Decimate signal using polyphase IIR halfband filter
Filtering/Filter Designs
dspfdesign
The IIR Halfband Decimator block performs polyphase decimation of the input signal by a factor of two. To design the halfband filter, you can specify the block to use an elliptic design or a quasilinear phase design. The block uses these design methods to compute the filter coefficients. To filter the inputs, the block uses a polyphase structure. The allpass filters in the polyphase structure are in a minimum multiplier form.
Elliptic design introduces nonlinear phase and creates the filter using fewer coefficients than quasi linear design. Quasilinear phase design overcomes phase nonlinearity at the cost of additional coefficients.
Alternatively, instead of designing the halfband filter using a design method, you can specify the filter coefficients directly. When you choose this option, the allpass filters in the two branches of the polyphase implementation can be in a minimum multiplier form or in a wave digital form.
You can also use the block to implement the analysis portion of a twoband filter bank to filter a signal into lowpass and highpass subbands.
The input signal can be a real or complexvalued column vector or matrix. If the input signal is a matrix, each column of the matrix is treated as an independent channel. The number of rows in the input signal must be a multiple of 2.
Parameters used to design the IIR halfband filter. Because the filter design has only two degrees of freedom, you can specify only two of the three parameters:
Transition width and stopband attenuation
(default)
— Design the filter using Transition width (Hz) and
Stopband attenuation (dB). This design is the minimum order
design.
Filter order and transition width
— Design
the filter using Filter order and Transition width
(Hz).
Filter order and stopband attenuation
—
Design the filter using Filter order and Stopband
attenuation (dB).
Coefficients
— Specify the filter
coefficients directly using the enabled parameters.
Transition width of the IIR halfband filter, specified as a real positive scalar in
Hz. The transition width must be less than half the input sample rate. This parameter
applies when Filter specification is set to Filter
order and transition width
or Transition width and stopband
attenuation
. The default is 4.1e3
.
Filter order, specified as a finite positive integer. If you set Design
method to Elliptic
, then Filter
order must be an odd integer greater than one. If you set Design
method to Quasilinear phase
, then
Filter order must be a multiple of four. This parameter applies
when Filter specification is set to Filter order and
transition width
or Filter order and stopband
attenuation
. The default is 9
.
Minimum attenuation needed in the stopband of the IIR halfband filter, specified as
a real positive scalar in dB. This parameter applies when Filter
specification is set to Filter order and stopband
attenuation
or Transition width and stopband
attenuation
. The default is 80
.
Design method for the IIR halfband filter.
Elliptic
(default) — The filter has
nonlinear phase and uses few coefficients.
Quasilinear phase
— The first branch of the
polyphase filter structure is a pure delay, which results in an approximately
linear phase response.
This parameter applies when you set Filter
specification to any option except
Coefficients
.
Internal allpass filter implementation structure, specified as Minimum
multiplier
or Wave Digital Filter
. This
parameter applies when you set Filter specification to
Coefficients
. Each structure uses a different coefficients
set, independently stored in the corresponding coefficients property. The default is
Minimum multiplier
.
When you select this check box, the first branch of the polyphase filter structure
becomes a pure delay, and the Branch 1 allpass polynomial
coefficients and Branch 1 Wave Digital coefficients
parameters do not apply. This parameter applies when you set Filter
specification to Coefficients
.
By default, this check box is selected.
Length of the first branch delay, specified as a finite positive scalar. This
parameter applies when you set Filter specification to
Coefficients
and select Make the first branch a
pure delay. The default is 1
.
When you select this check box, the branch 1 allpass polynomial coefficients and branch 2 allpass polynomial coefficients are input through the input ports coeffs1 and coeffs2. When you clear this check box, the coefficients are specified on the block dialog through the Branch 1 allpass polynomial coefficients and Branch 2 allpass polynomial coefficients parameters.
This parameter applies when you set Filter specification to
Coefficients
and Internal allpass
structure to Minimum multiplier
.
Allpass polynomial filter coefficients of the first branch, specified as an
Nby1
or
Nby2
matrix of N
firstorder or secondorder allpass sections. This parameter applies only when you set
Filter specification to Coefficients
and Internal allpass structure to Minimum
multiplier
. The default is [0.1284563;
0.7906755
].
This parameter applies when you set Filter specification to
Coefficients
, set Internal allpass
structure to Minimum multiplier
, and clear the
Specify coefficients from input port parameter.
This parameter is tunable. That is, you can change its value during simulation.
Allpass polynomial filter coefficients of the second branch, specified as an
Nby1
or
Nby2
matrix of N
firstorder or secondorder allpass sections. This parameter applies only when you set
Filter specification to Coefficients
and Internal allpass structure to Minimum
multiplier
. The default is 0.4295667
.
This parameter applies when you set Filter specification to
Coefficients
, set Internal allpass
structure to Minimum multiplier
, and clear the
Specify coefficients from input port parameter.
This parameter is tunable. That is, you can change its value during simulation.
Allpass filter coefficients of the first branch in Wave Digital Filter form,
specified as an Nby1
or
Nby2
matrix of N
firstorder or secondorder allpass sections. This parameter applies only when you set
Filter specification to Coefficients
and Internal allpass structure to Wave Digital
Filter
. The default is [0.1284563; 0.7906755
].
This parameter applies when you set Filter specification to
Coefficients
and Internal allpass
structure to Wave Digital Filter
.
Allpass filter coefficients of the second branch in Wave Digital Filter form,
specified as an Nby1
or
Nby2
matrix of N
firstorder or secondorder allpass sections. This parameter applies only when you set
Filter specification to Coefficients
and Internal allpass structure to Wave Digital
Filter
. The default is 0.4295667
.
This parameter applies when you set Filter specification to
Coefficients
and Internal allpass
structure to Wave Digital Filter
.
When you select this check box, the last section of the second branch is treated as
a first order section. This parameter applies only when you set Filter
specification to Coefficients
. When the
coefficients of the second branch are in an Nby2
matrix, the block ignores the second element of the last row of the matrix. The last
section of the second branch then becomes a firstorder section.
When this check box is cleared, the last section of the second branch is treated as
a secondorder section. When the coefficients of the second branch are in an
Nby1
matrix, the block ignores this
parameter.
By default, this check box is cleared.
When you select this check box, the block acts as an analysis filter bank, producing two powercomplementary outputs. When you clear this check box, the block acts as an IIR halfband decimator and accepts a single vector or matrix as input. By default, this check box is cleared.
When you select this check box, the block inherits its sample rate from the input signal. The block calculates the sample rate based on the sample time of the input port. When you clear this check box, you specify the sample rate in Input sample rate (Hz).
This parameter applies when you set Filter specification to any
option except Coefficients
.
Input sample rate, specified as a scalar in Hz. The default is
44100
. You can specify an input sample rate when the
Inherit sample rate from input check box is cleared.
Opens the Filter Visualization Tool FVTool and displays the magnitude/phase response of the IIR Halfband Decimator. The response is based on the block dialog box parameters. Changes made to these parameters update FVTool.
To update the magnitude response while FVTool is running, modify the dialog box parameters and click Apply.
Type of simulation to run. You can set this parameter to:
Code generation
(default)
Simulate model using generated C code. The first time you run a simulation,
Simulink^{®} generates C code for the block. The C code is reused for subsequent
simulations, as long as the model does not change. This option requires additional
startup time but provides faster simulation speed
than Interpreted execution
.
Interpreted execution
Simulate model using the MATLAB^{®} interpreter. This option shortens startup time but has slower
simulation speed than Code generation
.
Port  Supported Data Types 

Input 

Output 

dsp.IIRHalfbandInterpolator  DSP System Toolbox 
dsp.IIRHalfbandDecimator  DSP System Toolbox 
IIR Halfband Interpolator  DSP System Toolbox 
FIR Halfband Interpolator  DSP System Toolbox 
FIR Halfband Decimator  DSP System Toolbox 
This block brings the capabilities of the dsp.IIRHalfbandDecimator
System
object™ to the Simulink environment.
For information on the algorithms used by this block, see the Algorithms section of
dsp.IIRHalfbandDecimator
.