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| R2011b Documentation → Signal Processing Toolbox | |
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D= fdesign.arbmag
D= fdesign.arbmag(SPEC)
D = fdesign.arbmag(SPEC,specvalue1,specvalue2,...)
D = fdesign.arbmag(specvalue1,specvalue2,specvalue3)
D = fdesign.arbmag(...,Fs)
D= fdesign.arbmag constructs an arbitrary magnitude filter specification object D.
D= fdesign.arbmag(SPEC) initializes the Specification property to SPEC. The input argument SPEC must be one of the strings shown in the following table. Specification strings are not case sensitive.
'N,F,A' — Single band design (default)
'F,A,R' — Single band minimum order design *
'N,B,F,A' — Multiband design
'N,B,F,A,C' — Constrained multiband design *
'B,F,A,R' — Multiband minimum order design *
'Nb,Na,F,A' — Single band design *
'Nb,Na,B,F,A' — Multiband design *
The string entries are defined as follows:
A — Amplitude vector. Values in A define the filter amplitude at frequency points you specify in f, the frequency vector. If you use A, you must use F as well. Amplitude values must be real. For complex values designs, use fdesign.arbmagnphase.
B — Number of bands in the multiband filter
C — Constrained band flag. This enables you to constrain the passband ripple in your multiband design. You cannot constrain the passband ripple in all bands simultaneously.
F — Frequency vector. Frequency values in specified in F indicate locations where you provide specific filter response amplitudes. When you provide F, you must also provide A.
N — Filter order for FIR filters and the numerator and denominator orders for IIR filters.
Nb — Numerator order for IIR filters
Na — Denominator order for IIR filter designs
R — Ripple
By default, this method assumes that all frequency specifications are supplied in normalized frequency.
F and A are the input arguments you use to define the filter response desired. Each frequency value you specify in F must have a corresponding response value in A. The following example creates a filter with two passbands (B = 4) and shows how F and A are related. This example is for illustration only. It is not an actual filter.
Define the frequency vector F as [0 0.1 0.2 0.4 0.5 0.6 0.9 1.0]
Define the response vector A as [0 0.5 0.5 0.1 0.1 0.8 0.8 0]
These specifications connect F and A as shown in the following table.
F (Normalized Frequency) | A (Response Desired at F) |
|---|---|
0 | 0 |
0.1 | 0.5 |
0.2 | 0.5 |
0.4 | 0.1 |
0.5 | 0.1 |
0.6 | 0.8 |
0.9 | 0.8 |
1.0 | 0.0 |
A response with two passband—one roughly between 0.1 and 0.2 and the second between 0.6 and 0.9—results from the mapping between F and A.
Different specifications can have different design methods available. Use designmethods to get a list of design methods available for a given specification string and filter specification object.
Use designopts to get a list of design options available for a filter specification object and a given design method. Enter help(D,METHOD) to get detailed help on the available design options for a given design method.
D = fdesign.arbmag(SPEC,specvalue1,specvalue2,...) initializes the specifications with specvalue1, specvalue2. Use get(D,'Description') for descriptions of the various specifications specvalue1, specvalue2, ... specvalueN.
D = fdesign.arbmag(specvalue1,specvalue2,specvalue3) uses the default specification string 'N,F,A', setting the filter order, filter frequency vector, and the amplitude vector to the values specvalue1, specvalue2, and specvalue3.
D = fdesign.arbmag(...,Fs) specifies the sampling frequency in Hz. All other frequency specifications are also assumed to be in Hz when you specify Fs.
Design a single band arbitrary-magnitude equiripple FIR filter.
n = 120; f = linspace(0,1,100); % 100 frequency points. as = ones(1,100)-f*0.2; absorb = [ones(1,30),(1-0.6*bohmanwin(10))',... ones(1,5), (1-0.5*bohmanwin(8))',ones(1,47)]; a = as.*absorb; d = fdesign.arbmag('N,F,A',n,f,a); hd1 = design(d,'equiripple');
If you have the DSP System Toolbox, you can design a minimum-phase equiripple filter.
hd2 = design(d,'equiripple','MinPhase',true); hfvt = fvtool([hd1 hd2],'analysis','polezero'); legend(hfvt,'Equiripple Filter','Minimum-phase Equiripple Filter');
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The following example requires the DSP System Toolbox. Design a multiband minimum order filter with notches at 0.25π and 0.55π radians/sample.
d = fdesign.arbmag('B,F,A,R');
d.NBands = 5;
d.B1Frequencies = [0 0.2];
d.B1Amplitudes = [1 1];
d.B1Ripple = 0.25;
d.B2Frequencies = 0.25;
d.B2Amplitudes = 0;
d.B3Frequencies = [0.3 0.5];
d.B3Amplitudes = [1 1];
d.B3Ripple = 0.25;
d.B4Frequencies = 0.55;
d.B4Amplitudes = 0;
d.B5Frequencies = [0.6 1];
d.B5Amplitudes = [1 1];
d.B5Ripple = 0.25;
Hd = design(d,'equiripple');
fvtool(Hd)
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The following example requires the DSP System Toolbox. Design a multiband constrained arbitrary magnitude FIR filter. Force the frequency response at 0.15π radians/sample to 0 dB.
d = fdesign.arbmag('N,B,F,A,C',82,2);
d.B1Frequencies = [0 0.06 .1];
d.B1Amplitudes = [0 0 0];
d.B2Frequencies = [.15 1];
d.B2Amplitudes = [1 1];
% Design a filter with no constraints
Hd1 = design(d,'equiripple','B2ForcedFrequencyPoints',0.15);
% Add a constraint to the first band to increase attenuation
d.B1Constrained = true;
d.B1Ripple = .001;
Hd2 = design(d,'equiripple','B2ForcedFrequencyPoints',0.15);
hfvt = fvtool(Hd1,Hd2);
legend(hfvt,'Original Design','Design with Constrained Stopband Ripple');
design | designmethods | fdesign
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