JPS5952874B2 - 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 we take this into account and control the gain of the amplifier circuit 2', The occurrence of howling can be effectively prevented.

In view of this, the present invention aims to propose a howling prevention device that can reliably prevent howling with a simple configuration.

An embodiment of the present invention will be described in detail below with reference to FIGS. 2 and 3.

FIG. 2 is a system diagram of an embodiment of the howling prevention device of the present invention.

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) VQ, it rises at a predetermined time. A rectangular wave signal as shown in FIG. 3B, which gradually falls with a constant constant, is generated.

This is to prevent consonants from being lost at the beginning of the voice and to make the ending of the voice clear.

Voltage dividing resistors 7 and 8 are connected in series between the output terminal of the control signal generating 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, 6 dB, between each syllable.

Therefore, the occurrence of howling can be reliably prevented by suppressing the gain of the variable gain amplification 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 until the howling grows.

By the way, in rare cases where a human voice continues almost without interruption, the gain of the amplification system for the audio signal becomes a large value over a long period of time, making it easy for howling to occur.

FIG. 4 is a circuit diagram of another embodiment of the present invention in which the occurrence of howling is more effectively prevented by taking this point into consideration.

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

That is, in the circuit shown in FIG.
The middle point of the connection between the resistor 9 and the capacitor 10 is connected to the cathode of the Zener diode 12.

The anode of the Zener diode 12 is connected to the midpoint between the voltage dividing resistors 13 and 14, and the other end of the resistor 13 is connected to the inverting input terminal of the operational amplifier 15 and connected to the negative terminal through the resistor 16. It is connected to the power supply (-B), the non-inverting input terminal of the operational amplifier 15 is grounded, and the output end of the operational amplifier 15 is connected to one end of the resistor 8 .

With this configuration, the output voltage of the control signal generating circuit 6 as shown in FIG. 3B is supplied to the integrating circuit 11, integrated, and then supplied to the Zener diode 12.

In addition, resistor 16.13,1 is connected to Zener diode]2.
A predetermined voltage is applied by 4.

As a result, when the output voltage of the integrating circuit 11 exceeds a certain voltage regulated by the Zener diode 12, the Zener diode 12 becomes conductive and reduces the DC voltage supplied from the operational amplifier 15 to the variable gain amplifier circuit 2. By doing so, the gain of the amplification circuit system is gradually reduced, and even if the audio signal continues almost without interruption, howling can be reliably prevented from occurring.

In this way, the circuit of the embodiment shown in FIG.
Howling prevention effect was obtained.

It was also confirmed that this circuit can significantly reduce noise other than voice.

Furthermore, since howling often occurs in the frequency range of 500 to 3kB of audio signals, a method of switching only the signal components in this frequency range as shown in Figure 5 can also achieve almost the same effect as described above. It will be done.

Next, a circuit according to another embodiment of the present invention shown in FIG. 5 will be explained.

In FIG. 5, parts corresponding to those in FIG. 4 are denoted by the same reference numerals, and redundant explanation thereof will be omitted.

That is, in the circuit shown in Fig. 5, the passband frequency is 500 to 3.
A kHz bandpass filter 20 is newly inserted between the microphone 1 and the variable gain amplification circuit 2.

Then, the audio signal from the microphone 1 is transmitted to the operational amplifier 2.
2, the output from the bandpass filter 20 is supplied to the inverting input terminal of the operational amplifier 22, and the output of the operational amplifier 22 is connected to one input of the summing amplifier 23. The output of the variable gain amplification circuit 2 is supplied to the other input terminal of the summing amplifier 23, and the output of the summing amplifier 23 is supplied to the speaker 3.

The circuit shown in FIG. 5 can also improve the howling prevention effect in the same way as the circuit shown in FIG. 4 described above.

In addition, in the circuit shown in FIG. 5, the signal processed in this way and the unprocessed signal are supplied to the summing amplifier 23 through the bandpass filter 20, and the output of the summing amplifier 23 is sent to the speaker 3.
This has the advantage that when the variable gain amplification circuit 2 switches the audio signal using the control signal, the audio signal does not fluctuate much, and the noise caused by this fluctuation in the audio signal is less likely to be audible. be.

Thus, according to the howling prevention device of the present invention, the audio signal from the microphone is supplied to the variable gain amplification circuit and also to the rectifier circuit, and the rectified output of the rectifier circuit is supplied to the control signal generation circuit to produce the rectified output. obtains a rectangular wave signal that rises steeply when the level is above a first reference level and falls with a predetermined time constant when it is below a second reference level, and supplies this rectangular wave signal to a variable gain amplification circuit. Since the output audio signal from the variable gain amplification circuit is supplied to the speaker by controlling its gain, the occurrence of howling can be suppressed very effectively with a very simple circuit configuration.

Also, the output of the gain control signal generation circuit is integrated, and when this integrated output exceeds the regulation voltage given by the Zener diode, a control signal is sent from the polarity inversion amplifier to the variable gain amplifier circuit to reduce the amplification gain. If this is supplied, it is possible to reliably prevent howling from occurring even when the audio signal continues almost without interruption.

Furthermore, 50% of the audio signal often causes howling.
Only signal components in the frequency range from 0 to 3 kHz are extracted and supplied to a variable gain amplification circuit, where the amplification gain is controlled,
Next, if this signal is added to an unprocessed audio signal and supplied to the speaker, howling can be suppressed more reliably and effectively, and when switching the audio signal, the audio signal It does not fluctuate much, and noise caused by this fluctuation in the audio signal is difficult to hear.

[Brief explanation of the drawing]

FIG. 1 is a system diagram of an amplification circuit system for explaining the present invention, FIG. 2 is a circuit diagram of an embodiment of the howling prevention device of the present invention,
FIG. 3 is a waveform diagram for explaining the operation of FIG. 2, and FIGS. 4 and 5 are circuit diagrams of other embodiments of the howling prevention device of the present invention. 1 is a microphone, 2 is a variable gain amplification circuit, 3 is a speaker, 5 is a rectifier circuit, and 6 is a control signal generation circuit.

Claims (1)

[Claims] 1. A variable gain amplification circuit to which the audio signal from the microphone is supplied; a rectifier circuit to which the audio signal is supplied;
Generates a rectangular wave signal that is supplied with the rectified output of the rectifier circuit and that rises steeply when the level of the rectified output is above a first reference level and falls with a predetermined time constant when the level of the rectified output is below a second reference level. a control signal generation circuit that supplies a rectangular wave signal from the control signal generation circuit to the variable gain amplification circuit to control its gain, and outputs an output audio signal from the variable gain amplification circuit to a speaker. Howling prevention device configured to supply to.