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Octave Filter

Design octave filter

    Note:   The Octave Filter block will be removed from DSP System Toolbox™ in a future release. Existing instances of the block continue to run. For new code, use the Octave Filter block from Audio System Toolbox™ instead.


Filtering / Filter Designs



This block brings the filter design capabilities of the filterBuilder function to the Simulink® environment.

Dialog Box

See Octave Filter Design Dialog Box — Main Pane for more information about the parameters of this block. The Data Types and Code Generation panes are not available for blocks in the DSP System Toolbox Filter Designs library.

Parameters of this block that do not change filter order or structure are tunable.

View filter response

This button opens the Filter Visualization Tool (fvtool) from the Signal Processing Toolbox™ product. You can use the tool to display:

  • Magnitude response, phase response, and group delay in the frequency domain.

  • Impulse response and step response in the time domain.

  • Pole-zero information.

The tool also helps you evaluate filter performance by providing information about filter order, stability, and phase linearity. For more information on FVTool, see the Signal Processing Toolbox documentation.

Filter Specifications


Specify filter order. Possible values are: 4, 6, 8, 10.

Bands per octave

Specify the number of bands per octave. Possible values are: 1, 3, 6, 12, 24.

Frequency units

Specify frequency units as Hz or kHz.

Input Fs

Specify the input sampling frequency in the frequency units specified previously.

Center Frequency

Select from the drop-down list of available center frequency values.


Design Method

Butterworth is the design method used for this type of filter.

Scale SOS filter coefficients to reduce chance of overflow

Select the check box to scale the filter coefficients.

Filter Implementation


Specify filter structure. Choose from:

  • Direct-form I SOS

  • Direct-form II SOS

  • Direct-form I transposed SOS

  • Direct-form II transposed SOS

Use basic elements to enable filter customization

Select this check box to implement the filter as a subsystem of basic Simulink blocks. Clear the check box to implement the filter as a high-level subsystem. By default, this check box is cleared.

The high-level implementation provides better compatibility across various filter structures, especially filters that would contain algebraic loops when constructed using basic elements. On the other hand, using basic elements enables the following optimization parameters:

  • Optimize for zero gains — Terminate chains that contain Gain blocks with a gain of zero.

  • Optimize for unit gains — Remove Gain blocks that scale by a factor of one.

  • Optimize for delay chains — Substitute delay chains made up of n unit delays with a single delay by n.

  • Optimize for negative gains — Use subtraction in Sum blocks instead of negative gains in Gain blocks.

Optimize for unit-scale values

Select this check box to scale unit gains between sections in SOS filters. This parameter is available only for SOS filters.

Input processing

Specify how the block should process the input. The available options may vary depending on he settings of the Filter Structure and Use basic elements for filter customization parameters. You can set this parameter to one of the following options:

  • Columns as channels (frame based) — When you select this option, the block treats each column of the input as a separate channel.

  • Elements as channels (sample based) — When you select this option, the block treats each element of the input as a separate channel.

    Note:   The Inherited (this choice will be removed — see release notes) option will be removed in a future release. See Frame-Based Processing in the DSP System Toolbox Release Notes for more information.

Rate options

When the Filter type parameter specifies a multirate filter, select the rate processing rule for the block from following options:

  • Enforce single-rate processing — When you select this option, the block maintains the sample rate of the input.

  • Allow multirate processing — When you select this option, the block adjusts the rate at the output to accommodate an increased or reduced number of samples. To select this option, you must set the Input processing parameter to Elements as channels (sample based).

Use symbolic names for coefficients

Select this check box to enable the specification of coefficients using MATLAB® variables. The available coefficient names differ depending on the filter structure. Using symbolic names allows tuning of filter coefficients in generated code. By default, this check box is cleared.

Supported Data Types

PortSupported Data Types


  • Double-precision floating point

  • Single-precision floating point


  • Double-precision floating point

  • Single-precision floating point

Introduced in R2007a

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