JPH0145769B2 - - Google Patents

Description

[Detailed Description of the Invention] (1) Technical Field of the Invention The present invention uses a high-sensitivity microphone to detect environmental sounds (noise noise) that enter a loudspeaker telephone or the like that is capable of transmitting a call from a distance of approximately 50 cm or more when the telephone is not transmitting a call. This invention relates to a microphone amplification circuit with a gain switch that improves the S/N ratio of a transmitted signal by suppressing the noise.

(2) Conventional technology and problems Conventionally, in microphone amplification circuits such as loudspeaker telephones, the level difference between stationary noise such as air conditioning noise and voice is used to increase the level of noise higher than the stationary noise level.
There is a method that has a variable gain switch with a threshold set at a point lower than the audio level.

FIG. 1 shows a microphone amplification circuit as an example. In the figure, a voice S, an environmental sound (stationary noise) N, etc. pass through a microphone M, a preamplifier P, and are outputted to a telephone circuit or the like via a variable loss circuit L. Therefore, the rectifier circuit R, the time constant circuit T, τ
The output level of the variable loss circuit L is controlled by setting a threshold value using the difference between the stationary noise level and the voice level from the voice level detection circuit V consisting of the above.

FIG. 2 shows the input/output characteristics of the circuit shown in FIG. 1, where the horizontal axis shows the input level of voice and noise, and the vertical axis shows the output level. When the input level becomes higher than the threshold value A, the audio level detection circuit V controls the variable loss circuit L to a low loss of 1, and when the input level becomes lower than the threshold value A, that is, when there is no audio input and the noise level becomes a constant noise level, the audio level detection circuit V controls the variable loss circuit L to a low loss of 1. The circuit V controls the variable loss circuit L to have a high loss of 2. In this way, by changing the output level in stages between the time of voice input and the time of no voice, as shown in 3, with level A as the boundary, the stationary noise when the speaker is not speaking can be suppressed to a low level. .

However, the level of stationary noise is not constant depending on the location, and if the threshold of the audio level detection circuit is fixed, the location where the microphone can be installed is restricted, and it is difficult to reset the threshold depending on the environment where the microphone is installed. The problem was that the noise in the environment had to be measured in advance, and that it was not possible to deal with fluctuations in noise economically.

(3) Purpose of the Invention In order to solve the above conventional problems, the present invention makes the threshold variable according to the level of stationary noise, and reduces the need for human intervention according to changes in the environment and changes over time. The purpose of the present invention is to provide a microphone amplification circuit that enables output at an optimum level in real time without any noise.

(4) Structure of the Invention In order to achieve the above object, the present invention includes a first variable loss circuit, an audio level detection circuit, and a steady noise level detection circuit on the output side of a microphone preamplifier, and The detection circuit has a second
A variable loss circuit is provided, the second variable loss circuit is controlled from a low loss state to a high loss state in proportion to the output of the steady noise level detection circuit, and the first variable loss circuit is controlled from a low loss state to a high loss state in proportion to the output of the steady noise level detection circuit. The output of the circuit is controlled stepwise from a high loss state to a low loss state with a preset threshold as the boundary, and the setting of the threshold is variable in proportion to the amount of loss of the second variable loss. do.

(5) Examples of the invention The present invention will be explained in detail below using examples.

FIG. 3 shows an example of a circuit that makes the threshold variable depending on the steady noise level. The difference from the circuit in FIG. 1 is that a second variable loss circuit L2 is provided at the input section of the audio level detection circuit V, and the control of the variable loss circuit L2 is such that a part of the output of the audio level detection circuit V is The point is that this is performed using a signal fed back via constant circuits T2 and _τ2 . That is, the variable loss circuit L1 at the rear stage of the microphone preamplifier is controlled to be in a high loss state when there is no audio input, and is controlled to be in a low loss state when the audio input exceeds a certain threshold, as in the conventional case. However, the steady noise component with little level fluctuation contained in the output of the audio level detection circuit V is further discriminated by the time constant circuit T2, _τ2 , and the variable loss L2 is changed from a low loss state to a high loss state in proportion to the level. The aforementioned threshold value is made variable by controlling the loss state.

Therefore, the variable loss circuit L1 gradually shifts to the first threshold value when the steady noise level is low, and gradually shifts to the second threshold value higher than the first threshold value when the overall steady noise level is high. For such a circuit, in the input/output characteristics shown in FIG.
Between the second threshold b, from low loss 1 to high loss 2,
Or vice versa, the variable loss circuit L1 is controlled to make a gradual transition as shown in 4.

This feedback method uses thresholds a, b
However, when the steady noise level becomes relatively high (for example, as shown at point C), the switching control of the variable loss circuit L1 will not result in high loss, as shown by the output level difference 5. First, there remains the problem that it is difficult to turn as a gain switch.

Accordingly, the present invention has been further improved to solve the conventional problems and the problems with the above-described feedback system by adopting the circuit configuration shown in FIG.

In Fig. 4, a rectifier circuit R, a time constant circuit T1, τ1 that responds quickly to audio level changes and has an appropriate hangover, and a time constant circuit T1, _τ1 that does not respond to fast level changes and outputs a constant output in response to a stationary input. Steady noise level detection circuit consisting of time constant circuit T2, τ ₂
A VT is provided in parallel with the sound level detection circuit V, and the variable loss circuit L2 is subjected to feedforward control by the output of the steady noise level detection circuit VT. The time constant circuits T1 and T2 are, for example, τ ₁ charging time tr several ms, discharging time tf ~ several tens to several hundreds m
s, the charging time tr ~ several seconds for τ ₂ , and the discharging time tf ~ several seconds
It has a function of holding (bottom hold) the low level range of voice plus steady noise level, that is, it discriminately outputs only the steady noise level component.

In response to this output, the variable loss circuit L2 is controlled to have a low loss when the steady noise level is low, and to have a high loss as the steady noise level increases.

Therefore, when the steady noise input level is sufficiently small, the variable loss circuit L2 has a low loss, so the gain of the audio level detection circuit V is sufficiently large, and the variable loss circuit L1 can easily change from high loss to low loss depending on the audio input. Switch to . That is, the threshold value remains at the preset minimum value (threshold value a). As the steady noise level increases, the variable loss circuit L2 gradually shifts to high loss, so the gain of the audio level detection circuit V gradually becomes smaller and the threshold value of the variable loss circuit L1 gradually becomes higher. As a result, the variable loss circuit L1 cannot switch to low loss only with its steady noise input, and only switches to low loss when a higher level audio input is received (transition to threshold b). The higher the steady noise level is, the higher the threshold value of the variable loss circuit L1 becomes, so that the steady noise when there is no voice input can be suppressed over a wide range of levels.

As shown in FIG. 5, the input/output characteristics of the circuit configuration of the present invention have a sufficiently low stationary noise (threshold a), and as the stationary noise level increases, the threshold gradually increases. The noise level at that time is sufficiently suppressed by the loss amount of the first variable loss, and the loss is quickly switched to low loss only by a voice input higher than the noise level.

Therefore, compared to the feedback method described earlier, the gain switching of the variable loss circuit L1 is effectively controlled, and the output of the amplifying circuit is better controlled and the level of the voice of the other party is improved. It is also possible to smoothly operate the voice switch that controls voice switching.

(6) Effects of the Invention As explained above, according to the present invention, a variable loss circuit is provided on the output side of the microphone preamplifier, and when the output of the audio level detection circuit exceeds a preset threshold, the loss changes from high loss to low loss. and the variable loss circuit of the audio level detection circuit controls the threshold according to steady-state noise level fluctuations, providing a sharp gain switch whether the steady-state noise is relatively low or high. This makes it possible to prevent malfunctions caused by constant noise such as voice switches of loudspeaker telephones, etc., and to make it possible to make calls smoothly.

[Brief explanation of drawings]

Figure 1 is a configuration diagram of a conventional microphone amplification circuit.
2 is an input/output characteristic diagram of FIG. 1, FIG. 3 is a configuration diagram of a microphone amplification circuit improved from the conventional circuit shown in FIG. 1, FIG. 4 is a configuration diagram of a microphone amplification circuit of the present invention, and FIG. This figure is a diagram explaining the input/output characteristics of the circuit configurations of FIGS. 3 and 4. M: Microphone, VT: Steady noise level detection circuit, V: Audio level detection circuit, L1, L2: Variable loss circuit, T1, T2: Time constant circuit.

Claims (1)

[Claims] 1 A microphone preamplifier is provided with a first variable loss circuit, an audio level detection circuit, and a steady noise level detection circuit on the output side, the audio level detection circuit is provided with a second variable loss circuit, and the second variable loss circuit is provided with a second variable loss circuit. The variable loss circuit is controlled from a low loss state to a high loss state in proportion to the output of the steady noise level detection circuit,
The first variable loss circuit is controlled stepwise from a high loss state to a low loss state with a threshold set in advance based on the output of the audio level detection circuit, and the setting of the threshold determines the loss of the second variable loss. A microphone amplification circuit characterized by being variable in proportion to the amount.