S2 is switched by the switching operation of the user. When reproducing with
the headphone 26, the audio signal output from the audio signal switching means 12 is amplified
by the headphone amplifier 28 through the switch S2, and supplied to the headphone 26
through the headphone jack 30 for reproduction. Since the reproduction band of the headphones
26 extends to a low band, a low cut filter corresponding to the low cut filter 14 disposed in the
reproduction signal path of the speaker 18 is not disposed in the reproduction signal path of the
headphones 26. Therefore, even if the audio signal is switched by the audio signal switching
means 12, no undershoot occurs in the audio signal, and no petit noise as perceived by the ear
occurs. The audio signal supplied from the audio source 10 is reproduced by the headphone 26
in the same band. Even when the audio signal is switched by the audio signal switching means 12
and the cutoff frequency of the variable low cut filter 22 is temporarily shifted up, the audio
signal switching is completed. For example, since the cutoff frequency is immediately shifted
down to the original frequency, Hi-Fi reproduction is maintained.
[0018]
Another application of the audio reproduction system of FIG. 1 is shown in FIG. In this audio
reproduction system, the speaker 18 of FIG. 1 is a tweeter, and a woofer is separately added to
constitute a two-way speaker system. A signal path 32 is branched from between the audio signal
switching means 12 and the low cut filter 14 (high pass filter) on the rear stage side. The lowpass component of the audio signal supplied to the signal path 32 is extracted by the low pass
filter 34 formed of the resistor R 4 and the capacitor C 4, amplified by the power amplifier 36,
and reproduced by the woofer 38. The lower limit of the pass band of the low cut filter 14 and
the upper limit of the reproduction band of the low pass filter 34 cross over each other. Since the
low pass filter 34 is disposed in the reproduction signal path of the woofer 38, no petit noise is
generated even if the audio signal switching means 12 switches the audio signal. Further, even if
the switching of the audio signal is performed by the audio signal switching means 12 and the
cutoff frequency of the variable low cut filter 22 is temporarily shifted up, the cutoff frequency is
immediately set to the original once the switching of the audio signal is completed. The low
frequency component contained in the original sound supplied from the audio source 10 is
supplied as it is to the reproduction signal path of the woofer 38, so that the Hi-Fi reproduction is
secured.
10-04-2019
9
[0019]
In the above embodiment, the cutoff frequency of the variable low-cut filter 22 is switched in two
steps: a low frequency that is always used when audio signal switching is not performed, and a
high frequency that is temporarily used when audio signal switching is performed. However, you
can switch to more stages. For example, in addition to the above two levels of cutoff frequency,
prepare a cutoff frequency of 0 Hz (that is, pass the audio signal as it is), hold the cutoff
frequency at 0 Hz when not switching audio signals, and switch audio signals When switching to
a higher cutoff frequency via a lower (intermediate) cutoff frequency, and after audio signal
switching is completed, the cutoff frequency is returned to a lower (intermediate) cutoff
frequency to a 0 Hz cutoff frequency. Frequency switching can be performed.
[0020]
Although the variable low cut filter 22 is configured by an analog filter in the above embodiment,
it can be configured by a digital filter when disposed in an audio reproduction system configured
by a digital circuit. Further, although the variable low cut filter 22 is provided only for the
application of the present invention in the above embodiment, when it is necessary to arrange
the filter at the position for other applications, the filter may be used for both applications. The
characteristics of the filter can be temporarily changed and used for the application of the
present invention at the time of audio signal switching.
[0021]
The sound reproduction system shown in the above-described embodiment may be integrated
into a single device, or may be divided into a plurality of devices. When divided and configured,
for example, in the audio reproduction system of FIG. 1, the audio source 10 to the audio signal
switching means 12 are incorporated in one device, and the low cut filter 14 to the speaker 18
are incorporated in another device. Can be connected to each other by a signal cable.
[0022]
12: audio signal switching means, 14: low cut filter (low cut filter disposed downstream of the
variable low cut filter, low cut filter on the subsequent stage), 18: electroacoustic transducer
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(speaker), 20: switching instruction means, 22 ... Variable low cut filter, 24 ... control circuit
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