JPH01181209A - Microphone audio signal processing circuit - Google Patents

Abstract

PURPOSE:To suppress the recording of microphone noise and to reduce unpleasant noise at reproduction by controlling the circuit in a manner of decreasing the amplification factor of the variable gain amplifier circuit amplifying a microphone audio signal when the audio signal outputted from a microphone is small and noise from the microphone is dominant. CONSTITUTION:An audio signal from the microphone 1 is given to a band pass filter 2 where an undesired band signal such as wind noise is attenuated and the result is given to a variable gain amplifier circuit 11. Then the output signal of the band pass filter 2 is amplified by a preamplifier circuit 12 and fed to a microphone audio signal level detection circuit 13. A DC signal in response to the microphone audio signal level is outputted from the microphone audio signal level detection circuit 13 and fed to an amplification factor control circuit 14. When the audio signal outputted from the microphone is small and noise from the microphone is dominant, the amplification factor control circuit 14 detects by using the output signal from the microphone audio signal level detection circuit 13 and gives an output signal to command the decrease of the amplification factor to the variable gain amplifier circuit 11.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is applicable to, for example, a camera-integrated video tape recorder (hereinafter referred to as a camera-integrated VTR) and an audio/video tape recorder (hereinafter referred to as a camera-integrated VTR).
The present invention relates to a microphone audio signal processing circuit that is applied to processing microphone audio signals in magnetic recording and reproducing devices that record and reproduce audio signals input from a microphone (hereinafter referred to as a microphone) such as a tape recorder.

[Prior Art] FIG. 2 shows a conventional microphone audio signal processing circuit that is used, for example, in a camera-integrated VTR and records audio signals input from a microphone on a magnetic tape with an AC bias using a fixed magnetic head for recording and reproducing audio signals. It is a block diagram showing the configuration of (1) a microphone, (2) a band pass filter, (3) an amplifier circuit for amplifying the microphone audio signal, (4) an audio signal recording processing circuit, ( 5) is an AC bias circuit, and (6) is a fixed magnetic head for recording and reproducing audio signals.

(7) is a magnetic tape.

Next, the operation of the above configuration will be explained.

The audio signal output from the microphone (1) is passed through a bandpass filter (2) to attenuate unnecessary band signals, and then
The microphone audio signal amplification circuit (3) amplifies the signal and then sends it to the audio signal recording processing circuit (4).

This audio signal recording processing circuit (4) prevents clipping caused by the circuit and deterioration of distortion of the audio signal in the process of recording and reproducing it on the magnetic tape (7) using a predetermined tributary bias when recording a large amplitude audio signal. When a large-amplitude audio signal is input, automatic level control is used to automatically control the signal level so that it does not exceed a predetermined level, or magnetic tape (7)
In order to prevent a drop in the high frequency signal level of the audio signal during the recording/reproduction process using the fixed magnetic head (6) for recording and reproducing audio signals, processing such as equalization is performed in advance to emphasize and record the high frequency signal.

Audio signal recording processing circuit (4) subjected to such processing
The output signal is superimposed on the AC bias signal outputted by the AC bias circuit (5), and AC bias recording is performed on the magnetic tape (7) by the fixed magnetic head (6) for recording and reproducing audio signals.

[Problems to be Solved by the Invention] Since the conventional microphone audio signal processing circuit is configured as described above, it has the following first-order problem.

In other words, when looking at camera-integrated VTRs, an electret microphone is generally used as the built-in microphone for this camera-integrated VTR, and although it varies somewhat depending on the diameter of the microphone, the S/N ratio of the microphone alone is approximately 40 dB.
That's about it. In addition, since such a built-in microphone is attached directly to the VTR body, it eliminates noise generated from the VTR's mechanical system, such as the sound of the magnetic tape running and the noise generated when the magnetic tape comes into contact with the rotating magnetic head. As a result, the S/N ratio of the microphone audio signal becomes worse than the S/N ratio of the microphone alone.

In addition, the band pass filter (2) placed before the microphone audio signal amplification circuit (3) mainly attenuates signal components below 100Hz, reducing wind noise caused by wind hitting the microphone (1). Although the band is limited to balance this and reduce unnecessary high-frequency components, the noise generated from the 1a structure may also contain mid-range frequency components of several KH2. Such noise cannot be removed by the bandpass filter (2).

On the other hand, since the S/N ratio of the reproduced audio signal when viewed from the input of the audio signal recording processing circuit (4) is generally about 40 to 50 dB, the noise generated from the mechanical system picked up by the above-mentioned microphone can also be magnetically absorbed. If you end up recording on tape (7) and the audio signal output from the microphone is relatively small, such as when the surrounding audio is low when shooting with a camera, when you play it back after recording, the built-in microphone may pick up the audio signal. The reproduced noise generated from the mechanical system becomes noticeable.

In addition, the noise generated from the mechanical system picked up by the built-in microphone also depends on the position of the built-in microphone on the VTR body. In other words, the mounting position of the built-in microphone is generally limited by the design and external shape of the camera-integrated VTR, and it cannot be said that the built-in microphone is mounted at the optimal position to prevent noise from being picked up from the mechanical system.

This invention was made to solve the above-mentioned problems.When the sound picked up by the microphone is small and there is noticeable noise such as noise from the mechanical system or noise from the microphone itself, the sound to be reproduced becomes An object of the present invention is to provide a microphone audio signal processing circuit capable of making internal noise inconspicuous.

[Means to solve the problem].

The microphone audio signal processing circuit according to the present invention includes a microphone audio signal level detection circuit,
The present invention is also characterized in that it is provided with an amplification control circuit that operates to lower the amplification degree of the variable gain amplification circuit that amplifies the microphone audio signal when microphone noise becomes dominant.

[Effect]

According to this invention, when the level of the audio signal output from the microphone is low and microphone noise becomes dominant,
By operating the amplification control circuit and controlling the amplification of the variable gain amplifier circuit that amplifies the microphone audio signal according to the output signal of the microphone audio signal level detection circuit to lower the amplification degree, recording of microphone noise is suppressed during playback. Reduces harsh noises at times.

[Embodiment of the Invention] An embodiment of the present invention will be described below based on the drawings.

FIG. 1 is a block diagram showing the configuration of a microphone audio signal processing circuit according to an embodiment of the present invention.
1) is a microphone, (2) is a band pass filter, (4) is an audio signal recording processing circuit, (5) is an AC bias circuit, (6)
) is a fixed magnetic head for recording and reproducing audio signals, and (7) is a magnetic tape, which are the same as the conventional one shown in FIG.

In FIG. 1, (11) is a variable gain amplifier circuit composed of, for example, a voltage-controlled amplifier circuit, (12) is a preamplifier circuit, and (13) is an audio signal level detection circuit that detects the level of a microphone audio signal. , (14) is an amplification control circuit.

Next, the operation of the above configuration will be explained.

The audio signal output from the microphone (1) is passed through a band pass filter (2) to attenuate unnecessary band signals such as wind noise, and then input to a variable gain amplifier circuit (11).

On the other hand, the output signal of the bandpass filter (2) is amplified by the preamplifier circuit (12) and sent to the microphone audio signal level detection circuit (13). The microphone audio signal level detection circuit (13) outputs a DC signal corresponding to the microphone audio signal level and sends it to the amplification control circuit (14). In this amplification degree control circuit (14), when the microphone voice signal is small and microphone noise becomes dominant, this is detected by the output signal of the microphone voice signal level detection circuit (13), and the variable gain amplifier circuit detects this using the output signal of the microphone voice signal level detection circuit (13). (moon), send an output signal that lowers its amplification.

The output audio signal of the variable gain amplifier circuit (11) is subjected to automatic level control processing and equalization processing in the audio signal recording processing circuit (4) as in the conventional example, and is superimposed on the AC bias signal to record and reproduce the audio signal. Fixed magnetic head (6) for magnetic tape (7)
recorded in

Figure 3 shows the microphone (1) in the above embodiment and the conventional example.
2 is a diagram for explaining the relationship between the output audio signal level and the input signal level to the audio signal recording processing circuit (4). For example, in a relatively small electret microphone used in a camera-integrated VTR, The S/N of the microphone itself is about 40 dB, and when the noise from the mechanical system is large, it is about 30-odd dB. On the other hand, the S/ of the reproduced audio signal seen from the input to the audio signal recording processing circuit (4)
In the case of AC bias recording as shown in the above embodiment, N is normally about 50-oddB. Therefore, in the case of the conventional example shown by the dotted line in FIG. Noise level (-
bdB) is -a>-b, and the noise from the microphone, including the noise from the mechanical system, will be recorded and played back on the magnetic tape (7), and if the sound is small when recording with the camera, A harsh noise will become noticeable in the reproduced sound.

However, in the case of the above embodiment, when the microphone audio signal level approaches the microphone noise level, the variable gain amplifier circuit (11
), the input/output characteristics will be as shown by the solid line in the MS3 diagram, for example. Therefore, the microphone noise level (-cdB
) and the input conversion reproduction noise level (-bdB) to the audio signal recording processing circuit (4) is -c>-b, which indicates that the audio to be recorded during camera recording is small and noise is dominant. When this happens, it becomes almost impossible to record it on the magnetic tape (7).

[Effects of the Invention] As described above, according to the present invention, when the audio signal level from the microphone is low and microphone noise such as noise generated from a mechanical system picked up by the microphone becomes dominant,
By lowering the amplification degree of the microphone audio signal, recording of microphone noise on magnetic tape can be suppressed, which improves the S/N ratio of the microphone audio signal and allows playback when the audio signal from the microphone is small. It is possible to reduce the harsh noise at times.

In addition, since the above-mentioned noise reduction can be achieved by controlling the amplification degree of the microphone audio signal, the microphone mounting is restricted to a position where it is easy to pick up noise generated from the mechanical system, such as a camera-body fiVTR. This has the effect that it can be effectively applied to, etc.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of a microphone audio signal processing circuit according to an embodiment of the present invention, FIG. 2 is a block diagram showing the configuration of a conventional microphone audio signal processing circuit, and FIG. 3 is an embodiment of the present invention. FIG. 3 is a characteristic diagram showing an example of the characteristics of the microphone output audio signal level and the input signal level to the audio signal recording processing circuit in the conventional example. (1) --- Microphone, (11) --- Variable gain amplification circuit, (13) --- Microphone audio signal level detection circuit, (14) --- Amplification degree control circuit. Note that the same reference numerals in the figures indicate the same or Wako parts.

Claims (1)

[Claims] (1) A variable gain amplifier circuit that amplifies the audio signal output from the microphone, an audio signal level detection circuit that detects the level of the audio signal, and output from the microphone according to the output signal of the audio signal level detection circuit. and an amplification control circuit that operates to lower the amplification degree of the variable gain amplification circuit by detecting that the voice signal to be transmitted is small and noise has become dominant. .