Filter
Foreign code  F110003619 

File No.  A22114WO 
Posted date  Jun 30, 2011 
Country  EPO 
Application number  09762379 
Gazette No.  2315353 
Gazette No.  2315353 
Date of filing  Jun 1, 2009 
Gazette Date  Apr 27, 2011 
Gazette Date  Jul 29, 2020 
International application number  JP2009059953 
International publication number  WO2009150949 
Date of international filing  Jun 1, 2009 
Date of international publication  Dec 17, 2009 
Priority data 

Title  Filter 
Abstract  Provided is an FIR filter capable of obtaining predetermined characteristics with a small number of input taps, delay circuits, and multipliers and achieving an improved response and low cost. In a lowpass filter, a bandpass filter, and a highpass filter based on an FIR filter, a basic filter is configured that gives a basic impulse response function and has a filter coefficient determined from the impulse response function. Filters having different frequency characteristics are configured by changing the time scale or frequency scale of the basic filter. These filters having different frequency characteristics are combined in a cascade form or a step form, thereby constructing an FIR filter having a small number of taps. 
Outline of related art and contending technology 
Background Art With recent digitalization of multimedia information, the importance of arithmetic processing algorithms and arithmetic processing circuits that implement digital signal processing has been increasing. Especially, digital filters are used to eliminate noise, adjust frequency characteristics, separate signals, and so on. FIR filters having finite impulse response ensure stability always against bounded input. If the impulse response is symmetrical, the FIR filters can implement perfect linear phases. The linear phases are important characteristics in waveform transmission, measurement, sound reproduction, and so on. However, because of the tradeoff between precision required in the frequency characteristics of the filter and the scale of the filter, highorder transfer characteristics are required to obtain steeper attenuation characteristics, and the circuit requires a large number of multipliers and delay elements. To create a passband filter with a very narrow band, for example, a very high degree of transfer characteristics is required, and a large number of multipliers are required accordingly. In that context, many propositions have been made. Nonpatent literature 1 describes the Remez method as a conventional method that has been used usually. Nonpatent literature 2 describes a method of configuring a lowsensitivity linearphase FIR filter by cascade connection, and the cascade connection allows the number of multipliers to be reduced. Patent literature 1 describes an FIR filter provided to vary the delay of the filter output signal. Moreover, FIR filters for various uses such as filters for separating acoustic signal bands and filters for eliminating noise in image have been studied. There are more conventional technologies. For example, nonpatent literature 3 discloses the design and implementation of oversampled filter banks for multilevel FIR filters with low aliasing. Nonpatent literature 4 discloses a filter bank with desired frequency response and low aliasing noise. Nonpatent literature 5 discloses a realization of oversampled cosine modulated filter banks with perfect reconstruction. Patent literature 2 discloses a filter bank and filtering method that can efficiently implement critically sampled filter banks for realvalue signals and especially a cosine modulated filter bank. Nakamura et al: "Design of FIR filter with small number of coefficients based on compactly supported fluency sampling function" (2003 IEEE Pacific Rim Conference on Communications, computers and signal processing) discloses the general concept of upsampling a basic lowpass and highpass filter having an impulse response function configured by a piecewise polynomial with a finite support and using a value of the impulse response function at a connection point between the polynomials as a filter coefficient. EP 1 533 898 A1 discloses lowpass and highpass filter units with a common center frequency designed on the basis of a basic unit filter having a predetermined basic numeric string as a filter coefficient. The lowpass and highpass filter units are connected in cascade in order to design a band pass filter. 
Scope of claims 
[claim1] 1. A filter having desired passband characteristics, in particular a desired cutoff frequency (f1), a desired passband attenuation degree (A) and a desired frequency width Δfb(M) of the passband, comprising ) a basic lowpass filter (L0) with passband width Δfb(0) and having an impulse response function (ψ) configured by a piecewise polynomial with a finite support and using values of the impulse response function (ψ) at connection points between the polynomials as filter coefficients c(k), ) a basic highpass filter (H0) with passband width Δfb(0) and having filter coefficients b(k) obtained by inverting the sign of the filter coefficients c(k) alternately, and ) upsampled lowpass filters (Lp) and upsampled highpass filters (Hq) combined in a cascade connection, characterized in that ) the upsampled lowpass filters (Lp) are formed by upsampling the basic lowpass filter (L0) by all values of p (p=1,2,...P1), with p being an upsampling factor in the time domain and P1 being smaller than M, and ) the upsampled highpass filters (Hq) are formed by upsampling the basic highpass filter (H0) by all values of q (q=1,2,... Q1), with q being an upsampling factor in the time domain and Q1 being smaller than M, with the maximum upsampling factor M+1 being determined by the ratio of the passband width Δfb(0) of the basic filter to the desired passband width Δfb(M). [claim2] 2. A filter according to claim 1, characterized in that the upsampling factor P is the minimum value of p that satisfies the following equation: f3p=f30/1+p<f3 with f3(p) being the cutoff frequency of the upsampled lowpass filter (LP) defined by a passband attenuation degree of 3dB, f3(0) being the cutoff frequency of the basic lowpass filter (L0) defined by a passband attenuation degree of 3dB and f3 being the desired cutoff frequency (f1) of the cascade connection stage defined by a desired passband attenuation degree (A) of 3dB. [claim3] 3. A method for configuring an FIR filter to provide desired passband characteristics, in particular a desired passband width Δfb(M), the filter comprising a cascade connection of ) a basic lowpass filter (L0) with a passband width Δfb(0) and having an impulse response function (ψ) configured by a piecewise polynomial with a finite support and using a value of the impulse response function (ψ) at a connection point between the polynomials as filter coefficients c(k), ) a basic highpass filter (H0) with a passband width Δfb(0) and having filter coefficients b(k) obtained by inverting the sign of the filter coefficients c(k) alternately, and ) upsampled lowpass filters (Lp) and upsampled highpass filters (Hq), the method comprising the step of inputting a desired center frequency (fc), a desired cutoff frequency (f1), a desired stopband frequency (f2), a desired passband attenuation degree (A) at the cutoff frequency (f1), a desired stopband attenuation degree (Ad) at the stopband frequency (f2), and a sampling frequency (fs), characterized in that the method further comprises the steps of ) calculating the desired passband width Δfb(M) as (f1fC)∗2 and obtaining the maximum upsampling factor M+1 by calculating the ratio of the passband width Δfb(0) of the basic filter to the desired passband width Δfb(M), ) sequentially selecting a number of αp (αp =0,1,2,...) of the upsampled lowpass filters (Lp) formed by upsampling the basic lowpass filter (L0) by all values of p (p=1,2,...P1), with p being an upsampling factor in the time domain and P1 being smaller than M, ) and sequentially selecting a number of βq (βq =0,1,2,...) of the upsampled highpass filters (Hq) formed by upsampling the basic highpass filter (H0) by all values of q (q=1,2,... Q1), with q being an upsampling factor in the time domain and Q1 being smaller than M, and ) determining whether a cascade connection of the selected upsampled lowpass filters (Lp) and upsampled highpass filters (Hq) conforms to the desired passband attenuation degree (A) at the cutoff frequency (f1) and to the desired stopband attenuation degree (Ad) at the stopband frequency (f2). [claim4] 4. A method according to claim 3, characterized in that the selection of upsampled lowpass filters (Lp) and upsampled highpass filters (Hq) is made depending on whether the attenuation degree at the stopband frequency (f2) is lower than or equal to the desired stopband attenuation degree (Ad). [claim5] 5. A method according to claim 3 or 4, characterized in that the selection of upsampled lowpass filters (Lp) and upsampled highpass filters (Hq) is made depending on whether the attenuation degree at the cutoff frequency (f1) is higher than or equal to a predetermined permissible level below the desired passband attenuation degree (A). [claim6] 6. A method according to one of the claims 3 to 5, characterized in that an optimum combination of p, q, αp, and βq is selected by selecting αp and βq in descending order from stored maximum values and selecting p and q in descending order from the maximum upsampling factor M. [claim7] 7. A method according to one of the claims 3 to 6, characterized in that αp and βq are increased, if this causes a filter gain at the desired stopband frequency (f2) of the filter to decrease. 




IPC(International Patent Classification) 

Specified countries  Contracting States: AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO SE SI SK TR 
Reference ( R and D project )  CREST New HighPerformance Information Processing Technology Supporting InformationOriented Society  Aiming at the Creation of New HighSpeed, LargeCapacity Computing Technology Based on Quantum Effects, Molecular Functions, Parallel Processing, etc. AREA 
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