Filter Design Toolbox

mfilt.fftfirinterp

Construct a an overlap-add FIR polyphase interpolator filter object

Syntax

• hm = mfilt.fftfirinterp(l,num,bl)
  

Description

hm = mfilt.fftfirinterp(l,num,bl) returns a discrete-time FIR filter object that uses the overlap-add method for filtering input data.

The number of FFT points is given by [bl+ceil(length(num)/l)-1]. It is to your advantage to choose bl such that the number of FFT points is a power of two--using powers of two can improve the efficiency of the FFT and the associated interpolation process.

Input Arguments

The following table describes the input arguments for creating hm.

Input Argument	Description
l	Interpolation factor for the filter. l specifies the amount to increase the input sampling rate. 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 interpolation. When num is not provided as an input, fftfirinterp uses a lowpass Nyquist filter with gain equal to l and cutoff frequency equal to /l by default.
bl	Length of each block of input data used in the filtering. bl must be an integer. When you omit input bl, it defaults to 100

mfilt.fftfirinterp 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.fftfirinterp objects.The next table describes each property for an mfilt.fftfirinterp filter object.

Name	Values	Description
FilterStructure		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 containing the coefficients of the FIR lowpass filter used for interpolation.
InterpolationFactor		Interpolation factor for the filter. It specifies the amount to increase the input sampling rate. It must be an integer.
BlockLength		Length of each block of input data used in the filtering.
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 zero any state that the filter changes during processing. States that the filter does not change are not affected.
States		Stored conditions for the filter, including values for the interpolator states.
NumSamplesProcessed		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

Interpolation by a factor of 8. Notice that this object removes the spectral replicas in the signal after interpolation.

• l = 8;                        % Interpolation factor
  hm = mfilt.fftfirinterp(l);   % We use the default filter
  n = 8192;                     % Number of points
  hm.blocklength = n;           % Set block length to number of points
  fs = 44.1e3;                  % Original sampling frequency: 44.1 KHz
  n = 0:n-1;                    % 0.1858 secs of data
  x = sin(2*pi*n*22e3/fs);      % Original signal, sinusoid at 22 
  KHz
  y = filter(hm,x);             % Interpolated sinusoid
  xu = l*upsample(x,8);         % Upsample to compare--the spectrum 
                                % does not change
  [px,f]=periodogram(xu,[],65536,l*fs);% Power spectrum of 
  original                                      % signal
  [py,f]=periodogram(y,[],65536,l*fs); % Power spectrum of 
                                       % interpolated signal
  plot(f,10*log10(([fs*px,l*fs*py])))
  legend('22  KHz sinusoid sampled at 44.1  KHz',...
  '22  KHz sinusoid sampled at 352.8  KHz')
  xlabel('Frequency (Hz)'); ylabel('Power Spectrum');
  

See Also

mfilt.firinterp, mfilt.holdinterp, mfilt.linearinterp, mfilt.firfracinterp, mfilt.cicinterp, mfilt.cicinterpzerolat

mfilt.cicinterpzerolat

mfilt.firdecim