design

Apply design method to filter specification object

Syntax

filt = design(designSpecs,'Systemobject',true)

filt = design(designSpecs,method,'Systemobject',true)

filt = design(designSpecs,method,PARAM,VALUE,...,'Systemobject',true)

filt = design(designSpecs,method,opts,'Systemobject',true)

Description

filt = design(designSpecs,'Systemobject',true) uses the filter specification object, designSpecs, to generate a filter System object™, filt. When you do not provide a design method as an input argument, design uses the default design method. Use designmethods(designSpecs,'default') to see the default design method for your filter design specification object. For more information on filter design specifications, see Design a Filter in Fdesign — Process Overview.

filt = design(designSpecs,method,'Systemobject',true) uses the design method specified by method. method must be one of the options returned by designmethods.

filt = design(designSpecs,method,PARAM,VALUE,...,'Systemobject',true) specifies design method options. Use designoptions(designSpecs,method) to see a list of available design method options to choose from. For detailed help on each of these options, type help(designSpecs,method) in the MATLAB^® command prompt.

example

filt = design(designSpecs,method,opts,'Systemobject',true) specifies design method options using the structure opts. opts is usually obtained from the designopts function and then specified as an input to the design function. Use help(designSpecs,method) for more information on optional inputs.

Examples

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Design FIR Equiripple Lowpass Filters

Open Live Script

Design an FIR equiripple lowpass filter. Specify a passband edge frequency of 0.2π rad/sample and a stopband edge frequency of 0.25π rad/sample. Set the passband ripple to 0.5 dB and the stopband attenuation to 40 dB..

designSpecs = fdesign.lowpass('Fp,Fst,Ap,Ast',0.2,0.25,0.5,40)

designSpecs = 
  lowpass with properties:

               Response: 'Lowpass'
          Specification: 'Fp,Fst,Ap,Ast'
            Description: {4×1 cell}
    NormalizedFrequency: 1
                  Fpass: 0.2000
                  Fstop: 0.2500
                  Apass: 0.5000
                  Astop: 40

Use the default Equiripple method to design the filter.

filt = design(designSpecs,'SystemObject',true)

filt = 
  dsp.FIRFilter with properties:

            Structure: 'Direct form'
      NumeratorSource: 'Property'
            Numerator: [-0.0057 -0.0010 0.0010 0.0040 0.0065 0.0073 0.0055 0.0014 -0.0033 -0.0066 -0.0067 -0.0029 0.0033 0.0091 0.0114 0.0083 4.2874e-04 -0.0091 -0.0158 -0.0154 -0.0070 0.0069 0.0202 0.0259 0.0193 5.1049e-04 -0.0239 … ] (1×69 double)
    InitialConditions: 0

  Show all properties

Determine the available design methods by running the designmethods function on the filter design specification object, designSpecs.

designmethods(designSpecs,'SystemObject',true)

Design Methods that support System objects for class fdesign.lowpass (Fp,Fst,Ap,Ast):


butter
cheby1
cheby2
ellip
equiripple
ifir
kaiserwin
multistage

You can also specify the design options used in designing the filter. To see a list of available options, run the designoptions function on designSpecs.

designoptions(designSpecs,'equiripple')

ans = struct with fields:
           FilterStructure: {'dffir'  'dffirt'  'dfsymfir'  'fftfir'}
             DensityFactor: 'double'
                  MinPhase: 'bool'
                  MaxPhase: 'bool'
                  MinOrder: {'any'  'even'  'odd'}
             StopbandShape: {'flat'  'linear'  '1/f'}
             StopbandDecay: 'double'
               UniformGrid: 'bool'
              SystemObject: 'bool'
    DefaultFilterStructure: 'dffir'
      DefaultDensityFactor: 16
           DefaultMaxPhase: 0
           DefaultMinOrder: 'any'
           DefaultMinPhase: 0
      DefaultStopbandDecay: 0
      DefaultStopbandShape: 'flat'
       DefaultSystemObject: 0
        DefaultUniformGrid: 1

Design a minimum-phase FIR equiripple filter by setting 'MinPhase' to true.

filtMin = design(designSpecs,'equiripple','MinPhase',true,'SystemObject',true)

filtMin = 
  dsp.FIRFilter with properties:

            Structure: 'Direct form'
      NumeratorSource: 'Property'
            Numerator: [0.0163 0.0381 0.0724 0.1143 0.1562 0.1875 0.1981 0.1814 0.1378 0.0750 0.0071 -0.0499 -0.0824 -0.0846 -0.0593 -0.0181 0.0236 0.0514 0.0571 0.0412 0.0117 -0.0188 -0.0385 -0.0411 -0.0272 -0.0040 0.0182 0.0308 0.0295 … ] (1×59 double)
    InitialConditions: 0

  Show all properties

Display pole-zero plots of the default and minimum-phase designs.

filterAnalyzer(filt,filtMin,Analysis='polezero',...
    FilterNames=["DefaultDesign","MinimumPhaseDesign"]);

Redesign the filter using the elliptic method. Determine the available design options for the elliptic method.

designoptions(designSpecs,'ellip')

ans = struct with fields:
           FilterStructure: {'df1sos'  'df2sos'  'df1tsos'  'df2tsos'  'cascadeallpass'  'cascadewdfallpass'}
              SOSScaleNorm: 'ustring'
              SOSScaleOpts: 'fdopts.sosscaling'
              MatchExactly: {'passband'  'stopband'  'both'}
              SystemObject: 'bool'
    DefaultFilterStructure: 'df2sos'
       DefaultMatchExactly: 'both'
       DefaultSOSScaleNorm: ''
       DefaultSOSScaleOpts: [1×1 fdopts.sosscaling]
       DefaultSystemObject: 0

Match the passband exactly by setting 'MatchExactly' to 'passband'.

filt = design(designSpecs,'ellip','MatchExactly','passband','SystemObject',true)

filt = 
  dsp.SOSFilter with properties:

            Structure: 'Direct form II'
    CoefficientSource: 'Property'
            Numerator: [3×3 double]
          Denominator: [3×3 double]
       HasScaleValues: true
          ScaleValues: [0.2868 0.1982 0.2819 1]

  Show all properties

You can specify the Pth norm scaling on the second-order sections. Use L-infinity norm scaling in the time domain.

filtL = design(designSpecs,'ellip','MatchExactly','passband','SOSScaleNorm','linf', ...
    'SystemObject',true)

filtL = 
  dsp.SOSFilter with properties:

            Structure: 'Direct form II'
    CoefficientSource: 'Property'
            Numerator: [3×3 double]
          Denominator: [3×3 double]
       HasScaleValues: false

  Show all properties

Display the frequency responses of the filters.

filterAnalyzer(filt,filtL,Analysis="magnitude",...
    FilterNames=["DefaultScaling","LInfinityNormScaling"]);

Input Arguments

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`designSpecs` — Filter design specification object
`fdesign.response` object

fdesign returns a filter design specification object. Every filter design specification object has these properties.

Property Name	Default Value	Description
`Response`	Depends on the chosen type	Defines the type of filter to design, such as an interpolator or bandpass filter. This is a read-only value.
`Specification`	Depends on the chosen type	Defines the filter characteristics used to define the desired filter performance, such as the cutoff frequency `Fc` or the filter order `N`.
`Description`	Depends on the filter type you choose	Contains descriptions of the filter specifications used to define the object, and the filter specifications you use when you create a filter from the object. This is a read-only value.
`NormalizedFrequency`	Logical `true`	Determines whether the filter calculation uses a normalized frequency from 0 to 1, or the frequency band from 0 to F_s/2, the sampling frequency. Accepts either `true` or `false` without single quotation marks. Audio weighting filters do not support normalized frequency.

In addition to these properties, filter design specification objects may have other properties as well, depending on whether they design single-rate filters or multirate filters.

Added Properties for Multirate Filters	Description
`DecimationFactor`	Specifies the amount to decrease the sampling rate. Always a positive integer.
`InterpolationFactor`	Specifies the amount to increase the sampling rate. Always a positive integer.
`PolyphaseLength`	Polyphase length is the length of each polyphase subfilter that composes the decimator or interpolator or rate-change factor filters. Total filter length is the product of `pl` and the rate change factors. `pl` must be an even integer.

`method` — Design method
character vector

Design method, specified as a character vector. The design method you provide as the input argument must be one of the methods returned by:

designmethods(designSpecs,'Systemobject',true)

The table lists all the design methods. A subset of these become available depending on the filter design specification object, designSpecs.

Design methods	Description
`butter`	Butterworth filter
`cheby1`	Chebyshev Type I filter
`cheby2`	Chebyshev Type II filter
`ellip`	Elliptic filter
`equiripple`	Equiripple FIR filter
`firls`	Least-square linear-phase FIR filter
`freqsamp`	Frequency-sampled FIR filter
`ifir`	Interpolated FIR filter
`iirlinphase`	Quasi-linear phase IIR filter
`iirlpnorm`	Least P-norm optimal IIR filter
`iirls`	Least-squares IIR filter
`fircls`	FIR constrained least squares filter
`kaiserwin`	Kaiser window filter
`maxflat`	Maxflat FIR filter
`multistage`	Multistage filter
`window`	FIR filter using windowed impulse response

To help you design filters more quickly, the input argument method accepts a variety of special keywords that force design to behave in different ways. This table presents the keywords you can use for method and how design responds to the keyword:

Design Method Keyword	Description of the Design Response
`'FIR'`	Forces `design` to produce an FIR filter. When no FIR design method exists for object `d`, `design` returns an error.
`'IIR'`	Forces `design` to produce an IIR filter. When no IIR design method exists for object `d`, `design` returns an error.
`'ALLFIR'`	Produces filters from every applicable FIR design method for the specifications in `d`, one filter for each design method. As a result, `design` returns multiple filters in the output object.
`'ALLIIR'`	Produces filters from every applicable IIR design method for the specifications in `d`, one filter for each design method. As a result, `design` returns multiple filters in the output object.
`'ALL'`	Designs filters using all applicable design methods for the specifications object `d`. As a result, `design` returns multiple filters, one for each design method. `design` uses the design methods in the order that `designmethods(D,'Systemobject',true)` returns them.

Keywords are not case sensitive.

When design returns multiple filters in the output object, use indexing to see the individual filters. For example, to see the third filter in filt, enter:

filt(3)

Example: filt = design(designSpecs,'butter','SystemObject',true)

Example: filt = design(designSpecs,'ALLFIR','SystemObject',true)

`opts` — Specify design options
structure

Specify design options by passing opts structure as an input to the design function. The opts structure is obtained by running designopts(designSpecs,method).

designSpecs = fdesign.notch
opts = designopts(designSpecs,'butter')
opts.FilterStructure = 'df1sos'
filt = design(designSpecs,'butter',opts,'SystemObject',true)

Version History

Introduced in R2009a

design

Syntax

Description

Examples

Design FIR Equiripple Lowpass Filters

Input Arguments

`designSpecs` — Filter design specification object
`fdesign.response` object

`method` — Design method
character vector

`opts` — Specify design options
structure

Version History

See Also

Topics

design

Syntax

Description

Examples

Design FIR Equiripple Lowpass Filters

Input Arguments

designSpecs — Filter design specification object fdesign.response object

method — Design method character vector

opts — Specify design options structure

Version History

See Also

Topics

`designSpecs` — Filter design specification object
`fdesign.response` object

`method` — Design method
character vector

`opts` — Specify design options
structure