Filter Design Toolbox™

dfilt - Discrete-time filter

Syntax

hd = dfilt.structure(input1,...)
hd = [dfilt.structure(input1,...), dfilt.structure(input1,...),...]
hd = design(d,'designmethod')

Description

hd = dfilt.structure(input1,...) returns a discrete-time filter, hd, of type structure. Each structure takes one or more inputs. When you specify a dfilt.structure with no inputs, a default filter is created.

Note   You must use a structure with dfilt.

hd = [dfilt.structure(input1,...), dfilt.structure(input1,...),...] returns a vector containing dfilt filters.

Structures

Structures for dfilt.structure specify the type of filter structure. Available types of structures for dfilt are shown below.

For more information on each structure, refer to its reference page.

hd = design(d,'designmethod') returns the dfilt object hd resulting from the filter specification object d and the design method you specify in designmethod. When you omit the designmethod argument, design uses the default design method to construct a filter from the object d.

With this syntax, you design filters by

1. Specifying the filter specifications, such as the response shape (perhaps highpass) and details (passband edges and attenuation).

2. Selecting a method (such as equiripple) to design the filter.

3. Applying the method to the specifications object with design(d,'designmethod).

Using the specification-based technique can be more effective than the coefficient-based filter design techniques.

Design Methods for design Syntax

When you use the hd = design(d,'designmethod') syntax, you have a range of design methods available depending on d, the filter specification object. The table below lists all of the design methods in the toolbox.

As specifications object d changes, the methods that apply for designing filters from d change. For instance, if d is a lowpass filter, these are the applicable methods:

% Create an object to design a lowpass filter.
d=fdesign.lowpass 
 
d =
 
               Response: 'Lowpass'      
          Specification: 'Fp,Fst,Ap,Ast'
            Description: {4x1 cell}     
    NormalizedFrequency: true           
                  Fpass: 0.45           
                  Fstop: 0.55           
                  Apass: 1              
                  Astop: 60             
                                        
designmethods(d) % What design methods apply to object d?

Design Methods for class fdesign.lowpass (Fp,Fst,Ap,Ast):

butter
cheby1
cheby2
ellip
equiripple
ifir
kaiserwin
multistage

When d is a bandstop filter, the design methods change.

% Create default bandstop specifications 
d=fdesign.bandstop 
object.
 
d =
 
               Response: 'Bandstop'                     
          Specification: 'Fp1,Fst1,Fst2,Fp2,Ap1,Ast,Ap2'
            Description: {7x1 cell}                     
    NormalizedFrequency: true                           
                 Fpass1: 0.35                           
                 Fstop1: 0.45                           
                 Fstop2: 0.55                           
                 Fpass2: 0.65                           
                 Apass1: 1                              
                  Astop: 60                             
                 Apass2: 1                              
                                                        
designmethods(d) % Show design methods that apply to d.

Design Methods for class fdesign.bandstop 
(Fp1,Fst1,Fst2,Fp2,Ap1,Ast,Ap2):

butter
cheby1
cheby2
ellip
equiripple
kaiserwin

Theifir and multistage design methods do not apply to this bandstop specifications object d.

Analysis Methods

Methods provide ways of performing functions directly on your dfilt object without having to specify the filter parameters again. You can apply these methods directly on the variable you assigned to your dfilt object.

For example, if you create a dfilt object, hd, you can check whether it has linear phase with islinphase(hd), view its frequency response plot with fvtool(hd), or obtain its frequency response values with h = freqz(hd). You can use all of the methods described here in this way.

Note   If your variable hd is a 1-D array of dfilt filters, the method is applied to each object in the array. Only freqz, grpdelay, impz, is*, order, and stepz methods can be applied to arrays. The zplane method can be applied to an array only if zplane is used without outputs.

Some of the methods listed here have the same name as functions in Signal Processing Toolbox™ or Filter Design Toolbox™ software. They behave similarly.

Viewing Properties

As with any object, use get to view a dfilt properties. To see a specific property, use

 get(hd,'property') 

To see all properties for an object, use

get(hd)

Note   If you have Filter Design Toolbox software, dfilt objects include an arithmetic property. You can change the internal arithmetic of the filter from double- precision to single-precision using: hd.arithmetic = 'single'. If you have both Filter Design Toolbox software and \&tm_fixedpointtoolbox; software Toolbox, you can change the arithmetic property to fixed-point using: hd.arithmetic = 'fixed'

Changing Properties

To set specific properties, use

set(hd,'property1',value,'property2',value,...) 

Note that you must use single quotation marks around the property name. Use single quotation marks around the value argument when the value is a string, such as specifyall or fixed.

Copying an Object

To create a copy of an object, use the copy method.

h2 = copy(hd)

Note   Using the syntax H2 = hd copies only the object handle and does not create a new, independent object.

Converting Between Filter Structures

To change the filter structure of a dfilt object hd, use

hd2 = convert(hd,'structure_string');

where structure_string is any valid structure name in single quotation marks. If hd is a cascade or parallel structure, each stage is converted to the new structure.

Using Filter States

Two properties control the filter states:

• states — stores the current states of the filter. Before the filter is applied, the states correspond to the initial conditions and after the filter is applied, the states correspond to the final conditions. For df1, df1t, df1sos and df1tsos structures, states returns a filtstates object.

• PersistentMemory — controls whether filter states are saved. The default value is 'false', which causes the initial conditions to be reset to zero before filtering and turns off the display of states information. Setting PersistentMemory to 'true' allows the filter to use your initial conditions or to reuse the final conditions from a previous filtering operation as the initial conditions of the next filtering operation. The true setting also displays information about the filter states.

Note   If you set the states and want to use them for filtering, you must set PersistentMemory to 'true' before you use the filter.

Examples

Create a direct-form I filter and use a method to see if it is stable.

[b,a] = butter(8,0.25);
hd = dfilt.df1(b,a)
 
hd = 
         FilterStructure: 'Direct-Form I'
               Numerator: [1x9 double]
             Denominator: [1x9 double]
        PersistentMemory: false
    
isstable(hd)
ans =
     1

If a dfilt's numerator values do not fit on a single line, a description of the vector is displayed. To see the specific numerator values for this example, use

get(hd,'numerator')

ans =
Columns 1 through 6 
    0.0001  0.0009  0.0030  0.0060  0.0076  0.0060
  Columns 7 through 9 
    0.0030  0.0009  0.0001

Create an array containing two dfilt objects, apply a method and verify that the method acts on both objects, and use a method to test whether the objects are FIR objects.

b = fir1(5,.5);
hd = dfilt.dffir(b);         % Create an FIR object
[b,a] = butter(5,.5);
hd(2) = dfilt.df2t(b,a);	   % Create DF2T object and place
                           % in the second column of hd.

[h,w] = freqz(hd);
size(h)								% Verify that resulting h is
ans =								% 2 columns.
        8192           2
size(w)								% Verify that resulting w is
ans =								% 1 column.
        8192           1

test_fir = isfir(hd)
test_fir =
     1     0								% hd(1) is FIR and hd(2) is not.

Refer to the reference pages for each structure for more examples.

See Also

dfilt, design, fdesign, realizemdl, sos, stepz

dfilt.cascade, dfilt.df1, dfilt.df1t, dfilt.df2, dfilt.df2t, dfilt.dfasymfir, dfilt.dffir, dfilt.dffirt, dfilt.dfsymfir, dfilt.latticeallpass, dfilt.latticear, dfilt.latticearma, dfilt.latticemamax, dfilt.latticemamin, dfilt.parallel, dfilt.statespace, filter, freqz, grpdelay, impz, zplane in Signal Processing Toolbox documentation

designopts

dfilt.allpass