JPH03768Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a circuit for reducing noise in a stereo receiver, particularly an audio output signal, such as an FM stereo receiver.

Background Art and Its Problems For example, in an FM receiver, when the input electric field becomes weak, the S/N of the audio output signal deteriorates rapidly. Therefore, in cases such as mobile reception where the input electric field fluctuates rapidly, the quality of the audio output signal is significantly degraded. Furthermore, when a stereo receiver uses stereo, the noise level increases significantly compared to when monaural. Therefore, in a stereo receiver, when the signal electric field strength decreases, the high frequency components of the left and right audio output signals are cut off to suppress the noise level, or the high frequency components of the left and right audio output signals are mixed in the stereo receiver. It is required to calibrate the noise by canceling the in-phase noise components.

Purpose of the invention In view of this point, the present invention aims to simultaneously and continuously change the high-frequency cutoff characteristics and high-frequency mixing characteristics of the left and right audio output signals using a single control voltage source using a simple circuit. This makes it possible to reduce noise when the input electric field is weak or during stereo.

Summary of the invention In the invention, the emitter of a first transistor whose collector is grounded is connected to a first input terminal via a first capacitive element and a first resistor, and a second transistor whose collector is grounded is connected to a first input terminal via a first capacitive element and a first resistor. The emitter of the transistor is connected to a second input terminal via a second capacitor and a second resistor, and the base of each of the first and second transistors is connected to a control voltage source via a third resistor. , the first output terminal is derived from the connection point between the first capacitive element and the first resistor, and the second output terminal is derived from the connection point between the second capacitive element and the second resistor. A stereo demodulation circuit includes a high-frequency cut-off mixing circuit which is derived and supplies a power supply voltage to the emitters of the first and second transistors through fourth and fifth resistors that are larger than the first and second resistors, respectively. is provided on the output side of the stereo demodulation circuit, supplies the left and right audio signals of the output of the stereo demodulation circuit to the first and second input terminals, respectively, and takes out the left and right audio output signals from the first and second output terminals, respectively. A low-pass filter is configured to block high-frequency components using the impedance seen from the output side of the transistor whose collector is grounded, and the mixing amount of the high-frequency components is determined by the impedance seen from the input side.

Embodiment Figure 1 shows an example of the stereo receiver of the present invention.
In the case of an FM stereo receiver, an FM broadcast signal received by an antenna 1 is supplied to a front end 2 and converted to an intermediate frequency signal, which is supplied to an FM detection circuit 4 through an intermediate frequency amplification circuit 3.
FM detection is performed, and the detected signal is supplied to the stereo demodulation circuit 5 to generate left and right channel audio signals.
_SL and _SR are obtained.

In the present invention, a special high frequency cutoff mixing circuit 6 is provided on the output side of the stereo demodulation circuit 5. This high-frequency cutoff mixing circuit 6 includes first and second transistors.
Q ₁ and Q ₂ , the collectors of transistors Q ₁ and Q ₂ are respectively grounded AC, and the transistor
The emitters of Q ₁ and Q ₂ are the first and second capacitive elements C ₁ ,
_C2 and the first and second input terminals 6a and 6b via the first and second resistors _R1 and _R2 , and the base is connected to the control terminal _6c via the third resistor R3. ,
The first and second output terminals 6 are connected to the connection point between the first and second capacitive elements C ₁ and C ₂ and the first and second resistors R ₁ and R ₂ .
d and 6e are derived, and the emitters of transistors Q ₁ and Q ₂ are connected to bias terminal 6f via resistors R ₅ and R ₆ .
The output signals S _L and S _R of the stereo demodulation circuit 5 are supplied to the input terminals 6 a and 6 b,
A voltage source 7 of a control voltage V _C is connected to the control terminal 6c, and a power supply voltage V _B is applied to the bias terminal 6f. Each of the capacitive elements C ₁ and C ₂ is made to exhibit low impedance at frequencies of 2 to 3 kHz or higher. Resistors R ₅ and R ₆ are made sufficiently large.

In this high-frequency cutoff mixed circuit 6, capacitive elements C ₁ and C ₂
The impedance of X _C1 , X _C2 , transistor Q ₁ ,
The emitter resistance of Q ₂ is r _e1 , r _e2 , the base resistance is r _b1 ,
r _b2 , the base voltages are V _b1 , V _b2 , the mutual conductances are g _n1 , g _n2 , and the output admittances are g _p1 , g _p2 , the signals S _L and _SR are the junction capacitances and feedback of transistors Q ₁ and Q ₂ In the case of a frequency where capacitance etc. can be ignored, this circuit 6 is equivalently represented as shown in Fig. 2, and the bias by voltages V _B and V _C brings transistors Q ₁ and Q ₂ into the cut-off region. when,
The impedance between the output terminals 6d, 6e and the ground is significantly larger than that of the resistors _R1 , _R2 , and the signals S _L , S _R themselves appear at the original levels at the output terminals 6d, 6e.

Then, the control voltage V _C is lowered and the transistor Q ₁ ,
When Q ₂ is set as the operating region, the components S _d (S _L ) and Se (S _R ) of the signals S _L and S _R appearing at the output terminals 6d and 6e are as follows.
If the common collector small signal current amplification factors of transistors Q ₁ and Q ₂ are h _fc1 and h _fc2 , then S _d (S _L ) = X _C1 + r _e1 + {1/g _p1 r _b1 + R ₃ [r _b2 + h
_fc2 {1/g _p2 (r _e2 +X _C2 +R ₂ )}])/h _fc1 }/R ₁ +
X _C1 r _e1 +{1/g _p1 r _b1 +R ₃ [r _b2 +h _fc2 {1/g _p2
(r _e2 +X _{C2 + R 2} ) _} ] ₎ / _{h fc1} } _{S L} S _e (S _R ) =
_fc1 {1/g _p1 (r _e1 +X _C1 +R ₁ )}])/h _fc2 }/R ₂ +
X _C2 +r _e2 {1/g _p1 r _b2 +(R ₃ [r _b1 +h _fc1 {1/g _p1
(r _e1 +X _C1 +R ₁ )})/h _fc2 }S _R. Furthermore, if the components of the signals S _L and S _R appearing at point C are S _C (S _L ) and S _C (S _R ), then S _C (S _L )=1/g _p1 r _b1 + (R ₃ [r _b2 +h _fc2 {1/g
_p2 (r _e2 +X _C2 +R ₂ )})/h _fc1 /X _C1 +r _e1 +{1/
g _p1 r _b1 + (R ₃ [r _b2 +h _fc2 {1/g _p2 (r _e2 +X _C2 +
R ₂ )})/h _fc1 } ×R ₃ [r _b2 +h _fc2 {1/g _p2 (r _e2 +X _C2 +R ₂ )}]
/r _b1 + (R ₃ [r _b2 +h _fc2 {1/g _p2 (r _e2 +X _C2 R ₂ )
})×S _d (S _L ) S _C (S _R )=1/g _p2 r _b2 + (R ₃ [r _b1 +h _fc1 {1/g
_p1 (r _e1 +X _C1 +R ₁ )})/h _fc2 /X _C2 +r _e2 +{1/
g _p2 r _b2 + (R ₃ [r _b1 +h _fc1 {1/g _p1 (r _e1 +X _C1 +
R ₁ )}])/h _fc2 } ×R ₃ [r _b1 +h _fc1 {1/g _p1 {r _e1 +X _C1 R ₁ )}]/
r _b2 + (R ₃ [r _b1 + h _fc1 {1/g _p1 (r _e1 +X _C1 +R ₁ )
})×S _e (S _R ), and conversely the components of the signals S _R and S _L at the output terminals 6d and 6e are also S _d (S _R )=h _fc1 {1/g _p1 (r _e1 +X _C1 +R ₁ )}／r _b1
+h _fc1 {1/o _o1 (r _e1 +X _C1 +R ₁ )}×R ₁ /R ₁ +X _C1 +
r _e1 ×S _C (S _R ) S _e (S _L )=h _fc2 {1/g _p2 (r _e2 +X _C2 +R ₂ )}/r _b2
+h _fc2 {1/g _p2 (r _e2 X _C2 +R ² )}×R ₂ /R ₂ +X _C2 +r _e
2 ×S _C (S _L ) appears.

Therefore, in the end, the output terminal 6d has S _L ′=S _d (S _L )
+S _d (S _R ), the signal S _L passes through a low-pass filter with its high-frequency components cut off, and the signal S _R
A _signal with _mixed high _{- frequency} components _of
A signal is obtained in which the high-frequency components of the signal S _R are filtered through a low-pass filter and the high-frequency components of the signal S _L are mixed. By changing parameters such as the emitter resistances r _e1 and r _e2 by changing the control voltage V _C , the high-frequency cutoff characteristics of the signals S _L and S _R at the output terminals 6d and 6e and the signals S _R and S The high _- frequency mixing characteristics of _L are continuously changed _{, and} the lower the control _{voltage V} The amount of high frequency component mixed increases.

Therefore, if the control voltage V _C is lowered when the signal field strength decreases, the audio signals S _L ,
At the same time, the high-frequency components of S _R are blocked and the noise level is suppressed, and at the same time, the high-frequency components of the audio signals S _R and S _L of other channels are mixed, canceling out the in-phase noise components of the left and right channels. Can reduce noise.

In practice, it is preferable that the control voltage V _C be automatically changed according to the signal electric field strength so that the high-frequency cutoff characteristics and the high-frequency mixing characteristics can be automatically changed. Third
The figure shows an example of such a case, in which the intermediate frequency signal passed through the intermediate frequency amplifier circuit 3 is supplied to the amplitude detection circuit 8 and amplitude detected, thereby obtaining a detected voltage proportional to the signal electric field strength, which is then applied to the control voltage described above. It is applied as V _C to the control terminal 6c of the high frequency cutoff mixing circuit 6. Therefore, as the signal electric field strength decreases, the amount of blocking of the high-frequency components of the audio signals S _L and S _R of that channel increases, suppressing the noise level that increases with the decrease of the signal electric field strength, and blocking the high frequency components of the audio signals S L and S R of that channel. The amount of mixing of the high-frequency components of the audio signals S _R and S _L increases, and the in-phase noise components of the left and right channels are canceled out, and the noise is reduced.

Fig. 4 shows an example of the high-frequency cutoff characteristic when the above-mentioned high-frequency cutoff mixing circuit 6 is connected in two stages, and Fig. 5 shows an example of the high-frequency mixing characteristic.The high-frequency mixing characteristic is expressed by the degree of separation in stereo. There is. Parameter control voltage
V _C has the relationship of V ₁ > V ₂ > V ₃ > V ₄ > V ₅ > V ₆ > V ₇ .

Effects of the Invention According to the stereo receiver of the present invention, the emitter of the first transistor _Q1 whose collector is grounded is connected to the first input terminal 6a via the first capacitive element _C1 and the first resistor R1. At the same time, the emitter of the second transistor Q ₂ whose collector is grounded is connected to the second input terminal 6b via the second capacitive element C ₂ and the second resistor R ₂ . The bases of the second transistors Q _{1 and} Q ₂ are connected to the voltage source of the control voltage Vc via the third resistor R 3 , and the first capacitive element C ₁ and the first resistor R ₁ are connected to each other. The first output terminal 6d is derived from the point, and the second output terminal 6d is derived from the connection point between the second capacitive element _C2 and the second resistor _R2 .
The output terminal 6e of the transistor Q ₁ and the emitter of the second transistor Q ₂ are connected to fourth and fifth low resistors R ₅ which are larger than the first and second resistors R ₁ and R ₂ , respectively. A high-frequency cut-off mixing circuit that supplies the power supply voltage through R6 and _R6 is provided on the output side of the stereo demodulation circuit 5, and the left and right audio signals S _L and S _R from the stereo demodulation circuit 5 are connected to the first and second input terminal 6a of
and 6b, respectively, and take out the left and right audio output signals from the first and second output terminals 6d and 6e, and the output sides of the first and second transistors _Q1 and _Q2 whose collectors are grounded, i.e. , constitute a low-pass filter that blocks high-frequency components using the impedance seen from the emitter side, and the input side, that is,
Since the amount of high-frequency component mixing is determined by the impedance seen from the base side, the high-frequency cutoff characteristics and high-frequency mixing characteristics of the left and right audio output signals can be changed simultaneously and continuously using a single control voltage source. This makes it possible to reduce noise when the input electric field is weak or during stereo. Therefore, the auditory performance is significantly improved, especially in cases such as mobile reception where the input electric field changes rapidly.

[Brief explanation of the drawing]

Fig. 1 is a connection diagram of an example of the stereo receiver of the present invention, Fig. 2 is a diagram showing an equivalent circuit of the main part of the high-frequency cutoff mixing circuit, and Fig. 3 is another example of the stereo receiver of the present invention. The connection diagram, FIGS. 4 and 5 are characteristic curve diagrams showing an example of high frequency cutoff characteristics and high frequency mixing characteristics in the stereo receiver of the present invention. In the figure, 5 is a stereo demodulation circuit, 6 is a high frequency cutoff mixing circuit, Q ₁ and Q ₂ are first and second transistors, C ₁ and C ₂ are first and second capacitive elements, R ₁ ,
R ₂ and R ₃ are the first, second and third resistors, 6a and 6b are the first and second input terminals, 6d and 6e
are first and second output terminals, and 7 is a control voltage source.

Claims (1)

[Scope of utility model registration request] The collectors of each of the first and second transistors are AC grounded, and the emitter of the first transistor is connected to a first input terminal via a first capacitor and a first resistor. , an emitter of the second transistor is connected to a second input terminal via a second capacitor and a second resistor, and a base of each of the first and second transistors connects to a third resistor. A first output terminal is connected to a control voltage source via a control voltage source, and a first output terminal is derived from a connection point between the first capacitive element and the first resistor;
A second output terminal is led out from a connection point between the second capacitive element and the second resistor, and a second output terminal is connected to the emitters of the first and second transistors, respectively. A high-frequency cutoff mixing circuit to which the power supply voltage is supplied through the fourth and fifth resistors is provided on the output side of the stereo demodulation circuit, and the left and right audio signals from the stereo demodulation circuit are connected to the first and fifth resistors, respectively. supplied to a second input terminal;
A stereo receiver from which left and right audio output signals are respectively taken out from the first and second output terminals.