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JP2005033307

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DESCRIPTION JP2005033307
The present invention provides a digital filter in which a composite output frequency
characteristic is formed by a plurality of filters used in graphic equalizers and the like, wherein
the influence of the adjacent band is extremely small without changing the characteristic of the
conventional filter alone. Provided is a digital filter design method that can easily form
approximate frequency characteristics. An inverse convolution operation is performed on a
desired target frequency characteristic formed by a plurality of filters alone and a frequency
characteristic of each of the filters alone, and each of the filters alone is determined by the
inverse convolution. The coefficients of each filter are set according to the frequency
characteristic setting value, and based on that, the combined output frequency characteristic is
formed. [Selected figure] Figure 1
Digital filter design method
The present invention relates to a digital filter used in circuits of audio equipment, video
equipment, communication equipment and the like, and relates to a method of designing a digital
filter which can obtain a desired frequency characteristic. [0002] Digital filters, for example,
graphic equalizers used in audio circuits of audio equipment, have long been used as convenient
tools for changing the frequency characteristics. However, it is also a fact that there is a big
difference between the set value and the measured value of the frequency characteristic. In the
prior art, the cause of the occurrence of the error of the frequency characteristic will be
described in detail based on the frequency characteristic setting value graph and the actual value
graph of the graphic equalizer configured by a plurality of digital filters. Conventional Example 1
When the amplitude characteristics of each filter unit 14 of the graphic equalizer 13 as shown in
FIG. 10A are set as a frequency characteristic setting value graph, the measured values of FIG.
10B are obtained. It can be seen that a large error occurs in the curve of the frequency
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characteristic 15 according to the conventional design method realized as shown in the graph.
Especially in the portion where the curve changes rapidly, the error is large and the set
characteristics can not be realized. (Conventional Example 2) As in the frequency characteristic
setting value graph shown in FIG. 11 (a), the amplitude is increased by only one band of the filter
unit 14 according to the conventional design method of FIG. 11 (b) actual value graph. The
frequency characteristic 16 can be confirmed. It can be seen from the frequency characteristics
of this measured value graph that the amplitudes of not only the center frequency f0 but also the
adjacent bands increase. Therefore, even if the amplitude of only one band of the filter unit 14 is
raised, it can be confirmed that the amplitude increases in the range of several bands before and
after and causes an error. (Conventional Example 3) As in the frequency characteristic setting
value graph shown in FIG. 12 (a), when the amplitudes of the seven filter units 14 are equalized,
the time of one band of the filter unit 14 and Similarly, the adjacent band is affected, and the
characteristics on both sides are raised as shown in the frequency characteristic 17 according to
the conventional design method of the actually measured value graph in FIG. Also, when the
amplitudes of a plurality of bands are raised, the amplitude of the center frequency of each filter
also becomes larger than the set value. This is because the bands affect each other and increase
the amplitude. As described above, even if the frequency characteristic of the filter unit 14 is set
as in the frequency characteristic setting value graph, not only the setting frequency but also the
adjacent bands are respectively added, as shown in the actual value graph. The amplitudes of
adjacent bands also increase or decrease, and desired frequency characteristics can not be
obtained.
In addition, even if a filter in which adjacent bands do not change is designed, a high-order filter
is required and the filter becomes complicated, resulting in a large apparatus and an expensive
cost burden. SUMMARY OF THE INVENTION In view of the above-described current situation, a
method of designing a digital filter is provided which can obtain a desired combined output
frequency characteristic without changing the characteristics of a conventional filter alone.
Means for Solving the Problems In view of the above, the inventor of the present invention has
solved the problem by the following means as a result of earnest research. (1) In a digital filter
design method for forming a combined output frequency characteristic with a plurality of filters
used for graphic equalizers etc., a desired target frequency characteristic formed with the
plurality of filters alone, and a plurality of the plurality of filters. The inverse convolution
operation is performed with the frequency characteristics of each filter alone, the coefficients of
each filter alone are set according to the respective frequency characteristic setting values of the
filter alone determined by the inverse convolution operation, and the combined output frequency
characteristics are based thereon In the method of designing a digital filter, it is possible to easily
form a characteristic similar to a target frequency characteristic, which has very little influence of
adjacent bands, without changing the characteristic of a conventional filter alone. (2) A digital
filter for setting a coefficient of each filter unit according to each frequency characteristic setting
value of the filter unit determined by the inverse convolution calculation and forming a combined
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output frequency characteristic based on the set value is an audio signal. The method of
designing a digital filter according to (1), which is applicable to a digital filter incorporated in an
electronic device such as a device, a video device, and a communication device. DETAILED
DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in
detail based on the drawings. FIG. 1 is an explanatory view for obtaining the frequency
characteristic setting value of each filter by “inverse convolution calculation (* <− 1> mark)”
from the target frequency characteristic of the cascade connection type filter of the embodiment
of the present invention. Explanatory drawing which sets the coefficient of each cascade
connection type | mold filter according to the frequency characteristic setting value of each filter
calculated | required by "inverse convolution calculation" of an example, FIG. Fig. 5 is an
explanatory diagram of a cascade connection type filter, and Fig. 6 is a flow chart of each filter of
the cascade connection type filter from the target frequency characteristic of the cascade
connection type filter for obtaining the frequency characteristic setting value of each filter by
"inverse convolution operation" An explanatory view for obtaining a combined output frequency
characteristic by “Convolution calculation (* mark)” from the frequency characteristic setting
value, and FIG. 7 shows “Inverted from the combined output frequency characteristic of the
cascade connection type filter obtained in FIG. Explanatory drawing which calculates | requires
the frequency characteristic setting value of each filter by calculation (* <-1> mark), FIG. 8: the
frequency characteristic of each filter calculated | required by the "inverse convolution
operation" of the cascade connection type filter of this invention invention Example Comparison
graph of actual measurement result 1 of synthetic output frequency characteristic by setting
value and synthetic output frequency characteristic by setting the frequency characteristic of
each filter by the conventional method, FIG. 9 is “reverse convolution of cascade connection
type filter according to the embodiment of the present invention It is a comparison graph of the
measurement result 2 of the synthetic | combination output frequency characteristic by the
frequency characteristic setting value of each filter calculated | required by calculation ", and the
synthetic | combination output frequency characteristic by the frequency characteristic setting
value of each filter by a conventional system.
First, based on the signal processing of the current graphic equalizer, a method of calculating the
filter characteristic in consideration of the influence of the adjacent band will be described. The
present embodiment will be described using an example of an IIR (Infinite Impulse Response /
Recursive) filter system in which a single filter is composed of an adder, a plurality of delay units,
and a multiplier based on digital signal processing. In FIG. 5, transfer functions H 1 (z) and H 2
(z) to Hn (z) of the cascade connection type filter 3 of 1 to n filters alone 1 are set. For example, a
second-order band boost filter with a quality factor Q = 5 and a peak of +12 dB is set. Assuming
that the input signal amplitude is X (z), the output signal amplitude is Y (z), and the transfer
function of the filter is Hn (z), the transfer function H (z) can be expressed by the product of the
transfer functions of the individual filters. Here, when the transfer function is decomposed into
the amplitude and the phase, the amplitude characteristic M (ω) is in the form of a product, The
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phase characteristic θ (ω) can be expressed in the form of a sum. As described above, the
amplitude characteristic at a certain frequency can be expressed by the product of the amplitude
characteristics of all the filters. Here, the amplitude value is converted to a logarithmic scale to
reduce the amount of calculation. By this conversion, the amplitude characteristic becomes the
sum of all filter characteristics. <Img class = "EMIRef" id = "198843278-000005" /> [0018] Here,
considering the frequency characteristic setting value of each filter and the frequency
characteristic of a single filter, the synthesis of the graphic equalizer is performed. The output
frequency characteristic is understood to be one in which the frequency characteristic of a single
filter is moved and integrated on the frequency axis for each central frequency. Therefore, by
converting the amplitude value of each filter into a logarithmic scale, it can be treated as an
integral, and as shown in FIG. 6, the frequency characteristic setting value of each filter and the
frequency characteristic 5 of the single filter unit have a relationship of "convolution". I
understand that
That is, based on the frequency characteristic setting values 4a to i of each filter shown in the
frequency characteristic setting value graph of each filter in FIG. 6A and the frequency
characteristic 5 of the single filter in FIG. If "*" is selected, the composite output frequency
characteristic 6 is calculated as a result of the "convolution" calculation result shown in FIG. 6 (c).
Here, if the frequency characteristic of the graphic equalizer is determined by the relation of
“folding”, it becomes possible to obtain the frequency characteristic setting value of each filter
from the combined output frequency characteristic. Since “combined” (the “*” mark)
indicates that the frequency characteristic setting values 4a to i of each filter and the frequency
characteristic 5 of the filter unit are “combined”, the composite output frequency
characteristic 6 is obtained. It is obvious that the result of “reversing” (characters * <− 1>) in
the characteristic 5 is the frequency characteristic setting values 4a to i of each filter. The
situation is shown in FIG. Based on the combined output frequency characteristic 6 according to
the “convoluted” calculation result shown in FIG. 7C and the frequency characteristic 5 of the
single filter of FIG. 7B, “inverse operation (* <− 1> mark 6A, the frequency characteristic
setting of each filter of FIG. 7A is the same as the frequency characteristic setting values 4a to i
of each filter shown in the frequency characteristic setting value graph of each filter of FIG. 6A.
Frequency characteristic set values 4a to i of the respective filters shown in the value graph are
calculated. Here, based on the combined output frequency characteristic, “inverse convolution
operation (* <− 1> mark)” is performed based on the target frequency characteristic 7 of FIG. 1
(a) and the frequency characteristic 5 of the filter unit of FIG. 1 (b). For example, as in the
frequency characteristic setting value graph of each filter in FIG. 1C, the frequency characteristic
setting values 8a to 8i of each filter considering the influence on the adjacent band are obtained.
Furthermore, each filter of the frequency characteristic setting value graph of each filter of FIG.
2A obtained by “inverse convolution calculation (* <− 1> mark)” from the target frequency
characteristic 7 of FIG. 1A The coefficients of each filter unit 1 of the cascade connection type
filter 3 are set as shown in FIG. 2 (b) according to the frequency characteristic setting values 8a
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to i of FIG. Can be realized. That is, in FIG. 3, as one example, the filter unit 1 is an input of the
multiplier a1 and the multiplier a1 in which the input signal X (z) is connected in series with the
multiplier a0 and the delay device Z <−1>. Is connected to the adder 18 via each of the
multipliers a2 connected in series to the delay device Z <−1> connected to the output signal Y
(z) connected in series to the delay device Z <−1> The multiplier b1 is connected to the adder 18
via the multiplier b2 connected in series to the delay device Z <-1> connected to the input of the
multiplier b1.
Here, as described above, the frequency characteristic setting value graph of each filter of FIG. 2
(a) obtained from the target frequency characteristic 7 of FIG. 1 (a) by "inverse convolution
calculation (* <-1> mark)" By setting the frequency characteristic setting values 8a to 8 of the
respective filters as the coefficients of the multipliers a0, a1, a2, b1 and b2 forming the
characteristics of the single filter 1, it is possible to realize a filter which does not include an
error. FIG. 4 is a flowchart for obtaining the frequency characteristic setting value of each filter
by “inverse convolution operation” from the target frequency characteristics of the cascade
connection type filter according to the embodiment of the present invention, wherein “inverse
convolution operation” is cascaded The procedure applied to the filter is as follows. After the
setting procedure “START”, “set target frequency characteristics” and “convert amplitude
values to logarithms” and “set frequency characteristics of filter alone” and “convert
amplitude values to logarithms”. Subsequently, the frequency characteristics to be set for each
filter obtained by “inverse convolution operation” of the two are input to a graphic equalizer
or other IIR digital filter design tool or the like, and finally “filter coefficients of each filter
Calculate "" and "STOP". As a result, it is possible to realize a combined output frequency
characteristic that approximates the target frequency characteristic. The measurement result of
the filter which realized the complicated transfer function by this design method is shown.
(Measured result 1) In FIG. 8, this figure is actually measured data in the case of setting to raise
the amplitude of seven bands of single filter, and compared with synthesized output frequency
characteristic 9 by the conventional design method. It can be seen that the frequency
characteristic 10 has a frequency characteristic similar to the target frequency characteristic in
which the influence of the adjacent band is extremely small. (Result of Measurement 2) In FIG. 9,
the figure shows the measurement data when setting the amplitude of 5 bands out of 10 bands
of the filter alone to lower the amplitude of 5 bands in the same manner as described above. It
can be seen that the combined output frequency characteristic 12 according to the design
method of the present invention has a frequency characteristic similar to the target frequency
characteristic with very little influence of the adjacent band compared to the combined output
frequency characteristic 11 according to the conventional design method. Although the example
of the IIR filter system has been described above, the present invention may be applied to a
design tool of an FIR (Finite Impulse Response / non-recursive) filter system. Moreover, it is also
possible to apply to a parallel connection type filter. According to the design method of the
digital filter, the coefficients of each filter unit are set according to the respective frequency
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characteristic setting values of the filter unit determined by the inverse convolution operation,
and the combined output frequency characteristic is formed based on the set coefficients. The
present invention is applicable to all digital filters incorporated in electronic devices such as
audio devices, video devices, communication devices and the like.
According to the present invention, the following effects are exhibited. 1) Even if the target
transfer function is complicated, filter coefficients can be easily calculated. 2) The operation time
does not depend on the complexity of the transfer function. 3) A desired combined output
frequency characteristic can be obtained only by changing the frequency characteristic setting
value of each filter unit. From the above, it is possible to easily form a digital filter having a
frequency characteristic close to the target frequency characteristic, which is hardly influenced
by the adjacent band, without changing the characteristic of the conventional filter alone. 4) The
digital filter design method can be applied to all digital filters incorporated in electronic devices
such as audio devices, video devices, communication devices and the like. BRIEF DESCRIPTION
OF THE DRAWINGS [FIG. 1] A description for obtaining the frequency characteristic setting value
of each filter by "inverse convolution operation (* <-1> mark)" from the target frequency
characteristic of the cascade connection type filter of the embodiment of the present invention.
Figure. FIG. 2 is an explanatory view for setting coefficients of cascade-connected filters
according to frequency characteristic setting values of the filters obtained by “inverse
convolution operation” in the embodiment of the present invention. FIG. 3 is a circuit example
block diagram of a single filter. FIG. 4 is a flow chart for obtaining a frequency characteristic
setting value of each filter by “inverse convolution operation” from the target frequency
characteristic of the cascade connection type filter of the embodiment of the present invention.
FIG. 5 is an explanatory view of a cascade connection type filter. FIG. 6 is an explanatory diagram
for obtaining a combined output frequency characteristic by “convolution calculation (* mark)”
from the frequency characteristic setting value of each filter of the cascade connection type filter.
FIG. 7 is an explanatory view for obtaining frequency characteristic setting values of the
respective filters by “inverse convolution calculation (* <− 1> mark)” from the combined
output frequency characteristics of the cascade connection type filter obtained in FIG. 6; FIG. 8 is
a combined output frequency characteristic according to the frequency characteristic setting
value of each filter determined by “inverse convolution operation” of the cascade connection
type filter according to the embodiment of the present invention, and a combined output
according to the frequency characteristic setting value according to the conventional method The
comparison graph of measurement result 1 with a frequency characteristic. FIG. 9 is a combined
output frequency characteristic according to the frequency characteristic setting value of each
filter determined by “inverse convolution operation” of the cascade connection type filter
according to the embodiment of the present invention, and a combined output according to the
frequency characteristic setting value according to the conventional method The comparison
graph of measurement result 2 with a frequency characteristic. FIG. 10 is an explanatory diagram
of a frequency characteristic setting value graph (a) of a graphic equalizer according to
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Conventional Example 1 and an actually measured value graph (b). FIG. 11 is an explanatory
diagram of a frequency characteristic setting value graph (a) of a graphic equalizer according to
Conventional Example 2 and an actually measured value graph (b).
FIG. 12 is an explanatory diagram of a frequency characteristic set value graph (a) of a graphic
equalizer according to Conventional Example 3 and an actually measured value graph (b).
[Description of the code] 1: Filter alone 2: Transfer function 3: cascaded filter 4: Frequency
characteristics setting value 5: Frequency characteristics of a single filter 6: Synthetic output
frequency characteristics 7: Target frequency characteristics 8: Frequency characteristics setting
values 9 11, 15, 16, 17: Synthesized output frequency characteristic by the conventional design
method 10, 12: Synthesized output frequency characteristic by the present invention design
method 13: Graphic equalizer 14: Filter unit 18: Adder
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