JP2003179465A - Filter device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an analog modeling filter which operates stably in frequency bands extending from the zero frequency to the Nyquist frequency. <P>SOLUTION: A band-elimination section 150 for restricting the frequency bands of the passing signals with the cut-off characteristics and a peaking section 200 for adjusting the gain only near a cut-off frequency fc are provided differently, and are constituted by using digital filters. As the result, the overall characteristics can be maintained at any frequency, and the stability can be secured in the case where the resonance is increased extending from the zero frequency to the Nyquist frequency. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter device in which an analog filter used in, for example, an analog synthesizer is modeled by a digital filter.

[0002]

2. Description of the Related Art When an analog filter, which is a main component of an analog synthesizer, is modeled by a digital filter, it has been difficult to realize it for the following reasons. First, in the conventional digital filter, when the resonance (corresponding to the Q value, which indicates the sharpness of the pass shoulder characteristic in the low pass filter) is increased in the frequency band from the frequency 0 to the Nyquist frequency, the coefficient sensitivity becomes large. This is because the operation becomes unstable because it changes greatly. Secondly, since the analog filter causes waveform distortion due to the non-linearity of the constituent elements, it is possible to easily realize a unique effect addition or the like, but it is not easy to realize this with a digital filter.

[0003]

Therefore, the above-mentioned first problem is solved.
As a method of solving the problem of the operation instability, it is proposed that the cutoff characteristic is configured to have an internal limitation between the parameters that determine the resonance characteristic, or the processing for appropriately correcting the filter coefficient is added. However, these are not the fundamental countermeasures against unstable operation, and their configuration becomes complicated. Moreover, almost no method has been proposed that focuses on the second problem and effectively solves it.

The present invention has been made in order to solve such a conventional problem, and an object thereof is to provide an analog modeling type filter which stably operates in a frequency band from the frequency 0 to the Nyquist frequency. And Another object of the present invention is to make it possible to easily reproduce waveform distortion even in such a filter.

[0005]

In order to achieve the above-mentioned object, the present invention is a filter device which filters an input signal to produce an output signal, wherein a pass signal of a pass signal is set with a set cut-off characteristic. It is characterized in that a band rejection unit for limiting a frequency band and a peaking unit for adjusting a gain only near the cutoff frequency for defining the cutoff characteristic are separately configured by using a digital filter.

Further, the band stop unit is composed of a plurality of first-order IIs.
An R-type low-pass filter is cascaded, and the peaking unit is configured to input a feedback signal of an output signal to a band-pass filter including a first-order IIR low-pass filter and a first-order IIR high-pass filter. However, the filter device can be realized by making the cutoff frequencies of the plurality of first-order IIR low-pass filters of the band rejection unit coincide with the center frequency of the band-pass filter of the peaking unit.

Further, there is provided a configuration in which the peaking section further includes a non-linear input / output section having a non-linear input / output characteristic. The nonlinear input / output unit can be configured to be able to input an addition signal of the output signal from the band rejection unit and the output signal from the bandpass filter of the peaking unit.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) (Structure) FIG. 1 is a block diagram of an analog modeling digital filter 1000 according to a first embodiment of the present invention. This analog modeling filter 1000 includes a first-order IIR (Infinite Impulse Response) type low-pass filter 100 and a first-order IIR type low-pass filter 11.
0 and a peaking unit 200.

A signal input through the input terminal 300 receives two first-order IIR type low-pass filters 100 and 110.
The frequency components below the cut-off frequency fc are passed by and the frequency components above this are blocked. Thus, the two first-order IIR type low-pass filters 100 and 110 serve as a band rejection unit 150 that limits the frequency band of the pass signal with the set cutoff characteristic.

The output signal output from the output terminal 310 is fed back to the bandpass filter 220 with a feedback amount determined by the feedback coefficient β230. The bandpass filter 220 is realized by, for example, a first-order IIR lowpass filter and a first-order IIR highpass filter, and the center frequency of its passband is set to the same frequency as the cutoff frequencies of the filters 100 and 110. There is. The signal that has passed through the bandpass filter 220 is configured to be added and output by the adder 210 with the signal from the band rejection unit 150 side.

FIG. 2 shows a more specific example of the configuration of FIG. 1 with a digital filter. In the first-order IIR filters 100 and 110, 101 and 111 are adders,
102 and 112 are multipliers of coefficient k, 103 and 113 are delay elements, and 104 and 114 are subtractors. In addition,
The transfer function of each of the first-order IIR type filters 100 and 110 is represented by the expression "low-pass characteristic" in FIG.

In the bandpass filter 220, 221 is a delay element, 222 is a multiplier of coefficient a, and 22 is a multiplier.
3 is a subtractor, 224 is an adder, 225 is a delay element, 22
Reference numeral 6 is a multiplier of coefficient a, 227 and 228 are adders.
Further, 229 is a subtractor and 231 is a delay element. As can be seen by referring to FIG. 2, this bandpass filter 2
220 is a first-order IIR low-pass filter and a first-order II
And an R-type high-pass filter.

The coefficient table 400 shown in FIG. 2 stores the values of the coefficients k and a prepared according to the frequency of the input signal. FIG. 3 is an explanatory diagram of the stored coefficients k and a, in which the horizontal axis represents the normalized frequency normalized by the sampling frequency (point 0.5 corresponds to the Nyquist frequency),
The vertical axis represents the value of the coefficient k that changes according to the normalized frequency indicated by the line 3a on the upper side of the drawing, and the value of the coefficient a that changes according to the normalized frequency indicated by the line 3b on the lower side of the drawing. There is. The coefficient table 400 thus stores the coefficients k and a that change depending on the frequency.

The coefficient control unit 500 retrieves the corresponding coefficients k and a from the coefficient table 400 according to the frequency of the input signal and supplies them to the filters 100, 110 and 220. In response to this, each filter 100, 110, 22
The coefficient of 0 is updated and the characteristic is appropriately changed to perform the filtering operation.

FIG. 4A shows characteristics when β is “1.95” and the value of Fc obtained by normalizing the cutoff frequency fc with the sampling frequency is “0.0015”, and FIG. 4B shows the cutoff. The characteristic is shown when the frequency is multiplied by 10, that is, when the value of Fc is “0.015”. In addition,
In FIG. 4, the vertical axis represents power and the horizontal axis represents frequency normalized by the sampling frequency.

Dotted lines indicated by symbols a and d in FIGS. 4A and 4B show the power-frequency characteristics of the band rejection unit 150, and are indicated by symbols b and e in FIGS. 4A and 4B. The dotted line shown indicates the power-frequency characteristic of the peaking unit 200, and the lines indicated by reference signs c and f in FIGS. 4A and 4B show the overall power-frequency characteristic.

As can be seen with reference to FIG. 4, a band rejection unit 150 that limits the frequency band of the passing signal with the set cutoff characteristic and a peaking unit 200 that adjusts the gain only near the cutoff frequency fc are provided. By dividing them and configuring them with digital filters, the overall characteristics are maintained at any frequency,
It is possible to ensure stability when the resonance (the sharpness of the peak value in the peaking portion) is increased within the band from the frequency 0 to the Nyquist frequency.

(Second Embodiment) (Structure) FIG. 5 is a block diagram of an analog modeling digital filter 1001 according to the second embodiment. Figure 1
As can be seen by comparing with the analog modeling digital filter 1001, in the peaking unit 200 of the analog modeling digital filter 1000 of FIG. 1, the nonlinear input / output unit 2 is provided between the adder 210 and the output terminal.
Since there is a feature only in that 40 is provided and there is no change in other points, only the part relating to the change will be described.

FIG. 6 is a detailed configuration diagram of the analog modeling digital filter 1001. As can be seen by comparing with FIG. 2, the nonlinear input / output unit 240 is newly added to the adder 21.
It is provided between 0 and the output terminal 310. The nonlinear input / output unit 240 includes a table 242 and a signal processing unit 240. The table 242 stores data for generating an output signal in a non-linear manner with respect to the input, and its non-linear characteristic is shown in FIG. 7, for example. As the input x becomes larger, the output γ becomes flatter and has non-linearity as a whole. Then, the signal processing unit 240 is configured to obtain the output signal corresponding to the input signal x by referring to the table 242 and output the obtained output signal.

The transfer function showing the characteristic of the peaking section 200 including the non-linear input / output section 240 is represented by the peaking characteristic of FIG. 8, and the overall characteristic of the analog modeling filter 1001 is shown by the overall characteristic of FIG. It becomes like a formula.

An amplitude of 12.5 is shown in FIGS. 9, 10 and 11.
(%) (Total 2 16 = 65536 peak t
12.5 (%) of the peak value
sine wave (frequency f = 240)
The output signal of the non-linear input / output unit 240 when (Hz) is input is shown. In this case, the sampling frequency fs = 48 k (Hz) and the cutoff frequency fc = 240
As Hz, β is changed to “0% (case 1)”, “50% (case 2)”, and “100% (case 3)”.

On the other hand, FIGS. 12, 13 and 14 are explanatory views of the “power-frequency characteristics” in cases 1, 2 and 3, respectively. In case 1 (see FIG. 12), one harmonic wave h1 appears with respect to the fundamental wave f1, and in case 2 (see FIG. 13), two harmonic wave h2 appears with respect to the fundamental wave f1. Then, in case 3 (see FIG. 14) in which the feedback coefficient is maximized, a large number of harmonics h1, h2, h3, ... Appear with respect to the fundamental wave f1.

Therefore, according to the second embodiment, by providing the non-linear output section 240 in the peaking section 200, it is possible to increase or decrease the resonance and easily reproduce the distorted waveform characteristic of the analog filter.

Although the embodiments of the present invention have been described above, various modifications and changes can be made to the above embodiments without departing from the scope of the present invention. Further, the configuration of the analog modeling digital filters 1000 and 1001 can be realized by hardware such as a dedicated LSI as much as possible, or the function can be realized by the CPU (or DSP) executing the operation program as much as possible. It is also possible to do so. Further, a series of operations may be performed by an algorithm program without using each table. Further, in the above embodiment, the band rejection unit is configured by cascading a plurality of first-order IIR low-pass filters, but the band rejection unit may be configured by cascading a plurality of first-order IIR high-pass filters, In this case, the transfer function showing the high-pass characteristic may be the one shown at the bottom of FIG.

As described above, according to the present invention, it is possible to obtain an effect that an analog modeling type filter that realizes stable operation in the frequency band from frequency 0 to the Nyquist frequency can be realized. Further, another object of the present invention is to obtain the effect of being able to easily reproduce the waveform distortion even in such a filter.

[Brief description of drawings]

FIG. 1 is a block configuration diagram of an analog modeling digital filter 1000 according to a first embodiment.

FIG. 2 is a specific configuration diagram of an analog modeling digital filter 1000 according to the first embodiment.

FIG. 3 is an explanatory diagram of coefficients.

FIG. 4 Analog modeling digital filter 1000
It is explanatory drawing of the characteristic of.

FIG. 5 is a block configuration diagram of an analog modeling digital filter 1001 according to a second embodiment.

FIG. 6 is a specific configuration diagram of an analog modeling digital filter 1001 according to the first embodiment.

FIG. 7 is an explanatory diagram of filter characteristics.

FIG. 8 is a block configuration diagram of an oscillator 200 according to a second embodiment.

FIG. 9 is an explanatory diagram of input signals.

FIG. 10 is an explanatory diagram of input signals.

FIG. 11 is an explanatory diagram of input signals.

FIG. 12 is an explanatory diagram of an output power spectrum.

FIG. 13 is an explanatory diagram of an output power spectrum.

FIG. 14 is an explanatory diagram of an output power spectrum.

[Explanation of symbols]

100 IIR type low pass filter 110 IIR type low pass filter 200 Peaking Club 210 adder 220 bandpass filter 230 Feedback coefficient 240 Non-linear input / output section 241 table 242 Signal processing unit 112 tables 300 input terminals 310 output terminal 400 coefficient table 500 coefficient control unit 1000 analog modeling digital filter 1001 analog modeling digital filter

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] March 18, 2003 (2003.3.1)
8)

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 1

[Correction method] Change

[Correction content]

[Figure 1]

Claims (4)

[Claims] 1. A filter device for filtering an input signal to obtain an output signal, wherein a band stop unit for limiting a frequency band of a passing signal with a set cutoff characteristic and the cutoff characteristic are defined. A filter device comprising a peaking section for adjusting a gain only in the vicinity of a cutoff frequency and a digital filter for separating them. 2. The filter device according to claim 1, wherein the band rejection unit includes a plurality of first-order IIR low-pass filters that are cascaded, and the peaking unit includes a first-order IIR low-pass filter and a first-order IIR low-pass filter. An IIR high-pass filter is used to input the output signal fed back to a band-pass filter, and the cut-off frequencies of the plurality of first-order IIR low-pass filters of the band stop unit are set to the band of the peaking unit. A filter device characterized in that it matches the center frequency of a pass filter. 3. The filter device according to claim 1, wherein the peaking unit further includes a nonlinear input / output unit having a nonlinear input / output characteristic. apparatus. 4. The filter device according to claim 3, wherein the nonlinear input / output unit can input an addition signal of an output signal from the band rejection unit and an output signal from a bandpass filter of the peaking unit. A filter device having a structure.