Filter Design Toolbox

mfilt.firdecim

Construct a a direct-form FIR polyphase decimator filter object

Syntax

• hm = mfilt.firdecim(m)
  hm = mfilt.firdecim(m,num)
  

Description

hm = mfilt.firdecim(m) returns a direct-form FIR polyphase decimator object hm with an decimation factor of m. A low-pass Nyquist filter of gain 1 and cutoff frequency of /m is designed by default.

hm = mfilt.firdecim(m,num) uses the coefficients specified by num for the decimation filter. This lets you specify more completely the FIR filter to use for the decimator.

Input Arguments

The following table describes the input arguments for creating hm.

Input Argument	Description
m	Decimation factor for the filter. m specifies the amount to reduce the sampling rate of the input signal. It must be an integer. When you do not specify a value for l it defaults to 2.
num	Vector containing the coefficients of the FIR lowpass filter used for decimation. When num is not provided as an input, mfilt.firdecim constructs a lowpass Nyquist filter with gain equal to 1 and cutoff frequency equal to /m by default.

mfilt.firdecim Object Properties

Every multirate filter object has properties that govern the way it behaves when you use it. Note that many of the properties are also input arguments for creating mfilt.firdecim objects.The next table describes each property for an mfilt.firdecim filter object.

Name	Values	Description
FilterStructure	String	Reports the type of filter object, such as a decimator or fractional integrator. You cannot set this property--it is always read only and results from your choice of mfilt object.
Numerator	Vector	Vector containing the coefficients of the FIR lowpass filter used for interpolation.
DecimationFactor	Integer	Decimation factor for the filter. m specifies the amount to reduce the sampling rate of the input signal. It must be an integer.
NonProcessedSamples	0	Shows the values of the samples that were not processed during the filtering operation. Note that this is not the number of samples that were not processed.
ResetBeforeFiltering	'off' or 'on'	Determine whether the filter states get restored to their starting values for each filtering operation. The starting values are the values in place when you create the filter if you have not changed the filter since you constructed it. ResetBeforeFiltering returns to their default values any state that the filter changes during processing. States that the filter does not change are not affected.
States	Matrix	Stored conditions for the filter, including values for the decimator states.
NumSamplesProcessed	Integer	Returns the number of samples processed during filtering. As a check, the number of samples reported processed plus the number of nonprocessed samples should be the total number of input samples.

Examples

Convert an input signal from 44.1 KHz to 22.05 KHz using decimation by a factor of 2.

• m = 2;                        % Decimation factor
  hm = mfilt.firdecim(m);       % 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 1khz
  y = filter(hm,x);             % 5120 samples, still 0.232 seconds
  stem(n(1:44)/fs,x(1:44))      % Plot original sampled at 44.1 KHz
  hold on                       % Plot decimated signal (22.05 KHz) 
                                % in red
  stem(n(1:22)/(fs/m),y(13:34),'r','filled')
  xlabel('Time (sec)');ylabel('Signal Value')
  

See Also

mfilt.firtdecim, mfilt.firfracdecim, mfilt.cicdecim, mfilt.cicdecimzerolat

mfilt.fftfirinterp

mfilt.firfracdecim