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Discrete-time filter


Hd = dfilt.structure(input1,...)


Hd = dfilt.structure(input1,...) returns a discrete-time filter, Hd, of type structure. Each structure takes one or more inputs. If 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.


Available structures for the dfilt object are shown below. The target block for the block method depends on the filter structure. Depending on the target block, the DSP System Toolbox™ software may be required.



Coefficient Mapping Support in realizemdl

Target Filter Block for block Method



Not supported


Requires DSP System Toolbox


Direct-form I


Discrete Filter


Direct-form I, second-order sections


Discrete Filter

Requires DSP System Toolbox


Direct-form I transposed


Discrete Filter


Direct-form I transposed, second-order sections


Biquad Filter

Requires DSP System Toolbox


Direct-form II


Discrete Filter


Direct-form II, second-order sections


Discrete Filter


Direct-form II transposed


Discrete Filter


Direct-form II transposed, second-order sections


Biquad Filter

Requires DSP System Toolbox


Direct-form FIR


Discrete FIR Filter


Direct-form FIR transposed


Discrete FIR Filter


Direct-form symmetric FIR


Discrete FIR Filter


Direct-form antisymmetric FIR


Discrete FIR Filter


Overlap-add FIR

Not supported

Overlap-Add FFT Filter

Requires DSP System Toolbox


Lattice allpass


Not supported


Lattice autoregressive (AR)


Allpole Filter

Requires DSP System Toolbox


Lattice autoregressive moving- average (ARMA)


Not supported


Lattice moving-average (MA) for maximum phase


Not supported


Lattice moving-average (MA) for minimum phase


Discrete FIR Filter




Not supported


Scalar gain object



Requires DSP System Toolbox


Filters arranged in series


Target blocks depend on filter structures in the series


Filters arranged in parallel


Target blocks depend on filter structures in the parallel system

For more information on each structure, use the syntax help diflt.structure at the MATLAB® prompt or refer to its reference page.


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 below in this way.

    Note   If your variable 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 it is used without outputs.

Some of the methods listed below have the same name as Signal Processing Toolbox™ functions and they behave similarly. This is called overloading of functions.

Available methods are:




Adds a stage to a cascade or parallel object, where a stage is a separate, modular filter. See dfilt.cascade and dfilt.parallel.


block(Hd) creates a Simulink® filter block of the dfilt object. The target filter block depends on the filter structure. You must have Simulink to use this method. Additionally, the DSP System Toolbox may be required depending on the filter structure. See Structures for a mapping between the target blocks and filter structures.

The block method can specify these properties/values:

'MapCoeffstoPorts' indicates whether to map the filter coefficients to constant blocks connected to the generated block. Default value is 'off'. Setting 'MapCoeffstoPorts' to 'on' turns on the mapping and enables the 'CoeffNames' property, which defines the constant block parameter names. 'CoeffNames' is a cell array of character vectors. Default values are {'Num'} for Direct form FIR filters, {'K'} for lattice filters, {'Num','Den'} for IIR filters, and {Num','Den','g'} for biquad filters. Variables, defined by 'CoeffNames', are created in the MATLAB workspace and have the same data type as the filter's 'Arithmetic' property. Any existing variable with the same name is overwritten. Note that you can use either 'Link2Obj' or 'MapCoeffstoPorts', but not both simultaneously.

'InputProcessing' specifies sample-based, 'elementsaschannels', frame-based, 'columnsaschannels', processing, or 'inherited'. The default is frame-based processing. If you do not have the DSP System Toolbox software, explicitly set the 'InputProcessing' property to 'elementsaschannels' to avoid a runtime error. Setting 'InputProcessing' to 'inherited' targets the Digital Filter block regardless of structure.


Returns the series combination of two dfilt objects. See dfilt.cascade.


Returns the filter coefficients in a structure containing fields that use the same property names as those in the original dfilt.


Converts a dfilt object from one filter structure to another filter structure.


Writes a filter coefficient ASCII file. The file can contain a single filter or a vector of objects. Default filename is untitled.fcf.

fcfwrite(Hd,filename) writes to a disk file named filename in the current working directory. The .fcf extension is added automatically.

fcfwrite(...,fmt) writes the coefficients in the format fmt, where fmt can be one of the following:

'hex' for hexadecimal

'dec' for decimal

'bin' for binary representation.


Returns the frequency-domain coefficients used when filtering with a dfilt.fftfir.


Performs filtering using the dfilt object.

y = filter(Hd,x) filters x using the Hd filter and returns the filtered data in y. See Using Filter States for information on using initial conditions. If x is a matrix, each column is filtered as an independent channel. If x is a multidimensional array, filter operates on the first nonsingleton dimension.

y = filter(Hd,x,dim) operates along the dimension dim. If x is a vector or matrix and dim is 1, every column of x is a channel. If dim is 2, every row is a channel.


Returns the type (1-4) of a linear phase FIR filter.


Plots the frequency response in fvtool. Note that unlike the freqz function, this dfilt freqz method has a default length of 8192.


Plots the group delay in fvtool.


Plots the impulse response in fvtool.


Returns the length of the impulse response.


Displays brief dfilt information, such as filter structure, length, stability, linear phase, and, when appropriate, lattice and ladder length. To display detailed information about the design method, options, etc, use info(Hd, 'long'). The default display is 'short'. For multistage filters (cascade and parallel), use info(Hd.Stage(x)), where x is the stage number, to see information about that stage.


Returns a logical 1 (i.e., true) if the dfilt object in an allpass filter or a logical 0 (i.e., false) if it is not.


Returns a logical 1 if the dfilt object is cascaded or a logical 0 if it is not.


Returns a logical 1 if the dfilt object has finite impulse response (FIR) or a logical 0 if it does not.


Returns a logical 1 if the dfilt object is linear phase or a logical 0 if it is not.


Returns a logical 1 if the dfilt object is maximum-phase or a logical 0 if it is not.


Returns a logical 1 if the dfilt object is minimum-phase or a logical 0 if it is not.


Returns a logical 1 if the dfilt object has parallel stages or a logical 0 if it does not.


Returns a logical 1 if the dfilt object has real-valued coefficients or a logical 0 if it does not.


Returns a logical 1 if the dfilt object is a scalar or a logical 0 if it is not scalar.


Returns a logical 1 if the dfilt object has second-order sections or a logical 0 if it does not.


Returns a logical 1 if the dfilt object is stable or a logical 0 if it are not.


Returns the number of sections in a second-order sections filter. If a multistage filter contains stages with multiple sections, using nsections returns the total number of sections in all the stages (a stage with a single section returns 1).


Returns the number of stages of the filter, where a stage is a separate, modular filter.


Returns the number of states for an object.


Returns the filter order. If Hd is a single-stage filter, the order is given by the number of delays needed for a minimum realization of the filter. If Hd has multiple stages, the order is given by the number of delays needed for a minimum realization of the overall filter.


Returns the parallel combination of two dfilt filters. See dfilt.parallel.


Plots the phase response in fvtool.


(Available only with Simulink software.)

realizemdl(Hd) creates a Simulink model containing a subsystem block realization of your dfilt.

realizemdl(Hd,p1,v1,p2,v2,...) creates the block using the properties p1, p2,... and values v1, v2,.. specified.

The following properties are available:

'Blockname' specifies the name of the block. The default value is 'Filter'.

'Destination' specifies whether to add the block to a current Simulink model, create a new model, or place the block in an existing subsystem in your model. Valid values are 'current', 'new', or the name of an existing subsystem in your model. Default value is 'current'.

'OverwriteBlock' specifies whether to overwrite an existing block that was created by realizemdl or create a new block. Valid values are 'on' and 'off' and the default is 'off'. Note that only blocks created by realizemdl are overwritten.

The following properties optimize the block structure. Specifying 'on' turns the optimization on and 'off' creates the block without optimization. The default for each of the following is 'on'.

'OptimizeZeros' removes zero-gain blocks.

'OptimizeOnes' replaces unity-gain blocks with a direct connection.

'OptimizeNegOnes' replaces negative unity-gain blocks with a sign change at the nearest summation block.

'OptimizeDelayChains' replaces cascaded chains of delay block with a single integer delay block set to the appropriate delay.


Removes a stage from a cascade or parallel dfilt. See dfilt.cascade and dfilt.parallel.


Overwrites a stage of a cascade or parallel dfilt. See dfilt.cascade and dfilt.parallel.


Converts the dfilt to a second-order sections dfilt. If Hd has a single section, the returned filter has the same class.

sos(Hd,flag) specifies the ordering of the second-order sections. If flag='UP', the first row contains the poles closest to the origin, and the last row contains the poles closest to the unit circle. If flag='down', the sections are ordered in the opposite direction. The zeros are always paired with the poles closest to them.

sos(Hd,flag,scale) specifies the scaling of the gain and the numerator coefficients of all second-order sections. scale can be 'none', 'inf' (infinity-norm) or 'two' (2-norm). Using infinity-norm scaling with up ordering minimizes the probability of overflow in the realization. Using 2-norm scaling with down ordering minimizes the peak roundoff noise.


Converts the dfilt to state-space. To see the separate A,B,C,D matrices for the state-space model, use [A,B,C,D]=ss(Hd).


Plots the step response in fvtool.

stepz(Hd,n) computes the first n samples of the step response.

stepz(Hd,n,Fs) separates the time samples by T = 1/Fs, where Fs is assumed to be in Hz.


Converts the dfilt to a filter System object. See the reference page for a list of supported objects. To use this method, you must have DSP System Toolbox software installed.


Converts the dfilt to a transfer function.


Plots the zero-phase response in fvtool.


Converts the dfilt to zeros-pole-gain form.


Plots a pole-zero plot in fvtool.

For more information on each method, use the syntax help diflt/method at the MATLAB prompt.

Viewing Properties

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


To see all properties for an object, use


Changing Properties

To set specific properties, use


Note that you must use single quotation marks around the property name.

Alternatively, you can get or set a property value with

b = [0.05 0.9 0.05];
Hd = dfilt.dffir(b);
% Lowpass direct-form I FIR filter
Hd.arithmetic % get arithmetic property
% returns double
Hd.arithmetic = 'single';
% Set arithmetic property to single precision

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 object.

Converting Between Filter Structures

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


where structure_name is any valid structure name in single quotes. If Hd is a cascade or parallel structure, each of its stages 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 filtstate 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 of a previous filtering operation as the initial conditions of the next filtering operation. It also displays information about the filter states.

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


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)

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


or alternatively


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

Introduced before R2006a

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