JPS5952875B2 - Howling prevention device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to howling prevention devices for various acoustic devices,
The purpose of this invention is to propose a device that can reliably prevent howling with a particularly simple configuration.

First, the mechanism of howling is generated will be explained below with reference to FIG.

Figure 1 is a system diagram of a conventional amplification circuit system, where 1 is a microphone, 2' is an amplification circuit to which the audio signal from the microphone is supplied, and 3 is an audio signal amplified by the amplification circuit 2'. In this case, the microphone 1 and the speaker 3 are arranged in the same space.

In this way, when the audio signal of the human voice from the microphone 1 is sufficiently amplified by the amplifying circuit 2' having a large gain and is emitted from the speaker 3 arranged in the same space as the microphone 1, the speaker 3 The sound emitted from the microphone 1 is returned to the microphone 1 and becomes more likely to oscillate.

By the way, this system oscillates after a considerable amount of time has elapsed from the rising edge of the audio signal, so if the gain of the amplification circuit 2' is controlled with this point in mind, howling can be suppressed. The occurrence can be effectively prevented.

Therefore, the present applicant previously proposed a howling prevention device that can reliably prevent howling with a simple configuration.

The howling prevention device will be explained below with reference to FIGS. 2 and 3.

FIG. 2 is a system diagram of an example of the howling prevention device.

In FIG. 2, parts corresponding to those in FIG. 1 will be described with the same reference numerals.

First, an audio signal from a microphone 1 is supplied to a variable gain amplification circuit 2, and is also supplied to a rectifier circuit (in this embodiment, a double-wave rectifier circuit) 5, where it is rectified.

This rectified voltage is applied to an amplitude discriminator 6 as a control signal generation circuit.
Here, when the level of the rectified output voltage is above the first reference level VO, it rises steeply, and when it is below the second reference level (equal to the first reference level in this example) VO, it rises to a predetermined level. A rectangular wave signal as shown in FIG. 3B that gradually falls with a time constant is generated.

This is to prevent the beginning of a word, especially a consonant, from being lost at the beginning of a voice, and to make the end of a voice clear.

A voltage dividing resistor 7.8 is connected in series between the output terminal of the control signal generation circuit 6 and the power supply element B, and a voltage with a waveform as shown in FIG. 3C is generated from the midpoint of the connection. is supplied to the control terminal of the variable gain amplification circuit 2.

The gain of the variable gain amplification circuit 2 is maintained at a predetermined level for each syllable of the audio signal shown in FIG. 3A, but is kept sufficiently low, for example, 5 dB, between each syllable.

Therefore, the occurrence of howling can be reliably prevented by suppressing the gain of the variable gain amplifier circuit 2 to a sufficiently low level before reaching the so-called build-up time, which is the time from the rise of the audio signal to the occurrence of howling.

However, since the rectangular wave signal which is the output of the control signal generation circuit as shown in FIG. 3B rises rapidly at its rising edge, discontinuous portions occur in the audio signal waveform, which tends to be audibly unnatural.

In addition, the level of background noise of the audio signal to be amplified changes as if it were modulated with the gain control of the audio signal.
This becomes easy to hear.

In view of these points, the present invention uses a simple circuit configuration to ensure that the audio signal does not become audibly unnatural, and that background noise of the audio signal does not become difficult to hear, thereby reliably preventing the occurrence of howling. The purpose of this paper is to propose a howling prevention device that can prevent howling.

Next, an embodiment of the howling prevention device of the present invention will be described with reference to FIG.

In FIG. 4, parts corresponding to those in FIG. 2 are designated by the same reference numerals, and redundant explanation thereof will be omitted.

In FIG. 4, the audio signal from the microphone 1 is supplied to a pre-emphasis circuit 9, and the high frequency components of the audio signal are emphasized.

The output of the pre-emphasis circuit 9 is then supplied to the variable gain amplification circuit 2 and the rectifier circuit 5.

This rectified output is supplied to the control signal generation circuit 6, and when the level of the rectified output exceeds the first reference level, it rises with a predetermined time constant, and when it is below the second reference level, it rises with a predetermined time constant or more. A falling rectangular wave signal is generated.

This rectangular wave signal is then set to a predetermined DC potential by a resistor 7.8 and supplied to the variable gain amplification circuit 2.

Next, the output audio signal of the variable gain amplification circuit 2 is supplied to a de-emphasis circuit 10, and its high frequency components are suppressed and supplied to the speaker 3.

Incidentally, the pre-emphasis circuit 9 as described above is configured as shown in FIG. 5, for example, and its frequency characteristics are expressed as shown in FIG. 6.

In FIG. 5, the input terminal of the pre-emphasis circuit 9 is connected to the inverting input terminal of an operational amplifier 25 through a series circuit of a resistor 21 of Ω at 10 and a resistor 23 of Ω at 100. A 1200PF capacitor 22 is connected in parallel with the 1200PF capacitor 22 .

The non-inverting input terminal of the operational amplifier 25 is directly grounded, and the output terminal thereof is connected to the output terminal 26 of the pre-emphasis circuit 9 and connected to the inverting input terminal of the operational amplifier 25 through a 100KΩ feedback resistor 24. Connected to the input terminal.

Further, the de-emphasis circuit 10 as described above may be used, for example, in the seventh
It is constructed as shown in the figure, and its frequency characteristics are expressed as shown in FIG.

In FIG. 7, input terminal 2 of the de-emphasis circuit 10
7 is connected to the inverting input terminal of the operational amplifier 32 through a 100Ω resistor 28, and its non-inverting input terminal is directly grounded.

The output terminal of the operational amplifier 32 is connected to the de-emphasis circuit 10.
10 connected in series to the output terminal 33 of
0 to the inverting input terminal of the operational amplifier 32 via the feedback resistors 31 and 29 of Ω and IOKΩ, and the resistor 31
A 1200PF capacitor 30 is connected in parallel with the 1200PF capacitor 30.

With this configuration, the high frequency components of the audio signal are first emphasized and then supplied to the variable gain amplification circuit 2, so that when the audio signal is gain controlled here, the signal waveform is discontinuous. This is alleviated, and the sound emitted from the speaker becomes audibly natural.

In addition, the noise generated in the variable gain amplification circuit is reduced as a whole, and the background noise of the audio signal to be amplified changes in level as if it were modulated with the gain control of the audio signal, and this becomes audible. It never gets easier.

[Brief explanation of drawings]

1 and 2 are system diagrams of the amplification circuit system to explain the present invention, FIG. 3 is a waveform diagram to explain the operation of FIG. 2, and FIG. 4 is a system diagram of an embodiment of the present invention. , Figure 5 is a circuit diagram of an example of a pre-emphasis circuit, Figure 6 is a frequency characteristic diagram of a pre-emphasis circuit, Figure 7 is a circuit diagram of a de-emphasis circuit, and Figure 8 is a frequency characteristic diagram of a de-emphasis circuit. be. 1 is a microphone, 2 is a variable gain amplification circuit, 3 is a speaker, 5 is a rectifier circuit, 6 is a control signal generation circuit, 9 is a pre-emphasis circuit, and 10 is an emphasis circuit.

Claims (1)

[Claims] 1. A pre-emphasis circuit to which an audio signal from a microphone is supplied, a variable gain amplification circuit to which the output of the pre-emphasis circuit is supplied, a de-emphasis circuit to which the output of the variable gain amplification circuit is supplied; A rectifier circuit to which the output of the pre-emphasis circuit is supplied, and a rectifier output of the rectifier circuit, which rises sharply when the level of the rectifier output is above a first reference level and at a predetermined time when it is below a second reference level. a control signal generation circuit that generates a rectangular wave signal that falls at a constant rate; the rectangular wave signal from the control signal generation circuit is supplied to the variable gain amplification circuit to control its gain; A howling prevention device configured to supply an output audio signal from a de-emphasis circuit to a speaker.