mfilt.firdecim
Directform FIR polyphase decimator
mfilt.firdecim
has been removed. Use dsp.FIRDecimator
instead.
Syntax
hm = mfilt.firdecim(m)
hm = mfilt.firdecim(m,num)
Description
hm = mfilt.firdecim(m)
returns a directform
FIR polyphase decimator object hm
with a decimation factor of
m
. A lowpass Nyquist filter of gain 1 and cutoff frequency of
π/m
is designed by default. This filter allows some aliasing in
the transition band but it very efficient because the first polyphase component is a
pure delay.
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.
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');
Input Arguments
The following table describes the input arguments for creating
hm
.
Input Argument  Description 

 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

 Vector containing the coefficients of the FIR lowpass
filter used for decimation. When 
Object Properties
This section describes the properties for both floatingpoint filters (doubleprecision and singleprecision) and fixedpoint filters.
FloatingPoint Filter 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 


 Defines the arithmetic the filter uses. Gives you the
options 
 Integer  Decimation factor for the filter. 
 Character vector  Reports the type of filter object. You cannot set this
property — it is always read only and results from your
choice of 
 Integers  Contains a value derived from the number of input samples
and the decimation factor — 
 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.

 0 in  Differentiates between the adders in the filter that work
in full precision at all times
( 

 This property contains the filter states before, during, and after filter operations. States act as filter memory between filtering runs or sessions. Double is the default setting for floatingpoint filters in double arithmetic. 
FixedPoint Filter Properties
This table shows the properties associated with the fixedpoint implementation of
the filter. You see one or more of these properties when you set Arithmetic to
fixed. Some of the properties have different default values when they refer fixed
point filters. One example is the property PolyphaseAccum
which
stores data as doubles when you use your filter in doubleprecision mode, but stores
a fi
object in fixedpoint mode.
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 hm
is a
filter.
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. 
 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 [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. 

 This property contains the filter states before, during,
and after filter operations. States act as filter memory between
filtering runs or sessions. The states use 
Filter Structure
To provide decimation, mfilt.firdecim
uses the following structure.
At the input you see a commutator that operates counterclockwise, moving from position 0
to position 2, position 1, and back to position 0 as input samples enter the
filter.
The following figure details the signal flow for the direct form FIR filter
implemented by mfilt.firdecim
.
Notice the order of the states in the filter flow diagram. States 1 through 9 appear
in the diagram above each delay element. State 1 applies to the first delay element in
phase 2. State 2 applies to the first delay element in phase 1. State 3 applies to the
first delay element in phase 0. State 4 applies to the second delay in phase 2, and so
on. When you provide the states for the filter as a vector to the
States
property, the above description explains how the filter
assigns the states you specify.
In property value form, the states for a filter hm
are
hm.states=[1:9];
Examples
Convert an input signal from 44.1 kHz to 22.05 kHz using decimation by a factor of 2. In the figure that appears after the example code, you see the results of the decimation.
m = 2; % Decimation factor. hm = mfilt.firdecim(m); % Use the default filter. fs = 44.1e3; % Original sample freq: 44.1kHz. n = 0:10239; % 10240 samples, 0.232 second long % signal. x = sin(2*pi*1e3/fs*n); % Original signalsinusoid at 1kHz. y = filter(hm,x); % 5120 samples, 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')