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JP2015095743

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DESCRIPTION JP2015095743
PROBLEM TO BE SOLVED: To obtain an equalizer capable of changing frequency characteristics
in a low frequency domain while suppressing an increase in the amount of calculation of filter
processing. The FIR filter processing unit 105 that changes the frequency characteristics of the
sound signal input in step b), the IIR filter processing unit 104 that changes the frequency
characteristics of the sound signal using the IIR filter, and the sound signal whose frequency
characteristics are changed using the FIR filter processing unit Is input, and a high-pass filter
processing unit 107 which passes and outputs a frequency component higher than the cutoff
frequency, and an acoustic signal whose frequency characteristic is changed by the IIR filter
processing unit is input and a frequency component lower than the cutoff frequency And a low
pass filter processing unit 106 that outputs the signal by passing It comprises an adder 108
which adds the output signal to output the playback audio signal from the signal low-pass filter
processing section that is output from. [Selected figure] Figure 1
イコライザ
[0001]
The present invention relates to an acoustic signal processing technique for changing frequency
characteristics at the time of reproduction of an acoustic signal.
[0002]
BACKGROUND ART In an audio reproduction system that reproduces an audio signal such as
music or announcement sound, “audience” is improved by changing the frequency
07-05-2019
1
characteristic of the audio signal to be reproduced by signal processing.
Such signal processing is called equalization, and a device that performs equalization is called an
equalizer. As digital filter systems for processing an acoustic signal in an equalizer, two types of
systems are known: Infinite Impulse Response (IIR) filters and Finite Impulse Response (FIR)
filters.
[0003]
While the IIR filter has the advantage of excellent adjustability in determining the desired
frequency characteristics, the sound quality is degraded when the acoustic signal after the
frequency characteristics change is reproduced because calculation errors are accumulated due
to the influence of the feedback loop. It has the disadvantage of being Further, the FIR filter has a
drawback that the adjustability is poor, but there is an advantage that the sound quality is not
easily deteriorated since the operation error is not accumulated.
[0004]
As a method of the equalizer which compensates for the drawbacks of the above two methods, at
the stage of determining the desired frequency characteristics, the frequency characteristics are
changed by the IIR filter to determine the coefficients of the IIR filter, and after the desired
frequency characteristics are determined, the FIR filter To perform signal processing by the FIR
filter with the coefficients of the FIR filter obtained based on the coefficients of the IIR filter,
thereby alleviating the sound quality deterioration due to the process of changing the frequency
characteristic while maintaining the good adjustability of the IIR filter A method is proposed
(Patent Document 1).
[0005]
WO 2010/041381 (Figure 1)
[0006]
In order to change the characteristic (for example, the intensity) of a certain frequency to the
desired characteristic by the FIR filter, the length of the impulse response (the impulse response
length is referred to as the response length) is longer than one wavelength at the frequency A FIR
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filter is required.
For this reason, generally, the amount of calculation when changing the characteristics with the
FIR filter is larger than the amount of calculation when changing the characteristics with the IIR
filter.
In particular, when changing the characteristics of the low frequency band, a large amount of
calculation is required because the period becomes long.
[0007]
On the other hand, when the frequency at which the response length is one wavelength or more
is increased to reduce the amount of calculation of the process of the FIR filter, it is difficult to
change the frequency region lower than the frequency to a desired frequency characteristic.
[0008]
In the above-mentioned conventional equalizer, after the desired frequency characteristic is
determined, the frequency characteristic is changed by the FIR filter, and the frequency
characteristic can be changed with high sound quality as compared with the equalizer using the
IIR filter, There is a problem that the amount of calculation of filter processing is increased.
In particular, this problem becomes noticeable when changing the characteristics in the low
frequency region, but if the operation amount is reduced by increasing the frequency at which
the response length is 1 wavelength or more, the response length is less than 1 wavelength The
problem arises that the frequency can not be changed to the desired characteristics.
[0009]
The present invention has been made to solve the above problems, and has as its object to obtain
an equalizer capable of changing frequency characteristics in a low frequency region while
suppressing an increase in the amount of calculation of filter processing.
[0010]
The equalizer according to the present invention is an FIR filter process for changing the
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frequency characteristic of an acoustic signal input by an FIR (Finite Impulse Response) filter of
an order in which the length of the impulse response is one or more wavelengths at a
predetermined threshold frequency And an IIR filter processing unit that changes the frequency
characteristic of the sound signal with an IIR (Infinite Impulse Response) filter, and an acoustic
signal whose frequency characteristic is changed by the FIR filter processing unit is input, and a
predetermined cutoff frequency is input. Also, a high-pass filter processing unit that passes high
frequency components and outputs the sound signal, and an acoustic signal whose frequency
characteristic is changed by the IIR filter processing unit is input and low-pass filter processing
that passes frequency components lower than the cutoff frequency And the signal output from
the high pass filter processing unit and the low pass filter processing unit An adder for
outputting the reproduced acoustic signal by adding the output signal, is obtained as comprising
a.
[0011]
According to the equalizer of the present invention, the equalizer includes an FIR filter
processing unit having an FIR filter of an order having a length of impulse response of one or
more wavelengths at a predetermined threshold frequency and an IIR filter processing unit
having an IIR filter. A component of a band in which the frequency characteristic of the input
acoustic signal is changed by the FIR filter processing unit and a band higher than a
predetermined cutoff frequency and a signal whose frequency characteristic of the same acoustic
signal is changed by the IIR filter processing unit Among them, since an acoustic signal for
reproduction including a component of a band below the cutoff frequency is output, the IIR filter
is also applied to a low frequency band below the threshold frequency while suppressing an
increase in the amount of calculation due to the FIR filter processing. It is possible to change the
frequency characteristics by
[0012]
It is a block diagram which shows the structure of the equalizer of Embodiment 1 of this
invention.
It is a graph explaining the characteristic of the LPF processing part and the HPF processing part
of the equalizer of Embodiment 1 of this invention.
It is a graph explaining another example of the characteristic of the LPF processing part of the
equalizer of Embodiment 1 of this invention, and the HPF processing part.
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It is a schematic diagram explaining the setting example of the LPF cutoff frequency in the
equalizer of Embodiment 1 of this invention, a HPF cutoff frequency, and a threshold frequency.
It is a block diagram which shows the structure of the equalizer of Embodiment 2 of this
invention. It is a schematic diagram explaining the example of the frequency which changes a
characteristic in the IIR filter process part of the equalizer of Embodiment 2 of this invention, and
the frequency which changes a property in a FIR filter process part. It is a block diagram which
shows the example of IIR filter and FIR filter.
[0013]
Hereinafter, embodiments of the present invention will be described with reference to the
drawings. The same or corresponding parts in the drawings to be referred to are denoted by the
same reference numerals. Embodiment 1 FIG. 1 is a block diagram showing a functional
configuration of an equalizer according to Embodiment 1 of the present invention. The equalizer
of this embodiment includes a setting unit 101, an IIR coefficient calculation unit 102, an FIR
coefficient calculation unit 103, an IIR filter processing unit 104, an FIR filter processing unit
105, a low pass filter processing unit (LPF processing unit) 106, and a high pass filter
processing. A unit (HPF processor) 107 and an adder 108 are provided.
[0014]
The setting unit 101 receives frequency characteristic information input from the outside. The
frequency characteristic information is information defining the frequency characteristic after
the change by the equalizer. The setting unit 101 notifies the IIR coefficient calculation unit 102
of the frequency characteristic (the requested frequency characteristic) defined in the received
frequency characteristic information. The IIR coefficient calculation unit 102 calculates the
coefficient (IIR filter coefficient) of the IIR filter of the IIR filter processing unit 104 so as to
obtain the frequency characteristic notified from the setting unit 101, and notifies the IIR filter
processing unit 104.
[0015]
Further, the IIR filter coefficients calculated by the IIR coefficient calculation unit 102 are also
input to the FIR coefficient calculation unit 103, and the FIR coefficient calculation unit 103
calculates the coefficients of the FIR filter of the FIR filter processing unit 105 based on the IIR
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filter coefficients (FIR filter The coefficient is calculated and notified to the FIR filter processing
unit 105.
[0016]
The digital acoustic signal input from the outside to the equalizer of this embodiment is input to
the IIR filter processing unit 104 and the FIR filter processing unit 105.
The IIR filter processing unit 104 includes an IIR filter, sets the IIR filter coefficient notified from
the IIR coefficient calculation unit 102 in this IIR filter, and performs processing to change the
frequency characteristic of the input acoustic signal. Output. Further, the LPF processing unit
106 includes a low pass filter (LPF), performs low pass processing (low pass filter processing) on
the signal output from the IIR filter processing unit 104, and outputs the signal.
[0017]
The FIR filter processing unit 105 includes an FIR filter, sets the FIR filter coefficient notified
from the FIR coefficient calculation unit 103 in this FIR filter, and performs processing to change
the frequency characteristic of the input acoustic signal. Output. The HPF processing unit 107
includes a high pass filter (HPF), performs high-pass processing (high pass filter processing) on
the signal output from the FIR filter processing unit 105, and outputs it.
[0018]
The signals output from the LPF processing unit 106 and the HPF processing unit 107 are input
to the adder 108, and the adder 108 adds these two signals and outputs the result as an audio
signal for reproduction.
[0019]
FIG. 7 shows an example of a second-order IIR filter and an S-order FIR filter.
In this figure, X (n) and x (n) are input signals, Y (n) and y (n) are output signals, and n represents
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time in digital processing. Also, a0, a1, b shown together with the multiplier represent filter
coefficients to be multiplied by the signal in the multiplier. The response length of the FIR filter is
determined by the order of the FIR filter.
[0020]
Each block of the setting unit 101, IIR coefficient calculation unit 102, FIR coefficient calculation
unit 103, IIR filter processing unit 104, FIR filter processing unit 105, LPF processing unit 106,
HPF processing unit 107, and adder 108 performs digital signal processing It can be realized by
hardware comprising a processor and a memory and a program operating on the hardware.
Alternatively, it can be realized by hardware using an application specific integrated circuit
(ASIC) or the like, or can be realized by combining them.
[0021]
Next, the operation will be described. Frequency characteristic information is input to the setting
unit 101 from the outside, and the setting unit 101 notifies the IIR coefficient calculation unit
102 of the frequency characteristic requested by the frequency characteristic information. The
item defined by the frequency characteristic information may be any item as long as it includes
information that enables calculation of IIR filter coefficients. For example, information such as a
frequency to be changed in characteristics, sampling frequency, quality factor, gain, and the like.
[0022]
The IIR coefficient calculation unit 102 calculates the IIR filter coefficient of the IIR filter
processing unit 104 based on the notified frequency characteristic. Here, it is assumed that the
IIR coefficient calculation unit 102 calculates the coefficients of the IIR filter by the linear
transformation method, which is a known method, using the above-described frequency
characteristic information.
[0023]
Generally, it is possible to widen the adjustable frequency by making the second-order IIR filter in
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a multistage configuration, so here, the IIR filter processing unit 104 is configured by a plurality
of second-order IIR filters It is assumed that Since the present invention is not limited by the
number of stages of the IIR filter, hereinafter, the number of stages of the IIR filter will be
described as N (N is a natural number of 2 or more). However, the IIR filter processing unit 104
is not limited to this configuration, and may have another configuration such as a single (that is,
N = 1) first-order IIR filter, a single high-order IIR filter, or a combination of a plurality of these. It
may be.
[0024]
The IIR coefficient calculation unit 102 calculates N sets of IIR filter coefficients corresponding to
the N stages of IIR filters of the IIR filter processing unit 104.
[0025]
The IIR coefficient calculation unit 102 outputs the calculated IIR filter coefficient to the IIR filter
processing unit 104 and the FIR coefficient calculation unit 103.
The FIR coefficient calculation unit 103 calculates FIR filter coefficients based on the received IIR
filter coefficients. As a method of obtaining the FIR filter coefficient from the IIR filter coefficient,
a method of obtaining the coefficient of the FIR filter by measuring an impulse response of the
IIR filter having the IIR filter coefficient, Inverse Discrete Fourier Transform, or the like You may
use the method etc. which are calculated | required by calculation.
[0026]
The order of the FIR filter is such that the FIR filter has a response length of one or more
wavelengths at a predetermined threshold frequency. In order to obtain the FIR filter coefficient
corresponding to this order, for example, if the coefficient of the FIR filter is found by finding the
impulse response of the IIR filter, the number of samples of the impulse response up to the order
found above You should do it.
[0027]
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The FIR coefficient calculation unit 103 outputs the FIR filter coefficient thus obtained to the FIR
filter processing unit 105. With such a configuration, each filter can be set so that the IIR filter of
IIR filter processing unit 104 and the FIR filter of FIR filter processing unit 105 have similar
characteristics based on the same frequency characteristic information. . The frequency
characteristics of the IIR filter of the IIR filter processing unit 104 and the FIR filter of the FIR
filter processing unit 105 do not have to match.
[0028]
The acoustic signal input to the equalizer of this embodiment is branched, and one is sent to the
IIR filter processing unit 104 and the other is sent to the FIR filter processing unit 105. The IIR
filter processing unit 104 performs N-stage IIR filter processing on the input sound signal
according to the IIR filter coefficient received from the IIR coefficient calculation unit 102, and
outputs the processed sound signal to the LPF processing unit 106. . Further, the FIR filter
processing unit 105 performs FIR filter processing on the input acoustic signal according to the
FIR filter coefficient received from the FIR coefficient calculation unit 103, and outputs the
processed acoustic signal to the HPF processing unit 107.
[0029]
The LPF processing unit 106 performs low-pass filter processing for passing frequency
components lower than a predetermined cut-off frequency to the sound signal after frequency
characteristic change received from the IIR filter processing unit 104 and processing the sound
signal after processing Are output to the adder 108. Further, the HPF processing unit 107
performs high-pass filter processing for passing frequency components higher than the cutoff
frequency to the acoustic signal after frequency characteristic change received from the FIR filter
processing unit 105, and adds the processed acoustic signal. Output to the control unit 108.
[0030]
FIG. 2 shows an example of filter characteristics of the LPF processing unit 106 and the HPF
processing unit 107. As shown in FIG. 2A, the LPF processing unit 106 is adjusted so that the
frequency at which the intensity is attenuated by 6 dB (here, the frequency at which the 6 dB is
attenuated is a cutoff frequency) is matched with the frequency attenuated by 6 dB in the HPF
processing unit 107. ing. Further, as shown in FIG. 2B, the frequency characteristic of the phase
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is also adjusted to match.
[0031]
The design of a filter having such a feature can be easily performed by configuring both the LPF
and the HPF as FIR filters and aligning the order of the filters with each other. In addition, when
the LPF and the HPF are configured by an IIR filter, it can also be realized by providing an APF
(All Pass Filter) for phase adjustment at the subsequent stage of the LPF and the HPF. However,
in the case of configuring with an IIR filter, it is preferable to configure the LPF and HPF with an
FIR filter because there is a possibility that the operation error in the processing of the LPF and
HPF can not be ignored depending on the value of the cutoff frequency. A well-known method
may be used as a design method of these filters.
[0032]
The LPF processing unit 106 and the HPF processing unit 107 may be adjusted so that the
characteristics of the cutoff frequency and the phase become equal as shown in FIG. In FIG. 3A,
the cutoff frequency (LPF cutoff frequency) of the LPF processing unit 106 and the cutoff
frequency (HPF cutoff frequency) of the HPF processing unit 107 are similar (for example, within
20 percent of the cutoff frequency) Shows an example adjusted to be However, it is desirable that
the HPF cutoff frequency be equal to or lower than the LPF cutoff frequency. Further, FIG. 2B
shows an example in which the frequency characteristics of the phase of the filters of the LPF
processing unit 106 and the HPF processing unit 107 are adjusted to be substantially the same
(for example, within 10 degrees). By doing this, the filters of the LPF processing unit 106 and the
HPF processing unit 107 can be configured more easily.
[0033]
The following shows an example of the order and threshold frequency of the FIR filter in the FIR
filter processing unit 105, the LPF cutoff frequency and the HPF cutoff frequency, and the order
in the case where the LPF processing unit 106 and the HPF processing unit 107 are composed of
the FIR filter. . Here, the sampling frequency of the sound signal is 44100 Hz (sampling
frequency of music compact disc).
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[0034]
The LPF cutoff frequency and the HPF cutoff frequency are 500 Hz. The sound quality
deterioration due to the calculation error of the IIR filter is clearly recognized at a frequency of
about 500 Hz or more, and the component of the frequency of 500 Hz or less of the acoustic
signal to be reproduced includes the calculation error of the IIR filter Even so, it can be said that
the influence on the sound quality when the acoustic signal is reproduced is small.
[0035]
The order of the FIR filters of the LPF processing unit 106 and the HPF processing unit 107 is a
128 (> 44100 ÷ 500 = 88.2) order as a value whose response length is one or more
wavelengths at the cutoff frequency.
[0036]
The order of the FIR filter of the FIR filter processing unit 105 is the 128th order similar to that
of the LPF processing unit 106 and the HPF processing unit 107 as an order capable of changing
the characteristics of the frequency of 500 Hz or more.
When the order of the FIR filter of the FIR filter processing unit 105 is 128th, the threshold
frequency is 345 Hz. FIG. 4 is a schematic diagram for explaining the LPF cutoff frequency, the
HPF cutoff frequency, and the threshold frequency in the case of the example shown here. The
frequency range of the component included in the reproduction acoustic signal among the
acoustic signals whose frequency characteristics have been changed by the IIR filter processing
unit 104, and the reproduction acoustic signal among the sound signals whose frequency
characteristics have been changed by the FIR filter processing unit 105. And the range of
frequencies where the response length of the FIR filter processing unit 105 is 1 wavelength or
more (that is, the frequency characteristic can be changed to a desired characteristic).
[0037]
In this example, the LPF cut-off frequency and the HPF cut-off frequency are the same. However,
as described above, they do not have to exactly match. In this example, the threshold frequency is
lower than the HPF cutoff frequency (and LPF cutoff frequency), but may be the same or higher.
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However, from the viewpoint of changing the characteristics of the requested frequency, it is
preferable to set the threshold frequency to a low value as in this example, because an
unchangeable frequency band does not occur.
[0038]
The adder 108 adds the signal output from the LPF processing unit 106 and the signal output
from the HPF processing unit 107, and outputs the added signal to the outside as a reproduction
acoustic signal. With such a configuration, the component of the band below the LPF cutoff
frequency of the output signal of the IIR filter processing unit 104 and the component of the
band above the HPF cutoff frequency of the output signal of the FIR filter processing unit 105
are combined Can be output. The cutoff frequencies of the LPF processing unit 106 and the HPF
processing unit 107 are adjusted to be the same frequency, and the phase characteristics with
respect to the frequency of the LPF processing unit 106 and the HPF processing unit 107 are the
same. Because of the adjustment, it is possible to combine these two signals without frequency
excess or deficiency.
[0039]
Next, the effect of the equalizer of this embodiment will be described. The order of the FIR filter
of the FIR filter processing unit 105 is 128, the order of the IIR filter of the IIR filter processing
unit 104 is 2, and the number of stages is 10 (N = 10). It is assumed that both the LPF processing
unit 106 and the HPF processing unit 107 are configured by an FIR filter whose cutoff frequency
is 500 Hz and whose order is 128. Further, the sampling frequency of the acoustic signal is
44100 Hz.
[0040]
As in the example of the FIR filter shown in FIG. 7, in the Sth-order FIR filter, processing of
multiplication and addition S times per sample of an acoustic signal (multiplication and addition
are counted and counted as one operation process) Need to). Therefore, in order for the FIR filter
processing unit 105 of the equalizer of this embodiment to process an acoustic signal with a
sampling frequency of 44100 Hz, it is necessary to perform arithmetic processing of 5.7 × 10 6
times at 44100 × 128 per second. There is. Further, since the LPF processing unit 106 and the
HPF processing unit 107 are also 128th-order FIR filters, it is necessary to perform the same
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arithmetic processing as the FIR filter processing unit 105 in one second. In addition, for the IIR
filter processing unit 104, if it is necessary to perform arithmetic processing five times per
sample (if it is considered in the example of FIG. 7, actually, five multiplications and four
additions), Since the filter is provided, 44100 × 5 × 10 and 2.2 × 10 6 or so arithmetic
operations are required per second. From the above, the equalizer of this embodiment needs the
ability to perform 19.3 × 10 6 operations per second in the IIR filter processing unit.
[0041]
On the other hand, in the case of an equalizer that changes the frequency characteristics with an
FIR filter, for example, when changing the characteristics of a 20 Hz signal, the order of the FIR
filter is required to make the response length of the FIR filter be 1 wavelength or more at 20 Hz.
Since 2205 needs to be performed next, it is necessary to be able to perform 97.2 × 10 6
operations per second. Therefore, when the equalizer of this embodiment tries to change the
frequency characteristic of a 20 Hz signal as compared to an equalizer whose frequency
characteristic is changed by the FIR filter, about 77.9 × 10 <6> times of calculation processing
per second It can be reduced.
[0042]
Further, in the case of the equalizer configured only by the 128th-order FIR filter, the response
length is less than one wavelength, and therefore the frequency characteristic can not be
changed to a desired characteristic for 88 Hz or less. On the other hand, in the equalizer of this
embodiment, the 20 Hz component of the acoustic signal whose frequency characteristic is
changed by the IIR filter processing unit 104 passes through the LPF processing unit 106 and is
output as the reproduction acoustic signal via the adder 108. Therefore, it is possible to change
the characteristics of the 20 Hz frequency component.
[0043]
Although the acoustic signal whose frequency characteristic has been changed by the IIR filter
processing unit 104 includes an operation error due to the IIR filter processing, the frequency
band higher than the LPF cutoff frequency can not pass through the LPF processing unit 106,
and the reproduction acoustic signal There is no impact on that. Also, the sensitivity of the
human ear is the best in the range of about 1,000 to 4,000 Hz of the audio frequency band, and
07-05-2019
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the sensitivity is reduced in the other bands. Even if the component of the frequency band of 500
Hz or less includes the calculation error of the IIR filter, it can be said that the influence of the
calculation error on the sound quality is minor.
[0044]
As described above, the equalizer of this embodiment includes the FIR filter processing unit 105
having the FIR filter configured to have a response length of one wavelength or more at the
threshold frequency and the IIR filter processing unit 104 having the IIR filter. And the frequency
characteristic of the input acoustic signal is changed by the FIR filter processing unit 105. The
component of the band above the predetermined cutoff frequency and the frequency
characteristic of the input acoustic signal are subjected to IIR filtering. The reproduction acoustic
signal including the component of the band below the cutoff frequency among the signals
changed in the section 104 is output.
[0045]
As a result, the amount of operation required for the equalizer to change the frequency
characteristics of the acoustic signal is reduced, and in the frequency band higher than the cutoff
frequency, an acoustic signal with good sound quality is output with the frequency
characteristics changed by the FIR filter, In the frequency band below the cutoff frequency, an
acoustic signal whose frequency characteristic is changed by the IIR filter is output. Therefore,
while suppressing an increase in the amount of computation due to the FIR filter processing, the
FIR filter is used for the frequency band above the cutoff frequency It is possible to obtain an
equalizer capable of changing the frequency characteristic of high sound quality and changing
the frequency characteristic even in a frequency band (low frequency band) below the cutoff
frequency.
[0046]
Second Embodiment FIG. 5 is a block diagram showing a functional configuration of an equalizer
according to a second embodiment of the present invention.
The difference between the equalizer according to this embodiment and the equalizer according
to the first embodiment is coefficient information different from the IIR filter coefficient output
from the IIR coefficient calculation unit 102 b to the IIR filter processing unit 104 to the FIR
coefficient calculation unit 103 It is a point to be
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[0047]
Next, the operation of the equalizer of the second embodiment will be described.
The operations of setting unit 101, FIR coefficient calculation unit 103, FIR filter processing unit
105, LPF processing unit 106, HPF processing unit 107, and adder 108 are the same as in the
first embodiment. The differences from the first embodiment will be mainly described below.
[0048]
As in the first embodiment, the IIR coefficient calculation unit 102b calculates N sets of IIR filter
coefficients for changing the frequency characteristic to a desired characteristic. Then, among
the calculated N sets of IIR filter coefficients, filter coefficients of J sets (where J is a natural
number less than or equal to N) including IIR filter coefficients for frequencies below the
threshold frequency are selected for IIR filter processing Output to unit 104. On the other hand,
N sets of IIR filter coefficients are output to FIR coefficient calculation section 103 as in the first
embodiment. The IIR filter processing unit 104 sets a J-stage IIR filter based on the J sets of IIR
filter coefficients received from the IIR coefficient calculation unit 102b. With such a
configuration, the number of stages of the IIR filter processing unit 104 can be reduced from N
to J in addition to the effects of the equalizer described in the first embodiment. The amount of
calculation required for the filter processing can be further reduced than that of the one
equalizer.
[0049]
Alternatively, among the N sets of IIR filter coefficients calculated from the IIR coefficient
calculation unit 102b to the FIR coefficient calculation unit 103, the IIR filter coefficients for
frequencies above the threshold frequency are selected (this time selection It is also conceivable
to notify K sets (where K is a natural number less than or equal to N) of notified IIR filter
coefficients. Then, the FIR coefficient calculation unit 103 determines the order of the FIR filter
of the FIR filter processing unit 105 so that the response length is at least one wavelength at the
lowest frequency among the frequencies corresponding to the K sets of IIR filter coefficients
received. Alternatively, FIR filter coefficients may be determined.
07-05-2019
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[0050]
At this time, N sets of IIR filter coefficients may be notified to the IIR filter processing unit 104 as
in the first embodiment, or N−K sets for frequencies below the threshold frequency. N or less
sets of filter coefficients including IIR filter coefficients may be selected and notified.
[0051]
With such a configuration, it is possible to further lower the order of the FIR filter of the FIR filter
processing unit 105 from the order corresponding to the threshold frequency in accordance with
the frequency at which the FIR filter processing unit 105 changes the characteristics. It is
possible to further reduce the amount of calculation.
[0052]
6 selects and notifies the IIR filter processing unit 104 of IIR filter coefficients for frequencies
below the threshold frequency, and targets the FIR coefficient calculation unit 103 for
frequencies above the threshold frequency. It is a schematic diagram explaining an example at
the time of selecting and notifying the IIR filter coefficient to assume.
Here, the cutoff frequency (the LPF cutoff frequency and the HPF cutoff frequency) and the
threshold frequency are set to the same value.
As shown in FIG. 6, to change the characteristics of 10 frequencies from F1 to F10, it is assumed
that F4 to F4 are below the threshold frequency and F5 to F10 are above the threshold
frequency.
[0053]
In the IIR filter processing unit 104, four sets of IIR filter coefficients corresponding to the
respective frequencies for changing the characteristics of four frequencies from F1 to F4 are set.
On the other hand, FIR filter processing section 105 sets FIR filter coefficients for changing the
characteristics of six frequencies from F5 to F10. In this case, the IIR filter processing unit 104
performs four-stage IIR filter processing. Further, the FIR filter of the FIR filter processing unit
105 has an order in which the response length becomes one wavelength or more in F5.
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[0054]
As described above, according to the equalizer of the second embodiment, in addition to the
effects of the equalizer shown in the first embodiment, the IIR filter processing unit 104 is
adjusted according to the number of frequencies whose frequency characteristics are to be
changed by the IIR filter processing unit 104. The amount of calculation can be further reduced
by changing the number of stages. Further, it is possible to further lower the order of the FIR
filter from the order corresponding to the threshold frequency in accordance with the frequency
at which the characteristic is changed in the FIR filter processing unit 105, and the amount of
calculation can be further reduced.
[0055]
DESCRIPTION OF SYMBOLS 101 setting part, 102, 102b IIR coefficient calculation part, 103 FIR
coefficient calculation part, 104 IIR filter processing part, 105 FIR filter processing part, 106 low
pass filter processing part (LPF processing part), 107 high pass filter processing part (HPF
processing part ), 108 adders
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