JPH0770951B2 - Octave multiple filter - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an octave multiple filter in which a pair of digital filters having a low pass characteristic and a band pass characteristic are regularly combined to form each filter stage.

[Outline of Invention]

It is an octave multiplex filter in which a filter stage consisting of a pair of digital filters (FIR type filters) each having a bandpass characteristic and a lowpass characteristic is arranged in parallel between the input and output according to a predetermined rule, over a wide band in the frequency domain, The characteristics can be specified at almost logarithmically uniform intervals.

[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 over a wide band in a frequency domain at logarithmically uniform intervals in order to solve the problems of the prior art.

[Means for solving problems]

According to the present invention, in order to achieve the above object, each filter stage is composed of a pair of digital filters having a bandpass characteristic and a lowpass characteristic which operate with a sampling time interval of an input signal as a clock at the time interval. The inputs of the filter stages are connected in parallel. In each of the filter stages, the delay time per stage of the filter having the bandpass characteristic is an integer with respect to the reference time interval, with the sampling time interval as the reference time interval. The digital filter having a low-pass characteristic has a delay time per stage, which is composed of a delay element that operates at one time interval of the double time-interval group, and the delay time per delay element of the band-pass digital filter. Each of the filter stages operates with a longer delay time. If the stage of a digital filter having a bandpass characteristic and / or a lowpass characteristic having a delay element to be defined is defined as an upper stage, the output of the digital filter having a bandpass characteristic of each filter stage is one stage higher than that stage. In the octave multiplex filter which is connected so that it is added to the output of the digital filter and added by the adder to the digital filter of the low-pass characteristic of its own stage, the low-pass cutoff of the digital filter of the band-pass characteristic of the predetermined filter stage The gist is that the roll-off characteristic has the same Nyquist slope as the high-frequency cutoff characteristic of the digital filter having the bandpass characteristic one step above.

[Action]

As the digital filter having the low-pass and band-pass characteristics, for example, an FIR type filter is used, and the digital filter having the low-pass characteristic operates so as to eliminate the odd-numbered pass band of the digital filter having the band-pass characteristic, and both are paired. Each of the filter stages is configured to be used multiplex by changing the clock frequency unit delay time of the delay element constituting the digital filter of each stage.

〔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 (5 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, S ₀
S ₄ is a digital filter having a band pass characteristic using, for example, a FIR type filter, F _{0 to} F ₄ are digital filters having a low pass characteristic using the same type of filter, and D _{0 to} D ₄ are adding circuits. .

The FIR type filter is composed of a delay element SR, a coefficient multiplier ML and an adder AD as shown in FIG. 2 and is shown in FIG.
The coefficient a _{-5 to} a ₆ of the coefficient multiplier ML so that the target frequency characteristic can be obtained by the DFT (Discrete Fourier Transform) method.
Has been set.

FIG. 3 (a) shows the target frequency characteristic (low-pass characteristic).
, And FIG. 6B shows a function in the time domain obtained by performing an inverse Fourier transform on the function.

Each pair of digital filters (S ₀ , F ₀ ) to (S ₄ ,
Each filter stage is composed of F ₄ ) and is arranged in parallel between the input and output.

In each of the filter stages, each of the digital filters S _{0 to} S ₄ having the band pass characteristic has a delay time nT per stage which is the sampling time interval of the input signal in the A / D converter A / D as a reference time interval T. as has delay elements S ₀ -1~S ₄ -5 operating in one of the time intervals of the time interval group T, 2T, 3T ... integral multiple with respect to the reference time interval. Further, the digital filters F _{0 to} F _{4 having the} above low-pass characteristics
Is a delay element F ₀ -1 that operates with a delay time which is an integer fraction of the delay time per stage of the delay elements of the digital filters S _{0 to} S _{4 having} the above bandpass characteristics corresponding to the delay time per stage.
It has F ₄ -5.

Further, each filter stage is coupled as shown via the adder circuits D ₀ -D ₅ . That is, each filter stage has a bandpass characteristic or low-pass characteristic digital filter stage having a delay element that operates with a longer delay time, and is defined as an upper stage (in the order of S _{4 to} S ₀ , F _{4 to} F ₀ ). The output of the digital filter of the band pass characteristic of each filter stage is added to the output of the digital filter of the low pass characteristic of one stage higher than that stage, and is connected so as to be input to the digital filter of the low pass characteristic of the own stage. There is.

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 f ₀ (sampling rate 1 / f
It is AD converted in ₀ sec) and added to the digital filters S _{0 to} S ₄ having band pass characteristics. This filter, for example, S _4, has a delay time of 1 / f ₀ 2 ^-4 s per stage of delay elements that compose it.
ec. Usually digital filter of this configuration is used to clock frequency as f ₀ 2 ^-4 Hz, the sampling frequency of the input signal is also used as f ₀ · 2 ^-4 Hz. However, since the digital filter has a different clock frequency from the sampling frequency, it cannot be processed as it is. Therefore, in the present invention, the unit delay time is first set to 1 / f ₀ 2 ^-4 sec to determine the coefficient of the FIR type filter, and the clock frequency is set to f ₀ for the input signal at the next actual sampling interval 1 / f ₀ sec. The frequency is set to Hz and the delay time per stage of the delay element S ₄ −1 is set to nT = 1 / f ₀ 2 ^-2 sec with the coefficient kept unchanged. In the same way, set the other digital filters.

The output characteristic of the digital filter S ₄ is as shown in FIG. 4 (a), and the delay time per delay element is 1 / f ₀ 2 ^-3 in order to erase the next odd pass band. Input to digital filter F ₄ with low pass characteristic of sec. The output characteristic of F ₄ is as shown in FIG. 4 (b). The output of this F ₄ is the addition of the output of the digital filter S _{3 with} the bandpass characteristic that the delay time per stage of delay element is 1 / f ₀ 2 ^-3 sec.
Added by D ₄ . FIG. 4 (c) shows this added characteristic.

Further, in order to erase the odd-numbered pass band of S ₃ , it is input to a digital filter F ₃ having a low-pass characteristic with a delay time per stage of 1 / f ₀ 2 ^-2 sec. The output characteristic of F ₃ is as shown in FIG. 4 (d).

The same process is repeated thereafter, and the final digital filter is repeated.
From F ₀ , the output of the desired characteristic as shown in FIG. 5 is obtained.
At that time, the roll-off with the same Nyquist slope as the digital filter of the band pass characteristic of each filter stage, for example, the cut-off characteristic of the low range of S ₁ and the cut-off characteristic of the digital filter S ₀ of the band pass characteristic of one stage higher than that. It has characteristics.

By doing this, when the phase and amplitude of each stage are made the same, flat characteristics can be obtained as a whole, and when the frequency characteristics are changed by changing the phase and amplitude, the roll-off characteristics are changed to You can adjust the amount of the effect of the octave filter of the stage. That is, if the roll-off is made gentle and the influence is increased, the joint of the octave filter becomes smooth, and on the contrary, if the roll-off is made steep to reduce the influence, the joint of the octave filter becomes steep and the discontinuity approaches.

〔The invention's effect〕

As is apparent from the above description, according to the present invention, the above-described configuration allows the characteristics to be specified at logarithmically uniform intervals in the frequency domain (on the frequency axis), and widens the bandwidth in the low frequency direction. Since the resolution can be improved, the amplitude and phase can be specified independently. Further, by changing the roll-off characteristic and adjusting the amount of overlap between the filters, the smoothness of the characteristic can be made variable.

[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, FIG. 3 is a characteristic diagram of the filter, and FIG. FIG. 5 is an output characteristic diagram of each digital filter in the above embodiment, and FIG. 5 is an output characteristic diagram of the target characteristic. S _{0 to} S ₄ ...... Digital filter having band pass characteristic, F _{0 to} F ₄ ...... Digital filter having low pass characteristic, D _{0 to} D ₅ ...... Adding circuit, A / D ...... A-D converter, D / A ...
... DA converter.

Claims (1)

[Claims] 1. A sampling time interval of an input signal,
Each filter stage is composed of a pair of digital filters having a band-pass characteristic and a low-pass characteristic that operate with the time interval as a clock, and the input of each filter stage is connected in parallel. The delay time per stage of the digital filter having the path characteristic is the sampling time interval as the reference time interval,
The digital filter having a low-pass characteristic is constituted by a delay element that operates at one time interval of a time interval group that is an integral multiple of the reference time interval, and the delay time per stage is a digital filter having the band-pass characteristic. A delay element operating with a delay time which is an integer fraction of the delay time per stage, each of the filter stages having a delay element operating with a longer delay time has a bandpass characteristic and / or a lowpass characteristic. The stages of the digital filters are arranged so as to be the upper stages, and the output of the digital filter having the bandpass characteristic of each filter stage is added by the adder to the output of the digital filter having the lowpass characteristic one stage higher than that stage. Then, in the octave multiplex filter connected so as to be input to the low-pass characteristic digital filter of its own stage, The low-frequency cutoff characteristic of the digital filter with the constant bandpass characteristic and the high-frequency cutoff characteristic of the digital filter with the upper bandpass characteristic have the same roll-off characteristic with the same Nyquist slope. An octave multiple filter characterized by constructing a multi-stage digital filter.