JPS5952876B2 - 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 microphone 1 is supplied, and 3 is the audio amplified by the amplification circuit 2'. This is a speaker to which a signal is supplied, and 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 amplifier circuit 2' is controlled taking this point into consideration, howling can be prevented. can be effectively prevented from occurring.

Therefore, the applicant of the present application 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.
, where the level of the rectified output voltage is a first reference level V.

When it is above, it rises steeply, and when it is below the second reference level (equal to the first reference level in this example), it falls slowly with a predetermined time constant as shown in Figure 3B. A square wave signal 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.

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 between each syllable C, for example, by 6 dB.

Therefore, it is possible to reliably prevent the occurrence of howling by suppressing the gain of the variable gain amplification circuit 2 to a sufficient level (p) before reaching the so-called build-up time, which is the time from the rise of the audio signal to the occurrence of howling. can.

However, the level of the audio signal shown in FIG. 4A actually varies widely depending on the person standing in front of the microphone 1 and the distance between the person speaking and the microphone 1.

Therefore, it is not possible to generate a rectangular wave signal by comparing the rectified output of the audio signal with a fixed reference level. There is a risk that the supply may not be possible.

In view of these points, the present invention proposes a howling prevention device that can reliably prevent howling regardless of the level G of the audio signal.

An embodiment of the howling prevention device of the present invention will be described below with reference to FIGS. 4 and 5.

1 is a microphone, and 20 is an operational amplifier for polarity inversion.

An amplifier 20, diodes 10 and 11, a resistor 12, and a capacitor 13 constitute a double-wave rectifier circuit 14 to which the audio signal from the microphone 1 is supplied.

Further, 17 is an integrating circuit to which the rectified output of the rectifying circuit 14 is supplied, and is composed of a resistor 15 and a capacitor 16.

Reference numeral 18 denotes an operational amplifier as a control signal generating circuit, where `` stands up sharply when the level of the rectified output becomes larger than the level of the integrated output, and rises sharply when the level of the rectified output becomes smaller than the level of the integrated output, and rises with a predetermined time constant when the level of the rectified output becomes smaller than the level of the integrated output. generates a rectangular wave signal that gradually falls.

Next, the operation shown in FIG. 4 will be explained with reference to FIG. 5 as well.

The audio signal from the microphone 1 (FIG. 5A) is supplied to the variable gain amplification circuit 2, and is also supplied to the rectifier circuit 14 for double-wave rectification, and the double-wave rectified voltage (FIG. 5B)
The voltage (solid line) is supplied to the non-inverting input terminal of the operational amplifier 18 and is also supplied to the integrating circuit 17 consisting of a resistor 15 and a capacitor 16, where the integrated voltage (broken line in FIG. 6B) is applied to the operational amplifier 18. is supplied to the inverting input terminal of the circuit 18.

In the operational amplifier 18, the integrated output voltage shown by the broken line in FIG. 6B becomes a reference level, and the level is compared with the rectified output voltage shown by the solid line in FIG. 5B.

When the rectified output voltage becomes larger than the integrated output voltage at time t1, the output voltage of the operational amplifier 18 rises sharply from -Eb to +Eb, and when the rectified output voltage becomes smaller than the integrated output voltage at time t2, the operational amplifier 18 increases. The output voltage of the amplifier 18 is configured to gradually drop from +Eb to -Eb with a predetermined time constant (the operational amplifier 18 is provided with a time constant circuit, not shown).

Then, by supplying the rectangular wave signal voltage shown in FIG. 5C to the variable gain amplification circuit 2, the amplification circuit 2
Since the gain of the audio signal is controlled, the occurrence of howling is reliably prevented even when the level of the audio signal fluctuates significantly.

In this manner, the howling prevention device of the present invention reliably prevents howling from occurring even when the level of the audio signal fluctuates significantly.

[Brief explanation of the drawing]

1 and 2 are system diagrams of the amplification circuit system for explaining the present invention, and FIG. 3 is a waveform diagram for explaining the circuit in FIG. 2.
FIG. 4 is a circuit diagram of an embodiment of the present invention, and FIG. 5 is a waveform diagram for explaining the operation of FIG. 4. 1 is a microphone, 2 is a variable gain amplification circuit, 14 is a rectifier circuit, 17 is an integration circuit, 18 is a reference signal forming circuit, 1
8 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;
When the rectified output of the rectifying circuit and the integral output of the integrating circuit are supplied and compared with an integrating circuit to which the rectified output of the rectifying circuit is supplied, and the level of the rectified output becomes higher than the level of the integral output. and a control signal generation circuit that generates a rectangular wave signal that falls with a predetermined time constant when the rise becomes small, and the rectangular wave signal from the control signal generation circuit is transmitted to the variable gain amplification circuit. 1. A howling prevention device which controls the gain of the variable gain amplification circuit and supplies the output audio signal from the variable gain amplification circuit to a speaker.