Filter Design Toolbox

mfilt.firinterp

Construct an FIR filter-based interpolator mfilt object

Syntax

• hm = mfilt.firinterp(l)
  hm = MFILT.FIRINTERP(l,num)
  

Description

hm = mfilt.firinterp(l) returns an FIR-based interpolator object hm with an interpolation factor of l. A low-pass Nyquist filter of gain l and cutoff frequency of /l is designed by default when you do not include l as an input.

hm = mfilt.firinterp(l,num) uses the coefficients specified by num for the numerator coefficients of the interpolation filter.

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, firinterp uses a lowpass Nyquist filter with gain equal to l and cutoff frequency equal to /l by default.

mfilt.firinterp 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.firinterp objects.The next table describes each property for an mfilt.firinterp 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	double vector	Vector containing the coefficients of the FIR lowpass filter used for interpolation
InterpolationFactor	Positive integers	Interpolation factor for the filter. l specifies the amount to increase the input sampling rate
ResetBeforeFiltering	'off' or 'on'	Determine whether the filter states get restored to zero for each filtering operation
States	Double array	Filter states. states defaults to a vector of zeros that has length equal to nstates(hm)
NumSamplesProcessed	Integer	Returns the number of samples processed during filtering

Examples

In this example, use mfilt.firinterp to double the sample rate of a 22.05 KHz input signal. The output signal ends up at 44.1 KHz. Although l is set explicitly to 2, this represents the default value for mfilt.firinterp objects.

• l = 2;                       % Interpolation factor
  hm = mfilt.firinterp(l);     % We use the default filter
  fs = 22.05e3;                % Original sampling frequency: 22.05 KHz
  n = 0:5119;                   % 5120 samples, 0.232 second long signal
  x  = sin(2*pi*1e3/fs*n);      % Original signal, sinusoid at 1 KHz
  y = filter(hm,x);             % 10240 samples, still 0.232 seconds
  stem(n(1:22)/fs,x(1:22),'filled') % Plot original sampled at 
                                    % 22.05 KHz 
  hold on;
  
  % Plot interpolated signal (44.1 KHz) in red
  stem(n(1:44)/(fs*l),y(25:68),'r')
  xlabel('Time (sec)');ylabel('Signal Value')
  

See Also

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

mfilt.firfracinterp

mfilt.firsrc