Exploring the Process Flow Diagram

The process flow diagram shown in the following figure lists the steps and shows the order of the filter design process.

The first four steps of the filter design process relate to the filter Specifications Object, while the last two steps involve the filter Implementation Object. Both of these objects are discussed in more detail in the following sections. Step 5 - the design of the filter, is the transition step from the filter Specifications Object to the Implementation object. The analysis and verification step is completely optional. It provides methods for the filter designer to ensure that the filter complies with all design criteria. Depending on the results of this verification, you can loop back to steps 3 and 4, to either choose a different algorithm, or to customize the current one. You may also wish to go back to steps 3 or 4 after you filter the input data with the designed filter (step 7), and find that you wish to tweak the filter or change it further.

The diagram shows the help command for each step. Enter the help line at the MATLAB^® command prompt to receive instructions and further documentation links for the particular step. Not all of the steps have to be executed explicitly. For example, you could go from step 1 directly to step 5, and the interim three steps are done for you by the software.

The following are the details for each of the steps shown above.

Select a Response

If you type:

help fdesign/responses

You must select a response to initiate the filter. In this example, a bandpass filter Specifications Object is created by typing the following:

d = fdesign.bandpass

Select a Specification

A specification is an array of design parameters for a given filter. The specification is a property of the Specifications Object.

When you select a filter response, there are a number of different specifications available. Each one contains a different combination of design parameters. After you create a filter Specifications Object, you can query the available specifications for that response. Specifications marked with an asterisk require the DSP System Toolbox™.

d = fdesign.bandpass; % step 1 - choose the response
set (d, 'specification')

ans =

  16×1 cell array

    'Fst1,Fp1,Fp2,Fst2,Ast1,Ap,Ast2'
    'N,F3dB1,F3dB2'
    'N,F3dB1,F3dB2,Ap'
    'N,F3dB1,F3dB2,Ast'
    'N,F3dB1,F3dB2,Ast1,Ap,Ast2'
    'N,F3dB1,F3dB2,BWp'
    'N,F3dB1,F3dB2,BWst'
    'N,Fc1,Fc2'
    'N,Fc1,Fc2,Ast1,Ap,Ast2'
    'N,Fp1,Fp2,Ap'
    'N,Fp1,Fp2,Ast1,Ap,Ast2'
    'N,Fst1,Fp1,Fp2,Fst2'
    'N,Fst1,Fp1,Fp2,Fst2,C'
    'N,Fst1,Fp1,Fp2,Fst2,Ap'
    'N,Fst1,Fst2,Ast'
    'Nb,Na,Fst1,Fp1,Fp2,Fst2'

d = fdesign.arbmag; 
set(d,'specification')

ans =

  7×1 cell array

    'N,F,A'
    'F,A,R'
    'Nb,Na,F,A'
    'N,B,F,A'
    'N,B,F,A,C'
    'B,F,A,R'
    'Nb,Na,B,F,A'

The set command can be used to select one of the available specifications as follows:

d = fdesign.lowpass;
% step 1: get a list of available specifications
set (d, 'specification') 

ans =

  18×1 cell array

    'Fp,Fst,Ap,Ast'
    'N,F3dB'
    'Nb,Na,F3dB'
    'N,F3dB,Ap'
    'N,F3dB,Ap,Ast'
    'N,F3dB,Ast'
    'N,F3dB,Fst'
    'N,Fc'
    'N,Fc,Ap,Ast'
    'N,Fp,Ap'
    'N,Fp,Ap,Ast'
    'N,Fp,F3dB'
    'N,Fp,Fst'
    'N,Fp,Fst,Ap'
    'N,Fp,Fst,Ast'
    'N,Fst,Ap,Ast'
    'N,Fst,Ast'
    'Nb,Na,Fp,Fst'

% step 2: set the required specification
 set (d, 'specification', 'N,Fc') 

Select an Algorithm

The availability of algorithms depends the chosen filter response, the design parameters, and the availability of the DSP System Toolbox. In other words, for the same lowpass filter, changing the specification entry also changes the available algorithms. In the following example, for a lowpass filter and a specification of 'N, Fc', only one algorithm is available—window.

% step 2: set the required specification
set (d, 'specification', 'N,Fc') 
% step 3: get available algorithms
designmethods (d,'Systemobject',true) 

Design Methods that support System objects for class fdesign.lowpass (N,Fc):


window

set(d, 'specification', 'Fp,Fst,Ap,Ast')
designmethods(d,'Systemobject',true)

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


butter
cheby1
cheby2
ellip
equiripple
ifir
kaiserwin
multistage

filt = design(d,'butter','Systemobject',true)

filt = 

  dsp.SOSFilter with properties:

            Structure: 'Direct form II'
    CoefficientSource: 'Property'
            Numerator: [13×3 double]
          Denominator: [13×3 double]
       HasScaleValues: true
          ScaleValues: [0.4151 0.3718 0.3374 ...]

  Show all properties

If you do not perform this step explicitly, design automatically selects the optimum algorithm for the chosen response and specification.

Customize the Algorithm

The customization options available for any given algorithm depend not only on the algorithm itself, selected in Select an Algorithm, but also on the specification selected in Select a Specification. To explore all the available options, type the following at the MATLAB command prompt:

help (d, 'algorithm-name')

The application of these customization options takes place while Design the Filter, because these options are the properties of the filter Implementation Object, not the Specification Object.

If you do not perform this step explicitly, the optimum algorithm structure is selected.

Design the Filter

To create a filter, use the design command:

% Design filter without specifying the algorithm
filt = design(d,'Systemobject',true);

To apply the algorithm chosen in Select an Algorithm, use the same design command, but specify the algorithm as follows:

filt = design(d,'butter','Systemobject',true)

filt = 

  dsp.SOSFilter with properties:

            Structure: 'Direct form II'
    CoefficientSource: 'Property'
            Numerator: [13×3 double]
          Denominator: [13×3 double]
       HasScaleValues: true
          ScaleValues: [0.4151 0.3718 0.3374 … ]

  Show all properties

To obtain help and see all the available options, type:

help fdesign/design

filt = design(d, 'butter', 'filterstructure', 'df2sos','Systemobject',true)

filt = 

  dsp.SOSFilter with properties:

            Structure: 'Direct form II'
    CoefficientSource: 'Property'
            Numerator: [13×3 double]
          Denominator: [13×3 double]
       HasScaleValues: true
          ScaleValues: [0.4151 0.3718 0.3374 … ]

  Show all properties

The filter design step, just like the first task of choosing a response, must be performed explicitly. A filter object is created only when design is called.

Design Analysis

After the filter is designed, you may wish to analyze it to determine if the filter satisfies the design criteria. Filter analysis is broken into these main sections:

For a list of analysis methods for a discrete-time filter, enter the following in the MATLAB command prompt:

dsp.<sysobjName>.helpFilterAnalysis

Replace <sysobjName> with the name of the System object™. Alternatively, you can see the list of analysis methods under the Filter Analysis category.

To analyze your filter, you must explicitly perform this step.

Realize or Apply the Filter to Input Data

After the filter is designed and optimized, it can be used to filter actual input data.

y = filt(x)

help realizemdl