JPH0921691A - Control of filter attenuation characteristic and filter device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a filter device, whose attenuation characteristic can be arbitrarily varied and controlled, with a small-scale constitution by variably setting the cut-off frequency of the output characteristics, which do not include aliasing noises, and the resonance degree of shoulder characteristics. SOLUTION: A filter 1 can variably control a cut-off frequency FC by setting a filter coefficient CFC in coefficient multipliers 10 and 11 and a resonance degree QC of shoulder characteristics by setting a filter coefficient CQC in a coefficient multiplier 12. A filter-coefficient setting part 2 sets the filter coefficients CFC and CQC corresponding to the received cut-off frequency FC and resonance degree QC in the filter 1. A parameter converting part 3 converts the inputted cut-off frequency fc and attenuation-gradient degree Kenv into the cut-off frequency FC and the resonance degree QC in accordance with parameter converting operation and supplies the values into the filter-coefficient setting part 2. A time-varing-parameter generating part 4 variably generates the envelope of musical sounds with a key-on signal as a trigger, changes the degree of the attenuation gradient and supplies the result into the converting part 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter attenuation characteristic control method and a filter device capable of variably controlling an attenuation characteristic in a filter output characteristic. When the filter device of the present invention is used, for example, in an electronic musical instrument or the like, a desired timbre can be created by filtering a musical tone signal.

[0002]

2. Description of the Related Art A change in the tone color of a natural sound generated by a natural musical instrument is attenuated faster as the order of the overtone is higher, and slower as the order of the overtone is lower. In order to realize such a tone color change in an electronic musical instrument, a musical tone signal is filtered by a low-pass filter having a required frequency characteristic. in this case,
In order to realize the frequency characteristic of the timbre in which the higher the order of the overtones, the faster the attenuation, and the lower the order of the overtones, the slower the attenuation,
It is necessary to control the attenuation characteristic of the low-pass filter to obtain the desired frequency characteristic of the tone color.

The low-pass filter used in electronic musical instruments is currently generally realized by a digital filter.
It is easy to variably control the cutoff frequency of the digital filter, but it is not easy to control the attenuation. FIG. 11 shows this explanation, and as shown in the figure, the conventional low-pass filter generally changes its cutoff frequency in the arrow direction, for example, as fc1 and fc2, but changes its attenuation slope. Little was done. The reason for this is that the digital filter that can control the attenuation slope has a large configuration of the filter itself,
This is because the control itself is very troublesome because there are many filter coefficients to control.

As an example for realizing a plurality of attenuation gradients, for example, as shown in Japanese Patent Application Laid-Open No. 5-94193, a plurality of digital filters having different attenuation gradients are provided and a desired tone color is selected from them. I choose one to use.

[0005]

However, the provision of a plurality of digital filters in this manner causes an increase in the hardware scale when the hardware is used to implement the digital filters. Further, even when the digital filter is realized by a DSP (digital signal processor), the hardware and software are increased, which makes the configuration of the filter device large and it is difficult to actually mount it on an electronic musical instrument or the like. .

The present invention has been made in view of the above problems, and an object of the present invention is to realize a filter device capable of arbitrarily variably controlling the attenuation characteristic with a small-scale configuration.

[0007]

In order to solve the above-mentioned problems, a method of controlling the attenuation characteristic of a filter device according to the present invention uses a cutoff frequency Fc of the output characteristic which does not include aliasing noise and a resonance degree Qc of the shoulder characteristic thereof. And a digital filter capable of variably setting the total output characteristic including the folding noise in the output characteristic of the digital filter, by changing the degree of the folding noise characteristic, the degree of the attenuation gradient of the output characteristic to be realized. The attenuation characteristic is variably controlled by variably adjusting the cutoff frequency and the resonance degree of the output characteristic of the digital filter so as to have K _env and the target cutoff frequency fc.

The filter device of the present invention is, as one form, a digital filter capable of variably setting a cut-off frequency Fc of output characteristics not including aliasing noise and a resonance degree Qc of shoulder characteristics thereof, and an output to be realized. Input means for inputting the characteristic attenuation gradient degree K _env and the target cutoff frequency fc, and aliasing noise for the output characteristic of the digital filter by the attenuation gradient degree and the target cutoff frequency input by the input means. Conversion means for converting into a cutoff frequency and a resonance degree of the digital filter according to an arithmetic expression to be included, and setting means for setting a filter coefficient according to the cutoff frequency and the resonance degree converted by the conversion means in the digital filter. It is configured with.

The conversion means is such that the cutoff frequency of the output characteristic of the digital filter is Fc, the resonance degree of the shoulder characteristic thereof is Qc, the degree of the attenuation gradient of the output characteristic to be realized is K _env , and the target cutoff frequency thereof is fc, where fs is the sampling frequency of the digital filter, Fc (log) = fc (log) + [log (fs / 4) -fc (lo
g)] × K _env (however, Fc (log) and fc (log) represent a logarithmic scale), Fc (log) is logarithmically linearly converted to obtain a cutoff frequency Fc, and Qc = α + K _env However, α may be configured to perform conversion by obtaining the resonance designation value Qc with a coefficient such as 1.0.

The filter device may be configured to further include means for changing the degree of the attenuation gradient input by the input means with time.

[0011]

The present invention positively utilizes the aliasing characteristic as its operating principle. The aliasing noise is (1/2) fs (however, fs
Is higher than the sampling frequency) and does not satisfy the sampling theorem. FIG. 3 explains this aliasing noise. As shown in FIG. 3A, if the gain of the digital filter is completely attenuated by (1/2) fs, it exceeds (1/2) fs. Since there is no frequency component of the sampled signal, aliasing noise does not occur for low frequencies. On the other hand, as shown in FIG. 3B, when the gain of the digital filter exceeds (1/2) fs, (1
/ 2) Since there is a frequency component of the sampled frequency exceeding fs, aliasing noise shown by the shaded area in the figure occurs.

FIG. 1 illustrates the principle of the present invention. Now, it is assumed that a low-pass filter is made by a digital filter and the output characteristic (calculated value) that does not include aliasing noise is as shown in FIG. FIG. 1B shows the aliasing noise characteristic considering the sampling frequency fs with respect to this output characteristic. The overall output characteristics of the filter are shown in Fig. 1 (A).
And FIG. 1 (B) are combined, as shown in FIG. 1 (C). The output characteristics of this filter are such that the high frequency band cannot be sufficiently cut. However, focusing on the attenuation slope in section a in the figure, the cutoff frequency Fc and the shoulder characteristics of the output characteristics that do not include the aliasing noise of the digital filter. By changing the resonance degree Qc, the output characteristic and the folding characteristic can be changed, and as a result, the total output characteristic can be changed, whereby the attenuation slope of the total output characteristic can be changed.

FIG. 2 is a diagram for explaining a change in the attenuation characteristic of the total output characteristic. As shown in the figure, the cutoff frequency Fc of the output characteristic not including the aliasing noise of the digital filter is shown.
By changing the resonance degree Qc of the shoulder characteristic, the attenuation characteristic (attenuation slope and cutoff frequency) of the overall output characteristic can be changed in the direction indicated by the arrow in the figure.

When the present invention is applied, for example, to the processing of a musical tone signal of an electronic musical instrument, the degree K of the input attenuation gradient is input.
Simulate a change in the timbre of a natural sound generated by a natural musical instrument, where _env is time-varying according to the envelope change of the musical sound signal, the higher the harmonic order, the faster the attenuation, and the lower the harmonic order, the slower the attenuation. Will be possible.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 4 shows an embodiment in which the filter device of the present invention is used for processing a musical tone signal of an electronic musical instrument. In the figure, 1
Is a second-order state variable filter composed of a digital filter, and has a cutoff frequency Fc and a shoulder characteristic resonance degree Q as output characteristics not including folding noise characteristics.
You can variably set c and. The cut-off frequency Fc is the frequency at the point where it is lowered by 3 dB. The resonance degree Qc is a parameter mainly related to resonance,
It has an inverse relationship with Q (quality factor) in a so-called analog resonance circuit.

FIG. 5 is a diagram showing an example of the configuration of the second-order state variable filter 1, which is coefficient multipliers 10 to 13,
The 1-sample delay units 14 and 15 and the adders 16 to 19 are included. This filter 1 comprises coefficient multipliers 10 and 11
It is possible to variably control the cutoff frequency Fc by setting the filter coefficient C _{FC at,} and the resonance degree Qc of the shoulder characteristic by setting the filter coefficient C _QC at the coefficient multiplier 12.

Reference numeral 2 is a filter coefficient setting unit, and this filter coefficient setting unit 2 has a function of setting filter coefficients C _FC and C _QC corresponding to the received cutoff frequency Fc and resonance degree Qc in the filter 1.

Reference numeral 3 is a parameter conversion unit for supplying the cutoff frequency Fc and the resonance degree Qc to the filter coefficient setting unit 2. The parameter conversion unit 3 converts the input cutoff frequency fc and the attenuation gradient degree K _env into a cutoff frequency Fc and a resonance degree Qc according to a parameter conversion calculation described later.

Reference numeral 4 denotes a time-varying parameter generator, which is triggered by a key-on signal from a keyboard or the like and is generated while changing the envelope env of the musical sound with time, and the degree of the attenuation gradient corresponding to the change of the envelope env. K
_env is changed and supplied to the parameter conversion unit 3.

Here, fc is the cutoff frequency of the overall output characteristic of the filter 1 including the aliasing noise characteristic (filter output characteristic to be realized). K _env is a parameter that determines the degree of the damping slope of the overall output characteristic, and is 0
Time-varying in the range of ~ 1.0. Degree of this attenuation gradient K
_{The value of env} is set so that the closer the value is to 0, the larger the gradient is, and the closer it is to 1.0, the smaller the gradient is.

The above-mentioned parameter conversion calculation formula is shown below.
However, the meaning of each parameter is as follows. fs: sampling frequency (specifically, fs is 44.1)
Although it is selected to be about kHz, here, fs is made to correspond to a value of 2.0 to form a unit system. Therefore, for example, fs / 2 becomes 1.0. Fc: Cutoff frequency of the output characteristic that does not include the aliasing noise characteristic (however, it is displayed in a unit system where fs = 2.0) fc: Cutoff frequency of the total output characteristic ( However, fs =
2.0 a display in unit system to) Qc: resonance degree of output characteristics do not include the aliasing noise characteristics (Qc = 0~2.0) K _env: degree of attenuation slope of the total output characteristics (0-1.
0) Fc (log): value of cutoff frequency Fc in logarithmic domain fc (log): value of cutoff frequency fc in logarithmic domain

[Parameter conversion calculation formula] Fc (log) = fc (log) + [log (fs / 4) -fc (log)] × K _env (1) Fc (log) obtained by the formula (1) Is logarithmically (log-linear) converted to obtain the cutoff frequency Fc. _{Qc = 1. 0 + K env} (2) where, Qc + Fc ≧ _2. 0 _{If, Qc = 2. 0-Fc} (3)

[0023] (1) In expression, by varying the degree K _env attenuation gradient between 0 to 1.0, it is possible to change the cut-off frequency fc to the upper limit fs / 4. According to this equation (1), when K _env that defines the damping gradient changes in the direction in which the damping gradient becomes gentle, the cutoff frequency Fc is calculated to change in the higher direction. Expression (3) is a conditional expression for performing processing in the stable region, and is used to correct the resonance characteristic Qc so that the shoulder characteristic does not become sharper than the shoulder characteristic when the value is the reference value. According to the equation, Qc is corrected to a value smaller than 1.

This parameter conversion calculation may be carried out in real time by a DSP (digital signal processor), but the cut-off frequency F previously calculated from the cut-off frequency fc and the attenuation gradient degree K _env.
The c and the resonance degree Qc are stored in a memory such as a ROM in the form of a conversion table, and the cutoff frequency Fc and the resonance degree Qc are read from the conversion table for the input cutoff frequency fc and the degree K _env . It may be configured.

8 to 10 show the cutoff frequency Fc and the resonance degree Q in the output characteristic not including the aliasing noise characteristic.
It is a figure which shows the mode of change of c. In each figure, the horizontal axis represents frequency and the vertical axis represents level, and the cutoff frequency Fc is displayed in a unit system with the sampling frequency fs being 2.0. FIG. 8 is a characteristic diagram when the resonance degree Qc is fixed to 1.0 and the cutoff frequency Fc is changed, and FIG. 9 is a case where the cutoff frequency Fc is fixed to 0.5 and the resonance degree Qc is changed. Fig. 10 shows the cut-off frequency F
It is a characteristic view when c is fixed to 1.0 and the resonance degree Qc is changed. As is clear from these figures, changing the cutoff frequency Fc and the resonance degree Qc changes the output characteristics (not including the aliasing noise characteristics),
Therefore, since the folding noise characteristic obtained by folding the output characteristic also changes, it can be seen that the total output characteristic obtained by combining the output characteristic and the folding noise characteristic changes, and in this comprehensive output characteristic, as the cutoff frequency Fc becomes higher. , The attenuation gradient becomes gentle due to the influence of the frequency characteristic due to the folding back.

FIGS. 6 and 7 are graphs showing the total output characteristics including the aliasing noise characteristics output from the filter 1. In FIG. 6, the cutoff frequency fc of the total output characteristic is set to approximately fs / 32, and the degree of attenuation gradient K _env is set to 0.
FIG. 7 shows the characteristics when variously changed from 1.0 to 1.0. The cutoff frequency fc of the total output characteristics is approximately fs /
It is a characteristic when it is set to 16 and the degree of attenuation gradient K _env is variously changed from 0 to 1.0. When the cutoff frequencies Fc = fs / 4 and Qc = 1, the total output characteristic becomes flat. In order to obtain these characteristics, the cutoff frequency fc according to the above-mentioned parameter conversion formulas (1) to (3),
Degree of attenuation gradient K _env is cutoff frequency F
c and the degree of resonance Qc, and the corresponding filter coefficients C _FC and C _QC may be set in the filter 1.

In the above description, the parameter conversion unit
In 3, Fc and Qc have already been calculated with the filter coefficient.
Therefore, the filter set by the filter coefficient setting unit 2
Coefficient C _FCAnd C_QCIs input to the filter coefficient setting unit 2.
Value equal to the cutoff frequency Fc and the degree of resonance Qc
It has become.

In the attenuation range of the second-order state variable filter 1, a characteristic of 12 dB / oct can be obtained, but when the filter is opened and closed in accordance with the velocity of a piano sound, the attenuation of the high-order overtone is too large. "Too close"
State. The filter device of the present embodiment controls the damping gradient by correcting the resonance degree Qc so that the time-varying envelope env parameter is reflected in the damping gradient, and the higher the harmonic overtone, the faster the attenuation is. It simulates a characteristic similar to a timbre change of a natural sound in which the lower the attenuation, the slower the attenuation. For the purpose of changing the normal balance of the overtones, a range up to about -12 dB is a practical range, and further attenuation is "almost inaudible". Therefore, the characteristic between -12 dB and 0 dB is emphasized.

[0029]

As described above, according to the present invention, it is possible to realize a filter device capable of arbitrarily controlling the attenuation gradient with a small scale.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is a diagram illustrating the principle of the present invention.

FIG. 3 is a diagram illustrating a folding noise characteristic.

FIG. 4 is a block diagram of an embodiment in which the present invention is applied to a musical tone signal processing of an electronic musical instrument.

FIG. 5 is a diagram showing a configuration example of a second-order state variable filter in the embodiment.

FIG. 6 is a diagram showing an example of a total output characteristic of the apparatus of the embodiment.

FIG. 7 is a diagram showing another example of the filter total output characteristic of the embodiment apparatus.

FIG. 8 is a diagram showing an example of filter output characteristics not including aliasing noise.

FIG. 9 is a diagram showing another example of filter output characteristics not including aliasing noise.

FIG. 10 is a diagram showing another example of filter output characteristics not including folding noise.

FIG. 11 is a diagram illustrating output characteristics of a conventional filter.

[Explanation of symbols]

1 Digital Filter (Secondary State Variable Filter) 2 Filter Coefficient Setting Unit 3 Parameter Converting Unit 4 Time-Varying Parameter Generating Unit (Envelope Generating Unit) 10-13 Filter Coefficient Multiplying Unit 14, 15 1 Sample Delay Device 16-19 Adder

Claims (4)

[Claims] 1. A cutoff frequency (Fc) of an output characteristic not including a folding noise characteristic and a resonance degree (Qc) of its shoulder characteristic.
) And a digital filter that can be variably set, and the total output characteristic including the folding noise characteristic in the output characteristic of the digital filter is realized by changing the degree of the folding noise characteristic. Filter attenuation characteristic control method for variably controlling the attenuation characteristic by variably adjusting the cutoff frequency and the resonance degree of the output characteristic of the digital filter so as to have the degree (K _env ) and the target cutoff frequency (fc). . 2. A cutoff frequency (Fc) of output characteristics not including aliasing noise characteristics and a resonance degree (Qc) of shoulder characteristics thereof.
) And a digital filter that can be variably set, and the degree of the attenuation slope of the output characteristic that is the target of realization (K _env ).
And a target cutoff frequency (fc) are input, and the degree of the attenuation gradient and the target cutoff frequency input by the input means are calculated according to an arithmetic expression that includes a folding noise characteristic in the output characteristic of the digital filter. A filter device provided with a conversion means for converting the cutoff frequency and the resonance degree of the digital filter, and a setting means for setting a filter coefficient according to the cutoff frequency and the resonance degree converted by the conversion means in the digital filter. . 3. The conversion means comprises a cutoff frequency of the output characteristic of the digital filter as Fc, a resonance degree of its shoulder characteristic as Qc, a degree of attenuation gradient of the output characteristic as a realization target, and K _env as its target cutoff. When the frequency is fc and the sampling frequency of the digital filter is fs, Fc (log) = fc (log) + [log (fs / 4) -fc (lo
g)] × K _env (however, Fc (log) and fc (log) represent a logarithmic scale), Fc (log) is logarithmically linearly converted to obtain a cutoff frequency Fc, and Qc = α + K _env, where α is a constant and is configured to perform conversion by obtaining the resonance designated value Qc. 4. The filter device according to claim 2, further comprising means for changing the degree of the damping gradient input by the input means with the passage of time.