JPH0728201B2 - Octave multiple filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to an improvement of an octave multiplex filter formed by combining a plurality of digital filter groups having band-pass characteristics and low-pass characteristics.

SUMMARY OF THE INVENTION Each filter output of a first digital filter group having a low-pass characteristic connected in series is given to each filter of a digital filter group having a band-pass characteristic connected in parallel. Is added to each of the second digital filters having a low-pass characteristic connected in series via the amplitude / phase control circuit and taken out from the final stage thereof. The amplitude coefficient of each amplitude / phase control circuit , And an octave multiplex filter configured so that the phase coefficient is controlled independently and adaptively, over a wide band in the frequency domain, the characteristics can be specified at logarithmically uniform intervals, and the sampling time interval is sequentially lengthened, It is also possible to spread the input data and reduce the number of arithmetic circuits.

[Prior Art] Since a conventional digital filter processes discrete sample values at equal time intervals, it operates at a predetermined clock frequency. Therefore, characteristics in the frequency domain are specified at equal frequency intervals. It was being appreciated.

For this reason, a conventional filter that specifies characteristics over a wide band at logarithmic intervals has been 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. However, widening the band is very difficult.

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, it is possible to obtain accurate characteristics in the low range for the reasons described above. However, in the high frequency range, the characteristics were specified with higher precision than necessary, which was extremely inefficient in realizing the circuit. In particular, when trying to realize a complicated frequency characteristic like a graphic equalizer with a conventional FIR filter, such a constraint cannot be escaped, and a graphic equalizer using a FIR filter has not yet been put to practical use. .

On the other hand, since the latter filter adopts the addition or multiplication method of the analog active filter, there is interference between the filters and it is difficult to specify the phase and the amplitude independently. In particular, it was 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 the Problems] In order to achieve the above object, the present invention has a plurality of low-pass characteristics in which a sampling time interval of an input signal is clocked at a time interval equal to or longer than the sampling time interval. And a second digital filter group and a digital filter group having a band pass characteristic are provided, the first and second digital filter groups having the low pass characteristic are connected in series, and the digital filter group having the band pass characteristic are respectively connected in parallel. Connected, the respective outputs of the first digital filter group having the low-pass characteristic are given to the respective digital filter groups having the band-pass characteristic,
The respective outputs are added to the second digital filter group having the low-pass characteristic via the amplitude / phase control circuit and output from the final stage, and are attached to the respective filters of each filter group in the order of connection. Of the maximum number of digital filters, the delay time per unit delay element of the digital filter is set to be equal to or more than the sampling time interval of the input signal, and the filter number of each filter group decreases by one each time. The delay time per unit delay element is A ^B (A> 1, B> 0) times, and the delay time per unit delay element of a digital filter with a certain number of low-pass characteristics is equal to that of the band-pass characteristic with the same number. It is characterized in that it is configured to operate at an integral multiple of the delay time per unit delay element of the digital filter.

[Operation] In the octave multiplex filter having the above-described configuration according to the present invention, the output signals from the first digital filter group having the low-pass characteristic pass through the respective filters in series, so that the band is gradually narrowed and The low-pass output signal of is multiplied by the band-pass characteristic and given to the amplitude / phase control circuit to adaptively control its amplitude and phase, and is added to each of the second digital filter group of the low-pass characteristic to cut noise. It

[Examples] The present invention will be described below with reference to the examples 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 for analog signals such as audio signals, OUT is an output terminal, and A is an output terminal.
/ D A-D converter, D / A is a D-A converter, LPF is a normal analog type low-pass filter, LPF1-a to LPF3-a
And LPF1-b to LPF3-b are first and second digital filter groups having a low pass characteristic formed by using, for example, FIR type filters, and BPF1 to BPF4 are digital filters having a band pass characteristic formed by using similar filters. Flock, ADD1
~ ADD3 is an adder circuit, COEFF1 to COEFF4 are amplitude and phase control circuits, and CONT is an adaptive control circuit.

The adaptive control circuit CONT is constructed, for example, as shown in FIG. In the figure, DET is the detector of the band component, M
PU is a microprocessor.

Thus, the FIR type filter has a delay element S as shown in FIG.
RFT, coefficient multiplier ML, adder AD, DFT shown in FIG.
The coefficients a _{-5 to} a ₆ of the coefficient multiplier ML are set so that the desired frequency characteristic can be obtained by the (discrete Fourier transform) method.

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. First and second digital filter groups LPF1-a to LPF3-a, LPF1-- having low-pass characteristics
b to LPF3-b are respectively connected in series, digital filter groups BPF1 to BPF4 having bandpass characteristics are respectively connected in parallel, and each of the first digital filters LPF1-a to LPF3-a having low-pass characteristics is connected. The output is given to each of the band pass characteristic digital filter groups BPF1 to BPF4, and the amplitude / phase control circuits COEFF1 to
Low pass digital filter LPF1 via COEFF4
Each of -b to LPF3-b outputs is added and taken out.

Then, in each of the digital filter groups, the delay time nT per unit delay element of the digital filter LPF3-a, LPF3-b, BPF4 of the highest number among the numbers given to the respective filters in the order of connection is the sampling of the input signal. The delay time per unit delay element of the filter is set to A ^B (A> 1, B> 0) times each time the filter number is set to be equal to or more than the time interval T and the filter number decreases by one, and a certain number The delay time per unit delay element of the low-pass characteristic digital filter operates at an integral multiple of the delay time per unit delay element of the band-pass characteristic digital filter having the same number.

Further, the adaptive control circuit CONT is, for example, as shown in FIG. 2, each output BP1 to B of the bandpass characteristic digital filter.
By detecting P4 with the band component detector DET, the microprocessor MPU can be used according to the component within each band.
Calculates the amplitude coefficient and the phase coefficient of each of the amplitude / phase control circuits COEFF1 to COEFF4, and sends a predetermined control signal to each of the coefficient units to control the coefficient.

The amplitude / phase control circuit comprises, for example, as shown in FIG. 5, a RAM, a multiplier MUL, and a delay element SR, each coefficient being set in the RAM, in response to the control signal. The phase is changed by the offset of the timing of extracting the output from the delay element SR, and the amplitude is changed by multiplying each output of BPF1 to BPF4 by the multiplier MUL with the coefficient extracted from the RAM. SR is
It is also possible to share it with the delay element SR of the bandpass characteristic digital filter. In this embodiment, as described above, each coefficient is changed according to the output of BPF1 to BPF4 to change the output of each coefficient unit. However, in the configuration of FIG. 1, no feedback is applied and the system is stable. is there.

Now, in the octave multiplex filter having the above-mentioned configuration, the input analog signal given to the input terminal IN is AD
A / D conversion is performed at the sampling frequency f ₀ (sampling rate 1 / f ₀ sec) by the converter A / D, and the digital filter LPF3 with the highest number of low-pass characteristics and band-pass characteristics −
a, added to BPF4. This filter, for example, LPF1-a, has a delay time of 1 / (f ₀ ·.
2 ^-2 ) sec. Normally, the digital filter of this configuration is used with a clock frequency of f ₀ · 2 ^-2 Hz, and the sampling frequency of the input signal is also used with f ₀ · 2 ^-2 Hz.
However, since the clock frequency and the sampling frequency of the digital filter are different, the intended processing cannot be performed as it is. Therefore firstly determine the coefficients of the FIR filter unit delay time as described above as a ^{_{1 / (f 0 · 2 -2}} ) sec, then to the actual input signal sampling interval 1 / f ₀ sec, the clock frequency Is set to f ₀ Hz, and the delay time per unit delay element nT = 1 / (f ₀
・ Set it to be 2 ^-2 ) sec. Similarly, the digital filters of the other stages are also set.

Each digital filter BPF4-BPF1 and LPF3-a-LPF1-
The characteristics of b are as shown in FIGS. 4 (a) to (l),
For each output signal of the digital filters LPF3-a to LPF1-a of each low-pass characteristic whose band is narrowed sequentially,
The bandpass characteristics of BPF4 to BPF1 are multiplied, and the amplitude and phase of each are independently controlled by the amplitude / phase control circuits COEFF1 to COEFF4, and input to LPF1-b to LPF3-b to cut noise. As a result, an output having the desired characteristics as shown in FIG. 4 (m) can be obtained.

[Effects of the Invention] As is apparent from the above description, according to the present invention, the frequency domain (on the frequency axis) is set by the above-described configuration.
The characteristics can be specified logarithmically with a substantially uniform interval, and the resolution can be improved because the band can be widened in the low frequency direction. Moreover, the amplitude and phase can be adaptively controlled, and the sampling time interval can be gradually increased to obtain data. It is possible to reduce the number of calculations by skipping.

Thus, by combining these effects, various noise reductions can be achieved by preparing various coefficient calculation programs.
It can be applied to systems such as enhancers, and can have special effects and functions such as automatic sound field correction and surround sound field.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a block diagram showing an example of the configuration of an adaptive control circuit, and FIG. 3 is an example of the configuration of an FIR type filter used in the above-mentioned example. 4 and 5 are characteristic diagrams of the filter, FIG. 5 is a diagram showing an example of the configuration of the amplitude / phase control circuit, and FIG. 6 is an output characteristic diagram of each digital filter in the above embodiment. LPF1-a to LPF3-b ... Digital filter having low-pass characteristics, BPF1 to BPF4 ... Digital filter having band-pass characteristics, COEFF1 to COEFF4 ... Amplitude / phase control circuit, ADD1 to ADD3 ... Addition circuit, CONT. Adaptive control circuit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-63-260314 (JP, A) JP-A-63-252010 (JP, A) JP-A-63-51713 (JP, A) JP-A-63- 250209 (JP, A)

Claims (1)

[Claims] 1. A sampling time interval of an input signal,
First and second low-pass characteristics having a plurality of time intervals longer than the sampling time interval and operating with a clock
A digital filter group having a band-pass characteristic and a first digital filter group having a low-pass characteristic are connected in series, and a digital filter group having a band-pass characteristic are connected in parallel. The respective outputs of the first digital filter group having the low-pass characteristic are given to the respective digital filter groups having the band-pass characteristic, and the respective outputs are respectively passed through the amplitude / phase control circuit to the second digital filter group having the low-pass characteristic. Is added to the output from the final stage, and the delay time per unit delay element of the digital filter with the highest number among the numbers assigned to the filters in each filter group in the order of connection is the input signal. Is set to the sampling time interval of The delay time of the delay unit per stage of the filter every time the number of filters is reduced one Te is A
^B (A> 1, B> 0) times, and the delay time per unit delay element of a digital filter with a certain number of low-pass characteristics Delay per unit delay element of a digital filter with the same number of band-pass characteristics It is adapted to operate at an integral multiple of time, and is provided with a control means for controlling the amplitude coefficient and the phase coefficient of each of the amplitude / phase control circuits according to the output signal of the digital filter of each bandpass characteristic. Octave multi-filter.