designfilt

Design digital filters

Syntax

  • d = designfilt(resp,Name,Value) example
  • designfilt(d)

Description

example

d = designfilt(resp,Name,Value) designs a digitalFilter object, d, with response type resp. Specify the filter further using a set of Name,Value pairs. The allowed specification sets depend on the response type, resp, and consist of combinations of the following:

  • Frequency constraints correspond to the frequencies at which a filter exhibits a desired behavior. Examples include 'PassbandFrequency' and 'CutoffFrequency'. (See the complete list under Name-Value Pair Arguments.) You must always specify the frequency constraints.

  • Magnitude constraints describe the filter behavior at particular frequency ranges. Examples include 'PassbandRipple' and 'StopbandAttenuation'. (See the complete list under Name-Value Pair Arguments.) designfilt provides default values for magnitude constraints left unspecified. In arbitrary-magnitude designs you must always specify the vectors of desired amplitudes.

  • 'FilterOrder'. Some design methods let you specify the order. Others produce minimum-order designs. That is, they generate the smallest filters that satisfy the specified constraints.

  • 'DesignMethod' is the algorithm used to design the filter. Examples include constrained least squares ('cls') and windowing ('window'). For some specification sets, there are multiple design methods available to choose from. In other cases, you can use only one method to meet the desired specifications.

  • Design options are parameters specific to a given design method. Examples include 'Window' for the windowing method and optimization 'Weights' for arbitrary-magnitude equiripple designs. (See the complete list under Name-Value Pair Arguments.) designfilt provides default values for design options left unspecified.

  • 'SampleRate' is the frequency at which the filter operates. designfilt has a default sample rate of 2 Hz. Using this value is equivalent to working with normalized frequencies.

    Note:   If you specify an incomplete or inconsistent set of name-value pairs at the command line, designfilt offers to open a Filter Design Assistant. The assistant helps you design the filter and pastes the corrected MATLAB® code on the command line.

    If you call designfilt from a script or function with an incorrect set of specifications, designfilt issues an error message with a link to open a Filter Design Assistant. The assistant helps you design the filter, comments out the faulty code in the function or script, and pastes the corrected MATLAB code on the next line.

designfilt(d) lets you edit an existing digital filter, d. It opens a Filter Design Assistant populated with the filter's specifications, which you can then modify. This is the only way you can edit a digitalFilter object.

Examples

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Lowpass FIR Filter

Design a minimum-order lowpass FIR filter with normalized passband frequency 0.25π rad/sample, stopband frequency 0.35π rad/sample, passband ripple 0.5 dB, and stopband attenuation 65 dB. Use a Kaiser window to design the filter. Visualize its magnitude response. Use it to filter a vector of random data.

lpFilt = designfilt('lowpassfir','PassbandFrequency',0.25, ...
         'StopbandFrequency',0.35,'PassbandRipple',0.5, ...
         'StopbandAttenuation',65,'DesignMethod','kaiserwin');
fvtool(lpFilt)
dataIn = rand([1000 1]); dataOut = filter(lpFilt,dataIn);

Lowpass IIR Filter

Design an 8th-order lowpass IIR filter with passband frequency 35 kHz and passband ripple 0.2 dB. Specify a sample rate of 200 kHz. Visualize the magnitude response of the filter. Use it to filter a 1000-sample random signal.

lpFilt = designfilt('lowpassiir','FilterOrder',8, ...
         'PassbandFrequency',35e3,'PassbandRipple',0.2, ...
         'SampleRate',200e3)
fvtool(lpFilt)
dataIn = randn([1000 1]); dataOut = filter(lpFilt,dataIn);

Highpass FIR Filter

Design a minimum-order highpass FIR filter with normalized stopband frequency 0.25π rad/sample, passband frequency 0.35π rad/sample, passband ripple 0.5 dB, and stopband attenuation 65 dB. Use a Kaiser window to design the filter. Visualize its magnitude response. Use it to filter 1000 samples of random data.

hpFilt = designfilt('highpassfir','StopbandFrequency',0.25, ...
         'PassbandFrequency',0.35,'PassbandRipple',0.5, ...
         'StopbandAttenuation',65,'DesignMethod','kaiserwin');
fvtool(hpFilt)
dataIn = randn([1000 1]); dataOut = filter(hpFilt,dataIn);

Highpass IIR Filter

Design an 8th-order highpass IIR filter with passband frequency 75 kHz and passband ripple 0.2 dB. Specify a sample rate of 200 kHz. Visualize the filter's magnitude response. Apply the filter to a 1000-sample vector of random data.

hpFilt = designfilt('highpassiir','FilterOrder',8, ...
         'PassbandFrequency',75e3,'PassbandRipple',0.2, ...
         'SampleRate',200e3);
fvtool(hpFilt)
dataIn = randn([1000 1]); dataOut = filter(hpFilt,dataIn);

Bandpass FIR Filter

Design a 20th-order bandpass FIR filter with lower cutoff frequency 500 Hz and higher cutoff frequency 560 Hz. The sample rate is 1500 Hz. Visualize the magnitude response of the filter. Use it to filter a random signal containing 1000 samples.

bpFilt = designfilt('bandpassfir','FilterOrder',20, ...
         'CutoffFrequency1',500,'CutoffFrequency2',560, ...
         'SampleRate',1500);
fvtool(bpFilt)
dataIn = randn([1000 1]); dataOut = filter(bpFilt,dataIn);

Bandpass IIR Filter

Design a 20th-order bandpass IIR filter with lower 3-dB frequency 500 Hz and higher 3-dB frequency 560 Hz. The sample rate is 1500 Hz. Visualize the frequency response of the filter. Use it to filter a 1000-sample random signal.

bpFilt = designfilt('bandpassiir','FilterOrder',20, ...
         'HalfPowerFrequency1',500,'HalfPowerFrequency2',560, ...
         'SampleRate',1500);
fvtool(bpFilt)
dataIn = randn([1000 1]); dataOut = filter(bpFilt,dataIn);

Bandstop FIR Filter

Design a 20th-order bandstop FIR filter with lower cutoff frequency 500 Hz and higher cutoff frequency 560 Hz. The sample rate is 1500 Hz. Visualize the magnitude response of the filter. Use it to filter 1000 samples of random data.

bsFilt = designfilt('bandstopfir','FilterOrder',20, ...
         'CutoffFrequency1',500,'CutoffFrequency2',560, ...
         'SampleRate',1500);
fvtool(bsFilt)
dataIn = randn([1000 1]); dataOut = filter(bsFilt,dataIn);

Bandstop IIR Filter

Design a 20th-order bandstop IIR filter with lower 3-dB frequency 500 Hz and higher 3-dB frequency 560 Hz. The sample rate is 1500 Hz. Visualize the magnitude response of the filter. Use it to filter 1000 samples of random data.

bsFilt = designfilt('bandstopiir','FilterOrder',20, ...
         'HalfPowerFrequency1',500,'HalfPowerFrequency2',560, ...
         'SampleRate',1500);
fvtool(bsFilt)
dataIn = randn([1000 1]); dataOut = filter(bsFilt,dataIn);

FIR Differentiator

Design a 7th-order full-band differentiator filter. Display its zero-phase response. Use it to filter a 1000-sample vector of random data.

dFilt = designfilt('differentiatorfir','FilterOrder',7);
fvtool(dFilt,'MagnitudeDisplay','Zero-phase')
dataIn = randn([1000 1]); dataOut = filter(dFilt,dataIn);

FIR Hilbert Transformer

Design an 18th-order Hilbert transformer. Specify a normalized transition width of 0.25π rad/sample. Display in linear units the magnitude response of the filter. Use it to filter a 1000-sample vector of random data.

hFilt = designfilt('hilbertfir','FilterOrder',18, ...
        'TransitionWidth',0.25);
fvtool(hFilt,'MagnitudeDisplay','magnitude')
dataIn = randn([1000 1]); dataOut = filter(hFilt,dataIn);

Arbitrary-Magnitude FIR Filter

You are given a signal sampled at 1 kHz. Design an equiripple filter that stops frequencies from 100 Hz to 350 Hz and frequencies greater than 400 Hz. Specify a filter order of 60. Visualize the frequency response of the filter. Use it to filter a 1000-sample random signal.

mbFilt = designfilt('arbmagfir','FilterOrder',60, ...
         'Frequencies',0:50:500, ...
         'Amplitudes',[1 1 1 0 0 0 0 1 1 0 0], ...
         'DesignMethod','equiripple', ...
         'SampleRate',1000);
fvtool(mbFilt)
dataIn = randn([1000 1]); dataOut = filter(mbFilt,dataIn);

Input Arguments

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resp — Filter response and type'lowpassfir' | 'lowpassiir' | 'highpassfir' | 'highpassiir' | 'bandpassfir' | 'bandpassiir' | 'bandstopfir' | 'bandstopiir' | 'differentiatorfir' | 'hilbertfir' | 'arbmagfir'

Filter response and type, specified as a string. Click one of the possible values of resp to expand a table of allowed specification sets.

 'lowpassfir'

 'lowpassiir'

 'highpassfir'

 'highpassiir'

 'bandpassfir'

 'bandpassiir'

 'bandstopfir'

 'bandstopiir'

 'differentiatorfir'

 'hilbertfir'

 'arbmagfir'

Data Types: char

d — Digital filterdigitalFilter object

Digital filter, specified as a digitalFilter object generated by designfilt. Use this input to change the specifications of an existing digitalFilter.

Name-Value Pair Arguments

Specify optional comma-separated pairs of Name,Value arguments. Name is the argument name and Value is the corresponding value. Name must appear inside single quotes (' '). You can specify several name and value pair arguments in any order as Name1,Value1,...,NameN,ValueN.

Example: 'FilterOrder',20,'CutoffFrequency',0.4 suffices to specify a lowpass FIR filter.

Not all combinations of Name,Value pairs are valid. The valid combinations depend on the filter response that you need and on the frequency and magnitude constraints of your design.

Filter Order

'FilterOrder' — Filter orderpositive integer scalar

Filter order, specified as the comma-separated pair consisting of 'FilterOrder' and a positive integer scalar.

Data Types: double

'NumeratorOrder' — Numerator orderpositive integer scalar

Numerator order of an IIR design, specified as the comma-separated pair consisting of 'NumeratorOrder' and a positive integer scalar.

Data Types: double

'DenominatorOrder' — Denominator orderpositive integer scalar

Denominator order of an IIR design, specified as the comma-separated pair consisting of 'DenominatorOrder' and a positive integer scalar.

Data Types: double

Frequency Constraints

'PassbandFrequency', 'PassbandFrequency1', 'PassbandFrequency2' — Passband frequencypositive scalar

Passband frequency, specified as the comma-separated pair consisting of 'PassbandFrequency' and a positive scalar. The frequency value must be within the Nyquist range.

'PassbandFrequency1' is the lower passband frequency for a bandpass or bandstop design.

'PassbandFrequency2' is the higher passband frequency for a bandpass or bandstop design.

Data Types: double

'StopbandFrequency', 'StopbandFrequency1', 'StopbandFrequency2' — Stopband frequencypositive scalar

Stopband frequency, specified as the comma-separated pair consisting of 'StopbandFrequency' and a positive scalar. The frequency value must be within the Nyquist range.

'StopbandFrequency1' is the lower stopband frequency for a bandpass or bandstop design

'StopbandFrequency2' is the higher stopband frequency for a bandpass or bandstop design.

Data Types: double

'CutoffFrequency', 'CutoffFrequency1', 'CutoffFrequency2' — 6-dB frequencypositive scalar

6-dB frequency, specified as the comma-separated pair consisting of 'CutoffFrequency' and a positive scalar. The frequency value must be within the Nyquist range.

'CutoffFrequency1' is the lower 6-dB frequency for a bandpass or bandstop design.

'CutoffFrequency2' is the higher 6-dB frequency for a bandpass or bandstop design.

Data Types: double

'HalfPowerFrequency', 'HalfPowerFrequency1', 'HalfPowerFrequency2' — 3-dB frequencypositive scalar

3-dB frequency, specified as the comma-separated pair consisting of 'HalfPowerFrequency' and a positive scalar. The frequency value must be within the Nyquist range.

'HalfPowerFrequency1' is the lower 3-dB frequency for a bandpass or bandstop design.

'HalfPowerFrequency2' is the higher 3-dB frequency for a bandpass or bandstop design.

Data Types: double

'TransitionWidth' — Width of transition regionpositive scalar

Width of the transition region between passband and stopband for a Hilbert transformer, specified as the comma-separated pair consisting of 'TransitionWidth' and a positive scalar.

Data Types: double

'Frequencies' — Response frequenciesvector

Response frequencies, specified as the comma-separated pair consisting of 'Frequencies' and a vector. Use this variable to list the frequencies at which a filter of arbitrary magnitude response has desired amplitudes. The frequencies must be monotonically increasing and lie within the Nyquist range. The first element of the vector must be either 0 or –fs/2, where fs is the sample rate, and its last element must be fs/2. If you do not specify a sample rate, designfilt uses the default value of 2 Hz.

Data Types: double

'NumBands' — Number of bandspositive integer scalar

Number of bands in a multiband design, specified as the comma-separated pair consisting of 'NumBands' and a positive integer scalar not greater than 10.

Data Types: double

'BandFrequencies1', '...', 'BandFrequenciesN' — Multiband response frequenciesvectors

Multiband response frequencies, specified as comma-separated pairs consisting of 'BandFrequenciesi' and a numeric vector. 'BandFrequenciesi', where i runs from 1 through NumBands, is a vector containing the frequencies at which the ith band of a multiband design has the desired values, 'BandAmplitudesi'. NumBands can be at most 10. The frequencies must lie within the Nyquist range and must be specified in monotonically increasing order. Adjacent frequency bands must have the same amplitude at their junction.

Data Types: double

Magnitude Constraints

'PassbandRipple', 'PassbandRipple1', 'PassbandRipple2' — Passband ripple1 (default) | positive scalar

Passband ripple, specified as the comma-separated pair consisting of 'PassbandRipple' and a positive scalar expressed in decibels.

'PassbandRipple1' is the lower-band passband ripple for a bandstop design.

'PassbandRipple2' is the higher-band passband ripple for a bandstop design.

Data Types: double

'StopbandAttenuation', 'StopbandAttenuation1', 'StopbandAttenuation2' — Stopband attenuation60 (default) | positive scalar

Stopband attenuation, specified as the comma-separated pair consisting of 'StopbandAttenuation' and a positive scalar expressed in decibels.

'StopbandAttenuation1' is the lower-band stopband attenuation for a bandpass design.

'StopbandAttenuation2' is the higher-band stopband attenuation for a bandpass design.

Data Types: double

'Amplitudes' — Desired response amplitudesvector

Desired response amplitudes of an arbitrary magnitude response filter, specified as the comma-separated pair consisting of 'Amplitudes' and a vector. Express the amplitudes in linear units. The vector must have the same length as 'Frequencies'.

Data Types: double

'BandAmplitudes1', '...', 'BandAmplitudesN' — Multiband response amplitudesvectors

Multiband response amplitudes, specified as comma-separated pairs consisting of 'BandAmplitudesi' and a numeric vector. 'BandAmplitudesi', where i runs from 1 through NumBands, is a vector containing the desired amplitudes in the ith band of a multiband design. NumBands can be at most 10. Express the amplitudes in linear units. 'BandAmplitudesi' must have the same length as 'BandFrequenciesi'. Adjacent frequency bands must have the same amplitude at their junction.

Data Types: double

Design Method

'DesignMethod' — Design method'butter' | 'cheby1' | 'cheby2' | 'cls' | 'ellip' | 'equiripple' | 'freqsamp' | 'kaiserwin' | 'ls' | 'maxflat' | 'window'

Design method, specified as the comma-separated pair consisting of 'DesignMethod' and a string. The choice of design method depends on the set of frequency and magnitude constraints that you specify.

  • 'butter' designs a Butterworth IIR filter. Butterworth filters have a smooth monotonic frequency response that is maximally flat in the passband. They sacrifice rolloff steepness for flatness.

  • 'cheby1' designs a Chebyshev type I IIR filter. Chebyshev type I filters have a frequency response that is equiripple in the passband and maximally flat in the stopband. Their passband ripple increases with increasing rolloff steepness.

  • 'cheby2' designs a Chebyshev type II IIR filter. Chebyshev type II filters have a frequency response that is maximally flat in the passband and equiripple in the stopband.

  • 'cls' designs an FIR filter using constrained least squares. The method minimizes the discrepancy between a specified arbitrary piecewise-linear function and the filter's magnitude response. At the same time, it lets you set constraints on the passband ripple and stopband attenuation.

  • 'ellip' designs an elliptic IIR filter. Elliptic filters have a frequency response that is equiripple in both passband and stopband.

  • 'equiripple' designs an equiripple FIR filter using the Parks-McClellan algorithm. Equiripple filters have a frequency response that minimizes the maximum ripple magnitude over all bands.

  • 'freqsamp' designs an FIR filter of arbitrary magnitude response by sampling the frequency response uniformly and taking the inverse Fourier transform.

  • 'kaiserwin' designs an FIR filter using the Kaiser window method. The method truncates the impulse response of an ideal filter and uses a Kaiser window to attenuate the resulting truncation oscillations.

  • 'ls' designs an FIR filter using least squares. The method minimizes the discrepancy between a specified arbitrary piecewise-linear function and the filter's magnitude response.

  • 'maxflat' designs a maximally flat FIR filter. These filters have a smooth monotonic frequency response that is maximally flat in the passband.

  • 'window' designs an FIR filter using the windowing method. Window-based designs truncate the impulse response of an ideal filter and use a window function to attenuate the resulting truncation oscillations.

Data Types: char

Design Method Options

'Window' — Window@hamming (default) | vector | string | function handle | cell array

Window, specified as the comma-separated pair consisting of 'Window' and a vector of length N + 1, where N is the filter order. 'Window' can also be paired with a string or function handle that specifies the function used to generate the window. Any such function must take N + 1 as first input. Additional inputs can be passed by specifying a cell array. By default, 'Window' is an empty vector for the 'freqsamp' design method and @hamming for the 'window' design method.

For a list of available windows, see Windows.

Example: 'Window',hann(N+1) and 'Window',(1-cos(2*pi*(0:N)'/N))/2 both specify a Hann window to use with a filter of order N.

Example: 'Window','hamming' specifies a Hamming window of the required order.

Example: 'Window',@mywindow lets you define your own window function.

Example: 'Window',{@kaiser,0.5} specifies a Kaiser window of the required order with shape parameter 0.5.

Data Types: double | char | function_handle | cell

'MatchExactly' — Band to match exactly'stopband' | 'passband' | 'both'

Band to match exactly, specified as the comma-separated pair consisting of 'MatchExactly' and either 'stopband', 'passband', or 'both'. 'both' is available only for the elliptic design method, where it is the default. 'stopband' is the default for the 'butter' and 'cheby2' methods. 'passband' is the default for 'cheby1'.

Data Types: char

'PassbandOffset' — Passband offset0 (default) | positive scalar

Passband offset, specified as the comma-separated pair consisting of 'PassbandOffset' and a positive scalar expressed in decibels. 'PassbandOffset' specifies the filter gain in the passband.

Example: 'PassbandOffset',0 results in a filter with unit gain in the passband.

Example: 'PassbandOffset',2 results in a filter with a passband gain of 2 dB or 1.259.

Data Types: double

'ScalePassband' — Scale passbandtrue (default) | false

Scale passband, specified as the comma-separated pair consisting of 'ScalePassband' and a logical scalar. When you set 'ScalePassband' to true, the passband is scaled, after windowing, so that the filter has unit gain at zero frequency.

Example: 'Window',{@kaiser,0.1},'ScalePassband',true help specify a filter whose magnitude response at zero frequency is exactly 0 dB. This is not the case when you specify 'ScalePassband',false. To verify, visualize the filter with fvtool and zoom in.

Data Types: logical

'ZeroPhase' — Zero phasefalse (default) | true

Zero phase, specified as the comma-separated pair consisting of 'ZeroPhase' and a logical scalar. When you set 'ZeroPhase' to true, the zero-phase response of the resulting filter is always positive. This lets you perform spectral factorization on the result and obtain a minimum-phase filter from it.

Data Types: logical

'PassbandWeight', 'PassbandWeight1', 'PassbandWeight2' — Passband optimization weight1 (default) | positive scalar

Passband optimization weight, specified as the comma-separated pair consisting of 'PassbandWeight' and a positive scalar.

'PassbandWeight1' is the lower-band passband optimization weight for a bandstop FIR design.

'PassbandWeight2' is the higher-band passband optimization weight for a bandstop FIR design.

Data Types: double

'StopbandWeight', 'StopbandWeight1', 'StopbandWeight2' — Stopband optimization weight1 (default) | positive scalar

Stopband optimization weight, specified as the comma-separated pair consisting of 'StopbandWeight' and a positive scalar.

'StopbandWeight1' is the lower-band stopband optimization weight for a bandpass FIR design.

'StopbandWeight2' is the higher-band stopband optimization weight for a bandpass FIR design.

Data Types: double

'Weights' — Optimization weights1 (default) | positive scalar | vector

Optimization weights, specified as the comma-separated pair consisting of 'Weights' and a positive scalar or a vector of the same length as 'Amplitudes'.

Data Types: double

'BandWeights1', '...', 'BandWeightsN' — Multiband weights1 (default) | positive scalar | vectors

Multiband weights, specified as comma-separated pairs consisting of 'BandWeightsi' and a set of positive scalars or of vectors. 'BandWeightsi', where i runs from 1 through NumBands, is a scalar or vector containing the optimization weights of the ith band of a multiband design. If specified as a vector, 'BandWeightsi' must have the same length as 'BandAmplitudesi'.

Data Types: double

Sample Rate

'SampleRate' — Sample rate2 (default) | positive scalar

Sample rate, specified as the comma-separated pair consisting of 'SampleRate' and a positive scalar expressed in hertz. To work with normalized frequencies, set 'SampleRate' to 2, or simply omit it.

Data Types: double

Output Arguments

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d — Digital filterdigitalFilter object

Digital filter, returned as a digitalFilter object.

More About

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Filter Design Assistant

If you specify an incomplete or inconsistent set of design parameters, designfilt offers to open a Filter Design Assistant.

(In the argument description for resp there is a complete list of valid specification sets for all available response types.)

The assistant behaves differently if you call designfilt at the command line or within a script or function.

Filter Design Assistant at the Command Line

You are given a signal sampled at 2 kHz. You are asked to design a lowpass FIR filter that suppresses frequency components higher than 650 Hz. The "cutoff frequency" sounds like a good candidate for a specification parameter. At the MATLAB command line, you type the following.

Fsamp = 2e3;
Fctff = 650;
d = designfilt('lowpassfir','CutoffFrequency',Fctff, ...
               'SampleRate',Fsamp);

Something seems to be amiss because this dialog box appears on your screen.

You click Yes and get a new dialog box that offers to generate code. You see that the variables you defined before have been inserted where expected.

After exploring some of the options offered, you decide to test the corrected filter. You click OK and get the following code on the command line.

dee = designfilt('lowpassfir', 'FilterOrder', 10, ...
                 'CutoffFrequency', Fctff, 'SampleRate', Fsamp);

Typing the name of the filter reiterates the information from the dialog box.

dee
dee = 
 digitalFilter with properties:

    Coefficients: [1x11 double]
   Specifications:
    FrequencyResponse: 'lowpass'
      ImpulseResponse: 'fir'
           SampleRate: 2000
          FilterOrder: 10
      CutoffFrequency: 650
         DesignMethod: 'window'
 Use fvtool to visualize filter
 Use filter function to filter data

You invoke fvtool and get a plot of dee's frequency response.

fvtool(dee)

The cutoff does not look particularly sharp. The response is above 40 dB for most frequencies. You remember that the assistant had an option to set up a "magnitude constraint" called the "stopband attenuation". Open the assistant by calling designfilt with the filter name as input.

designfilt(dee)

Click the Magnitude constraints drop-down menu and select Passband ripple and stopband attenuation. You see that the design method has changed from Window to FIR constrained least-squares. The default value for the attenuation is 60 dB, which is higher than 40. Click OK and visualize the resulting filter.

dee = designfilt('lowpassfir', 'FilterOrder', 10, ...
                 'CutoffFrequency', Fctff, ...
                 'PassbandRipple', 1, 'StopbandAttenuation', 60, ...
                 'SampleRate', Fsamp);
fvtool(dee)

The cutoff still does not look sharp. The attenuation is indeed 60 dB, but for frequencies above 900 Hz.

Again invoke designfilt with your filter as input.

designfilt(dee)

The assistant reappears.

To narrow the distinction between accepted and rejected frequencies, increase the order of the filter or change Frequency constraints from Cutoff (6dB) frequency to Passband and stopband frequencies. If you change the filter order from 10 to 50, you get a sharper filter.

dee = designfilt('lowpassfir', 'FilterOrder', 50, ...
                 'CutoffFrequency', 650, ...
                 'PassbandRipple', 1, 'StopbandAttenuation', 60, ...
                 'SampleRate', 2000);
fvtool(dee)

A little experimentation shows that you can obtain a similar filter by setting the passband and stopband frequencies respectively to 600 Hz and 700 Hz.

dee = designfilt('lowpassfir', 'PassbandFrequency', 600, ...
                 'StopbandFrequency', 700, ...
                 'PassbandRipple', 1, 'StopbandAttenuation', 60, ...
                 'SampleRate', 2000);
fvtool(dee)

Filter Design Assistant in a Script or Function

You are given a signal sampled at 2 kHz. You are asked to design a highpass filter that stops frequencies below 700 Hz. You don't care about the phase of the signal, and you need to work with a low-order filter. Thus an IIR filter seems adequate. You are not sure what filter order is best, so you write a function that accepts the order as input. Open the MATLAB Editor and create the file.

function dataOut = hipassfilt(N,dataIn)
hpFilter = designfilt('highpassiir','FilterOrder',N);
dataOut = filter(hpFilter,dataIn);
end

To test your function, create a signal composed of two sinusoids with frequencies 500 and 800 Hz and generate samples for 0.1 s. A 5th-order filter seems reasonable as an initial guess. Create a script called driveHPfilt.m.

% script driveHPfilt.m
Fsamp = 2e3;
Fsm = 500;
Fbg = 800;
t = 0:1/Fsamp:0.1;
sgin = sin(2*pi*Fsm*t)+sin(2*pi*Fbg*t);
Order = 5;
sgout = hipassfilt(Order,sgin);

When you run the script at the command line, you get an error message.

The error message gives you the choice of opening an assistant to correct the MATLAB code. Click Click here to get the Filter Design Assistant on your screen.

You see the problem: You did not specify the frequency constraint. You also forgot to set a sample rate. After experimenting, you find that you can specify Frequency units as Hz, Passband frequency equal to 700 Hz, and Input Fs equal to 2000 Hz. The Design method changes from Butterworth to Chebyshev type I. You click OK and get the following.

The assistant has correctly identified the file where you call designfilt. Click Yes to accept the change. The function has the corrected MATLAB code.

function dataOut = hipassfilt(N,dataIn)
% hpFilter = designfilt('highpassiir','FilterOrder',N);
hpFilter = designfilt('highpassiir', 'FilterOrder', N, ...
                'PassbandFrequency', 700, 'PassbandRipple', 1, ...
                'SampleRate', 2000);
dataOut = filter(hpFilter,dataIn);
end

You can now run the script with different values of the filter order. Depending on your design constraints, you can change your specification set.

Filter Design Assistant Preferences

You can set designfilt to never offer the Filter Design Assistant. This action sets a MATLAB preference that can be unset with setpref:

  • Use setpref('dontshowmeagain','filterDesignAssistant',false) to be offered the assistant every time. With this command, you can get the assistant again after having disabled it.

  • Use setpref('dontshowmeagain','filterDesignAssistant',true) to disable the assistant permanently. You can also click Do not show this message again in the initial dialog box.

You can set designfilt to always correct faulty specifications without asking. This action sets a MATLAB preference that can be unset by using setpref:

  • Use setpref('dontshowmeagain','filterDesignAssistantCodeCorrection',false) to have designfilt correct your MATLAB code without asking for confirmation. You can also click Always accept in the confirmation dialog box.

  • Use setpref('dontshowmeagain','filterDesignAssistantCodeCorrection',true) to ensure that designfilt corrects your MATLAB code only when you confirm you want the changes. With this command, you can undo the effect of having clicked Always accept in the confirmation dialog box.

Troubleshooting

There are some instances in which, given an invalid set of specifications, designfilt does not offer a Filter Design Assistant, either through a dialog box or through a link in an error message.

  • You are not offered an assistant if you use code-section evaluation, either from the MATLAB Toolstrip or by pressing Ctrl+Enter. (See Divide Your File into Code Sections for more information.)

  • You are not offered an assistant if your code has multiple calls to designfilt, at least one of those calls is incorrect, and

    • You paste the code on the command line and execute it by pressing Enter.

    • You select the code in the Editor and execute it by pressing F9.

  • You are not offered an assistant if you run designfilt using an anonymous function. (See Anonymous Functions for more information.) For example, this input offers an assistant.

    d = designfilt('lowpassfir','CutoffFrequency',0.6)

    This input does not.

    myFilterDesigner = @designfilt;
    d = myFilterDesigner('lowpassfir','CutoffFrequency',0.6)
  • You are not offered an assistant if you run designfilt using eval. For example, this input offers an assistant.

    d = designfilt('lowpassfir','CutoffFrequency',0.6)

    This input does not.

    myFilterDesigner = ...
        sprintf('designfilt(''%s'',''CutoffFrequency'',%f)', ...
                            'lowpassfir',0.6);
    d = eval(myFilterDesigner)

The Filter Design Assistant requires Java® software and the MATLAB desktop to run. It is not supported if you run MATLAB with the -nojvm, -nodisplay, or -nodesktop options.

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