JPH0334244B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention automatically controls the playback volume according to ambient noise, and in particular, an automatic volume control system that prevents discomfort caused by sudden changes in volume. This invention relates to a correction device.

(Prior art) An automatic volume correction device detects ambient noise, such as noise inside a car, and increases or decreases the volume depending on the driving conditions. It makes it audible under certain conditions.

The conventional automatic volume correction device that has been generally used uses a microphone to capture in-vehicle noise, detects it, and performs volume control in accordance with the average value of the noise level.

(Problem to be Solved by the Invention) However, in the conventional automatic volume correction device described above, the volume of the reproduced sound is controlled based on the average level of the noise inside the car, so if the level of the noise inside the car suddenly changes, the average level changes. The volume changes suddenly and the volume is suddenly controlled, which makes me feel uncomfortable.

That is, for example, as shown in FIG. 1a, suppose that the noise signal increases by A times at time _t1 , and the noise voltage accordingly changes from N (V) to AN (V). Then, the control voltage VC for volume control obtained by a time constant circuit with a time constant τ to obtain the average level of the noise signal is VC=N+(AN-N)(1-e ^-t/ 〓)... As shown in FIG. 2B, it rises in a curved line at time _t1 . Here, the gain GVC' of the voltage control amplifier section for performing volume control using the control voltage VC obtained in this way is assumed to have an amplification coefficient K. GVC'=KN+K (AN-N) (1-e ^-t/ 〓) =KN+KN(A-1) (1-e ^-t/ 〓) =KN[1+(A-1)(1-e ^-t/ 〓)] , here, taking the logarithm for decibel conversion, GVC = 20log (GVC') = 20logK + 20logN + 20log
[1+(A-1)・(1-e ^-t/ 〓)]... It becomes.

Therefore, as can be seen from this equation, since the time variable is included in the logarithmic term, (1−e ^-t/ 〓)
Depending on the characteristics of the term, the changes may not be audible and constant, resulting in discomfort.

The present invention has been made in response to such circumstances, and an object of the present invention is to provide an automatic volume correction device that can perform volume control that does not cause discomfort due to sudden changes in ambient noise. .

(Means for Solving the Problems) In order to achieve the above object, the present invention provides an automatic system that captures and detects ambient noise using a microphone and controls the playback volume in accordance with the level of the detected noise signal. The volume correction device includes a filter means having a characteristic of making the detected noise signal have a gentle slope on the low frequency side in accordance with auditory characteristics and a steep slope on the high frequency side of the noise signal. , a logarithmic amplification means for logarithmically amplifying the signal matched to auditory characteristics by the filter means, a time constant circuit section for mitigating sudden changes in the output of the logarithmic amplification means, and a time constant circuit section for mitigating sudden changes in the output of the logarithmic amplification means. The apparatus is characterized by comprising a playback volume increase/decrease means for increasing/decreasing the playback volume based on the output.

(Function) In the automatic volume correction device of the present invention, ambient noise is taken in by the microphone, the filter means outputs a noise signal that corresponds to auditory characteristics, and the logarithmic amplification means outputs a signal matched to auditory characteristics. After applying logarithmic amplification to the signal, a time constant circuit section alleviates sudden changes in the output. Then, the playback volume increase/decrease means increases/decreases the playback volume based on the output of the time constant circuit section.

Therefore, even if the level of noise inside the vehicle suddenly changes, the output from the time constant circuit section to be controlled is made gentle, so that gradual volume control is performed.

(Example) Hereinafter, details of an example of the present invention will be described based on the drawings.

FIG. 2 shows an embodiment of the automatic volume correction device of the present invention, which includes a microphone M that captures noise inside a car and converts this noise into an electrical signal, a microphone amplifier MA that amplifies the noise signal, and a microphone amplifier MA that amplifies the noise signal. A low-pass filter LPF is provided that extracts only low-frequency components from the noise signal amplified by the microphone amplifier MA.

Here, the low-pass filter LPF is used to prevent a positive feedback phenomenon caused by reproduction sound, speech, etc. being taken in through the microphone M. In other words, if you analyze the noise inside the car, most of it is generated while driving, and the noise is several tens of Hz.
It is dominated by the following low frequency components:
Therefore, by extracting only these low frequency components, the negative effects caused by the positive feedback of the reproduced sound, speech, etc. described above can be prevented, so that appropriate volume correction can be performed. However, if volume control is performed using a noise signal obtained by simply extracting only this low frequency component, the corrected volume will be unnatural. This is because sensitivity decreases as the human auditory characteristics shift toward ultra-low frequencies, making it impossible to control the volume in response to the auditory noise level.

In other words, according to the Fletcher-Yarmanson curve (not shown), which expresses the intensity of sound that is perceived as the same loudness in phons and measures it as a parameter, 2~
Below 300 Hz, the auditory characteristics of the ear within the sound pressure range of 0 to 120 phon increase toward the lower range, for example, with a slope of 0 to 6 dB/octave (octave).

Therefore, even if noise exists in the very low frequency range of sound, it generally does not cause harshness to the ears, and in this case there is almost no need to correct the playback volume. However, in the low frequency range of about 50 Hz, for example, it is perceived as noise and becomes harsh to the ear, so it is necessary to correct it. Therefore, it is necessary to correct the reproduction volume according to the auditory characteristics.

Therefore, the low-pass filter LPF in this embodiment has a characteristic that the low frequency side has a gentle slope to match human hearing, and the high frequency side has a steep slope, so that it is not affected by the adverse effects of reproduced sound and speech. In addition to making sure that this is not the case, we also perform auditory correction.

The automatic volume correction device also includes a noise level detection unit DET that detects the noise level from the noise signal that has passed through the low-pass filter LPF, a logarithmic amplification unit LA that logarithmically amplifies the detected level, and a logarithmic amplification unit LA that logarithmically amplifies the detected level.
It is provided with a time constant circuit section TC that makes the output of LA gentle, and a voltage control amplification section VCA that changes the degree of amplification of the volume to be reproduced according to the output signal VC from the time constant circuit section TC. Note that this voltage-controlled amplification unit VCA has an input terminal IN into which the audio signal to be reproduced is input, and a time constant circuit unit to which the audio signal is input.
An output terminal OUT is provided to amplify and output the amplified signal VC according to the voltage value of the output signal VC from the TC.

The automatic volume correction device having such a configuration operates as follows.

First, when the noise inside the vehicle increases, the microphone M captures the noise and converts the captured sound into an electrical signal that is a noise signal. The output from the microphone M is amplified to a predetermined level in the microphone amplifier MA, and then supplied to the low pass filter LPF. low pass filter LPF
cuts out middle and high frequency components of the noise signal and passes only low frequency and very low frequency components of several tens of hertz. Here, the low pass filter LPF is, as mentioned above,
The low-frequency side has a gentle slope that matches human hearing, and the high-frequency side has a steep slope, so that it is not adversely affected by signal sounds and speech.

Here, when a peak occurs in the low frequency range due to resonance of the vehicle body, as the frequency of the peak becomes lower, an error occurs between the voltage detected by the microphone M and the auditory sound pressure. In other words, due to poor auditory characteristics, even if there is noise at low frequencies that would normally be unnoticeable, and no noise occurs in the mid-frequency range or above, it will be detected as a noise signal and the volume will be unnecessarily loud. Corrections are made. For this reason, it is necessary to make the detected noise signal have a gentle slope on the low frequency side in accordance with human hearing, and a steep slope on the high frequency side.

In fact, in-vehicle noise often exhibits resonance in the low range, and the resonance frequency also changes in a complex manner.
Therefore, when extracting low-frequency signals that are not affected by reproduced sounds or speech and making them into car interior noise, by making the low-frequency side part of the above-mentioned low-pass filter LPF correspond to the human auditory characteristics, The error between the detected voltage and the auditory sound pressure is reduced, and appropriate volume correction is performed.

Low pass filter corrected in this way
The output of the LPF is converted into a voltage in the noise level detector DET. Then, this converted voltage is logarithmically compressed in the logarithmic amplifier LA, and then
The sudden change is absorbed by the time constant circuit TC and supplied to the voltage control amplifier VCA as the control voltage VC. The voltage-controlled amplifier VCA changes its gain in response to the control voltage VC.
It changes the level of the audio signal to be reproduced supplied to IN and outputs it from the output terminal OUT.

Here, the audio signal is increased or decreased according to the control voltage VC corresponding to the in-vehicle noise level. Therefore,
For example, when the noise inside the vehicle increases, the level of the sound to be reproduced is appropriately controlled by increasing the level of the audio signal.

Further, here, the control signal VC to be supplied to the voltage control amplifier section VCA has a time constant section which is a noise voltage N
Since it acts on the difference between the logarithm of (V) and the logarithm of the increased noise voltage AN (V), it is expressed as the sum of the logarithm of the noise voltage and the difference between the above logarithm multiplied by a time constant term, VC= 20log N+(20log AN-20log N)・(1−e ^-t/ 〓)=
20log N+(1-e ^-t/ 〓)・(20logA).

Then, the control coefficient of the voltage control amplifier VCA is set to K
Gain GVC of voltage controlled amplifier VCA when
GVC=K20log N+K(1-e ^-t/ 〓) ・20logA...

As a result, the gain GVC of the voltage control amplifier VCA
Since the equation for determining , does not include a time variable in its logarithmic term, the change in volume becomes aurally constant regardless of the value of the magnification A of the ambient noise.

In other words, as shown in Figure 1a, for example, even if the ambient noise suddenly increases at time _t1 , the time constant circuit
Due to the operation of TC, the change in control signal VC is as shown in Figure 1c.
As shown in the figure, the slope is gentle, so natural volume control characteristics can be obtained.

Furthermore, if the natural volume control characteristic due to the gentle slope in this embodiment is compared with the conventional volume control characteristic, it will be as shown in FIG. 3, for example.

As can be seen from the figure, in the characteristics a and a' shown by the solid line in this example, the characteristic a' is approximately
Even if the volume is increased by 10 dB, the rate of increase in each corrected sound volume is expressed as a time constant, for example, 1 second. This means that the volume change is aurally constant regardless of the value of the magnification A of the ambient noise. On the other hand, in the conventional characteristics b and b' represented by the chain lines, the characteristic b' is approximately 10 dB relative to the characteristic b.
When it is turned up, the time constant for characteristic b is about 0.8
seconds, whereas the time constant in characteristic b′ is approximately
It is 0.4 seconds. This, as shown above,
Since the time constant exists in the logarithmic term, when the time constant is subjected to logarithmic transformation, the change in volume will not be audibly constant, leading to discomfort.

FIG. 4 shows a specific example of the automatic volume correction device shown in FIG. 3, and the output of the microphone M is as follows.
The signal is supplied via a capacitor C1 and a resistor R1 to an inverting input terminal of an operational amplifier OPA1 constituting a microphone amplifying section MA, and is then amplified. Here, the gain A (R2/R1) of the microphone amplifier MA changes by changing the value of the feedback resistor R2. When the output of the operational amplifier OPA1 is supplied to the operational amplifier OPA2 via resistors R3 to R5, the operational amplifier OPA2 cuts out the range of sounds such as those reproduced from the radio and speech, and also performs hearing correction. will be held. In addition, in the operational amplifier OPA3 of the band pass filter BPF, the low pass filter
The output of LPF is capacitor C2, C3 and resistor R
It is made into a non-inverting input via 6. At this time, the output of the operational amplifier OPA3 is fed back by the capacitor C4. The noise level detection section DET has
The output of bandpass filter BPF is connected to capacitor C.
5 and a resistor R7 as an inverting input, and a reference voltage divided by resistors R8 and R9 as a non-inverting input.
The output from the bandpass filter BPF is converted into a DC voltage, and then detected over a wide dynamic range. And the noise level detection section
The DET outputs a noise level detection signal with a corresponding value when a voltage signal higher than the reference value is input. In the logarithmic amplification section LA constituted by the operational amplifier OPA5, the input signal is logarithmized by inserting a transistor Q1 into the feedback path of the operational amplifier OPA5, and after being logarithmically compressed, the time constant circuit
Supplied to TC. Here, since the amplitude of the output signal is small when only the operational amplifier OPA5 is used, the reference input level also becomes low. For this reason, the operational amplifier
The level is raised by inserting OPA6. The time constant circuit TC includes an inverting transistor Q3 with a gain of 1 whose base input is the output of the logarithmic amplifier LA (which becomes smaller due to noise amplification) via a resistor R10.
A capacitor C6 and a collector resistor R11 are connected between the collector and the base input terminal of the transistor Q3, and these elements constitute a Miller integration circuit. The voltage control amplification unit VCA is constituted by an integrated circuit block IC, and its gain G changes in response to the control signal VC output from the time constant circuit TC. As a result, the level of the audio signal supplied to the input terminals IN1 and IN2 is changed in accordance with the noise level, and is outputted from the output terminals OUT1 and OUT2. The switches S1 and S2 are manual switches for directly taking out the audio signals supplied to the input terminals IN1 and IN2 when automatic volume correction is not required. Further, the symbol PS in the figure indicates a power supply section.

In this way, in this embodiment, the noise level detection signal that detects ambient noise is logarithmically compressed and a control signal is generated via a time constant circuit, and this control signal is used to control the voltage control amplifier for volume control. Since it is controlled, sudden changes in volume caused by sudden changes in noise level are alleviated, and natural volume correction is performed, so you do not feel uncomfortable.

(Effects of the Invention) As explained above, according to the automatic volume correction device of the present invention, even when the level of noise inside the vehicle suddenly changes, the output from the time constant circuit section to be controlled can be made smooth. Since the volume control is performed gradually, the volume control can be performed so that the user does not feel uncomfortable due to sudden changes in ambient noise.

[Brief explanation of drawings]

FIGS. 1a and 1b are waveform diagrams showing the relationship between conventional ambient noise changes and control signals, FIG. 1c is a waveform diagram showing the relationship between ambient noise changes and control signals according to this embodiment, and FIG. A block diagram showing an embodiment of the automatic volume correction device of the present invention, FIG. 3 is a characteristic diagram showing the volume control characteristics of the automatic volume correction device of FIG. 2 in comparison with conventional volume control characteristics, and FIG. FIG. 2 is a circuit diagram showing a specific circuit configuration thereof. M... Microphone, MA... Microphone amplification section, LPF... Low pass filter, DET... Noise level detection section, LA... Logarithmic amplification section, TC... Time constant circuit, VCA... Voltage control amplification section.

Claims (1)

[Scope of Claims] 1. In an automatic volume correction device that captures and detects ambient noise with a microphone and controls the playback volume in accordance with the level of the detected noise signal, with respect to the detected noise signal, A filter means having a characteristic that the low frequency side of the noise signal has a gentle slope in accordance with auditory characteristics and the high frequency side thereof has a steep slope, and a signal adjusted to the auditory characteristics by this filter means. a logarithmic amplification means for performing logarithmic amplification on the logarithmic amplification means; a time constant circuit section for alleviating sudden changes in the output of the logarithmic amplification means; and a playback volume increase/decrease means for increasing/decreasing the playback volume based on the output of the time constant circuit section. An automatic volume correction device comprising: