JPS5952877B2 - 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, the audio signal from the 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) for rectification.

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 ■.

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) Vo, it falls slowly with a predetermined time constant, as shown in FIG. 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, 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 to the occurrence of howling.

Incidentally, in rare cases, the audio signal may continue almost without interruption, but in the above-mentioned howling prevention device, the gain of the variable gain amplification circuit remains at a large value for a long time, so howling is prevented. becomes more likely to occur.

In view of this, the present invention has a very simple configuration, and
It is an object of the present invention to propose a howling prevention device that prevents howling from occurring even when an audio signal continues with almost no breaks.

An embodiment of the howling prevention device of the present invention will be described below 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.

That is, in FIG. 4, the control signal generation circuit 6 in FIG. The clamp diode 11 is connected in parallel to the clamp diode 11.

Then, the rectangular wave output voltage from the control signal generation circuit 6 is converted to an appropriate voltage by resistors 7 and 8 and is supplied to the variable gain amplification circuit 2.

The rest of the configuration is almost the same as that in FIG. 2, so a redundant explanation thereof will be omitted.

Next, the operation of the circuit shown in FIG. 4 described above will be explained with reference to the waveform diagram shown in FIG.

The audio signal from the microphone 1 (FIG. 5A) is rectified by the rectifier circuit 5 and then supplied to the Schmitt trigger circuit 12 of the control signal generating circuit 6.

The Schmitt trigger circuit 12 outputs a square wave output voltage waveform (FIG. 5B) depending on the level of the audio signal (FIG. 5A), for example, when the audio signal exceeds the level indicated by the broken line.

This square wave signal is supplied to a differentiating circuit 13 having a low frequency time constant τ1, which is composed of a capacitor 9 and a resistor 10, and is differentiated.

However, if the differential waveform as it is is applied as a control signal to the variable gain amplification circuit 2, the voltage at the top of the gain control signal will fluctuate because the DC component of the differential waveform is lost, and the variable gain amplification circuit The gain of 2 also fluctuates.

Therefore, this differential waveform is further clamped with a diode 11 to produce a DC component and a regenerated signal (Fig. 5C), and then resistors 7 and 8 are used to generate a signal with a predetermined DC potential (Fig. 5D). may be supplied to the variable gain amplification circuit 2.

Next, the differential circuit 1 described above with reference to the equal circuit shown in FIG.
3 and the operation of the diode 11 will be explained in more detail.

In FIG. 6, 9 is a capacitor of the differentiating circuit 13, and 20 is a forward resistance of the diode 11.

Reference numeral 21 denotes a DC power supply that supplies a rectangular wave voltage that is the output of the Schmitt trigger circuit 12, and this DC power supply 21 and switch 22 are connected in parallel to the capacitor 9 and the resistor 20.

The midpoint of the connection between the capacitor 9 and the switch 22 is connected to the control terminal of the variable gain amplification circuit 2.

Now, when the charge stored in the capacitor 9 of the differentiating circuit 13 is "0" and the output pulse of the Schmitt trigger circuit 12 suddenly becomes negative, the capacitor 9 is rapidly charged and supplied to the variable gain amplifier circuit 2. The potential immediately rises and reaches near Ovρ.

Next, when the output pulse of the Schmitt trigger circuit 12 turns positive, a voltage of opposite polarity is applied to both ends of the diode 11, so that the diode 11 is turned off.

Then, the square wave output voltage of the Schmitt trigger circuit 12 is discharged by the differential constant of the resistor 10 and the capacitor 9, given a negative slope characteristic in the time direction, and further set to a predetermined DC potential by the resistors 7 and 8. (FIG. 6D) is supplied to the variable gain amplification circuit 2 as a gain control signal.

As a result, even if the audio signal continues with almost no breaks, the gain of the variable gain amplification circuit 2 gradually decreases, so howling does not occur.

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 is also supplied to the rectification circuit for rectification, and the rectified output is supplied to the variable gain amplification circuit, generating a rectangular wave signal that rises steeply when the rectified output level is above a first reference level, falls with a predetermined time constant when the rectified output level is below a second reference level, and has a negative slope in the time direction; This rectangular wave signal is supplied to the variable gain amplification circuit to control its gain, and the output audio signal from the variable gain amplification circuit is supplied to the speaker, so the circuit configuration is simplified and the audio signal is almost Even if the signal continues without interruption, the gain of the variable gain amplification circuit gradually decreases, and howling does not occur.

In addition, since the rectangular wave signal supplied to the variable gain amplification circuit has a steep rise, a slow fall, and a negative slope in the time direction, it is particularly difficult to distinguish sounds such as consonants. By increasing the gain at the beginning of a word, which contains a lot of important information, and decreasing the gain at the subsequent part, the occurrence of howling can be reliably prevented without impairing the distinctiveness of the voice, thereby reducing the audible unnaturalness of the voice signal. can be reduced.

[Brief explanation of drawings]

1 and 2 are system diagrams for explaining the howling device of the present invention, FIG. 3 is a waveform diagram for explaining the operation of FIG. 2,
FIG. 4 is a system diagram of an embodiment of the howling device of the present invention, FIG. 5 is a waveform diagram for explaining the operation of FIG. 4, and FIG.
FIG. 3 is a circuit diagram for explaining the operation of the figure. 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 an audio signal from a microphone is supplied, a rectifier circuit to which the audio signal is supplied, and a rectified output of the rectifier circuit is supplied, and the level of the rectified output is equal to or higher than a first reference level. a control signal generation circuit that generates a rectangular wave signal that rises steeply and falls with a predetermined time constant when the signal is below a second reference level and has a negative slope in the time direction, the control signal generation circuit; A howling prevention device comprising: supplying a rectangular wave signal to the variable gain amplification circuit to control its gain; and supplying an output audio signal from the variable gain amplification circuit to a speaker.