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DESCRIPTION JPH1066197

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DESCRIPTION JPH1066197
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
loudspeaker howling elimination apparatus for automatically detecting and eliminating howling
generated by a loudspeaker.
[0002]
2. Description of the Related Art Heretofore, in the case where a sound picked up by a
microphone is amplified and used, howling occurs by picking up the sound generated from the
loudspeaker of the loudspeaker again by the microphone. Therefore, howling was prevented by
throttling the volume.
[0003]
SUMMARY OF THE INVENTION This has the following disadvantages. When the loud speaker is
used with a large volume, it is extremely unstable because the howling margin is insufficient
since the volume is used at a slightly lower volume than the volume at which the howling occurs.
Therefore, it was necessary to adjust the volume frequently. Also, when the volume was lowered
to a stable state where no howling occurred, a sufficient volume could not be obtained. The
present invention was made to solve the above-mentioned drawbacks.
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[0004]
A microphone amplifier (1), a filter amplifier with a detection circuit (2), and a circuit block of a
buffer amplifier (3) are provided. The present invention is an automatic howling detection /
removal circuit for a loudspeaker having the above configuration.
[0005]
BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be
described below. (A) AC coupled amplifier in which two stages of microphone amplifier
operational amplifier A1 and operational amplifier A2 are connected in series. (Ii) Filter amplifier
with filter circuit The circuit comprises a notch filter composed of a filter amplifier operational
amplifier A3, a resistor R7, a resistor R8, a resistor R9, a capacitor C4, a capacitor C5 and a
capacitor C6 and a level comparator of an operational amplifier A4. (Iii) Amplifier with
amplification degree switching comprised of buffer amplifier operational amplifier A5. The
present invention is configured as described above. When using the invention, it is connected to
the microphone input of the loudspeaker. Also, a microphone connects to the present invention.
The audio input from the microphone is amplified to a level at which audio processing can be
performed by the microphone amplifier (1). Here, howling is detected by the filter amplifier with
detection circuit (2) which forcibly generates howling and incorporates a notch filter having an
attenuation characteristic in which the audio frequency band is divided into nine parts by one
octave. Select a notch filter to be attenuated. The buffer amplifier (3) performs volume correction
before and after howling detection, and eventually attenuates to the microphone input level.
[0006]
The present invention will be described in more detail by way of the following examples. (A) As
shown in FIG. 2, in the microphone amplifier (1), the output voltage generated from the
microphone is input to the resistor R1 and the non-inverted input + of the operational amplifier
A1 via the capacitor C1. The resistor R2 is connected in parallel between the inverting input-and
the output of the operational amplifier A1, and the resistor R3 is connected to the resistor R2 and
the inverting input-of the operational amplifier A1. The output of the operational amplifier A1 is
input to the resistor R4 and the noninverting input + of the operational amplifier A2 via the
capacitor C2. The resistor R5 is connected in parallel between the inverting input-and the output
of the operational amplifier A2, the resistor R6 is connected to the resistor R5 and the inverting
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2
input of the operational amplifier A2, and the other end of the resistor R6 is connected to the
capacitor C3. (B) The filter amplifier with detection circuit (2) is also connected to the output of
the operational amplifier A2 by the resistor R8, the capacitor C4, the analog switch IC1, and the
analog switch IC3 as shown in FIG. , And capacitor C6. The other end of the capacitor C4 is
connected to the resistor R7 and the capacitor C5, and the other end of the resistor R9 and the
other end of the capacitor C5 are connected to the non-inverting input + of the operational
amplifier A3. The other end of the resistor R7 is connected to the inverting input-and the output
of the operational amplifier A3, and a notch filter is thus formed. Hereinafter, the level
comparator is connected from the connection point A to the resistor R10 and the capacitor C7
via the analog switch IC1, and further connected to the resistor R11 and the resistor R12 and the
inverting input-of the operational amplifier A4 via the capacitor C7. The connection point B is
connected to the resistors R13 and R14 via the analog switch IC2, and the other end of the
resistor R14 is connected by the non-inversion input + of the operational amplifier A4 and the
resistor R15. The other end of the resistor R15 is connected via the diode D1 to the output of the
operational amplifier A4, the resistor R16, the resistor R17, and the control input of the analog
switch IC3. The other end of the resistor R16 is connected to the resistor R18, and there is a
supply of a high level voltage. The other end of the resistor R17 is connected to the base of the
transistor Q1, and the light emitting diode D2 and the diode D3 are connected to the collector of
the transistor Q1. The other end of the light emitting diode D2 is connected to the other end of
the resistor R18. The emitter of the transistor Q1 is supplied with a low level voltage. In the
above configuration, 9 stages of series connection of the same circuit configuration which has
attenuation peaks of 63, 125, 250, 500, 1k, 2k, 4k, 8k and 16k hertz for each octave and which
are different constants respectively are connected Thus, the filter amplifier with detection circuit
(2) is configured.
(Iii) Also in the buffer amplifier (3), as shown in FIG. 2, the output from the filter amplifier with
detection circuit (2) is input to the noninverting input + of the operational amplifier A5, and the
output of the operational amplifier A5 and the inverting input A resistor R19 is connected in
parallel between them. The resistor R19 connected to the inverting input-side of the operational
amplifier A5 is also connected to the resistor R20 and the analog switch IC4. The output of the
operational amplifier A5 is connected to the resistors R21 and R22, and the other end of the
resistor R21 is connected to the analog switch IC4. The other end of the resistor R22 is
connected to the resistor R23, and this connection point is input to the loudspeaker as a
microphone output. Next, the reason and operation of howling is eliminated in the embodiment
circuit configured as described above will be described. The howling sound amplified by the
operational amplifier A1 and the operational amplifier A2 in FIG. 2 automatically detects the
howling frequency and automatically selects a notch filter for attenuating this frequency band by
the operational amplifier A3 and the operational amplifier A4 in the next stage. The howling
sound is generated in the sine wave waveform of FIG. 3 and is input to the notch filter formed of
the operational amplifier A3 and the level comparator formed of the operational amplifier A4. If
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the frequency of the sine wave input to the point shown in FIG. 2A matches the attenuation band
of the notch filter formed by the operational amplifier A3, the B point output level of the
operational amplifier A3 decreases. If they do not match, they are output at the same level as the
input level. The input and the output of the notch filter are input to the input terminals of the
operational amplifier A4 via the analog switch IC1 and the analog switch IC2. The inverting
input-of the operational amplifier A4 is level shifted in advance by the voltage dividing resistors
of the resistors R11 and R12, and the potential of the inverting input-of the operational amplifier
A4 is always higher than the noninverting input + of the operational amplifier A4 at point C in
the steady state. It has become. That is, the output of the operational amplifier A4 holds the low
level state. Here, when the frequency of the howling matches the attenuation band of the notch
filter, the output of the operational amplifier A3 is attenuated. At this time, since the levels
inputted to the non-inverted input + of the operational amplifier A4 and the inverted input-of the
operational amplifier A4 are different, the non-inverted input + level B of the operational
amplifier A4 is the inverted input-of the operational amplifier A4 from the waveform shown in
FIG. There is a section higher than level C. In this section, the output of the operational amplifier
A4 changes to high level, and a potential divided by the resistor R14 and the resistor R15 is input
to the non-inverting input + of the operational amplifier A4 via the diode D1.
At this time, the potential of the non-inverted input + of the operational amplifier A4 becomes
higher than the inverted input − of the operational amplifier A4, and the output of the
operational amplifier A4 is held at the high level. When the output of the operational amplifier
A4 becomes high level, the transistor Q1 is turned on to light the light emitting diode D2, thereby
informing that the filter of this band is operating. At this time, a low level signal is input to the
control input of the analog switch IC1 and the analog switch IC2 to shut off the switch, and no
signal is input to the input of the operational amplifier A4. When the output of the operational
amplifier A4 becomes high level, a high level signal is also input to the control input of the
analog switch IC3, and is configured by the operational amplifier A3 and the resistor R7, the
resistor R8, the resistor R9, the capacitor C4, the capacitor C5, and the capacitor C6. Select the
output side through the notch filter, and at this point howling is eliminated. The inverting inputof the operational amplifier A4 is cut off by the analog switch IC1, whereby the resistors R10,
R11 and R12 are removed from the load of the operational amplifier A2. That is, the volume of
the loudspeaker increases by changing from the state divided by the on resistance of the analog
switch IC1, the resistor R10, the resistor R11, and the resistor R12 to the undivided state, and the
analog switch IC4 for the purpose of correcting this. The low level signal is input to the inputs of
the analog switch IC1 and the analog switch IC2 at the same time as the input of the analog
switch IC1 and the analog switch IC2 to make the resistor R21 conductive, thereby reducing the
amplification of the operational amplifier A5. By changing the amplification degree of the
operational amplifier A5, the volume difference between before and after howling detection is
corrected. The output of the operational amplifier A5 is divided by the resistors R22 and R23 and
attenuated to the microphone input level. The frequency-output characteristic of the notch filter
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according to the invention is shown in FIG. Attenuation of -10 dB to -65 dB can be confirmed in
the band of 38 Hz to 27 k Hz. This means that a howling margin of 3.16 to 1778 times when the
present invention is not used is secured with respect to the howling frequency.
[0007]
Industrial Applicability By using the present invention, the loudness limit of the loudspeaker can
be expanded, and the use of the narrow band notch filter reduces the sound quality deterioration.
Moreover, the maximum effect can be obtained by howling automatic detection and removal
without receiving the characteristics of the microphone, loudspeaker and room.
[0008]
Brief description of the drawings
[0009]
Fig. 1 State of use of the present invention
[0010]
Fig. 2 Circuit diagram of the present invention
[0011]
Fig. 3 Notch filter input / output waveform diagram of the present invention
[0012]
Fig. 4 Notch filter frequency-output characteristics of the present invention
[0013]
Explanation of sign
[0014]
Reference Signs List 1 microphone amplifier 2 filter amplifier with detection circuit 3 buffer
amplifier C1 to 7 capacitor R1 to 23 resistor D1 and D3 diode D2 light emitting diode Q1
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transistor A1 to 5 operational amplifier IC1 to 4 analog switch
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