Filter Design Toolbox

mfilt

Construct a multirate filter object

Syntax

• hm = mfilt.structure(input1,input2,...)
  

Description

hm = mfilt.structure(input1,input2,...) returns the object hm of type structure. As with dfilt and adaptfilt objects, you must include the structure string to construct a multirate filter object. You can, however, construct a default multirate filter object of a given structure by not including input arguments in your calling syntax.

Multirate filters include decimators and interpolators, and fractional decimators and fractional interpolators, meaning the resulting interpolation or decimation factor is not an integer.

Structures

Each of the following multirate filter structures has a reference page of its own.

Filter Structure String	Description of Resulting Multirate Filter
mfilt.cicdecim	Cascaded integrator-comb decimator
mfilt.cicdecimzerolat	Zero-latency cascaded integrated-comb decimator
mfilt.cicinterp	Cascaded integrator-comb interpolator
mfilt.cicinterpzerolat	Zero-latency cascaded integrator-comb interpolator
mfilt.fftfirinterp	Overlap-add FIR polyphase interpolator
mfilt.firdecim	Direct-form FIR polyphase decimator
mfilt.firfracdecim	Direct-form FIR polyphase fractional decimator
mfilt.firfracinterp	Direct-form FIR polyphase fractional interpolator
mfilt.firinterp	Direct-form FIR polyphase interpolator
mfilt.firsrc	Direct-form FIR polyphase sample rate converter
mfilt.firtdecim	Direct-form transposed FIR polyphase decimator
mfilt.holdinterp	FIR hold interpolator
mfilt.linearinterp	FIR Linear interpolator

Examples

Create an FIR decimator that uses a decimation factor equal to three. In this case, the only input argument needed is m, the decimation factor. Other input arguments are available to you--refer to the reference page for the structure that interests you for more information.

m=3;

• hm=mfilt.firdecim(m)
   
  hm =
   
               FilterStructure: 'Direct-Form FIR Polyphase Decimator'
                     Numerator: [1x73 double]
              DecimationFactor: 3
           NonProcessedSamples: []
      NumberOfSamplesProcessed: 0
                   ResetStates: 'on'
                        States: [72x1 double]
  

To demonstrate a few of the methods that apply to multirate filters, here are two examples of using hm, your FIR decimator.

Use the Filter Visualization tool to review the magnitude response of your decimator.

Now check to see if your filter is stable.

• isstable(hm)
  
  ans =
  
       1
  

Finally, pass a signal through the filter to see if it indeed decimates by three.

• m = 3;                        % Decimation factor
  hm = mfilt.firdecim(m);       % We use the default filter
  fs = 44.1e3;                  % Original sampling frequency: 44.1kHz
  n = 0:10239;                  % 10240 samples, 0.232 second long 
                                % signal
  x  = sin(2*pi*1e3/fs*n);      % Original signal, sinusoid at 1 KHz
  y = filter(hm,x);            % 5120 samples, still 0.232 seconds
  stem(n(1:44)/fs,x(1:44))     % Plot original sampled at 44.1kHz 
  hold on                       % Plot decimated signal (22.05kHz) in red
  stem(n(1:22)/(fs/m),y(13:34),'r','filled')
  xlabel('Time (sec)');ylabel('Signal Value')
  

Here is the stem plot that shows the result of the decimation process.

• hm =
  

•          FilterStructure: 'Direct-Form FIR Polyphase Decimator'
                 Numerator: [1x73 double]
          DecimationFactor: 3
       NonProcessedSamples: 0.8963
      ResetBeforeFiltering: 'on'
                    States: [72x1 double]
       NumSamplesProcessed: 10239
  

Notice that the filter processed 10239 samples with 1 nonprocessed sample whose value is 0.8963. One nonprocessed sample results from dividing the number of samples, 10240, by the decimation factor, 3, to get 3413 output samples and one left over.

See Also

mfilt.firfracdecim,mfilt.firfracinterp,mfilt.firinterp,mfilt.firsrc,mfilt.firtdecim

maxstep

mfilt.cicdecim