JPS6134302B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a noise reduction device for automobile audio equipment.

Generally speaking, it is possible to prevent the generation of internal noise in audio equipment such as radios, cassette stereos, televisions, etc. that are specially equipped for automobiles, but due to the unique characteristics of the automobile, external noise may be generated. It is not possible to prevent the occurrence of noise caused by such causes.

In other words, there are various sources of noise in the outside world of a car, and these external noises are picked up while driving, and the field strength of broadcast waves received by the antenna temporarily decreases. Noise with a wide frequency spectrum occurs when receiving broadcasts. Furthermore, the electrical equipment of the vehicle itself, such as pulse noise caused by radiated interference from the engine's ignition system, noise caused by transient disturbances transmitted through the power wiring due to the opening and closing of electrical contacts, especially their continuous operation. Noise is generated from the Therefore, when these external noises enter the audio equipment, it causes discomfort to the listener, and among these noises, pulse noise that contains a lot of high frequency components is the most annoying.

Conventionally, as a countermeasure against noise that enters automobile audio equipment from the outside, so-called muting circuits have been installed that temporarily forcibly cut off the sound output based on the noise detection output, or devices that detect noise when it is mixed in. There is a noise canceling circuit that eliminates pulse noise components at a uniform rate during the mixing period. However, in the former case, the sound output is interrupted, which hurts the listener's feelings, and in the latter case, the overall circuit configuration is complex, and the sound quality deteriorates, giving the listener a sense of discomfort. There is a drawback.

Normally, audio equipment for cars makes some noise when the car is running, so in order to reduce the noise of the audio equipment that is actually felt by the listener, it is necessary to remove the high frequency components of the sound output, even if the sound quality is degraded to a certain extent. It is possible to reproduce sound that is not too disturbing and can be easily heard.

The present invention has been made in consideration of the above points, and
Based on the above idea, the noise component generated in the sound output is detected, and the low-pass filtering control is performed by changing the cutoff frequency of the sound output according to the detected signal. The present invention provides a noise reduction device for automobile audio equipment in which the acoustic output is temporarily lowered to some extent, but the acoustic output is continuously obtained without interruption, and acoustic characteristics that are actually comfortable are guaranteed.

The noise reduction device for automobile audio equipment according to the present invention mainly detects noise signals in acoustic output,
It consists of a noise detection circuit that converts the detection signal into a signal optimal for low-pass filter control, and a variable low-pass filter whose cutoff frequency is varied by the output of this noise detection circuit. For devices that receive broadcast radio waves using an antenna, such as in the case of a receiver, an electric field that detects the degree of electric field strength of the received broadcast radio waves and generates a voltage signal that is inversely proportional to the field strength. By providing an intensity detection circuit and an addition circuit that performs proportional addition of the output of the field intensity detection circuit and the output of the noise detection circuit, and using the addition output as a control signal for the variable low-pass filter. This is designed to more effectively suppress noise.

Hereinafter, one embodiment of the present invention will be described in detail with reference to the accompanying drawings.

Figure 1 shows a noise reduction device for example in an AM or FM radio receiver.
a high frequency or intermediate frequency amplification circuit 2 that amplifies the high frequency or intermediate frequency of the broadcast radio waves received by the amplifier; a detection amplification circuit 3 that performs detection and amplification of the output of the amplification circuit 2; Detection amplification circuit 3
In a radio receiver comprising an output amplification circuit 4 which amplifies the acoustic signal demodulated by the detector and sends it to the speaker 5, a noise detection circuit 6 detects a noise signal from the acoustic output of the detection amplification circuit 3. , a field strength detection circuit 7 that detects the degree of field strength of the radio wave received by the amplification circuit 2 and outputs a voltage signal inversely proportional to the detected field strength, a noise detection circuit 6, and a field strength detection circuit. According to the output of the adder circuit 8, the cutoff frequency of the output of the detection amplification circuit 3 is adjusted, and the low-pass filtered signal is sent to the output amplification circuit 3. 4 and a variable low-pass filter 9 to be sent to the filter.

The noise detection circuit 6, as shown in FIG.
It is comprised of a noise selection circuit 61, a peak value detection circuit 63 for rectifying and detecting the peak value of the noise signal selected by this circuit 61, and a coupling circuit 62 for coupling these two circuits.

The noise screening circuit 61 includes a resistor for high frequency stabilization.
R ₁ is connected in series with the first operational amplifier OP1, which is provided to form a differential circuit together with the capacitor C ₁ and the resistor R ₂ , and the same resistor R ₃ for high frequency stabilization. A second operational amplifier OP2 is connected to the second operational amplifier OP2 and is provided to form a differential circuit together with the capacitor _C2 and the resistor _R4 .

This noise selection circuit 61 selects pulse noise (containing many high frequency components) that has a particularly large amplitude and a steep change from among the noises mixed in the input signal E _IN . The output obtained by differentially processing E _IN and picking up the steep voltage changes (harmonic components) that are characteristic of pulse noise is used as a noise detection signal. Arithmetic amplifier
The outputs E ₁ and E ₂ of the differentiating circuits respectively constituted by the parts OP1 and OP2 are given by the following equations.

E ₁ = -R ₂ C ₁ dE _IN /dt ...(1) E ₂ = -R ₄ C ₂ dE _IN /dt ...(2) In addition, each of these differentiating circuits is used for stabilization to lower the high frequency gain. Since the resistor R ₁ or R ₃ is provided, each differentiator circuit has a cutoff frequency region expressed by the following equation.

High frequency shedding frequency _H1 = 1/2πR ₁ C ₁ ...(3) _H2 = 1/2πR ₃ C ₂ ...(4) Low frequency shedding frequency _L1 = 1/2πR ₂ C ₁ ...(5) _L2 = 1/2πR ₄ C ₂ ...(6) In addition, this low frequency cutting frequency _L1 or _L2
is used to limit extremely low frequency response; for example, in the case of FM radio, this is set to about 20KHz in practice in order to distinguish the pilot signal (19KHz) sent during stereo broadcasting from noise, and also limits the high frequency response. The cut-off frequency _H1 or _H2 imposes a restriction on the high frequency response, and is appropriately set so as not to respond to unnecessarily high frequency components of acoustic signals other than noise.

Further, the coupling circuit 62 is constituted by a parallel circuit of a capacitor _C3 and a resistor _R5 .

Furthermore, the peak value detection circuit 63 is limited by a diode D ₁ that rectifies the pulse noise detection signal detected by the noise screening circuit 61, a current limiting resistor R ₆ , and a current limiting resistor R ₆ . A capacitor that charges the noise signal and stores its peak value.
A circuit consisting of C ₄ and its discharging resistor R ₇ ;
The charging voltage of the capacitor _C4 is used as a positive input signal, the output is fed back through the diode _D2 , and the variable low-pass filter ₉ , which will be described later, is operated by comparing it with the voltage signal from the resistor R8 input to the negative side. and an operational amplifier OP3 which generates a control signal. Note that the resistor R ₆ for limiting the charging current of the capacitor C ₄ prevents the control voltage signal of the voltage-controlled variable low-pass filter 9 from changing too rapidly. Since the sudden change in the cut-off frequency gives a strange feeling to the listener, the rise response of the capacitor _C4 during charging is adjusted as appropriate in order to soften the change in the control voltage signal. In addition, the discharging resistor _R7 of the capacitor _C4 reduces the auditory discomfort caused by sudden changes in the cut-off frequency of the variable low-pass filter 9, and is variable in response to continuously occurring noise. In order to continuously maintain the cut-off frequency characteristics of the low-pass filter 9, the peak voltage of the noise charged (stored) in the capacitor C ₄ is gradually discharged with a sufficient discharge time constant to make it variable. The cut-off frequency of the low-pass filter 9 is changed slowly, and the settings are made so that the most comfortable acoustic characteristics can be obtained even when high-frequency noise is cut off.

In the noise detection circuit 6 configured as described above, for example, a pulse noise signal in the acoustic output of the detection amplification circuit 3 of a radio receiver is detected from a differentiating circuit having high and low frequency limits. Noise selection circuit 6
1, and the peak value of the detected noise is detected by a peak value detection circuit 63 having a constant charging/discharging characteristic.
While a noise signal is being detected, it operates to generate a control signal for the variable low-pass filter 9 according to its magnitude.

FIG. 3 shows a specific example of the configuration of the voltage-controlled variable low-pass filter 9. It is made up of a so-called controlled source type low-pass filter consisting of resistors R ₁ , R ₂ , capacitors C ₁₁ , C ₁₂ , C ₂ and an operational amplifier OP4. A photocoupler made of a combination of a sending diode LED and cadmium sulfide Cds is used as a voltage resistance element whose resistance changes according to a control voltage signal, and the amount of feedback by feedback capacitors C ₁₁ and C ₁₂ is determined by the voltage resistance element. It is designed to change the low-pass filtering characteristics by adjusting the amount.

Figure 4a shows the equivalent circuit of the voltage-controlled variable low-pass filter 9 shown in Figure 3, and Figure 4b shows the equivalent circuit when the voltage resistance element has a very high resistance or an infinite resistance. This shows the circuit, and the capacitor in the diagram
C ₁ is the series composite sum of C ₁₁ and C ₁₂ . In addition, Figure c shows the equivalent circuit when the voltage resistance element becomes zero, and it is a passive low-pass filter using two combinations of resistor R ₁ and capacitor C ₁₁ and resistor R ₂ and capacitor C ₂ . The circuit is in the front stage, and a capacitor _C12 is connected in parallel to the output side of the operational amplifier.

Now, if we compare the frequency characteristics of the two low-pass filter circuits shown in Fig. 4b and c, we can see that the low-pass filter circuit differs depending on the presence or absence of the feedback action by capacitor _C1 , as shown in Fig. 5. The shearing frequency characteristics change greatly, and the shearing frequency with the circuit shown in Figure 4b
_C1 has a higher cutting frequency due to the circuit shown in c in the same figure.
It will be higher than _c2 .

In the present invention, for example, during normal reception by a radio receiver (when no pulse noise is detected by the noise detection circuit 6), the light emitting diode
The LED is off and this causes the voltage resistance element Cds
shows a high resistance and maintains the circuit state shown in Figure 4, and the cut-off frequency _c1 at that time is the highest required frequency of the acoustic signal (for example, 10KHz for ordinary audio equipment).
The resistance value of the voltage resistance element Cds becomes zero when the light emitting intensity of the light emitting diode LED reaches its maximum according to the control signal from the noise detection circuit 6 due to the mixing of large pulse noise. As a result, the circuit state shown in Fig. 4c is reached, and the cutoff frequency _C2 at that time is the minimum frequency that can be heard sufficiently as noise-reduced voice (for example,
The circuit constants of this voltage-controlled variable low-pass filter circuit 9 are set so that the frequency is approximately 1 KHz). Therefore, the pulse noise mixed into the acoustic signal is detected by the noise detector 6, and a voltage is output via the light emitting diode LED by a control signal corresponding to the magnitude of the detected noise. When the resistance value of the resistive element Cds is changed, the cut-off frequency of this low-pass filter circuit 9 changes appropriately between _C1 and _C2 , thereby filtering the acoustic signal mixed with pulse noise into this low-pass filter. By passing the signal through the filter circuit 9, the noise can be effectively suppressed.

Furthermore, if the relationship between the feedback capacitors C ₁₁ and C ₁₂ is such that C ₁₁ > C ₁₂ , the cut-off frequency _C2 shown in FIG. 5 can be lowered, and the pulse noise can be reduced. This means that suppression can be carried out more effectively. Further, when it is necessary to improve the pulse noise suppression characteristics, it is more effective to use a plurality of voltage resistance elements and stack them in multiple stages. Further, in the above embodiment, a photocoupler variable resistance element combining cadmium CdS and a light emitting diode LED is used, but a voltage-controlled variable resistance element using a semiconductor element may be used instead.

The noise reduction device for automobile audio equipment according to the present invention can effectively suppress pulsed noise mixed into the acoustic signal by the combination of the noise detection circuit 6 and the variable reduction filter circuit 9 described below.
If this continues, pulse noise will be detected by the noise selection circuit 61 in the noise detection circuit 6, and a control signal for the variable low-pass filter circuit 9 will be obtained by the peak detection circuit 63 based on the detection signal. This will cause a time delay. In addition, the peak value detection circuit 63
As mentioned above, if the cutoff frequency of the variable low-pass filter circuit 9 is suddenly changed, the sound quality will suddenly change and the hearing sensation will be impaired. Although the noise is adjusted to provide a delay, even if it occurs continuously, the effect of suppressing pulse noise cannot be obtained for the first short period of time, and a portion of the noise comes out as output from the speaker. On the other hand, considering the causes of noise in radio receivers, etc., AM,
Noise occurs in the acoustic output when receiving broadcasts such as FM because the received field strength of the broadcast radio waves is weak, or because there is a source of noise electromagnetic waves inside or outside the car.
This is when the SN ratio decreases. Therefore, especially in broadcast radio receivers such as radios, in order to increase the reduced SN ratio and to compensate for the time delay in the noise detection circuit 6 and the lack of noise suppression effect at the initial stage of noise generation, As shown in FIG. 1, there is a field strength detection circuit 7 that detects the received field strength of broadcast radio waves and generates a voltage signal inversely proportional to the detected field strength, and the output of this field strength detection circuit 7 and the noise. An adder circuit 8 that performs proportional addition with the output of the detection circuit 6 is provided, and this adder circuit 8
The summed output is sent to the variable low-pass filter 9 as a control signal, so that noise can be suppressed more effectively.

This electric field strength detection circuit 7 detects the electric field strength according to the reception state of the broadcast wave of the high frequency or low frequency amplification circuit 2, but especially in the case of FM broadcast, the electric field strength is detected before the amplitude limiter stage of the receiver. It is provided to extract the received radio wave from the section and perform amplitude detection of the received radio wave.

FIG. 6 shows a specific example of the configuration of the adder circuit 8, in which the output voltages E _IN1 and E _IN2 of the noise detection circuit 6 and the electric field strength detection circuit 7 are
An operational amplifier OP5 which takes as an input, determines the addition ratio of these input voltages E _IN1 and E _IN2 using resistors R ₁ and R ₂ , and outputs the sum of the input voltages E _IN1 and E _IN2 with the same polarity. It is made up of Note that the output voltage E _OUT is given by the following equation.

E _OUT =K(E _IN1 /R ₁ +E _IN2 /R ₂ )K:
Constant...(7) Therefore, the electric field strength of the received radio wave is detected by the electric field strength detection circuit 7, and a voltage signal that is inversely proportional to the detected electric field strength and the voltage signal detected by the noise detection circuit 6 are detected. , and the ratio of addition to the noise signal processed by signal processing with an appropriate time constant is determined by the resistance.
By appropriately adjusting R ₁ and R ₂ and using the added output as a control signal for the cut-off frequency in the variable low-pass filter circuit 9, when the received broadcast radio wave becomes weak, the By relatively increasing the cut-off frequency of the variable low-pass filter circuit 9, noise can be suppressed more effectively to improve hearing sensation. In addition, when adding this system of field strength detection circuit 7 and addition circuit 8, the control amount of variable low-pass filter 9 by the output of field strength detection circuit 7 is compared with the control amount by the output of noise detection circuit 6. If the noise suppression effect is moderately adjusted and set, it will be more useful for the auditory sense. In other words, noise does not necessarily occur when the received broadcast radio waves have a weak electric field; for example, even in areas with a weak electric field, if you stop the car engine and eliminate radio wave interference from the ignition system, noise will not be mixed in. Therefore, there is no need to control and drive the variable low-frequency three-wave circuit 9 more than necessary using the output of the field strength detection circuit 7 to cut off the high frequency components of the received acoustic signal. Therefore, as mentioned above, only in the early stages of pulse noise generation when the noise suppression effect is not sufficiently exerted, the sound quality can be prevented from causing a sense of strangeness to the listener due to sudden changes, and a certain degree of noise suppression effect can be obtained intuitively. It goes without saying that this may be used as an auxiliary function.

As described above, in the noise reduction device for automobile audio equipment according to the present invention, the pulse noise mixed into the acoustic signal is detected by the noise detection circuit through differential processing, and the detected noise is This low-pass filter adjusts the rise and fall characteristics of the peak voltage signal as appropriate and controls a variable low-pass filter circuit using the output of this noise detection circuit to actually cut out high-frequency components in the acoustic signal. This is a device that automatically and relatively slowly changes the cut-off frequency of the circuit, so that when noise is mixed in, the sound output is not interrupted in any way, and the sound output is continuously obtained while effectively suppressing the noise. Also, unlike conventional noise canceling circuits that cut out the high frequency components of the acoustic signal at the time of noise detection, the cutting frequency can be adjusted according to the magnitude of the detected noise signal. By determining the frequency and taking sufficient care to prevent sudden changes in the cutoff frequency, it is possible to reliably eliminate the deterioration in sound quality during noise reduction and the discomfort that it gives to the listener.
Moreover, it has the excellent advantage that the overall structure of the device is simple.

In addition, in the noise reduction device for automobile audio equipment according to the present invention, especially in devices such as radio receivers that receive and reproduce broadcast waves, the electric field strength of the received broadcast waves may be adjusted as necessary. An electric field strength detection circuit is provided to detect and generate a signal proportional to the electric field strength detection circuit, and the output thereof is added to the control output of the noise detection circuit at an appropriate ratio via an addition circuit, and is received by this. When the broadcast radio waves become weak, the cut-off frequency of the variable low-pass filter circuit that cuts out high-frequency noise components in the acoustic signal is relatively increased, which reduces the sense of discomfort caused to the listener when the noise is reduced. It has the excellent advantage of being able to eliminate even more.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a configuration example when the noise reduction device for automobile audio equipment according to the present invention is applied to a radio receiver, and FIG. 2 is a specific configuration of a noise detection circuit in one embodiment of the present invention. An electrical wiring diagram showing an example, Fig. 3 is an electrical wiring diagram showing a specific example of the configuration of the voltage-controlled variable low-pass filter circuit in the same embodiment, and Fig. 4a is shown in Fig. 3. The equivalent circuit diagram of the variable low-pass filter circuit, Figure b is the equivalent circuit diagram when the resistance is high (infinite), Figure c is the equivalent circuit diagram when the resistance is zero, Figure 5 is the equivalent circuit diagram of Figure 4 b and Figure 5 is the equivalent circuit diagram when the resistance is zero. Cut-off frequency characteristic diagram in the circuit shown in c,
FIG. 6 is an electrical wiring diagram showing an example of the configuration of the adder circuit in the same embodiment. 1... Antenna, 2... High frequency or intermediate frequency amplification circuit, 3... Detection amplification circuit, 4... Output amplification circuit, 5... Speaker, 6... Noise detection circuit, 6
1... Noise screening circuit, 62... Combining circuit, 63...
...Peak value detection circuit, 7...Field strength detection circuit, 8
...Addition circuit, 9...Variable low-pass filter circuit, _C1 ,
_C2 ...Shrinking frequency.

Claims (1)

[Claims] 1. A noise screening circuit and its detection for detecting pulsed noise mixed into the acoustic signal in automotive audio equipment that generates acoustic output from the speaker based on the acoustic signal obtained by receiving broadcast radio waves. A noise detection circuit consists of a peak value detection circuit that detects and temporarily stores the peak value of the received noise, and a noise detection circuit that detects the electric field strength of the received broadcast radio wave and generates a signal that is inversely proportional to the detected electric field strength. a generating electric field strength detection circuit; an addition circuit that adds the output signal of the electric field strength detection circuit and the peak value signal output from the noise detection circuit at a predetermined ratio; A noise reduction device for automotive audio equipment that includes a variable low-pass filter circuit that is controlled to lower the cutoff frequency of a signal.