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DESCRIPTION JPS612496

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DESCRIPTION JPS612496
[0001]
TECHNICAL FIELD The present invention relates to a howling suppressor using a digital filter for
howling suppression of a loudspeaker. 2. Description of the Related Art In the recent
development of conference systems and teleconference systems, there has been a demand for a
howling suppressor capable of clear and natural sounding. The conventional howling suppressor
will be described below. Fig. 1 is a block diagram of a conventional howling suppressor using a
digital filter, in which 1 is a microphone, 2 is a microphone amplifier, 3 is a low pass filter (LPF),
4 is a high pass filter (HPF), and 6 is this high pass Cutoff frequency adjuster that controls the
cutoff frequency of the filter 4 to an arbitrary frequency higher than the cutoff frequency of the
low pass filter 3, 6 is a subtraction unit, 7 is a transfer characteristic estimation unit that
estimates and stores transfer characteristics in the form of impulse response 8 are switches for
controlling the operation of the transfer characteristic estimation unit 7. The estimation
operation of the transfer characteristic is performed when the switch is closed, and the
estimation operation is stopped when the switch is open. 9 is a transfer characteristic stored in
the transfer characteristic estimation unit, that is, a digital filter (hereinafter referred to as echo
signal estimation unit) for performing convolution of an impulse response and a speaker
amplifier input signal, 1 o is an addition unit, 11 is an addition unit A white noise generating unit,
12 are switches for disconnecting a loudspeaker system and inputting white noise when
estimating transfer characteristics, 13 is a speaker amplifier, and 14 is a speaker. The operation
of the conventional howling suppressor configured as described above will be described below.
The operation of the microphone 1 will be briefly described. The output of the microphone 1 is
divided by the microphone amplifier 2 after amplification by the mouth-pass filter 3 and the high
pass filter 4. The signal which has become only the low frequency band component by the low
pass filter 3 has its echo signal contained therein removed by the subtractor 6, the transfer
characteristic estimator 7 and the echo signal estimator 9. The low frequency band component
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from which the echo signal has been removed is added by the adding section 10 to the high
frequency band component separated by the high pass filter 4 and is amplified by the speaker
amplifier 13 and the speaker 14. The howling phenomenon of the loudspeaker system is the
microphone 1. Amplification system, speaker 14. Since the loop of the microphone 1 is formed,
by canceling the low-frequency echo signal flowing from the speaker 14 to the microphone 1,
howling in the low frequency can be suppressed. Next, the cancellation operation of the lowfrequency echo signal will be described in more detail.
First, prior to the amplification, the transfer characteristic between the speaker 14 and the
microphone 1, that is, the impulse response is estimated. In FIG. 1, since it is in the switch state
at the time of loud sounding, the switch 8 is closed and the switch 12 is connected to the B side
to make the transfer characteristic estimation state. The output of the white noise generator 11 is
amplified by the speaker amplifier 13 and the speaker 14, and the echo is collected by the
microphone 1. The output of the white noise generation unit 11 is also input to the echo signal
estimation unit 9. The echo signal estimation unit 9 calculates the convolution of the white noise
signal and the estimated impulse response of the bass region sent from the transfer
characteristic estimation unit 7 to obtain an estimated reverberation signal of the bass region,
and a low pass in the reduction x unit e. The output of the filter 3, that is, the low frequency
component of the output of the microphone 1 is reduced. The output of the subtracting unit 6 is
sent to the transfer characteristic estimating unit 7, and the transfer characteristic estimating
unit 7 doubles the estimated impulse response of the bass region stored therein in the direction
in which the output of the subtracting unit 6 decreases. Next, calculation of the estimated echo
signal is performed again using this corrected impulse response, and subtraction from the output
of the low pass filter 3 is performed to obtain a new output of the subtraction unit 6. The transfer
characteristic estimation unit 7 corrects the impulse response again in the direction in which the
output decreases. By repeating the above-described procedure a sufficient number of times, the
output of the subtractor 6 becomes zero, and as a result, the impulse response of the bass region
having a high degree of approximation is stored inside the transfer characteristic estimation unit
7. After the estimation of the transfer characteristic is completed in this manner, the switch 5 is
opened to inhibit correction of the impulse response. Next, the switch 12 is connected to the A
side to perform a loudspeaker. At the time of this sound amplification, the low frequency range
component of the echo signal which has been looped in from the speaker 14 included in the
output of the microphone 1 is completely canceled as in the case of the white noise described
above and does not appear in the output of the subtraction unit 6. As described above, in the
prior art, digital filtering is used to suppress howling in the low range. Howling in the high range
is less likely to occur for reasons to be described later, and howling mainly occurs only in the
middle frequency range. In this conventional example, the cutoff frequency adjusting section 6 is
provided to suppress howling in the middle range, and the cutoff frequency of the high pass filter
4 which is set to the cutoff frequency of the low pass filter 3 when the howling does not occur.
When the howling in the midrange occurs, the method is configured to suppress the howling in
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the midrange by setting the cutoff frequency above the band where the howling occurs.
This is explained by the measured value of the transfer characteristic between the speaker and
the microphone. Figure 2 shows an example of the frequency dependence of the absolute value
of the transfer characteristic between the speaker and the microphone measured at two sound
fields (using an auditorium) and at a microphone spacing of 1 m in FIG. 15 is a characteristic in a
sound field where sound absorption processing such as walls has a relatively short reverberation
time is relatively good, and 16 is a characteristic in a sound field long in reverberation time. In
each sound field, the absolute value of the transfer characteristic becomes smaller as the
frequency becomes higher due to the sound absorbing action of the wall or the like. Therefore,
the frequency at which howling occurs is mainly a low to mid frequency up to 3.5 KHz, and
howling suppression may be performed in this frequency band. Furthermore, in the low to mid
frequency band, the mid frequency band from 1.5 KHz to 3.6 KHz is a non-negligible value in a
sound field having a characteristic 16 with a long reverberation time, but a characteristic with a
short reverberation time The value is negligible at 15 sound fields. Therefore, howling
suppression should be performed only at a low frequency of 1.5 KHz i in the sound field of the
characteristic 15 in which the reverberation time is short. By limiting the frequency band in this
manner, howling suppression due to cancellation of the echo signal can be performed
inexpensively. On the other hand, as in the case of a sound field with a characteristic 16 with a
long reverberation time, the cutoff frequency of the high-pass filter 4 described above is used
because the quality of the loud sound is essentially deteriorated due to reverberation. The sound
quality is not noticeable even if it is raised to drop the mid frequency. However, in the abovedescribed conventional example, although there is no sense in general listening, if, for example,
the amplification factor of a loudspeaker system is increased to a certain extent or more to
enhance the loudspeaker effect by using it in a sound field with short reverberation time, The
loss of the mid-frequency occurs, so that the sound quality is noticeably deteriorated, and even
when listening to the medium reverberation time, the sound quality is strongly felt to deteriorate.
SUMMARY OF THE INVENTION The present invention solves the above-described conventional
problems, and when howling occurs at a mid-range frequency, howling with less deterioration in
sound quality is achieved by reducing the gain of this band by a necessary and sufficient amount.
The purpose is to provide a suppression device. The present invention comprises a low pass filter
for extracting a bass component of an output of a microphone, a high pass filter for extracting a
treble component of an output of the microphone, a band pass filter for extracting a mid tone
component of an output of the microphone, and the band pass A band pass filter output
attenuation unit for attenuating the output of the filter; a transfer characteristic estimation unit
for estimating the transfer characteristic of the low-pass echo signal that is transmitted from the
speaker to the microphone; and an echo signal estimator for approximating the low-pass echo
signal The subtraction unit that subtracts the output of the echo signal estimation unit from the
output of the low pass filter, the addition unit that adds the output of the high pass filter, the
output of the band pass filter attenuation unit, the output of the subtraction unit, The low
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frequency howling is suppressed by the cancellation of the echo signal as before, Range of
howling is high and configuration to which howling suppression effect of suppressing attenuated
by an amount necessary and sufficient gain in this band, but can provide an excellent low-cost /
N cowling suppressor sound quality. .
Description of the Embodiment FIG. 3 is a block diagram of the howling suppressor in the first
embodiment of the present invention. In FIG. 3, 21 is a microphone, 22 is a microphone
amplifier, 23 is a low pass filter (LPF), 24 is a band pass filter (BPF), 25 ? bypass filter (HPF), 26
is a reduction part, 27 is a transfer characteristic The estimation unit 28 is a switch for
controlling the transfer characteristic estimation unit 27, 29 is an echo signal estimation unit, 3o
is a band pass filter output attenuator, 31 is an addition unit, and 32 is disconnection of the
loudspeaker system and white noise at the time of transfer characteristic estimation. , A white
noise generator, 34 a speaker amplifier, and 35 a speaker. The operation of the howling
suppression apparatus of this embodiment configured as described above will be described
below. First of all, the operation of the microphone 21 is amplified by the microphone amplifier
22 and then divided into a low-pass component, a mid-pass component and a high-temperature
component by the low pass filter 23, the band pass filter 24 and the high pass filter 26. Ru. The
signal which has become only the bass component by the low pass filter 23 is removed by the
subtraction unit 26, the transfer characteristic estimation unit 27, and the echo signal estimation
unit 29 by the same operation as that of the conventional example. On the other hand, the signal
which appears only to the high-pitched component by the high-pass filter 26 is added by the
adding section 31 to the bass component from which the echo signal is removed as in the
conventional example. On the other hand, the mid-tone component between the cutoff frequency
of the low-pass filter 23 and the cutoff frequency of the high-pass filter 25 which were dropped
in the prior art is extracted by the bandpass filter 24 and attenuated by the bandpass filter
output attenuator 30 by a predetermined amount. After that, the addition unit 31 adds the low
frequency component and the high frequency component. This output is amplified by the
speaker amplifier 34 and the speaker 35. The estimation operation of the transfer characteristic
prior to the loud voice and the cancellation operation of the low-pass echo are exactly the same
as in the conventional example, and the switch 28.32 and the white noise generator 33 are used
in the estimation of the transfer characteristic as in the conventional example. It is a thing. The
principle of 71 ulling suppression of this howling suppressor will be described with reference to
FIGS. FIG. 4 shows a combination of the low pass filter 23 of the first embodiment of FIG. 3 and a
combination of a low pass filter 24 and a band pass filter output attenuating part 3 o (hereinafter
referred to as ?pantono (referred to as a filter part); The frequency characteristics of the input
and output of the I-pass filter 26 are shown. In FIG. 4, 41 is the characteristic of the low pass
filter, 42.44 is the characteristic of the band pass filter section, and 43 is the characteristic of the
high pass filter.
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In the case of the sound field having the characteristic 15 of FIG. 2, the frequency at which the
howling may occur is limited to the low frequency up to 1.5 KHz, and the low frequency howling
is suppressed by the cancellation of the echo signal described above. Sufficient effect is obtained.
Therefore, the gain of the mid-range frequency band, that is, the gain of the band pass filter does
not have to be attenuated and is set to the characteristic 42. Further, as described in the
explanation of the prior art, there is no possibility of howling occurring in the high frequency
region, and therefore, the characteristic 43 is set. As a result of the above, the output of the
addition unit 31 has a uniformly flat frequency characteristic and the sound quality does not
deteriorate. Next, consider the case of the sound field having the characteristic 16. In this case,
the transfer characteristic at the mid frequency band of 1.5 KHz to 3 and 5 KHz, that is, the echo
signal can not be neglected, and even flat characteristics can not increase the gain of the
loudspeaker system due to howling at the mid frequency. . In this case, the generation of howling
can be prevented by lowering the gain of the band pass filter section and setting the
characteristic 44. If the attenuation amount of the gain of the band pass filter unit, that is, the
attenuation amount of the band pass filter output attenuation unit 30 is set to e dB or less, the
deterioration of the sound quality of the speech can be extremely reduced. As described above,
according to this embodiment, by limiting howling suppression due to cancellation of the
reverberation signal to a low frequency, it is possible to provide an inexpensive howling
suppressor similar to the conventional example and at the same time greatly improve the sound
quality of the conventional example. It is. In the above example, the cutoff frequency of each
filter is set to 1.5 KHz and 3.5 KHz, but the characteristics differ from the transfer characteristics
shown in Fig. 2 depending on the characteristics of microphone 21 and speaker 35 and the
installation conditions If obtained, the cutoff frequency may be set according to the
characteristics. Next, a second embodiment of the present invention will be described. FIG. 5 is a
block diagram of a howling suppression apparatus according to a second embodiment of the
present invention. In FIG. 6, reference numeral 61 denotes a midrange howling detection unit, 62
denotes a band pass filter output attenuation unit 62 that increases the amount of attenuation
when howling is detected by the midrange howling detection unit 61; The configuration is
exactly the same as the embodiment. The howling suppression apparatus of the second
embodiment configured as described above automatically performs the attenuation amount of
the band pass filter output attenuation unit 52 performed in the first embodiment. As a method
of detecting howling, even if it is judged that howling is occurring due to the detection of a signal
at a certain level or more during silent speech, this embodiment can sufficiently achieve the
purpose, but other methods may be used. .
As described above, according to the present embodiment, howling at the midrange frequency is
automatically suppressed, and a howling suppressor that can be easily operated can be realized.
FIG. 6 is a block diagram of the howling suppression apparatus according to the third
embodiment of the present invention. In FIG. 6, reference numeral 61 denotes a band pass filter,
62 denotes a high pass filter, and 63 denotes a cutoff frequency adjustment unit. The cutoff
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frequency adjustment unit 63 uses the high pass cutoff frequency to high pass filter 62 of the
band pass filter 61. It is possible to change at the same time while keeping the cutoff frequency
of and equal. According to this embodiment of 0 or more configuration which is the same
configuration as the first embodiment except for the other configuration, it becomes easy to
determine the optimum divided frequency band adapted to the characteristics of the micropon
and the speaker and the installation conditions. A wide range of howling suppressor can be
realized. FIG. 7 is a block diagram of the howling suppression apparatus in the fourth
embodiment of the present invention. In FIG. 7, 71 is a high range howling detection unit, and 72
is a cutoff frequency adjustment that sends a signal to raise the cutoff frequency to the band pass
filter 61 and the high pass filter 62 when howling is detected by the high range howling
detection unit 71 It is a department. The band pass filter 61, the high pass filter 62 and the other
components are exactly the same as in the third embodiment. According to this embodiment
having the above configuration, the adjustment of the cut-off frequency of the third embodiment
can be performed automatically, and a howling suppressor having a wide application range and
easy operation can be realized. FIG. 8 is a block diagram of the howling suppression apparatus
according to the sixth embodiment of the present invention. In FIG. 8, reference numeral 81
denotes a howling detection unit for the low tone range, and 82 denotes an addition unit output
attenuation unit for increasing the amount of attenuation when howling is detected by the low
range feedback detection unit 81. The other configuration is exactly the same as that of the first
embodiment. According to the present embodiment having the above configuration, automatic
adjustment of a loudspeaker system gain is possible by focusing on howling in the low frequency
range which was impossible in the first embodiment, and a howling suppressor which is
extremely easy to operate. It is. The effects of the present invention are further enhanced by
using the combination of the embodiments of FIG. 5 and FIG. Be Effects of the Invention The
present invention uses howling suppression by the echo signal estimation unit in the bass region,
howling suppression for gain attenuation in the midrange region, and howling suppression by
the room sound absorption function in the high region, and has a high howling suppression
effect and sound quality. It is possible to realize an excellent howling suppressor at low cost, and
further to provide a howling suppressor having high operability by automatic suppression of
howling in combination with howling detection.
[0002]
Brief description of the drawings
[0003]
FIG. 1 is a block diagram of a conventional howling suppressor, FIG. 2 is a diagram of frequency
transfer characteristics between a speaker and a microphone of a loudspeaker measured in two
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sound fields, and FIG. 3 is a first embodiment of the present invention. Fig. 4 is a block diagram
of the howling suppressor in the example, Fig. 4 is a frequency characteristic diagram of the
respective parts, Fig. 5 is a block diagram of the howling suppressor in the second embodiment,
and Fig. 6 is howling in the third embodiment. FIG. 7 is a block diagram of a suppression
apparatus, FIG. 7 is a block diagram of a howling suppression apparatus in the fourth
embodiment, and FIG. 8 is a block diagram of a howling suppression apparatus in the fifth
embodiment.
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и и и и и и и и и и и и и и и и и и и и и и и и Subtractor, 27. и и и и и и Echo signal estimation unit, 3 o и и и и и и и и и и и и и и и
и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и mid range
howling detection unit 63 и и и и и и и и Cutoff frequency adjustment unit, 71 и и и и и и и и и и и и и и и и и и и и и и
и и и и и и и и и и и и cutoff frequency adjustment unit 71 и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и и
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Name of agent Attorney Nakao Toshio and 1 other person Fig. 1 Fig. 2 Fig. 2-Jl autumn EKHz)
Fig. 3 Fig. 4 Shoulder wave [MHz] Fig. 5 Fig. 6 Fig. 7
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