FIR filterbased interpolator
Hm = mfilt.firinterp(L)
Hm = mfilt.firinterp(L,num)
Hm = mfilt.firinterp(L)
returns
a FIR polyphase interpolator object Hm
with an
interpolation factor of L
and gain equal to L
. L
defaults
to 2 if unspecified.
Hm = mfilt.firinterp(L,num)
uses
the values in the vector num
as the coefficients
of the interpolation filter.
Make this filter a fixedpoint or singleprecision filter by
changing the value of the Arithmetic
property
for the filter Hm
as follows:
To change to singleprecision filtering, enter
set(hm,'arithmetic','single');
To change to fixedpoint filtering, enter
set(hm,'arithmetic','fixed');
The following table describes the input arguments for creating hm
.
Input Argument  Description 

 Interpolation factor for the filter. 
 Vector containing the coefficients of the FIR lowpass
filter used for interpolation. When 
This section describes the properties for both floatingpoint filters (doubleprecision and singleprecision) and fixedpoint filters.
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 


 Defines the arithmetic the filter uses. Gives you the
options 
 String  Reports the type of filter object. You cannot set this
property — it is always read only and results from your choice
of 
 Integer  Interpolation factor for the filter. 
 Vector  Vector containing the coefficients of the FIR lowpass filter used for decimation. 

 Determines whether the filter states get restored to
zeros 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. 

 Contains the filter states before, during, and after filter operations. States act as filter memory between filtering runs or sessions. 
This table shows the properties associated with the fixedpoint
implementation of the mfilt.firinterp
filter.
Note The table lists all of the properties that a fixedpoint filter can have. Many of the properties listed are dynamic, meaning they exist only in response to the settings of other properties. To view all of the characteristics for a filter at any time, use info(hm) where 
For further information about the properties of this filter
or any mfilt
object, refer to Multirate Filter Properties.
Name  Values  Description 

 Any positive or negative integer number of bits. [32]  Specifies the fraction length used to interpret data
output by the accumulator. This is a property of FIR filters and lattice
filters. IIR filters have two similar properties — 
 Any integer number of bits[39]  Sets the word length used to store data in the accumulator. 
 fixed for fixedpoint filters  Setting this to 
 [true], false  Specifies whether the filter automatically chooses the
proper fraction length to represent filter coefficients without overflowing.
Turning this off by setting the value to 
 Any integer number of bits [16]  Specifies the word length to apply to filter coefficients. 
 [FullPrecision], SpecifyPrecision  Controls whether the filter automatically sets the output
word and fraction lengths, product word and fraction lengths, and
the accumulator word and fraction lengths to maintain the best precision
results during filtering. The default value, 
 Any positive or negative integer number of bits [15]  Specifies the fraction length the filter uses to interpret input data. 
 Any integer number of bits [16]  Specifies the word length applied to interpret input data. 
 Any positive or negative integer number of bits [  Sets the fraction length used to interpret the numerator coefficients. 
 Any positive or negative integer number of bits [32]  Determines how the filter interprets the filter output
data. You can change the value of 
 Any integer number of bits [39]  Determines the word length used for the output data.
You make this property editable by setting 
 saturate, [wrap]  Sets the mode used to respond to overflow conditions
in fixedpoint arithmetic. Choose from either 
 [  Sets the mode the filter uses to quantize numeric values when the values lie between representable values for the data format (word and fraction lengths).
The choice you make affects only the accumulator and output arithmetic. Coefficient and input arithmetic always round. Finally, products never overflow — they maintain full precision. 
 [true], false  Specifies whether the filter uses signed or unsigned fixedpoint coefficients. Only coefficients reflect this property setting. 

 Contains the filter states before, during, and after
filter operations. States act as filter memory between filtering runs
or sessions. The states use 
To provide interpolation, mfilt.firinterp
uses
the following structure.
The following figure details the signal flow for the direct
form FIR filter implemented by mfilt.firinterp
.
In the figure, the delay line updates happen at the lower input rate.
The remainder of the filter — the sums and coefficients —
operate at the higher output rate.
This example uses 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 interpolation value for mfilt.firinterp
objects.
L = 2; % Interpolation factor. Hm = mfilt.firinterp(L); % Use the default filter. fs = 22.05e3; % Original sample freq: 22.05 kHz. n = 0:5119; % 5120 samples, 0.232s long signal. x = sin(2*pi*1e3/fs*n); % Original signal, sinusoid at 1 kHz. y = filter(Hm,x); % 10240 samples, still 0.232s. 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') legend('Original Signal','Interpolated Signal');
With interpolation by 2, the resulting signal perfectly matches the original, but with twice as many samples — one between each original sample, as shown in the following figure.
mfilt.cicinterp
 mfilt.fftfirinterp
 mfilt.firsrc
 mfilt.holdinterp
 mfilt.linearinterp