JPH0728197B2 - Octave multiple filter - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an octave multiple filter in which each filter stage is configured by regularly combining a digital filter having a high-pass or band-pass characteristic, an adder or a subtracter.

[Outline of Invention]

An octave in which filter stages consisting of digital filters (FIR type filters) having high-pass characteristics or band-pass characteristics are arranged in parallel between the input and output according to a predetermined rule and the outputs of the filters of each stage are added. It is a multi-filter and has a frequency characteristic of logarithmically uniform accuracy over a wide range in the frequency domain.

[Conventional technology]

Since the conventional digital filter processes discrete sample values at equal time intervals and operates at a predetermined clock frequency, the characteristics are specified in the frequency domain at equal frequency intervals.

For this reason, a conventional filter for designating characteristics at logarithmic intervals over a wide band is realized only by combining a plurality of analog filters.

[Problems to be solved by the invention]

Since the former digital filter adopts the characteristic designating method at equal frequency intervals as described above, the low frequency resolution is too coarse, while the high frequency resolution is too fine, and it is very difficult to widen the band.

Generally, the characteristics of audio equipment are evaluated or specified at logarithmic intervals in the frequency domain, so when using a conventional digital filter, such as a FIR type (finite impulse response type) filter, the characteristics are accurate in the low range for the reasons described above. However, it was not possible to specify, and on the other hand, in the high range, it became a characteristic specification with higher precision than necessary, which was extremely inefficient in realizing the circuit. Especially when trying to realize a complicated frequency characteristic like a graph equalizer with a conventional FIR filter, such a constraint cannot be avoided, and a graph equalizer using a FIR filter has not yet been put into practical use. Not not.

On the other hand, the latter filter adopts the addition or multiplication method of the analog active filter, so that interference between the filters occurs and it is difficult to specify the phase and the amplitude independently. Especially, it is very difficult to arbitrarily specify the phase characteristic.

SUMMARY OF THE INVENTION An object of the present invention is to provide an octave multiplex filter capable of designating characteristics at logarithmically uniform intervals over a wide band in the frequency domain in order to solve the problems of the prior art.

[Means for solving problems]

In order to achieve the above object, the present invention uses fo as a sampling rate. Each filter stage is composed of a digital filter having a high-pass or band-pass characteristic for processing an input signal of interval sample values, each filter stage is provided in parallel between input and output, and each digital filter has a unit delay time. The input signal is directly added to the first stage filter, but the signal after subtracting the output signal of that stage from the input signal of the previous stage is added to the input of the filters after the next stage. As described above, the respective filter stages are connected to each other, and the output signals of the respective filter stages are controlled so that the phase and the amplitude are controlled, respectively, and then added to form an overall output signal.

[Action]

As the digital filter having the above high-pass or band-pass characteristics, for example, a FIR type filter is used, and the input signal of each stage is the one obtained by subtracting the output signal of that stage from the input signal of the previous stage except the first stage. Output signals are added to form an overall output signal.

〔Example〕

The present invention will be described below with reference to the embodiments shown in the drawings.
FIG. 1 shows an embodiment of an octave multiple filter (4 octaves) according to the present invention. In the figure, IN is an input terminal, OUT is an output terminal, A / D is an AD converter, D / A is DA.
Converter, LPF is a normal analog low-pass filter, F ₀
~ F ₃ are digital filters each having a band pass characteristic formed by using, for example, an FIR type filter, D _{0 to} D ₂ are adders,
S _{0 to} S ₂ are sign inverters, B _{0 to} B ₃ are multipliers, ADD is an output adder, τ _{0 to} τ ₂ and ▲ τ ^′ ₀ ▼ to ▲ τ ^′ ₃ ▼ are delay elements.

As described above, the FIR type filter has a shift register SR as a delay element, a coefficient multiplier ML, and an adder AD as shown in FIG.
, And the coefficients a _{-5 to} a ₅ of the coefficient multiplier ML are determined so that the desired frequency characteristics can be obtained by the DFT (Discrete Fourier Transform) method.

Bandpass characteristics (or highpass characteristics) of such a configuration
Each of the filter stages is composed of a digital filter having the following, and is provided in parallel between the input and the output.

In each of the filter stages, the digital filters F _{0 to} F ₃ having the band pass characteristic have basically the same configuration and coefficient as described above, but differ only in the delay time Ti per shift register stage. That is, the input signal is If it is a sample value of the interval, the delay time is Is. Characteristics of the filter Fi is f ₀ 2 in the frequency domain of the respective accordingly ^- becomes a periodic function having a period of IHZ, part of the cut-off characteristics has roll-off characteristics of the Nyquist slope.

Further, each filter stage has adders D _{0 to} D ₃ and sign inverters S _{0 to} S ₃
Are connected as shown in the drawing. That is, each filter stage has a band-pass characteristic (or high-pass characteristic) digital filter stage having a shift register that operates with a shorter delay time. If the digital filter stages are defined as the upper stage (in the order of F _{0 to} F ₃ ), Although the input signal is directly applied to the filter F ₀ , the adder D _{0 to} D ₃ and the sign inverters S _{0 to} S ₃ add the input signal of the previous stage to the output signal of that stage to the input of the filter after the next stage. Is connected so that a signal obtained by subtracting is added.

Now, in the octave multiplex filter having the above-mentioned configuration, the input signal given to the input terminal IN is A / D converter A /
Sampling frequency by D Is AD converted and added to the digital filter F ₀ having the band pass characteristic.

FIGS. 3 (a) to 3 (d) show the frequency characteristics of the filters F _{0 to} F ₃ , and FIGS. 4 (a) to 4 (h) show the frequency spectrum of the input / output signals for each filter.

Input signals of the first-stage filter F ₀ has a frequency spectrum shown in Figure No. 4 (a), since the frequency characteristic of F ₀ is in the form shown in FIG. 3 (a), the output signal of the F ₀ Represents the establishment of the uppermost octave, as shown in FIG. 4 (b). When the output signal is subtracted from the input signal of F ₀ , it becomes a signal of the frequency spectrum shown in FIG. 4 (c), and this signal is input to the filter F ₁ of the next stage. The delay element τo in front of the adder D ₀ is for aligning the time bases between the input and output signals.

Similarly, as shown in FIGS. 4 (c) to 4 (h), the components of each octave are taken out by the filter of each stage, the output signal is subtracted from the input signal, and the result is input to the filter of the next stage. .

The output signals of the respective filter stages are multiplied by the coefficient bi by the multipliers b _{0 to} b ₃ , the phase characteristics thereof are designated by the delay element τ′i, and then added by the adder ADD to become the entire output signal.

Note that the filters after the second stage, such as F _1, have a delay time of one stage of the shift register. Is. Normally, the digital filter having this configuration is used with a clock frequency of f ₀ 2 ^-1 Hz and a sampling frequency of the input signal as f ₀ 2 ^-1 Hz. for that reason,
Since the clock frequency and the sampling frequency are different in the above digital filter, the intended processing cannot be performed as it is. Therefore, first set the unit delay time as described above. The FIR filter coefficient is defined as The clock frequency is set to f ₀ Hz for the input signal of and the delay time per stage of the shift register is kept as it is. To be set. In the same way, set the other digital filters.

〔The invention's effect〕

As is clear from the above description, according to the present invention, the above-mentioned configuration makes it possible to specify the characteristics at logarithmically uniform intervals in the frequency domain (on the frequency axis) and to broaden the band in the low frequency direction. Therefore, the resolution in the low range is improved, the amplitude and the phase can be designated independently, and the interference between the filters can be arbitrarily reduced.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing an example of the structure of an FIR type filter used in the embodiment, and FIGS. 3 (a) to 3 (d) are respectively the above-mentioned embodiments. FIGS. 4A to 4H are frequency spectrum diagrams of input / output signals to / from each filter in the example. F _{0 to} F ₃ ...... Digital filter having bandpass characteristics, D _{0 to} D ₂ ...... Adder, S _{0 to} S ₂ ...... Sign invertor, B _{0 to} B ₃
…… Multiplier, ADD …… Output adder, SR …… Shift register.

Claims (1)

[Claims] 1. Fo as a sampling rate Each filter stage is composed of a digital filter having a high-pass or band-pass characteristic for processing an input signal of interval sample values, each filter stage is provided in parallel between input and output, and each digital filter has a unit delay time. The input signal is directly added to the first stage filter, but the signal after subtracting the output signal of that stage from the input signal of the previous stage is added to the input of the filters after the next stage. The octave multiplex filter is characterized in that the respective filter stages are connected so that the output signals of the respective filter stages are controlled in terms of phase and amplitude and then added to form an overall output signal.