Use the following two steps to design a simple filter.
Create a filter specification object.
Design your filter.
Assume that you want to design a bandpass filter. Typically a bandpass filter is defined as shown in the following figure.
In this example,
a sampling frequency of
Fs = 48 kHz is used. This
bandpass filter has the following specifications, specified here using MATLAB® code:
A_stop1 = 60; % Attenuation in the first stopband = 60 dB F_stop1 = 8400; % Edge of the stopband = 8400 Hz F_pass1 = 10800; % Edge of the passband = 10800 Hz F_pass2 = 15600; % Closing edge of the passband = 15600 Hz F_stop2 = 18000; % Edge of the second stopband = 18000 Hz A_stop2 = 60; % Attenuation in the second stopband = 60 dB A_pass = 1; % Amount of ripple allowed in the passband = 1 dB
In the following two steps, these specifications are passed
fdesign.bandpass method as parameters.
To create a filter specification object, evaluate the following code at the MATLAB prompt:
d = fdesign.bandpass
Now, pass the filter specifications that correspond to the default
This example adds
fs as the final input argument
to specify the sampling frequency of 48 kHz.
>> BandPassSpecObj = ... fdesign.bandpass('Fst1,Fp1,Fp2,Fst2,Ast1,Ap,Ast2', ... F_stop1, F_pass1, F_pass2, F_stop2, A_stop1, A_pass, ... A_stop2, 48000)
The order of the filter is not specified, allowing a degree of freedom for the algorithm design in order to achieve the specification. The design will be a minimum order design.
The specification parameters, such as
Fstop1, are all given default
values when none are provided. You can change the values of the specification
parameters after the filter specification object has been created. For example, if
there are two values that need to be changed,
Fstop2, use the
set command, which takes
the object first, and then the parameter value pairs. Evaluate the following code at
>> set(BandPassSpecObj, 'Fpass2', 15800, 'Fstop2', 18400)
BandPassSpecObjis the new filter specification object which contains all the required design parameters, including the filter type.
You may also change parameter values in filter specification
objects by accessing them as if they were elements in a
Design the filter by using the
design command. You can access the design
methods available for you specification object by calling the
designmethods function. For example, in this case, you can
execute the command
>> designmethods(BandPassSpecObj) Design Methods for class fdesign.bandpass (Fst1,Fp1,Fp2,Fst2,Ast1,Ap,Ast2): butter cheby1 cheby2 ellip equiripple kaiserwin
>> BandPassFilt = design(BandPassSpecObj, 'equiripple') BandPassFilt = FilterStructure: 'Direct-Form FIR' Arithmetic: 'double' Numerator: [1x44 double] PersistentMemory: false
If you have the DSP System
Toolbox™ installed, you can
also design your filter with a filter System
object™. To create
a filter System object with the same specification object
you can execute the commands
>> designmethods(BandPassSpecObj,... 'SystemObject',true) Design Methods that support System objects for class fdesign.bandpass (Fst1,Fp1,Fp2,Fst2,Ast1,Ap,Ast2): butter cheby1 cheby2 ellip equiripple kaiserwin >> BandPassFiltSysObj = design(BandPassSpecObj,... 'equiripple','SystemObject',true) System: dsp.FIRFilter Properties: Structure: 'Direct form' NumeratorSource: 'Property' Numerator: [1x44 double] InitialConditions: 0 FrameBasedProcessing: true Show fixed-point properties
If you do not specify a design method, a default method will be used. For example, you can execute the command
>> BandPassFilt = design(BandPassSpecObj) BandPassFilt = FilterStructure: 'Direct-Form FIR' Arithmetic: 'double' Numerator: [1x44 double] PersistentMemory: false
To check your work, you can plot the filter magnitude response using the Filter Visualization tool. Verify that all the design parameters are met:
>> fvtool(BandPassFilt) %plot the filter magnitude response