IIR Halfband Decimator

Decimate signal using polyphase IIR halfband filter

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Libraries:
DSP System Toolbox / Filtering / Multirate Filters

Description

The IIR Halfband Decimator block performs polyphase decimation of the input signal by a factor of 2. To design the halfband filter, you can specify the block to use an elliptic design or a quasi-linear 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 the quasi-linear design. Quasi-linear 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 a wave digital form.

You can also use the block to implement the analysis portion of a two-band filter bank to filter a signal into lowpass and highpass subbands. For more details, see Algorithms.

Ports

Input

x — Input signal

column vector | matrix

Specify the input signal as a column vector or a matrix of size P-by-Q. If the input signal is a matrix, the block treats each column of the matrix as an independent channel. The number of rows in the input signal must be a multiple of 2.

This block supports variable-size input signals.

This port is unnamed until you select the Specify coefficients from input port parameter.

Data Types: single | double
Complex Number Support: Yes

coeffs1 — Branch 1 allpass polynomial coefficients

N-by-1 vector | N-by-2 matrix

Specify the allpass polynomial filter coefficients of the first branch as an N-by-1 vector or N-by-2 matrix of N first-order or second-order allpass sections.

Dependencies

To enable this parameter, set:

• Filter specification to Coefficients

• Internal allpass structure to Minimum multiplier

• Clear the Make the first branch a pure delay parameter

• Select the Specify coefficients from input port parameter

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fixed point

coeffs2 — Branch 2 allpass polynomial coefficients

N-by-1 vector | N-by-2 matrix

Specify the allpass polynomial filter coefficients of the second branch as an N-by-1 vector or N-by-2 matrix of N first-order or second-order allpass sections.

Dependencies

To enable this parameter, set:

• Filter specification to Coefficients

• Internal allpass structure to Minimum multiplier

• Select the Specify coefficients from input port parameter

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fixed point

Output

LP — Lowpass subband of decimator output

column vector | matrix

Lowpass subband of the decimator output, returned as a column vector or a matrix of size P/2-by-Q. As the filter is a halfband filter, the downsampling factor is always 2.

This port is unnamed until you select the Output highpass subband parameter.

Data Types: single | double
Complex Number Support: Yes

HP — Highpass subband of decimator output

column vector | matrix

Highpass subband of the decimator output, returned as a column vector or a matrix of size P/2-by-Q. As the filter is a halfband filter, the downsampling factor is always 2.

This port is unnamed until you select the Output highpass subband parameter.

Data Types: single | double
Complex Number Support: Yes

Parameters

Filter specification — Filter design parameters

Transition width and stopband attenuation (default) | Filter order and transition width | Filter order and stopband attenuation | Coefficients

Select the parameters that the block uses 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 (Hz) — Transition width in Hz

4.1e3 (default) | positive real scalar

Specify the transition width of the IIR halfband filter as a real positive scalar in Hz. The transition width must be less than 1/2 the sample rate of the input signal.

Dependencies

To enable this parameter, set Filter specification to Filter order and transition width or Transition width and stopband attenuation.

Filter order — Order of the IIR halfband filter

9 (default) | positive integer

Specify the filter order 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 Quasi-linear phase, then Filter order must be a multiple of four.

Dependencies

To enable this parameter, set Filter specification to Filter order and transition width or Filter order and stopband attenuation.

Stopband attenuation (dB) — Minimum attenuation needed in stopband

80 (default) | positive real scalar

Specify the minimum attenuation needed in the stopband of the IIR halfband filter as a real positive scalar in dB.

Dependencies

To enable this parameter, set Filter specification to Filter order and stopband attenuation or Transition width and stopband attenuation.

Design method — Design method

Elliptic (default) | Quasi-linear phase

Specify the design method for the IIR halfband filter.

• Elliptic — The filter has a nonlinear phase and uses few coefficients.

• Quasi-linear phase — The first branch of the polyphase filter structure is a pure delay, which results in an approximately linear phase response.

Dependencies

To enable this parameter, set Filter specification to any option except Coefficients.

Internal allpass structure — Internal allpass filter implementation structure

Minimum multiplier (default) | Wave Digital Filter

Specify the internal allpass filter implementation structure as Minimum multiplier or Wave Digital Filter. Each structure uses a different coefficients set, independently stored in the corresponding coefficients property.

Dependencies

To enable this parameter, set Filter specification to Coefficients.

Make the first branch a pure delay — Make first branch a pure delay

off (default) | on

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.

By default, this check box is not selected.

Dependencies

To enable this parameter, set Filter specification to Coefficients.

Delay length in samples for branch 1 — Length of delay

1 (default) | positive scalar

Specify the length of the first branch delay as a positive scalar.

Dependencies

To enable this parameter, set:

• Filter specification to Coefficients

• Select the Make the first branch a pure delay parameter

Specify coefficients from input port — Specify coefficients from input port

off (default) | on

When you select this check box, you can input the branch 1 allpass polynomial coefficients and branch 2 allpass polynomial coefficients through the input ports coeffs1 and coeffs2. When you clear this check box, you specify the coefficients in the block dialog box through the Branch 1 allpass polynomial coefficients and Branch 2 allpass polynomial coefficients parameters.

Dependencies

To enable this parameter, set:

• Filter specification to Coefficients

• Internal allpass structure to Minimum multiplier

Branch 1 allpass polynomial coefficients — Allpass polynomial filter coefficients of first branch

[0.1284563; 0.7906755] (default) | N-by-1 | N-by-2

Specify the allpass polynomial filter coefficients of the first branch as an N-by-1 vector or N-by-2 matrix of N first-order or second-order allpass sections.

This parameter is tunable, that is, you can change its value during simulation.

Tunable: Yes

Dependencies

To enable this parameter, set:

• Filter specification to Coefficients

• Internal allpass structure to Minimum multiplier

• Clear the Make the first branch a pure delay parameter

• Clear the Specify coefficients from input port parameter

Branch 2 allpass polynomial coefficients — Allpass polynomial filter coefficients of second branch

[0.4295667] (default) | N-by-1 | N-by-2

Specify the allpass polynomial filter coefficients of the second branch as an N-by-1 vector or N-by-2 matrix of N first-order or second-order allpass sections.

This parameter is tunable, that is, you can change its value during simulation.

Tunable: Yes

Dependencies

To enable this parameter, set:

• Filter specification to Coefficients

• Internal allpass structure to Minimum multiplier

• Clear the Specify coefficients from input port parameter

Branch 1 Wave Digital coefficients — Allpass filter coefficients of first branch in wave digital filter form

[0.1284563; 0.7906755] (default) | N-by-1 | N-by-2

Specify the allpass filter coefficients of the first branch in wave digital filter (WDF) form as an N-by-1 vector or N-by-2 matrix of N first-order or second-order allpass sections.

The magnitude of each WDF coefficient must not be greater than 1.

Dependencies

To enable this parameter, set:

• Filter specification to Coefficients

• Internal allpass structure to Wave Digital Filter

• Clear the Make the first branch a pure delay parameter

Branch 2 Wave Digital coefficients — Allpass filter coefficients of second branch in wave digital filter form

[0.4295667] (default) | N-by-1 | N-by-2

Specify the allpass filter coefficients of the second branch in wave digital filter form as an N-by-1 vector or N-by-2 matrix of N first-order or second-order allpass sections.

The magnitude of each WDF coefficient must not be greater than 1.

Dependencies

To enable this parameter, set:

• Filter specification to Coefficients

• Internal allpass structure to Wave Digital Filter

Last section of branch 2 is first order — Last section of branch 2 is first order

off (default) | on

When you select this check box, the block treats the last section of the second branch as a first order section. When the coefficients of the second branch are in an N-by-2 matrix, the block ignores the second element of the last row of the matrix. The last section of the second branch then becomes a first-order section.

When you clear this check box, the block treats the last section of the second branch as a second-order section. When the coefficients of the second branch are in an N-by-1 matrix, the block ignores this parameter.

Dependencies

To enable this parameter, set Filter specification to Coefficients.

Output highpass subband — Output highpass subband

off (default) | on

When you select this check box, the block acts as an analysis filter bank, producing two power-complementary outputs. When you clear this check box, the block acts as an IIR halfband decimator and accepts a single vector or matrix as input.

Inherit sample rate from input — Inherit sample rate from input signal

off (default) | on

When you select this check box, the block inherits its sample rate from the input signal. When you clear this check box, you specify the sample rate in Input sample rate (Hz).

Input sample rate (Hz) — Sample rate of input signal

44100 (default) | positive real scalar

Specify the sample rate of the input signal as a scalar in Hz.

Dependencies

To enable this parameter, clear the Inherit sample rate from input parameter.

View Filter Response — View Filter Response

button

Click this button to open the Filter Visualization Tool (FVTool) and display the magnitude and phase response of the IIR Halfband Decimator. The response is based on the values you specify in the block parameters dialog box. Changes made to these parameters update FVTool.

To update the magnitude response while FVTool is running, modify the dialog box parameters and click Apply.

Simulate using — Simulate using

Code generation (default) | Interpreted execution

Specify the 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.

Block Characteristics

Data Types	double | single
Direct Feedthrough	no
Multidimensional Signals	no
Variable-Size Signals	yes
Zero-Crossing Detection	no

Algorithms

Polyphase Implementation with Halfband Filters

When you filter your signal, the IIR halfband decimator uses an efficient polyphase implementation for halfband filters. You can use the polyphase implementation to move the downsample operation before filtering. This change enables you to filter at a lower sampling rate.

IIR halfband filters are generally modeled using two parallel allpass filter branches.

$$H(z)=0.5*[A_1(z^2)+z^{-1}{A}_2(z^2)]$$

Elliptic Design

The allpass filters for elliptic IIR halfband filter are given as

$$A_1(z)={\displaystyle \prod _{k=1}^{K_1}\frac{a_k^{(1)}+z^{-1}}{1+a_k^{(1)}{z}^{-1}}}$$

$$A_2(z)={\displaystyle \prod _{k=1}^{K_2}\frac{a_k^{(2)}+z^{-1}}{1+a_k^{(2)}{z}^{-1}}}$$

Quasi-Linear Phase Design

To achieve a near-linear phase response for IIR halfband filters, make one of the branches a pure delay. In this design, the cost of the filter increases.

The allpass filters for the quasi-linear phase IIR halfband filter are

$$A_1(z)=z^{-k}$$

where k is the length of the delay.

$$A_2(z)={\displaystyle \prod _{K=1}^{K_2^{(1)}}\frac{a_k+z^{-1}}{1+a_k{z}^{-1}}}{\displaystyle \prod _{K=1}^{K_2^{(2)}}\frac{c_k+b_k{z}^{-1}+z^{-2}}{1+b_k{z}^{-1}+c_k{z}^{-2}}}$$

where N is the order of the IIR halfband filter.

You can represent filtering the input signal and then downsampling it by 2 using this figure.

Using the multirate noble identity for downsampling, you can move the downsampling operation before the filtering operation. This change enables you to filter at a lower rate.

To implement the halfband decimator efficiently, this algorithm replaces the delay block and downsampling operator with a commutator switch.

Analysis Filter Bank

The transfer function of the complementary highpass filter branch of the analysis filter bank is given by

$$G(z)=0.5*[A_1(z^2)-z^{-1}{A}_2(z^2)]$$

You can represent the analysis filter bank as in this diagram.

The IIR halfband decimator generates two power-complementary output signals by adding and subtracting the two polyphase branch outputs respectively.

To summarize, the IIR halfband decimator:

• Decimates the input prior to filtering.

• Acts as an analysis filter bank.

• Has a nonlinear phase response and uses few coefficients with the elliptic design method.

• Has near-linear phase response at the cost of additional coefficients with the quasi-linear phase design method, where one of the branches is a pure delay

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2015b