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JPH04105859

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DESCRIPTION JPH04105859
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
audio signal processing circuit suitable for application to the output stage of portable audio
equipment such as headphone stereo.
[0002]
2. Description of the Related Art A headphone stereo is small and light and convenient for
carrying, so it is used for language practice on a train or a bus, music appreciation, and the like.
In this case, so-called squeaky sounds due to frequency components in the 5 to 7 kHz band leak
to the outside from the headphones. Therefore, use in a crowded train or the like often made
others uncomfortable.
[0003]
[0003] Therefore, in order to prevent others from feeling uncomfortable, it has been proposed to
provide a mode in which the frequency component of the 5 to 7 kHz band, which causes the
buzzing sound, is reduced and used. However, in this mode, frequency components in the 5 to 7
kHz band are uniformly reduced regardless of the volume. That is, the frequency component of
the 5 to 7 kHz band is greatly reduced even when listening at a small volume where it is unlikely
that leakage will occur from headphones. Although not described above, the reduction of
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frequency components in the 5 to 7 kHz band may lead to deterioration of the reproduced sound.
[0004]
Therefore, in the present invention, it is an object of the present invention to efficiently prevent
the leak of the squeaky sound from the headphones.
[0005]
SUMMARY OF THE INVENTION The present invention comprises filter means for reducing a
predetermined frequency component of an audio signal, and level detection means for detecting
the level of the audio signal, and the detection output of the level detection means is provided.
Accordingly, the reduction amount of the predetermined frequency component in the filter
means is controlled.
[0006]
By means of the filter means 10, for example, the frequency components which cause the
buzzing noise are reduced.
The amount of reduction is controlled according to the level of the audio signal. For example,
when the level of the audio signal is high, the amount of reduction is large, and when the level is
low, the amount of reduction is small.
Therefore, when the level of the audio signal is low and it is not likely that leakage will occur
from the headphones, degradation of the reproduced sound can be suppressed to a low level.
[0007]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present
invention will be described below with reference to FIG. This example is an example applied to
the output stage of headphone stereo. The left channel is described only because it is configured
similarly for the left and right channels.
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[0008]
In the figure, the left audio signal SL reproduced from a magnetic tape (not shown) is connected
to the output terminal 4 via a series circuit of a capacitor 1 for DC cut, a volume 2 for volume
control and a power amplifier 3. It is derived.
[0009]
A filter circuit 10 is connected to the connection point P between the capacitor 1 and the volume
2 for bypassing to the ground side a frequency component in the 5 to 7 kHz band that causes a
rattling noise and reducing it.
[0010]
That is, the connection point P is grounded via a series circuit of the capacitor 11, the collector /
emitter of the NPN transistor 12 and the resistor 13.
The connection point of the emitter of the transistor 12 and the resistor 13 is connected to the
base of the NPN transistor 16 via a series circuit of capacitors 14 and 15.
The junction of capacitors 14 and 15 is connected via resistor 17 to the emitter of transistor 16.
The emitter of the transistor 16 is grounded via a resistor 18, and its collector is connected to the
power supply terminal Vcc and to its base via a biasing resistor 19.
[0011]
In the filter circuit 10, the frequency is determined by the capacitors 11, 14, 15, and the resistors
17, 19. In this example, by adjusting the values of these capacitors and resistors, frequency
components in the 5 to 7 kHz band are bypassed to the ground side.
[0012]
Further, the connection point Q of the power amplifier 3 and the output terminal 4 is connected
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to the base of the NPN transistor 23 through a series circuit of a resistor 20, a connection switch
21 for reduction operation and a capacitor 22 for DC cut. Be done. The emitter of the transistor
23 is grounded, and its collector is connected to the power supply terminal Vcc via the resistor
24 and to its base via the bias resistor 25.
[0013]
The collector of the transistor 23 is grounded via a series circuit of a capacitor 31 and a cathodeanode of the diode 32 constituting the rectifying and smoothing circuit 30. The connection point
of the capacitor 31 and the diode 32 is grounded via a series circuit of the anode / cathode of the
diode 33 and the capacitor 34. The output terminal of the rectifying and smoothing circuit 30,
that is, the connection point R of the diode 33 and the capacitor 34 is connected to the base of
the transistor 12 of the filter circuit 10 via a series circuit of the diode 41 and the resistor 42.
[0014]
In the above configuration, the operation when the connection switch 21 is in the on state will be
described first. When the level of the audio signal at the output terminal 4 (connection point Q) is
large (at the time of large volume), the level of the DC signal obtained at the output terminal R of
the rectifying and smoothing circuit 30 becomes large. Therefore, the impedance of the
transistor 12 of the filter circuit 10 is lowered, the ratio of the frequency component of the 5 to 7
kHz band bypassed to the ground side through the filter circuit 10 is increased, and the
reduction amount of the frequency component is increased. On the other hand, when the level of
the audio signal at the output terminal 4 is small (at the time of small volume), the level of the DC
signal obtained at the output terminal R of the rectifying and smoothing circuit 30 becomes
smaller. Therefore, the impedance of the transistor 12 of the filter circuit 10 is increased, the
ratio of frequency components in the 5 to 7 kHz band bypassed to the ground side through the
filter circuit 10 is reduced, and the reduction amount of the frequency component is reduced.
[0015]
Next, the operation when the connection switch 21 is in the off state will be described. At this
time, the output terminal R of the rectifying and smoothing circuit 30 is at the ground level, and
the transistor 12 of the filter circuit 10 is turned off. Therefore, regardless of the level of the
audio signal at the output terminal 4, frequency components in the 5 to 7 kHz band will not be
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bypassed to the ground side through the filter circuit 10.
[0016]
FIG. 2 shows the relationship between the magnitude of the sound volume and the amount of
reduction of frequency components in the 5 to 7 kHz band. In the figure, solid lines a and b
respectively indicate frequency spectrums at high volume and low volume when the connection
switch 21 is turned off. On the other hand, broken lines a 'and b' indicate frequency spectrums at
high volume and low volume, respectively, when the connection switch 21 is turned on. As
apparent from this figure, the amount of reduction of frequency components in the 5 to 7 kHz
band increases as the volume increases.
[0017]
According to the present embodiment, the amount of reduction of frequency components in the
5 to 7 kHz band increases as the volume increases, so the amount of reduction decreases, for
example, when the volume is low enough that leaks are unlikely to occur compared to
headphones. Deterioration of reproduced sound can be suppressed to a low level.
[0018]
The configuration of the filter circuit 10 or the configuration of the transistor 23 for detecting
the audio signal level and the configuration of the rectifying / smoothing circuit 30 in the above
embodiment are merely an example, and it is a matter of course that the present invention is not
limited thereto.
[0019]
In the above embodiment, this invention is applied to the output stage of the headphone stereo,
but the invention can be applied to the output stage of a portable CD player, a radio receiver or
the like as well.
[0020]
According to the present invention, the amount of reduction of the specific frequency component
of the audio signal can be controlled in accordance with the level of the audio signal.
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For example, the amount of reduction of frequency components in the 5 to 7 kHz band can be
increased as high as at the time of high volume, thereby reducing the amount of reduction when
the volume is low enough that leaks are unlikely to leak from headphones. .
[0021]
Brief description of the drawings
[0022]
1 is a system diagram showing the configuration of the embodiment.
[0023]
2 is a diagram showing the relationship between the magnitude of the volume and the amount of
reduction.
[0024]
Explanation of sign
[0025]
2 Volume control 4 Volume adjustment output terminal 10 Filter circuit 21 Connection switch
for reduction operation 30 Rectification smoothing circuit
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