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DESCRIPTION JPS6028399

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DESCRIPTION JPS6028399
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
loudspeaker apparatus for amplifying a voice signal from a microphone and outputting it as a
speaker, and more particularly to a loudspeaker apparatus capable of suppressing harmful
howling. BACKGROUND OF THE INVENTION In general, in a loudspeaker system, there is a
problem that the speaker output feeds back to the microphone input to cause howling, and the
output level is limited. Conventionally, as a measure to prevent this howling, it is usually
practiced to make the speaker and the microphone have appropriate directivity and to make it
difficult for the howling to occur by the setting. However, this method has a disadvantage that it
can not be used when the positional relationship between the speaker and the microphone can
not be fixed, as in the case where the microphone is a hand microphone. Also, at the
experimental level, the input signal from the microphone is divided into a plurality of bands, the
signal energy of each divided band is compared to detect the occurrence of howling, and if
howling occurs, that band is passed Although attempts have been made to attenuate the signal,
etc., there is a problem that the sound quality changes. Moreover, although the device is
complicated, the improvement of the howling margin is not obtained so much. OBJECTS OF THE
INVENTION It is an object of the present invention to eliminate the above-mentioned drawbacks
of the prior art, and to provide a howling preventing device which has a sufficient effect with a
simple device and little change in sound quality. SUMMARY OF THE INVENTION Howling occurs
at a frequency where the output from the speaker feeds back to the microphone input and the
signal delay in this loop results in positive feedback. Therefore, howling does not occur unless
the speaker output feeds back to the microphone. This is a method of giving appropriate
directivity to the microphone and the speaker mentioned above and preventing howling by
setting. That is, no feedback occurs if the feedback loop is cut. Therefore, as a method of cutting
the feedback loop without limiting the positional relationship between the microphone and the
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speaker, the following is considered. It is assumed that the frequency ffO that generates howling.
At this time, the phase at the frequency fO in the feedback loose, including the spatial distance
between the speaker and the microphone, is approximately 660 °, which is positive feedback.
Therefore, the fO acid component of the sound input from the microphone is amplified, output
from the speaker, and again in phase with the fO acid component sound input first and input
from the microphone to start QDN at 10 frequencies .
Here, if a device for shifting the frequency is inserted in the signal path from the microphone to
the speaker through the amplifier, the feedback loop can be cut. That is, if the signal of the
frequency of fO input from the microphone is output from the speaker converted to fO + Δf (or
fO-Δf) by the device that shifts the frequency, the first microphone Since the frequency
component of fO is converted to a different frequency (fO + Δf) while rotating around the loop, a
positive feedback loop at fO does not occur. Thus, howling at the frequency of fO can be
prevented in this way. Here, as a device for shifting the frequency, for example, a digital sampling
is performed, and a pitch for performing sampling shift and shift to a low band by time axis
expansion, shifting to a high band by time axis compression and insertion of interpolation
samples There is a controller etc. Such a pitch controller can be used relatively inexpensively if it
is made into an LSI because it is digital processing. By the way, the above-mentioned frequency
shift amount may be slightly shifted if Q is high due to Q of the feedback loop which causes
howling, and the frequency shift amount may be increased accordingly if Q is low. However, if
the shift amount of the frequency is too large, there is too much difference between the
frequency input to the microphone and the frequency output from the speaker, causing a sense
of discord. In such a case, if a narrow band rejection filter in the stop band is inserted in the
signal path from the microphone to the speaker, the howling frequency shifted in frequency each
time in the loop will eventually become the stop band. Fell into the sky, and Ueuling stopped
immediately. Obviously, if the stopband is narrow, there will be less discord to the sound. If such
a filter is inserted at an appropriate frequency interval, it is possible to immediately drop into the
stop band no matter howling occurs at any frequency, which is effective in preventing snowing.
An embodiment of the present invention will be described below with reference to FIG. In FIG. 1,
it is assumed that the microphone 2 and the speaker 8 face each other at an appropriate distance
in the room indicated by 1. The signal from the microphone 2 is amplified by the microphone
amplifier 3 and supplied to the mixer 5 through the pitch conversion circuit 4. The mixer 5 sums
the output of another audio device 6 (for example, a tape recorder, a record player, etc.) and the
signal from the microphone, and drives the speaker 8 through the power amplifier 7.
Here, the signal of frequency f Hz input from the microphone 2 is amplified by the microphone
amplifier 3 and then input to the pitch conversion circuit 4. Then, the input signal of frequency f
Hz is converted to the frequency of KX f Hz and sent to the mixer 5. The signal is added to the
signal from the other audio equipment 6 by the mixer 5, and from the power amplifier 7 to the
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microphone 2 via the speaker 8. At this time, the frequency is changed to KXf Hz. Therefore,
even if howling occurs at fOHz when frequency shift is not performed in this system, if frequency
shift is performed by pitch conversion circuit 4 as in this example, feedback occurs at the next
moment even if feedback occurs at fOHz. Since the frequency of the signal changes to KXfO, it
does not grow as howling and stops immediately. If the constant of the pitch conversion circuit 4
is set to a value far from 1, a large frequency difference occurs between the input signal from the
microphone and the output signal from the speaker, giving a sense of discord. However, the
howling prevention effect is thicker if K is far from 1. If the value of K is within ± 2 to 6%, that
is, if it is about 097 ≦≦ 1.03, the change of the pitch is only a half pitch or less of semitone, so
there is little sense of discord, In FIG. 1 which also has a howling prevention effect, since no
frequency shift circuit is inserted in the signal path from the other audio equipment 6 to the
speaker in FIG. 1, degradation of the sound quality does not occur even if the music source is
reproduced. Further, it goes without saying that the effects of the present invention can be
obtained even in a system without the mixer 5 and other audio devices 6 that reproduces only
the sound of the microphone. Further, as shown in FIG. 2, in a system in which signals of a
plurality of microphones are added and output, if pitch conversion circuit 4 is added after mixer
5 which adds the outputs of each microphone amplifier 6, all the microphones The howling
prevention effect can be provided by one pitch conversion circuit 4 without individually adjusting
each of the configured loops. On the other hand, in the case where a sufficient effect can not be
obtained when the Q of the loop that generates howling is low and the amount of frequency shift
by the pitch conversion circuit 4 is small, as shown in FIG. A band rejection filter 9 having the
characteristics as shown in FIG. As a waveform is schematically shown in FIG. 6, a case is
considered in which a signal of the frequency from the microphone 2 of FIG. 3 to fl is input with
an amplitude as shown in FIG.
Assuming that the time required to turn the loop once is τ, after time elapses, the signal of the
frequency of fl becomes the frequency of f2 by the pitch conversion circuit 4, and as shown in
FIG. The amplitude increases and returns to the microphone 2. And owling begins to occur.
However, when time passes by τ further, the signal travels around the loop one more time, and
the frequency is further shifted by the pitch conversion circuit to f3. At this time, the band
rejection filter 9 blocks the frequency f3 as shown in FIG. 4 and returns to the microphone with a
small amplitude as shown in FIG. Therefore, here Ulling stops without getting bigger. As shown in
Fig. 4, if you make the filter comb-shaped, provide some stopbands, and reduce one stopband
width, it is possible to immediately generate 71 uling in any situation. The frequency reaches the
stop band of either of the band stop filters 9 by the pitch conversion circuit 4 and the ringing is
suppressed. Also, since the stop bandwidth is small, there is little sense of discord to the sound
reproduced from the speaker 8. Furthermore, as shown in FIG. 5 as another embodiment, the
center frequency of the stop band of the band rejection filter 9 can be externally controlled, and
this is periodically changed by the oscillator 10 as shown in FIG. By controlling in the same
manner, the sense of disorientation to the reproduced sound from the speaker 8 can be further
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reduced. The resonance due to the specific frequency causing the howling is monotonously
increased (or decreased) in frequency by the pitch conversion circuit 4. Therefore, even if the
stop band of the band stop filter 9 periodically changes, the signal due to the resonance is always
caught in the stop band and howling is prevented. In addition, since the stop band periodically
changes, it is possible to remove the sense of incongruity resulting from the fact that a specific
frequency is not reproduced. According to the present invention, it is possible to effectively
prevent howling with a relatively simple configuration, and to realize a loudspeaker system with
a slight sense of discord to the reproduced sound quality.
[0002]
Brief description of the drawings
[0003]
1 to 3 are each an embodiment of the present invention, FIG. 4 is a characteristic example of the
band elimination filter in the embodiment, FIG. 5 is another embodiment of -9, and FIG. It is a
schematic diagram of the waveform which shows the effect of the Example of FIG.
1 ... room 2 in which a loudspeaker is installed 2 ... microphone 3 ... microphone amplifier 4 ...
pitch conversion circuit 5 ... mixer 6 ... other audio equipment 7 ... power amplifier 8 ...・ Speaker
9 ・ ・ ・ Band lightening filter 1S ・ ・ ・ Oscillator 1 Fig. 2 Fig. 2 Fig. 4
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