JPH08234746A - Digital filter and electronic musical instrument using same - Google Patents

Abstract

PURPOSE: To provide the digital filter which can easily have through characteristics and the electronic musical instrument which uses the digital filter. CONSTITUTION: The digital filter 21 is provided with a characteristic control circuit which sets at least one of the output value, stored value, and input value of a delay register outputting data to the output path of the filter to zero according to external control information. Of the electronic musical instrument which has a waveform signal generating means, the digital filter 21 which filters a musical sound waveform signal, and a musical sound generation control means, the digital filter 21 includes the characteristic control circuit, and the musical sound generation control means when generating a musical sound signal that need not be passed through the filter supplies control information that makes the characteristic control circuit effective to the digital filter 21, which has flat characteristics.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital filter and an electronic musical instrument using the digital filter, and more particularly to a digital filter capable of controlling the filter in a through state and an electronic musical instrument using the filter.

[0002]

2. Description of the Related Art Conventionally, there has been an electronic musical instrument which controls a frequency characteristic by passing a generated musical tone waveform signal through a digital filter. For a tone signal that does not need to be passed through the digital filter, a digital filter bypass path and a selector that outputs either the output signal of the digital filter or the bypassed signal (input signal of the digital filter) are provided. I was bypassing the filter.

[0003]

In the conventional electronic musical instrument as described above, when the digital filter is realized by hardware, a bypass path is provided between the input terminal and the output terminal of the digital filter. There is a problem in that the configuration becomes complicated because it is necessary to provide a selector circuit, and when the digital filter is realized by a processing device such as a DSP and a program, whether the filter is passed or passed. There is a problem in that it is necessary to perform processing unsuitable for the time-divisional periodic processing, such as the processing contents and processing time greatly differ depending on the case, and different storage areas are required in each case. SUMMARY OF THE INVENTION It is an object of the present invention to improve the above-mentioned problems of the prior art and provide a digital filter which can easily realize a through characteristic and an electronic musical instrument using the digital filter.

[0004]

According to a first aspect of the present invention, in a digital filter, an output value, a stored value, or an input value of a delay register that outputs data to an output path of the filter is selected based on control information from the outside. Characteristic control means for setting at least one to zero is provided. A second invention is an electronic musical instrument including a waveform signal generating means for generating a tone waveform signal, a digital filter for filtering the tone waveform signal, and a tone generation control means, wherein the digital filter is the digital filter according to the first invention. There is a feature that the tone generation control means gives control information for enabling the characteristic control means to the digital filter when a tone signal which does not need to be filtered is generated.

[0005]

According to the first aspect of the invention, the digital filter is provided with the characteristic control means for setting at least one of the output value, the stored value and the input value of the delay register for outputting data to the output path of the filter to zero. By a simple means, the contents of the delay register do not affect the filter output, and the frequency characteristic of the filter becomes the same flat characteristic as in the case of bypass. Further, it is not necessary to provide a bypass path or a selector, and the processing has the same content and time as when the frequency characteristic is changed. In a second aspect of the invention, in the electronic musical instrument, when the digital filter according to the first aspect of the invention is used in the tone generation circuit and the tone signal that does not need to be filtered is generated, the characteristic control means is enabled. Since the control information is given to the digital filter, the structure is simplified and the processing contents are unified when the musical tone signals output from a plurality of independent musical tone generating channels are periodically time-division processed. It becomes possible and the program becomes easy.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a block diagram showing a circuit configuration of an electronic musical instrument to which the present invention is applied. CPU1 is ROM2
It is a central processing unit that controls the entire electronic musical instrument based on a control program stored in. The CPU 1 also has a built-in timer circuit that issues a timer interrupt to the CPU 1 at a set predetermined cycle. The ROM 2 stores tone color parameters as well as programs. The timbre parameters include waveform selection information, cutoff frequencies of digital filters, parameters such as resonance, and amplitude envelope base parameters. The RAM 3 is used as a work area and a buffer for storing performance information.
In addition, it has a battery backup so that it can retain the set information even when the main power is turned off.
The keyboard 4 includes, for example, a plurality of keys each having two switches, and has a key event and touch detection circuit 5
Is a circuit that scans the state of each switch and generates key events (key-on, key-off) and touch information. Panel 6 has various switches such as tone color selection switches,
Also, the panel interface circuit 7 scans the state of each switch of the panel 6 and drives the display device under the control of the CPU 1 by using a liquid crystal display device or a display device that displays characters or figures.

The tone generator circuit 8 reads musical tone waveforms from the waveform memory 9 in which digital musical tone waveform information is stored at address intervals proportional to the pitch to be generated, and multiplies the envelope signals to generate musical tone signals. In practice, one tone generation circuit is configured to be capable of independently generating a plurality of tone signals at the same time by time-division multiplexing operation of one tone generation circuit. D / A converter 1
0 performs D / A conversion of the digital tone signal, and the amplifier 11 amplifies the tone signal for driving the speaker 12. The bus 13 connects each circuit in the electronic musical instrument. In addition to this, if necessary, a memory card interface circuit,
An FDD (floppy disk drive) interface circuit, a MIDI interface circuit, etc. may be provided.

The tone generator circuit 8 is roughly divided into a waveform generating circuit, a digital filter, and an envelope applying circuit. The waveform generation circuit 20 includes an address accumulation circuit,
A read address signal is sequentially generated based on the waveform selection (tone color) information set by the CPU 1 and the read address interval information of the waveform memory corresponding to the pitch to be sounded, which is composed of a sample interpolation circuit or the like. Musical tone waveform data (samples) are sequentially read from the waveform memory 9 in accordance with the read address, subjected to interpolation processing by the waveform generation circuit 20, and output. The digital filter 21 is
The input signal is filtered according to various parameters supplied from the filter coefficient generation circuit 25. Any known method can be adopted as a method for realizing the digital filter. For example, a hardware method and D
A method using SP and software is possible. The through flag 22 is, for example, a 1-bit register for a tone generation channel, and is configured so that 0 or 1 can be set from the CPU 1 respectively. Then, for example, when the output is 1, the digital filter 21 is in the through state.

The cutoff value output circuit 23 and the resonance value output circuit 24 are each provided by the CPU 1 with a cutoff value ω which is a characteristic parameter of a filter for each sound generation channel.
0 and the resonance value Q are set, and the cutoff value ω0
And a function 1 / 2Q of the resonance value Q is output. Each circuit may be a simple register and conversion circuit,
A circuit having a function similar to that of an envelope generator that sequentially generates values that are asymptotic toward a target value may be provided so that the parameters change smoothly with time.
Further, the resonance value Q is usually a logarithm (dB
Since it is given as a parameter expressed as a value, it can be easily converted into an inverse number in the resonance value output circuit 24. The amplitude envelope generating circuit 26 generates an amplitude envelope signal for each channel based on the envelope parameter set by the CPU 1. The multiplier 27 multiplies the output signal of the digital filter 21 and the output envelope signal of the amplitude envelope generating circuit 26, and outputs a tone signal. Although not shown, tone signals of a plurality of tone generation channels are added and mixed after multiplication.

FIG. 4 shows the filter coefficient generation circuit 25 of FIG.
FIG. 3 is a block diagram showing the configuration of FIG. cos function generator 60
Is composed of, for example, a ROM that stores a conversion table for converting ω 0 to (1-cos ω 0), and an interpolation circuit. The sin function generator 61, for example, converts ω 0 to sin ω 0
It is composed of a ROM that stores a conversion table for converting to, and an interpolation circuit. The multipliers 62 and 63 are co
The output of the s-function generator 60 and the output of the sin-function generator 61 are respectively multiplied by 1 / 2Q output from the resonance value output circuit 24. This is a correction to prevent the output value of the filter from exceeding 1.

FIG. 2 is a block diagram showing a general configuration example of a transposed second-order IIR (recursive) filter. Multiplier 3
0 to 33 multiply signals by respective coefficients. 34-3
6 is an adder, and 37 and 38 are delay registers. Such a structure is obtained as follows. First, obtain the transfer function H (s) of the analog second-order low-pass filter,
The transfer function (Equation 1) of the digital filter is obtained by subjecting this to bilinear Z conversion. Note that ω 0 is the cutoff (resonance) frequency expressed by normalizing the sampling frequency,
Q is a parameter called quality factor or resonance, and 0 <ω0 <π and Q ≧ 1.

[0012]

[Equation 1] Note that α is set so that the maximum value of the amplitude-frequency characteristic is 1. From equation 1, the secondary I
A standard form of the IR digital filter is obtained, and by converting this into a transposed filter, the digital filter of FIG. 2 is obtained. Note that the transposed filter is a filter in which all signal directions are inverted in the block diagram of a digital filter, and a signal branch point is replaced by an adder and an adder is replaced by a branch point, which has the same characteristics as the original filter. Become.

Although a digital filter may be realized with the configuration of FIG. 2, in order to realize the filter of FIG. 2, complicated calculation is required to calculate the filter coefficients (α, β, γ).
Therefore, if b << 1, 1 / (1 + b) can be approximated by (1-b), and the expression 1 is transformed into the approximate expression 2.

[0014]

[Equation 2] The digital filter that realizes Expression 2 can be realized by multiplying the output of the digital filter corresponding to the fraction F on the right side of Expression 2 by the coefficient (1-b).
The coefficient of the feedback loop for feeding back the output of the filter of the portion F (first and second-order coefficients of Z in the denominator of F) also includes (1-b). Therefore, first, a signal is generated by multiplying the output of the filter of the portion F by the coefficient (1-b), and this signal is used as the output, and the feedback signal is generated by multiplying the signal by the remaining coefficient of the feedback loop. When set to, the calculation of the coefficient becomes simple.

FIG. 3 is a functional block diagram of a filter obtained by modifying the digital filter shown in FIG. 2 as described above and further applying the present invention. The filter coefficient generation circuit 25 of FIG.
The input signal multiplied by the coefficient a generated from is input to the adders 41, 49 and the 1-bit shift circuit 52 for doubling the signal. The output of the adder 41 is the multiplier 4
2 and the adder 43, and the signal multiplied by the coefficient b in the multiplier 42 is subtracted by the adder 43. With this configuration, the output signal of the adder 41 has the coefficient (1-b)
Can be applied. The signal becomes the output of the filter and
It is an input of two coefficient circuits. A coefficient 2 (1-A) is applied to the output signal by the first coefficient circuit, that is, the multiplier 44, the adder 45, and the 1-bit shift circuit 46, and the second coefficient circuit, that is, the multiplier 47 and the adder 48. Therefore, the coefficient (b-
1) is applied. The output of the second coefficient circuit is the adder 49.
Is added to the output signal of the multiplier 40 and input to the delay register 50. The output of the first coefficient circuit is added by the adder 51 to the outputs of the 1-bit shift circuit 52 and the delay register 50, and the result is input to the second delay register 52.

The second delay register 53 is a delay register for outputting data to the adder 41 in the output path of the filter, and the output signal is used for calculating the output signal of the filter. The output signal of the delay register 53 is an AND gate 54 that gates a signal of a plurality of bits.
The through flag signal is input to the adder 41 via one input terminal, and the through flag signal is input to the other input terminal of the AND gate 54 via the inverter 55. Therefore, when the through flag signal is 0, the output of the delay register 53 is directly input to the adder 41, but when it is 1, 0 is input to the adder 41. Therefore, the characteristic of the digital filter when the through flag is 1 is H
(Z) = a (1-b). Therefore, a = 1 and b = 0
If the resonance frequency ω 0 is set to approximately π and the resonance Q is set to 1 so that H (z) becomes approximately 1, the characteristic of the digital filter can be almost in the through state. Further, when the through flag is 1, the feedback loop of the digital filter is disconnected, so that the data in the delay registers 50 and 53 are rapidly attenuated and cleared. With the above configuration, it is possible to obtain a digital filter with a simple configuration that can easily realize the through state.

FIG. 5 is a block diagram showing the main part of the configuration of another embodiment of the digital filter of the present invention. Figure 5
The second embodiment shown in (a) is obtained by replacing the AND gate 54 of the first embodiment shown in FIG. 3 with a selector 70. The selector 70 outputs the signal from the delay register 53 when the through flag signal input to the control terminal is 0, and outputs 0 when the through flag signal is 1. FIG. 5B is a third embodiment in which AND gates 71 and 72 are provided on the input side of the delay registers 53 and 50. In such a configuration, when the through flag is 1, the contents of the two delay registers 50 and 53 become 0 after one operation cycle, and the filter characteristic is in the through state as in the first embodiment. At this time, the contents of the delay register are surely cleared regardless of the value of the input signal. Note that if only the filter characteristic is set to the through state, the AND gate 71 alone can be used, and the AND gate 72 may be omitted.

Although the embodiment has been described above, the following modifications are also possible. Although the example in which the output or the input of the delay register is set to 0 is disclosed in the embodiment, the delay register circuit itself may be provided with a reset function so that the content stored in the register is reset by the through flag signal. Although the transposed second-order IIR filter is disclosed, the delay register that outputs data to the output path of the filter, that is, the output of the delay register whose output value is directly related to the calculation of the output value of the filter becomes 0. If controlled, the present invention can be applied to any type of digital filter. Although the embodiment has disclosed the tone generation circuit of the waveform readout system, the tone waveform generation system is arbitrary and can be implemented in the sine synthesis system, for example.

[0019]

As described above, in the first invention, in the digital filter, at least one of the output value, the stored value and the input value of the delay register for outputting data to the output path of the filter is set to zero. Since the characteristic control means is provided, the content of the delay register does not affect the filter output by a simple means, and the frequency characteristic of the filter becomes the same flat characteristic as in the case of bypass. Further, it is not necessary to provide a bypass path or a selector, and there is an effect that the processing has the same content and time as when the frequency characteristic is changed. Furthermore, the value stored in the delay register can be cleared by disconnecting the feedback loop. In a second aspect of the invention, in the electronic musical instrument, when the digital filter according to the first aspect of the invention is used in the tone generation circuit and the tone signal that does not need to be filtered is generated, the characteristic control means is enabled. Since the control information is given to the digital filter, the structure is simplified and the processing contents are unified when the musical tone signals output from a plurality of independent musical tone generating channels are periodically time-division processed. This makes it possible to simplify the program. In addition, when the characteristics of the filter are suddenly changed such as at the start of sound generation, the contents of the delay register can be cleared by temporarily setting the through state.

[Brief description of drawings]

FIG. 1 is a block diagram showing a circuit configuration of an electronic musical instrument to which the present invention is applied.

FIG. 2 is a block diagram showing a general configuration example of a transposed second-order IIR filter.

FIG. 3 is a functional block diagram of a filter to which the present invention is applied by modifying the digital filter of FIG.

FIG. 4 is a block diagram showing a configuration of a filter coefficient generation circuit 25.

FIG. 5 is a block diagram showing the main part of the configuration of another embodiment of the digital filter of the present invention.

[Explanation of symbols]

1 ... CPU, 2 ... ROM, 3 ... RAM, 4 ... Keyboard, 5 ... Key event, touch detection circuit, 6 ... Panel,
7 ... Panel interface circuit, 8 ... Sound source circuit, 9 ...
Waveform memory, 10 ... D / A converter, 11 ... Amplifier, 12
… Speaker, 13… Bus

Claims (3)

[Claims] 1. An output value of a delay register for outputting data to an output path of a filter based on control information from the outside,
A digital filter comprising a characteristic control means for setting at least one of a stored value and an input value to zero. 2. The transposed secondary II
The digital filter according to claim 1, wherein the digital filter is an R filter. 3. An electronic musical instrument comprising a waveform signal generating means for generating a musical tone waveform signal, a digital filter for filtering a musical tone waveform signal, and a musical tone generation control means, wherein the digital filter is according to any one of claims 1 and 2. A digital filter, wherein the tone generation control means gives control information for enabling the characteristic control means to the digital filter when generating a tone signal that does not need to pass through the filter. Electronic musical instrument using.