JP2592047Y2 - Audio signal processing circuit - Google Patents

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 circuit suitable for an output stage of a portable audio device such as a headphone stereo.

[0002]

2. Description of the Related Art A headphone stereo is small and lightweight.
Because it is convenient to carry, it is used for language practice and music appreciation in trains and buses. In this case, 5-7 kHz
A so-called shaky sound due to the z-band frequency component leaks out of the headphones. For this reason, use on crowded trains and the like often gives discomfort to others.

[0003] Conventionally, in order to avoid discomfort to others, 5 to 7 kHz, which is a cause of shaky noise, is used.
One that has a mode to reduce and use the frequency component of the band has been proposed

However, in this mode, the frequency components in the 5 to 7 kHz band are uniformly reduced regardless of the volume level. In other words, even when listening at a low volume that does not seem to leak the shaky sound from the headphones, 5
Frequency components in the band of up to 7 kHz are greatly reduced. Even if not described above, the reduction of the frequency component in the 5 to 7 kHz band may cause deterioration of the reproduced sound.

[0004] In view of the above, it is an object of the present invention to efficiently prevent the leakage of a whispering sound from headphones.

[0005]

SUMMARY OF THE INVENTION The present invention provides a filter means for reducing a predetermined frequency component of an audio signal, an amplifier circuit for amplifying the audio signal, a rectifier circuit for rectifying an output signal of the amplifier circuit, and an output of the rectifier circuit. A level detecting means for detecting the level of the audio signal, comprising a smoothing circuit for smoothing the signal, and controlling a reduction amount of a predetermined frequency component in the filter means in accordance with a detection output of the level detecting means. It is.

[0006]

According to the filter means, for example, a frequency component causing a shaky noise is 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 of the audio signal is low, the amount of reduction is reduced. Therefore, when the level of the audio signal is low and it is unlikely that the sound is leaking from the headphones, the deterioration of the reproduced sound can be suppressed to a small level.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. This example is an example applied to an output stage of a headphone stereo. Since the left and right channels are similarly configured, only the left channel will be described.

In FIG. 1, a left audio signal SL reproduced from a magnetic tape (not shown) includes a DC cut capacitor 1, a volume control volume 2 and a power amplifier 3.
To the output terminal 4 through the series circuit of.

The connection point P between the capacitor 1 and the volume 2 has a frequency of 5 to 7 kHz, which causes shaky noise.
A filter circuit 10 is connected to bypass and reduce the z-band frequency component to the ground side.

That is, the connection point P is connected to the capacitor 11, N
It is grounded via the series circuit of the collector and emitter of the PN transistor 12 and the resistor 13. The connection point between the emitter of transistor 12 and resistor 13 is connected to the base of NPN transistor 16 via a series circuit of capacitors 14 and 15. The connection point between the capacitors 14 and 15 is connected to the transistor 16 via the resistor 17.
Connected to the emitter. The emitter of the transistor 16 is grounded via a resistor 18, and its collector is connected to a power supply terminal Vcc and to its base via a biasing resistor 19.

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, the frequency components in the 5 to 7 kHz band are configured to be bypassed to the ground side.

A connection point Q between the power amplifier 3 and the output terminal 4 is connected to a resistor 20 and a connection switch 2 for reducing operation.
1 and a base of a NPN transistor 23 through a series circuit of a DC cut capacitor 22.
The emitter of the transistor 23 is grounded, and the collector is connected to the power supply terminal Vcc via a resistor 24 and to the base via a resistor 25 for bias.

The collector of the transistor 23 is grounded through a series circuit of a cathode and an anode of a capacitor 32 and a diode 32 constituting a rectifying / 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 / 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.

In the above configuration, first, the connection switch 2
The operation when 1 is in the ON state will be described. Output terminal 4
When the level of the audio signal at the (connection point Q) is high (when the volume is high), the level of the DC signal obtained at the output terminal R of the rectifying / smoothing circuit 30 increases. Therefore, the impedance of the transistor 12 of the filter circuit 10 is reduced, and the ratio of the frequency component in the 5 to 7 kHz band that is 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 low (at a low volume), the level of the DC signal obtained at the output terminal R of the rectifying / smoothing circuit 30 is low. Therefore, the impedance of the transistor 12 of the filter circuit 10 increases, and the impedance of the transistor 12 bypassed to the ground side through the filter circuit 10 is increased.
The proportion of the frequency component in the band up to 7 kHz is reduced, and the reduction amount of the frequency component is reduced.

Next, the operation when the connection switch 21 is off will be described. At this time, the output terminal R of the rectifying / 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, 5 to 7k
The frequency component in the Hz band is not bypassed to the ground side through the filter circuit 10.

FIG. 2 shows the relationship between the magnitude of the sound volume and the amount of reduction of the frequency components in the 5 to 7 kHz band. In the figure, solid lines a and b represent frequency spectra when the connection switch 21 is turned off and when the volume is high and when the volume is low, respectively. On the other hand, broken lines a 'and b' indicate the frequency spectrums when the connection switch 21 is turned on and when the volume is high and when the volume is low, respectively. As is clear from this figure, the amount of reduction of the frequency components in the 5 to 7 kHz band increases as the volume increases.

According to this embodiment, the amount of reduction of the frequency components in the 5 to 7 kHz band is increased when the sound volume is large, so that the amount of reduction is small when the sound volume is small, for example, when the sound is unlikely to leak from the headphones. Deterioration of reproduced sound can be suppressed to a small level.

Note that the filter circuit 1 in the above embodiment is
The configuration of 0 or the configuration of the transistor 23 for detecting the audio signal level and the rectifying / smoothing circuit 30 is an example, and it is a matter of course that the configuration is not limited to this.

In the above embodiment, the present invention is applied to the output stage of a headphone stereo.
The present invention can be similarly applied to an output stage of a portable CD player, a radio receiver, and the like.

[0020]

According to the present invention, the amount of reduction of the specific frequency component of the audio signal can be controlled according to the level of the audio signal. For example, the reduction amount of the frequency component in the 5 to 7 kHz band can be increased as the volume increases, so that the reduction amount decreases when the volume of the sound is not likely to leak from the headphones and the deterioration of the reproduced sound can be suppressed to a small extent. . Further, since the level detecting means includes an amplifier circuit for amplifying the audio signal, the amount of reduction of the predetermined frequency component in the filter means can be favorably controlled by the detection output without being affected by the voltage drop of the diode or the like in the rectifying circuit.

[Brief description of the drawings]

FIG. 1 is a system diagram showing a configuration of an embodiment.

FIG. 2 is a diagram illustrating a relationship between a loudness of a sound volume and a reduction amount.

[Explanation of symbols]

 2 Volume control volume 4 Output terminal 10 Filter circuit 21 Connection switch for reduction operation 30 Rectification smoothing circuit

Continuation of front page (56) References JP-A-3-247098 (JP, A) JP-A-3-214999 (JP, A) JP-A-3-297209 (JP, A) JP-A-49-84347 (JP) , A) JP-A-53-33559 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G11B 20/00 G11B 5/00 H04R 3/00 H03G 5/02 H03H 7/12

Claims (1)

(57) [Scope of request for utility model registration] 1. A filter means for reducing a predetermined frequency component of an audio signal, an amplifier circuit for amplifying the audio signal, a rectifier circuit for rectifying an output signal of the amplifier circuit, and a smoothing circuit for smoothing an output signal of the rectifier circuit. And a level detecting means for detecting a level of the audio signal, wherein the amount of reduction of the predetermined frequency component in the filter means is controlled in accordance with a detection output of the level detecting means. circuit.