JPH05176211A - Sound circuit for video camera - Google Patents

Abstract

PURPOSE:To eliminate a wind break noise component of a microphone without sound quality deterioration by providing a low pass filter controlling a high cut-off frequency with an output of a low frequency component intensity detection circuit to the sound circuit. CONSTITUTION:An output signal V1 of a microphone 1 is amplified by a variable gain amplifier 2 and outputted through an HPF (high pass filter) 3 and an LPF (low pass filter) 4. On the other hand, a low frequency component (LF component) being a wind break noise component of the microphone signal V1 is detected by an LF level detection circuit 5, which converts the component into DC signals V11 and V12, they are applied respectively to the HPF 3 and the LPF 4, whose cutoff frequency is changed. For example, when a wind break noise sound is large, the signal V11 is increased to control the HPF 3 and the LPF 4 thereby increasing the cut-off frequency of the HPF 3 and decreasing the cut-off frequency of the LPF 4. Conversely when the wind break noise sound is small, the signal V11 is decreased to control the HPF 3 and the LPF 4 thereby decreasing the cut-off frequency of the HPF 3 and increasing the cut-off frequency of the LPF 4 and widening a band width of an output signal V2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microphone device, and more particularly to an audio circuit of a video camera provided with measures against wind noise of a microphone of the video camera.

[0002]

2. Description of the Related Art Since the wind noise of a microphone (microphone) becomes a serious problem in outdoor shooting with a video camera, many measures are taken as described below. For example, Japanese Utility Model Publication No. 62-121896 discloses 250 to 350 Hz.
It is described that the following filter for blocking low frequency components is selectively provided in the microphone output circuit to block wind noise of the microphone of the video camera. In addition, JP-A-63
In Japanese Patent Laid-Open No. 309097, the low-frequency component intensity of the microphone output of the video camera and the full-band component intensity are separately detected, and the two intensities are compared to determine the low-frequency component when the microphone output is determined to be wind noise. Is disclosed.

Further, in Japanese Patent Laid-Open No. 2-114708, as a microphone device to be mounted on an autobicycle helmet, a first microphone utilizing bone conduction and a second microphone for the mouth are provided, and a second microphone It is disclosed that the surrounding conditions are detected from the wind noise level signal obtained from the output, the engine speed / vehicle speed signal, etc. to control the output and / or frequency characteristics of the first and / or second microphones. .. Further, in Japanese Patent Application Laid-Open No. 2-174311, a microphone for wind noise detection is provided in the video camera in addition to the microphone for sound pickup, and when the wind noise level exceeds a predetermined level, the sound pickup microphone circuit has a high frequency range. The insertion of a pass filter is disclosed.

Further, in Japanese Patent Laid-Open No. 214400/1990, a microphone output of a video camera is separated into a low-pass filter and a high-pass filter, and the output level is automatically controlled according to the output level of the low-pass filter. Addition to the output of a high pass filter is disclosed. Further, Japanese Patent Laid-Open No. 3-46897 discloses controlling the cutoff frequency of a high-pass filter provided on the microphone output side according to the wind noise level. In addition, the actual Kaihei 3-66
Japanese Patent Publication No. 295 discloses that the low-pass filter output component of each microphone is subtracted from each output of the left microphone and the right microphone.

[0005]

In the above-mentioned prior art, although the wind noise component of the microphone can be removed if necessary, there is a problem that the sound quality is deteriorated because only the low frequency component is removed. In addition, the (variable) high-pass filter required to eliminate wind noise uses a relatively large-capacity capacitor, so the filter circuit becomes large,
In particular, in a video camera whose market value is evaluated by the degree of reduction in size and weight, the volume of the high-pass filter circuit has been a major obstacle. Furthermore, the ALC (Automatic
There is also a problem that the level control (automatic volume control) circuit malfunctions due to the wind noise component with a large energy level, and the voice volume decreases when the wind is strong. It is an object of the present invention to provide an audio circuit for a video camera that solves the above problems.

[0006]

In order to solve the above problems, a low pass filter whose high cutoff frequency is controlled by the output of the low frequency component intensity detection circuit is provided. Further, the high-pass filter is composed of a series circuit of a first capacitor element and a variable resistance element, a signal is input to one end of the first capacitor element, an output voltage is taken out from the variable resistance element end, and the low frequency component strength is obtained. The input terminal of the detection circuit is connected to the first capacitor element terminal, and the resistance value of the variable resistance element is controlled by the output signal thereof.

Further, the voltage of the first resistance element terminal of the series circuit of the first capacitor element and the first resistance element is amplified by the variable amplifier and negatively fed back to the first resistance element terminal, The input terminal of the frequency component intensity detection circuit is connected to the first capacitor element terminal and the output signal thereof controls the gain of the variable amplifier to form the high pass filter. Further, an input signal is applied to one end of the first capacitor element via a second resistance element, and a second capacitor element is connected to the first resistance element to connect the high-pass filter and the low-pass filter. And make up. Further, means for fixing the output of the low frequency component intensity detection circuit to a predetermined voltage is provided so that the function of the wind noise removing circuit can be stopped.

[0008]

The low-pass filter is controlled so as to raise its high cutoff frequency in proportion to the strength of the wind noise signal of the microphone detected by the low-frequency component strength detection circuit. Further, the high-pass filter composed of the first capacitor element and the variable resistance element is controlled so as to lower the low cutoff frequency thereof in proportion to the strength of the wind noise signal.

Further, the high-pass filter composed of the series circuit of the first capacitor element and the first resistance element, the variable amplifier and the negative feedback circuit cuts off the low frequency band in proportion to the strength of the wind noise signal. It is controlled to lower the frequency. Further, a circuit for applying an input signal to one end of the first capacitor element through a second resistance element and connecting the second capacitor element to the first resistance element is a low cutoff frequency of a high pass filter. And the high cutoff frequency of the low-pass filter are controlled simultaneously. Further, by fixing the output of the low frequency component intensity detection circuit to a predetermined voltage, the wind noise removing function is stopped.

[0010]

EXAMPLE FIG. 2 shows an example of a frequency spectrum of a microphone output signal when a wind having a wind speed of about 2 m is applied to the microphone.
From this, it can be seen that the wind noise component spreads to approximately 400 Hz, and is particularly concentrated in the band below 50 Hz. Therefore, in the present invention, the cutoff frequency is approximately 50 to 400 Hz.
The wind noise component of the microphone is removed by using the high-pass filter of. Also, the wind noise (low frequency)
If only the component is removed, the high frequency component is relatively emphasized and the sound quality becomes strange. Therefore, the high frequency component is also blocked in good balance in accordance with the above low frequency block.

FIG. 1 is a block diagram showing the principle of the present invention.
The output signal v ₁ of the microphone 1 is amplified by the variable gain amplifier 2 and output via the HPF (high-pass filter) 3 and the LPF (low-pass filter) 4. One of the characteristics of the present invention is that the HPF 3 is provided. On the other hand, the low frequency component (LF component) that is the wind noise component of the microphone signal v ₁ is L
DC signals V ₁₁ and V detected by the F level detection circuit 5
_It is converted to ₁₂ and applied to HPF3 and LPF4, respectively. HPF3 and LPF4 changes the respective cut-off frequency in accordance with the V ₁₁ and V _12. For example, when the wind noise is large, the DC signal V ₁₁ increases, the cutoff frequency of the HPF 3 increases, and the cutoff frequency of the LPF 4 decreases. On the contrary, when the wind noise is small, the DC signal V ₁₁ is lowered, and accordingly, the cutoff frequency of the HPF 3 is lowered, the cutoff frequency of the LPF 4 is raised, and the bandwidth of the output signal v ₂ is widened.

The ALC circuit 6 detects the level of the output signal v ₂ and controls the gain of the amplifier 2. Since the output signal v ₂ does not include the wind noise component, the ALC circuit does not malfunction due to the wind noise. Of course, the cutoff frequencies of the HPF 3 and the LPF 4 may be manually controlled by the operator of the video camera in view of the wind conditions. As is well known, since the size and weight of a video camera are especially important, it is necessary to make the wind noise removing circuit as small as possible. In particular, the capacitors used for the HPF 3 and the LPF 4 are difficult to be integrated into an IC and are larger than other parts. Therefore, there is a demand for a circuit capable of reducing the capacitance of these capacitors as much as possible and further reducing the number thereof.

FIG. 3 is one of the circuit diagrams of the embodiment of the present invention which achieves the reduction of the capacitance and the reduction of the number of the capacitors. In FIG. 3, the HPF 3 is composed of a capacitor C ₁ and a variable resistor Rv, and is moved to the input side of the amplifier 2. Since the high frequency component of the input signal v ₁ appears at the variable resistance Rv terminal, the amplifier 2 amplifies it. Further, since the low-frequency component of the input signal v ₁ appears at the end of the capacitor C ₁ , the LF level detection circuit 5 appropriately amplifies, detects and smoothes this, and applies the obtained DC signal V ₁₁ to the variable resistor Rv. Also, the DC signal V ₁₂ is applied to the LPF 4.

Normally, the LF level detecting circuit 5 needs an LPF circuit for extracting the above low frequency component from the input signal v ₁ , but by using the voltage at the capacitor C ₁ terminal as described above, The LPF circuit can be omitted. Further, the resistance value of the variable resistor Rv is changed in inverse proportion to the magnitude of the DC signal V ₁₁ . That is, when the wind noise is large, the resistance value of the variable resistor Rv decreases, so that the cutoff frequency of the HPF 3 increases and the removal ratio of the wind noise component increases.

The variable resistor Rv controlled by the DC signal V ₁₁ is usually a bipolar transistor or FET.
(Field effect transistor) or the like is used. As is well known, since the variable resistance characteristics of these transistors have nonlinearity, distortion increases rapidly when the applied signal voltage level is high. The HPF 3 formed by the capacitor C ₁ and the variable resistor Rv is inserted on the microphone side, and the applied signal voltage level is as low as several tens of mV, so that the distortion due to the nonlinearity can be sufficiently reduced to a negligible level. .. Further, by setting the resistance value range of the variable resistor Rv to the high resistance side, the capacitance of the capacitor C ₁ can be reduced.

FIG. 4 is a circuit diagram of another embodiment of the present invention for realizing the variable resistor Rv of FIG. 3 by another method. In FIG. 4, the resistance value of the resistor R ₁ is constant. The output of the amplifier 2 is negatively fed back to the lower side of the resistor R ₁ . Reference numeral 7 denotes a feedback circuit, the transfer function of which is B and the gain of the amplifier 2 is A. Assuming that the input voltage of the amplifier 2 is v ₃ and the output voltage thereof is v ₄ , v ₄ = (v ₃ −Bv ₄ ) A, so v ₄ / v ₃ = A / (1 + AB) (1) When the gain AB is sufficiently larger than 1, v ₄ / v ₃ ≈1 / B (2). Therefore, by keeping the transfer function B of the feedback circuit constant, the gains from v ₃ to v ₄ can be kept constant.

On the other hand, the voltage at the end of the resistor R ₁ is (v ₃ −Bv ₄ ).
Therefore, substituting equation (1) into this yields v ₃ −Bv ₄ = v ₃ {1-AB / (1 + AB)} = v ₃ {1 / (1 + AB)}, and the loop gain AB is smaller than 1. If it is sufficiently large, v ₃ −Bv ₄ ≈v ₃ / AB (3). That is, the voltage at the end of the resistor R ₁ is 1 by negative feedback.
/ AB, so the resistance value seen from v ₃ is increased to ABR ₁ .

The resistance value can be changed by changing the gain A of the amplifier 2. Therefore,
Capacitor C ₁ and with the resistor ABR ₁ can constitute a variable filter circuit similar to FIG. 3, further, since the value of the resistor ABR ₁ can be increased by setting a large round gain AB, the capacitor C ₁ The capacitance value can be reduced, which allows the filter circuit to be downsized. The gain A of the amplifier 2 is the DC signal V ₁₁
When the wind noise is loud, the cutoff frequency of the HPF 3 is increased. Further, since the resistor R ₁ has no nonlinearity, no distortion occurs regardless of the level of the signal voltage v ₃ .

FIG. 5 is a circuit diagram of another embodiment of the present invention in which the function of the LPF 4 in the circuit of FIG. 4 is included in the HPF 3 circuit composed of the capacitor C ₁ and the resistor R ₁ to further reduce the circuit configuration. is there. In FIG. 5, the fact that the cutoff frequency of the HPF3 and the cutoff frequency of the LPF3 of the microphone circuit are normally sufficiently separated is utilized. That is, in the low frequency band,
The resistor R ₁ can be ignored compared to the impedance of the capacitor C _{1, and} the impedance of the capacitor C ₂ can be ignored compared to the resistor R ₁ , so that FIG. 5 is equivalent to the HPF circuit of FIG. 4.

Further, in the high frequency band, Inpi capacitor C ₁ as compared to the resistance R ₁ - dance negligible, further the capacitor C ₂ Inpi - the resistance R ₁ is negligible compared to dance, 5 Figure 6 can be approximated.
Since the lower side of the capacitor C ₂ output v ₄ of the amplifier 2 is negative feedback, the capacitance of the capacitor C ₂ as seen from the input of the amplifier 2 is reduced to C ₂ / AB. Therefore, by changing the gain A of the amplifier 2 in proportion to the magnitude of the DC signal V ₁₁ , the cutoff frequency of the LPF 4 including the resistor R ₂ and the capacitor C ₂ can be changed.

For example, when the wind noise is large, the gain A of the amplifier 2 is increased due to the increase of the DC signal V ₁₁ , the capacitance value C ₂ / AB is decreased, and the cutoff frequency of the LPF 4 is lowered. At this time, since the cutoff frequency of the HPF 3 is high, the pass band width of the microphone signal is narrowed in a balanced manner. Conversely, when the wind noise is low, the pass band width of the microphone signal is widened in a balanced manner. In the circuits shown in FIGS. 4 and 5, the ALC circuit 6 shown in FIGS.
Is omitted. The gain of the circuit of FIGS.
It has been described above that it is determined by the transfer function B of Therefore, the ALC operation can be obtained by controlling the value of B by the output of the ALC circuit 6.

On the other hand, the cutoff frequency of the filter is the gain A
Since it is determined by B, the cutoff frequency changes with the ALC operation. That is, when the signal level rises and the output of the ALC circuit 6 increases, the pass band width of the filter circuit decreases. The ALC operation described above is convenient when such an increase in signal level is caused by wind noise. However, if the number of voice signals to be recorded is increased, the pass band width is narrowed corresponding to high frequencies, which is inconvenient. However, when such inconvenience is allowed to some extent, the ALC circuit 6 is
By adopting the method of controlling the transfer function B of the feedback circuit 7 by the output of, the advantage of miniaturizing the circuit can be obtained.

Further, since each circuit of the present invention does not distinguish between the wind noise signal and the low frequency component of the voice signal to be recorded, the low frequency component of the voice signal is suppressed in the no wind state. In many cases, such a low-frequency component is small, so that a practical harmful effect is small. However, if you want to record the low frequency component of the audio signal without omission,
It is necessary to stop the cutoff frequency control of the F and LPF circuits and fix the cutoff frequencies of these circuits to a predetermined value. Therefore, in the present invention, for example, the output of the LF level detection circuit 5 is fixed to a predetermined value based on a command from the microcomputer.

[0024]

According to the present invention, the wind noise component of the microphone is removed by the high-pass filter, and at the same time, the high frequency component of the microphone signal is removed by the low-pass filter in a well-balanced manner. Can be recorded. In addition, since the resistance used in the high-pass filter is increased by the negative feedback technique to control the value, the capacitor of the filter circuit can be downsized and downsized. Similarly, the negative feedback technique described above makes it possible to reduce the capacitance and size of the low-pass filter capacitor and control its capacitance value. By reducing the capacity of the capacitor of the filter circuit, the wind noise eliminating circuit and the sound quality automatic adjusting circuit of the video camera can be downsized. Further, since the function of the ALC circuit for suppressing the output fluctuation of the microphone circuit can also be performed by using the filter circuit, the audio circuit of the video camera can be further downsized.

[Brief description of drawings]

1 is a basic block diagram of an audio circuit according to the present invention.

FIG. 2 is an example of a wind noise spectrum of a microphone.

FIG. 3 is a specific partial circuit diagram of an audio circuit according to the present invention.

FIG. 4 is another specific partial circuit diagram of the audio circuit according to the present invention.

FIG. 5 is another specific partial circuit diagram of the audio circuit according to the present invention.

6 is an equivalent circuit in the high frequency band of FIG.

[Explanation of symbols]

 1 Microphone 2 Amplifier 3 HPF (High Pass Filter) 4 LPF (Low Pass Filter) 5 LF (Low Frequency) Level Detection Circuit 6 ALC (Automatic Volume Control) Circuit 7 Feedback Circuit

Claims (5)

[Claims] 1. An audio circuit for recording of a video camera for detecting a low frequency component strength of a microphone signal to control a low cutoff frequency of a high pass filter, wherein a high cutoff frequency is detected by an output of the low frequency component strength detection circuit. An audio circuit for a video camera, comprising a controlled low-pass filter. 2. The high-pass filter according to claim 1, wherein the high-pass filter is configured by connecting a first capacitor element and a variable resistance element in series, and a signal is input to one end of the first capacitor element to output from the variable resistance element end. A video camera, wherein an output voltage is taken out, an input terminal of the low frequency component intensity detection circuit is connected to a first capacitor element terminal, and a resistance value of the variable resistance element is controlled by an output signal thereof. Voice circuit. 3. The series circuit of the first resistance element and the first capacitor element, the variable gain amplifier for amplifying the voltage at the first resistance element end, and the output signal of the variable gain amplifier according to claim 1. And a feedback circuit for negatively feeding back to the first resistance element end, applying an input signal to the first capacitor element end, and connecting the input terminal of the low frequency component intensity detection circuit to the first capacitor element end. The audio circuit of the video camera, wherein the gain of the variable gain amplifier is controlled by the output signal of the audio amplifier. 4. The method according to claim 3, wherein an input signal is applied to one end of the first capacitor element via a second resistance element, and the second capacitor element is connected to the first resistance element. Characteristic video camera audio circuit. 5. The method according to any one of claims 1 to 4,
An audio circuit for a video camera, comprising means for fixing the output of the low frequency component intensity detection circuit to a predetermined voltage.