FIR Decimation

Filter and downsample input signals

Library

Filtering / Multirate Filters

dspmlti4

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Description

The FIR Decimation block resamples the discrete-time input at a rate K times slower than the input sample rate, where K is the integer value you specify for the Decimation factor parameter. To do so, the block implements a polyphase filter structure and performs the following operations:

1. Filters the data in each channel of the input using a direct-form FIR filter.

2. Downsamples each channel of filtered data by discarding K–1 consecutive samples following each sample that is retained.

The block uses a polyphase filter implementation because it is more efficient than straightforward filter-then-decimate algorithms. See Fliege [1] for more information.

You can use the FIR Decimation block inside triggered subsystems when you set the Rate options parameter to Enforce single-rate processing.

This block supports variable-size input. This means that while the block is simulating, the frame size (number of rows) can change. The output dimensions always equal those of the input signal.

Under specific conditions, this block also supports SIMD code generation. For details, see Code Generation.

Specifying the Filter Coefficients

To specify the filter coefficients, select the mode you want the FIR Decimation block to operate in. Select the mode in the Coefficient source group box.

• Dialog parameters — Enter information about the filter, such as structure and coefficients, in the block dialog box.

• Input port — Specify the filter coefficients as an input to the block. Coefficient values are tunable (can change during simulation), while their properties must remain constant.

• Filter object — Specify the filter using a dsp.FIRDecimator System object™.

• Auto (default) — Choose the filter coefficients of an FIR Nyquist filter, predesigned for the decimation factor specified in the block dialog box.

When you select Dialog parameters, you use the FIR filter coefficients parameter to specify the numerator coefficients of the FIR filter transfer function H(z).

$$H(z)=B(z)=b_1+b_2{z}^{-1}+\dots +b_m{z}^{-(m-1)}$$

You can generate the FIR filter coefficient vector, [b(1) b(2) ... b(m)], using one of the DSP System Toolbox™ filter design functions such as designMultirateFIR, firnyquist, firhalfband, firgr, or firceqrip.

The filter you specify must be a lowpass filter with a normalized cutoff frequency no greater than 1/K. The block internally initializes all filter states to zero.

When you select Auto, the block designs an FIR decimator with the decimation factor specified in Decimation factor. The designMultirateFIR function designs the filter and returns the coefficients used by the block. For more information on the filter design, see Orfanidis [2].

Frame-Based Processing

When you set the Input processing parameter to Columns as channels (frame based), the block resamples each column of the input over time. In this mode, the block can perform either single-rate or multirate processing. You can use the Rate options parameter to specify how the block resamples the input:

• When you set the Rate options parameter to Enforce single-rate processing, the input and output of the block have the same sample rate. To decimate the output while maintaining the input sample rate, the block resamples the data in each column of the input such that the frame size of the output (M_o) is 1/K times that of the input (M_o = M_i/K),

  In this mode, the input frame size, M_i, must be a multiple of the Decimation factor, K.

  For an example of single-rate FIR Decimation, see Example 1 — Single-Rate Processing.

• When you set the Rate options parameter to Allow multirate processing, the input and output of the FIR Decimation block are the same size, but the sample rate of the output is K times slower than that of the input. In this mode, the block treats an M_i-by-N matrix input as N independent channels. The block decimates each column of the input over time by keeping the frame size constant (M_i=M_o), and making the output frame period (T_fo) K times longer than the input frame period (T_fo = K*T_fi).

  See Example 2— Multirate Frame-Based Processing for an example that uses the FIR Decimation block in this mode.

Sample-Based Processing

When you set the Input processing parameter to Elements as channels (sample based), the block treats an M-by-N matrix input as M*N independent channels, and decimates each channel over time. The output sample period (T_so) is K times longer than the input sample period (T_so = K*T_si), and the input and output sizes are identical.

Latency

When you use the FIR Decimation block in sample-based processing mode, the block always has zero-tasking latency. Zero-tasking latency means that the block propagates the first filtered input sample (received at time t= 0) as the first output sample. That first output sample is then followed by filtered input samples K+1, 2K+1, and so on.

When you use the FIR Decimation block in frame-based processing mode with a frame size greater than one, the block may exhibit one-frame latency. Cases of one-frame latency can occur when the input frame size is greater than one, and you set the Input processing and Rate options parameters of the FIR Decimation block as follows:

• Input processing = Columns as channels (frame based)

• Rate options = Allow multirate processing

In cases of one-frame latency, you can define the value of the first M_i output rows by setting the Output buffer initial conditions parameter. The default value of the Output buffer initial conditions parameter is 0. However, you can enter a matrix containing one value for each channel of the input, or a scalar value to be applied to all channels. The first filtered input sample (first filtered row of the input matrix) appears in the output as sample M_i+ 1. That sample is then followed by filtered input samples K+ 1, 2K+ 1, and so on.

Note

For more information on latency and the Simulink^® tasking modes, see Excess Algorithmic Delay (Tasking Latency) and Time-Based Scheduling and Code Generation (Simulink Coder).

Fixed-Point Data Types

The following diagram shows the data types used within the FIR Decimation block for fixed-point signals.

You can set the coefficient, product output, accumulator, and output data types in the block dialog box as discussed in the Dialog Box section. This diagram shows that data is stored in the input buffer with the same data type and scaling as the input. The block stores filtered data and any initial conditions in the output buffer using the output data type and scaling that you set in the block dialog box.

When at least one of the inputs to the multiplier is real, the output of the multiplier is in the product output data type. When both inputs to the multiplier are complex, the result of the multiplication is in the accumulator data type. For details on the complex multiplication performed by this block, see Multiplication Data Types.

Note

When the block input is fixed point, all internal data types are signed fixed-point values.

Examples

Example 1 — Single-Rate Processing

In the ex_firdecimation_ref2 model, the FIR Decimation block decimates a single-channel input with a frame size of 64. Because the block is doing single-rate processing and the Decimation factor parameter is set to 4, the output of the FIR Decimation block has a frame size of 16. As shown in the following figure, the input, and output of the FIR Decimation block have the same sample rate.

Example 2— Multirate Frame-Based Processing

In the ex_firdecimation_ref1 model, the FIR Decimation block decimates a single-channel input with a frame period of one second. Because the block is doing multirate frame-based processing and the Decimation factor parameter is set to 4, the frame period of the output is 4 seconds. As shown in the following figure, the input, and output of the FIR Decimation block have the same frame size, but the sample rate of the output is four times that of the input.

Example 3

The ex_polyphasedec model illustrates the underlying polyphase implementations of the FIR Decimation block. Run the model, and view the results on the scope. The output of the FIR Decimation block matches the output of the Polyphase Decimation Filter block.

Example 4

The ex_mrf_nlp model illustrates the use of the FIR Decimation block in several multistage multirate filters.

Dialog Box

Coefficient Source

The FIR Decimation block can operate in four different modes. Select the mode in the Coefficient source group box.

• Dialog parameters — Enter information about the filter, such as structure and coefficients, in the block mask.

• Input port — Specify the filter coefficients with a Num input port. The Num input port appears when you select the Input port option. Coefficient values obtained through Num are tunable (can change during simulation), while their properties must remain constant.

• Filter object — Specify the filter using a dsp.FIRDecimator System object.

• Auto (default) — Choose the coefficients of an FIR Nyquist filter, predesigned for the decimation factor specified in the block dialog box.

Different items appear on the FIR Decimation block dialog box depending on whether you select Dialog parameters, Input port, Filter object, or Auto in the Coefficient source group box.

 Specify Filter Characteristics in Dialog Box

The Main pane of the FIR Decimation block dialog box appears as follows when you select Dialog parameters in the Coefficient source group box.

FIR filter coefficients

Specify the lowpass FIR filter coefficients, in descending powers of z. By default, designMultirateFIR(1,2) computes the filter coefficients.

Decimation factor

Specify the integer factor, K, by which to decrease the sample rate of the input sequence. The default is 2.

Filter Structure

Choose whether to implement a Direct form (default) or Direct form transposed filter.

Input processing

Specify how the block should process the input. You can set this parameter to one of the following options:

• Columns as channels (frame based) (default) — 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.

Rate options

Specify the method by which the block should decimate the input. You can select one of the following options:

• Enforce single-rate processing (default) — When you select this option, the block maintains the input sample rate and decimates the signal by decreasing the output frame size by a factor of K. To select this option, you must set the Input processing parameter to Columns as channels (frame based).

• Allow multirate processing — When you select this option, the block decimates the signal such that the output sample rate is K times slower than the input sample rate.

Output buffer initial conditions

In the case of one-frame latency, this parameter specifies the output of the block until the first filtered input sample is available. The default value of this parameter is 0, but you can enter a matrix containing one value for each channel, or a scalar value to be applied to all signal channels. This parameter appears only when you configure the block to perform multirate processing.

See Latency for more information about latency in the FIR Decimation block.

View filter response

This button opens the Filter Visualization Tool (fvtool) from the Signal Processing Toolbox™ product and displays the filter response of the filter defined in the block. For more information on FVTool, see the Signal Processing Toolbox documentation.

The Data Types pane of the FIR Decimation block dialog box appears as follows when you select Dialog parameters in the Coefficient source group box.

Rounding mode

Select the rounding mode for fixed-point operations. The default is Floor. The filter coefficients do not obey this parameter; they always round to Nearest.

Note

The Rounding mode and Saturate on integer overflow settings have no effect on numerical results when all the following conditions exist:

• Product output is Inherit: Inherit via internal rule

• Accumulator is Inherit: Inherit via internal rule

• Output is Inherit: Same as accumulator

With these data type settings, the block is effectively operating in full precision mode.

Saturate on integer overflow

When you select this parameter, the block saturates the result of its fixed-point operation. When you clear this parameter, the block wraps the result of its fixed-point operation. For details on saturate and wrap, see overflow mode for fixed-point operations.

Note

The Rounding mode and Saturate on integer overflow parameters have no effect on numeric results when all these conditions are met:

• Product output data type is Inherit: Inherit via internal rule.

• Accumulator data type is Inherit: Inherit via internal rule.

With these data type settings, the block operates in full-precision mode.

Coefficients

Specify the coefficients data type. See Fixed-Point Data Types and Multiplication Data Types for illustrations depicting the use of the coefficients data type in this block. You can set it to:

• A rule that inherits a data type, for example, Inherit: Same word length as input

• An expression that evaluates to a valid data type, for example, fixdt(1,16,0)

Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Coefficients parameter.

See Specify Data Types Using Data Type Assistant (Simulink) for more information.

Coefficients Minimum

Specify the minimum value of the filter coefficients. The default value is [] (unspecified). Simulink software uses this value to perform:

• Automatic scaling of fixed-point data types

Coefficients Maximum

Specify the maximum value of the filter coefficients. The default value is [] (unspecified). Simulink software uses this value to perform:

• Automatic scaling of fixed-point data types

Product output

Specify the product output data type. See Fixed-Point Data Types and Multiplication Data Types for illustrations depicting the use of the product output data type in this block. You can set it to:

• A rule that inherits a data type, for example, Inherit: Inherit via internal rule. For more information on this rule, see Inherit via Internal Rule.

• An expression that evaluates to a valid data type, for example, fixdt(1,16,0)

Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Product output parameter.

See Specify Data Types Using Data Type Assistant (Simulink) for more information.

Accumulator

Specify the accumulator data type. See Fixed-Point Data Types for illustrations depicting the use of the accumulator data type in this block. You can set this parameter to:

• A rule that inherits a data type, for example, Inherit: Inherit via internal rule. For more information on this rule, see Inherit via Internal Rule.

• An expression that evaluates to a valid data type, for example, fixdt(1,16,0)

Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Accumulator parameter.

See Specify Data Types Using Data Type Assistant (Simulink) for more information.

Output

Specify the output data type. See Fixed-Point Data Types for illustrations depicting the use of the output data type in this block. You can set it to:

• A rule that inherits a data type, for example, Inherit: Same as accumulator

• An expression that evaluates to a valid data type, for example, fixdt(1,16,0)

Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Output parameter.

See Control Signal Data Types (Simulink) for more information.

Output Minimum

Specify the minimum value that the block should output. The default value is [] (unspecified). Simulink software uses this value to perform:

• Simulation range checking (see Specify Signal Ranges (Simulink))

• Automatic scaling of fixed-point data types

Output Maximum

Specify the maximum value that the block should output. The default value is [] (unspecified). Simulink software uses this value to perform:

• Simulation range checking (see Specify Signal Ranges (Simulink))

• Automatic scaling of fixed-point data types

Lock data type settings against changes by the fixed-point tools

Select this parameter to prevent the fixed-point tools from overriding the data types you specify on the block mask.

 Provide Filter Coefficients Through Input Port

The Main pane of the FIR Decimation block dialog box appears as follows when you select Input port in the Coefficient source group box.

Decimation factor

Specify the integer factor, K, by which to decrease the sample rate of the input sequence. The default is 2.

Filter Structure

Choose whether to implement a Direct form (default) or Direct form transposed filter.

Input processing

Specify how the block should process the input. You can set this parameter to one of the following options:

• Columns as channels (frame based) (default) — 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.

Rate options

Specify the method by which the block should decimate the input. You can select one of the following options:

• Enforce single-rate processing (default) — When you select this option, the block maintains the input sample rate and decimates the signal by decreasing the output frame size by a factor of K. To select this option, you must set the Input processing parameter to Columns as channels (frame based).

• Allow multirate processing — When you select this option, the block decimates the signal such that the output sample rate is K times slower than the input sample rate.

Output buffer initial conditions

In the case of one-frame latency, this parameter specifies the output of the block until the first filtered input sample is available. The default value of this parameter is 0, but you can enter a matrix containing one value for each channel, or a scalar value to be applied to all signal channels. This parameter appears only when you configure the block to perform multirate processing.

See Latency for more information about latency in the FIR Decimation block.

The Data Types pane of the FIR Decimation block dialog box appears as follows when you select Input port in the Coefficient source group box.

Rounding mode

Select the rounding mode for fixed-point operations. The default is Floor. The filter coefficients do not obey this parameter; they always round to Nearest.

Note

The Rounding mode and Saturate on integer overflow settings have no effect on numerical results when all the following conditions exist:

• Product output is Inherit: Inherit via internal rule

• Accumulator is Inherit: Inherit via internal rule

• Output is Inherit: Same as accumulator

With these data type settings, the block is effectively operating in full precision mode.

Saturate on integer overflow

When you select this check box, the block saturates the result of its fixed-point operation. When you clear this check box, the block wraps the result of its fixed-point operation. By default, this check box is cleared. For details on saturate and wrap, see overflow mode for fixed-point operations. The filter coefficients do not obey this parameter; they are always saturated.

Product output

Specify the product output data type. See Fixed-Point Data Types and Multiplication Data Types for illustrations depicting the use of the product output data type in this block. You can set it to:

• A rule that inherits a data type, for example, Inherit: Inherit via internal rule. For more information on this rule, see Inherit via Internal Rule.

• An expression that evaluates to a valid data type, for example, fixdt(1,16,0)

Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Product output parameter.

See Specify Data Types Using Data Type Assistant (Simulink) for more information.

Accumulator

Specify the accumulator data type. See Fixed-Point Data Types for illustrations depicting the use of the accumulator data type in this block. You can set this parameter to:

• A rule that inherits a data type, for example, Inherit: Inherit via internal rule. For more information on this rule, see Inherit via Internal Rule.

• An expression that evaluates to a valid data type, for example, fixdt(1,16,0)

Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Accumulator parameter.

See Specify Data Types Using Data Type Assistant (Simulink) for more information.

Output

Specify the output data type. See Fixed-Point Data Types for illustrations depicting the use of the output data type in this block. You can set it to:

• A rule that inherits a data type, for example, Inherit: Same as accumulator

• An expression that evaluates to a valid data type, for example, fixdt(1,16,0)

Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Output parameter.

See Control Signal Data Types (Simulink) for more information.

Output Minimum

Specify the minimum value that the block should output. The default value is [] (unspecified). Simulink software uses this value to perform:

• Simulation range checking (see Specify Signal Ranges (Simulink))

• Automatic scaling of fixed-point data types

Output Maximum

Specify the maximum value that the block should output. The default value is [] (unspecified). Simulink software uses this value to perform:

• Simulation range checking (see Specify Signal Ranges (Simulink))

• Automatic scaling of fixed-point data types

Lock data type settings against changes by the fixed-point tools

Select this parameter to prevent the fixed-point tools from overriding the data types you specify on the block mask.

 Specify Multirate Filter Object

The Main pane of the FIR Decimation block dialog box appears as follows when you select Filter object in the Coefficient source group box.

Filter object

Specify the name of the multirate filter object that you want the block to implement. You must specify the filter as a dsp.FIRDecimator System object.

You can define the System object in the block mask or in a MATLAB^® workspace variable.

For information on creating System objects, see Define Basic System Objects.

Input processing

Specify how the block should process the input. You can set this parameter to one of the following options:

• Columns as channels (frame based) (default) — 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.

Rate options

Specify the method by which the block should decimate the input. You can select one of the following options:

• Enforce single-rate processing (default) — When you select this option, the block maintains the input sample rate and decimates the signal by decreasing the output frame size by a factor of K. To select this option, you must set the Input processing parameter to Columns as channels (frame based).

• Allow multirate processing — When you select this option, the block decimates the signal such that the output sample rate is K times slower than the input sample rate.

Output buffer initial conditions

In the case of one-frame latency, this parameter specifies the output of the block until the first filtered input sample is available. The default value of this parameter is 0, but you can enter a matrix containing one value for each channel, or a scalar value to be applied to all signal channels. This parameter appears only when you configure the block to perform multirate processing.

See Latency for more information about latency in the FIR Decimation block.

View filter response

This button opens the Filter Visualization Tool (fvtool) from the Signal Processing Toolbox product and displays the filter response of the System object specified in the Filter object parameter. For more information on FVTool, see the Signal Processing Toolbox documentation.

Note

If you specify a filter in the Filter object parameter, you must apply the filter by clicking the Apply button before using the View filter response button.

The Data Types pane of the FIR Decimation block dialog box appears as follows when you select Filter object in the Coefficient source group box.

The fixed-point settings of the filter object specified on the Main pane are displayed on the Data Types pane. You cannot change these settings directly on the block mask. To change the fixed-point settings, edit the filter object directly.

For more information on System objects, see What Are System Objects?.

 Choose Filter Coefficients Automatically

When you select Auto in the Coefficient source group box, the block chooses the filter coefficients automatically. For more information on the filter design algorithm the block uses, see Specifying the Filter Coefficients.

The Main pane of the FIR Decimation block dialog box appears as follows when you select Auto in the Coefficient source group box.

Decimation factor

Specify the integer factor, K, by which to decrease the sample rate of the input sequence. The default is 2.

Filter Structure

Choose whether to implement a Direct form (default) or Direct form transposed filter.

Input processing

Specify how the block should process the input. You can set this parameter to one of the following options:

• Columns as channels (frame based) (default) — 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.

Rate options

Specify the method by which the block should decimate the input. You can select one of the following options:

• Enforce single-rate processing (default) — When you select this option, the block maintains the input sample rate and decimates the signal by decreasing the output frame size by a factor of K. To select this option, you must set the Input processing parameter to Columns as channels (frame based).

• Allow multirate processing — When you select this option, the block decimates the signal such that the output sample rate is K times slower than the input sample rate.

Output buffer initial conditions

In the case of one-frame latency, this parameter specifies the output of the block until the first filtered input sample is available. The default value of this parameter is 0, but you can enter a matrix containing one value for each channel, or a scalar value to be applied to all signal channels. This parameter appears only when you configure the block to perform multirate processing.

See Latency for more information about latency in the FIR Decimation block.

View filter response

This button opens the Filter Visualization Tool (fvtool) from the Signal Processing Toolbox product and displays the filter response of the filter defined in the block. For more information on FVTool, see the Signal Processing Toolbox documentation.

The Data Types pane of the FIR Decimation block dialog box appears as follows when you select Auto in the Coefficient source group box.

Rounding mode

Select the rounding mode for fixed-point operations. The default is Floor. The filter coefficients do not obey this parameter; they always round to Nearest.

Note

The Rounding mode and Saturate on integer overflow settings have no effect on numerical results when all the following conditions exist:

• Product output is Inherit: Inherit via internal rule

• Accumulator is Inherit: Inherit via internal rule

• Output is Inherit: Same as accumulator

With these data type settings, the block is effectively operating in full precision mode.

Saturate on integer overflow

When you select this check box, the block saturates the result of its fixed-point operation. When you clear this check box, the block wraps the result of its fixed-point operation. By default, this check box is cleared. For details on saturate and wrap, see overflow mode for fixed-point operations. The filter coefficients do not obey this parameter; they are always saturated.

Coefficients

Specify the coefficients data type. See Fixed-Point Data Types and Multiplication Data Types for illustrations depicting the use of the coefficients data type in this block. You can set it to:

• A rule that inherits a data type, for example, Inherit: Same word length as input

• An expression that evaluates to a valid data type, for example, fixdt(1,16,0)

Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Coefficients parameter.

See Specify Data Types Using Data Type Assistant (Simulink) for more information.

Coefficients Minimum

Specify the minimum value of the filter coefficients. The default value is [] (unspecified). Simulink software uses this value to perform:

• Automatic scaling of fixed-point data types

Coefficients Maximum

Specify the maximum value of the filter coefficients. The default value is [] (unspecified). Simulink software uses this value to perform:

• Automatic scaling of fixed-point data types

Product output

Specify the product output data type. See Fixed-Point Data Types and Multiplication Data Types for illustrations depicting the use of the product output data type in this block. You can set it to:

• A rule that inherits a data type, for example, Inherit: Inherit via internal rule. For more information on this rule, see Inherit via Internal Rule.

• An expression that evaluates to a valid data type, for example, fixdt(1,16,0)

Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Product output parameter.

See Specify Data Types Using Data Type Assistant (Simulink) for more information.

Accumulator

Specify the accumulator data type. See Fixed-Point Data Types for illustrations depicting the use of the accumulator data type in this block. You can set this parameter to:

• A rule that inherits a data type, for example, Inherit: Inherit via internal rule. For more information on this rule, see Inherit via Internal Rule.

• An expression that evaluates to a valid data type, for example, fixdt(1,16,0)

Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Accumulator parameter.

See Specify Data Types Using Data Type Assistant (Simulink) for more information.

Output

Specify the output data type. See Fixed-Point Data Types for illustrations depicting the use of the output data type in this block. You can set it to:

• A rule that inherits a data type, for example, Inherit: Same as accumulator

• An expression that evaluates to a valid data type, for example, fixdt(1,16,0)

Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Output parameter.

See Control Signal Data Types (Simulink) for more information.

Output Minimum

Specify the minimum value that the block should output. The default value is [] (unspecified). Simulink software uses this value to perform:

• Simulation range checking (see Specify Signal Ranges (Simulink))

• Automatic scaling of fixed-point data types

Output Maximum

Specify the maximum value that the block should output. The default value is [] (unspecified). Simulink software uses this value to perform:

• Simulation range checking (see Specify Signal Ranges (Simulink))

• Automatic scaling of fixed-point data types

Lock data type settings against changes by the fixed-point tools

Select this parameter to prevent the fixed-point tools from overriding the data types you specify on the block mask.

References

[1] Fliege, N. J. Multirate Digital Signal Processing: Multirate Systems, Filter Banks, Wavelets. West Sussex, England: John Wiley & Sons, 1994.

[2] Orfanidis, Sophocles J. Introduction to Signal Processing. Upper Saddle River, NJ: Prentice-Hall, 1996.

Supported Data Types

Port	Supported Data Types
Input	Double-precision floating point Single-precision floating point Fixed point 8-, 16-, and 32-bit signed integers 8-, 16-, and 32-bit unsigned integers
Output	Double-precision floating point Single-precision floating point Fixed point 8-, 16-, and 32-bit signed integers 8-, 16-, and 32-bit unsigned integers

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using Simulink® Coder™.

Generated code relies on memcpy or memset functions (string.h) under certain conditions.

The FIR Decimation block supports SIMD code generation using Intel AVX2 technology under these conditions:

• Filter structure is set to Direct form.

• Input processing is set to Columns as channels (frame based).

• Rate options is set to Enforce single-rate processing.

• Input signal is real-valued with real filter coefficients.

• Input signal is complex-valued with real or complex filter coefficients.

• Input signal has a data type of single or double.

The SIMD technology significantly improves the performance of the generated code.

HDL Code Generation

Generate Verilog and VHDL code for FPGA and ASIC designs using HDL Coder™.

HDL Coder™ provides additional configuration options that affect HDL implementation and synthesized logic.

For a FIR decimation filter with hardware-friendly control signals and simulation of HDL latency in Simulink, or for complex data with complex coefficients, use the FIR Decimation HDL Optimized block instead of this block.

HDL Coder supports Coefficient source options Dialog parameters, Filter object, or Auto. Programmable coefficients are not supported.

Frame-Based Input Support

HDL Coder supports the use of vector inputs to FIR Decimation blocks, where each element of the vector represents a sample in time. You can use an input vector of up to 512 samples. The frame-based implementation supports fixed-point input and output data types, and uses full-precision internal data types. The output is a column vector of reduced size, corresponding to your decimation factor. You can use real input signals with real coefficients, complex input signals with real coefficients, or real input signals with complex coefficients.

1. Connect a column vector signal to the FIR Decimation block input port.

2. Specify Input processing as Columns as channels (frame based).

3. Set Rate options to Enforce single-rate processing.

4. Right-click the block and open HDL Code > HDL Block Properties. Set the Architecture to Frame Based. The block implements a parallel HDL architecture. See Frame-Based Architecture (HDL Coder).

Block Optimizations

To reduce area or increase speed, the FIR Decimator block supports block-level optimizations.

Right-click on the block or the subsystem to open the corresponding HDL Properties dialog box and set optimization properties.

Serial Architectures	To use block-level optimizations to reduce hardware resources, set Architecture to Fully Serial or Partly Serial. See HDL Filter Architectures (HDL Coder). When you specify SerialPartition for a FIR Decimator block, set Filter structure to Direct form. The Direct form transposed structure is not supported with serial architectures. Accumulator reuse is not supported for FIR Decimation filters.
Distributed Arithmetic	To minimize multipliers by replacing them with LUTs and shift registers, use a distributed arithmetic (DA) filter implementation. See Distributed Arithmetic for HDL Filters (HDL Coder). When you select the Distributed Arithmetic (DA) architecture and use the DALUTPartition and DARadix distributed arithmetic properties, set Filter structure to Direct form. The Direct form transposed structure is not supported with distributed arithmetic.
Pipelining	To improve clock speed, use AddPipelineRegisters to use a pipelined adder tree rather than the default linear adder. This option is supported for Direct form architecture. You can also specify the number of pipeline stages before and after the multipliers. See HDL Filter Architectures (HDL Coder).

HDL Filter Properties

AddPipelineRegisters	Insert a pipeline register between stages of computation in a filter. See also AddPipelineRegisters (HDL Coder).
CoeffMultipliers	Specify the use of canonical signed digit (CSD) optimization to decrease filter area by replacing coefficient multipliers with shift-and-add logic. When you choose a fully parallel filter implementation, you can set CoeffMultipliers to csd or factored-csd. The default is multipliers, which retains multipliers in the HDL. See also CoeffMultipliers (HDL Coder).
DALUTPartition	Specify distributed arithmetic partial-product LUT partitions as a vector of the sizes of each partition. The sum of all vector elements must be equal to the filter length. The maximum size for a partition is 12 taps. Set DALUTPartition to a scalar value equal to the filter length to generate DA code without LUT partitions. See also DALUTPartition (HDL Coder).
DARadix	Specify how many distributed arithmetic bit sums are computed in parallel. A DA radix of 8 (2^3) generates a DA implementation that computes three sums at a time. The default value is 2^1, which generates a fully serial DA implementation. See also DARadix (HDL Coder).
MultiplierInputPipeline	Specify the number of pipeline stages to add at filter multiplier inputs. See also MultiplierInputPipeline (HDL Coder).
MultiplierOutputPipeline	Specify the number of pipeline stages to add at filter multiplier outputs. See also MultiplierOutputPipeline (HDL Coder).
SerialPartition	Specify partitions for partly serial or cascade-serial filter implementations as a vector of the lengths of each partition. For a fully serial implementation, set this parameter to the length of the filter. See also SerialPartition (HDL Coder).

HDL Block Properties

ConstrainedOutputPipeline	Number of registers to place at the outputs by moving existing delays within your design. Distributed pipelining does not redistribute these registers. The default is 0. For more details, see ConstrainedOutputPipeline (HDL Coder).
InputPipeline	Number of input pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see InputPipeline (HDL Coder).
OutputPipeline	Number of output pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see OutputPipeline (HDL Coder).

Restrictions

• You must set Initial conditions to zero. HDL code generation is not supported for nonzero initial states.

• When you select Dialog parameters, the following fixed-point options are not supported for HDL code generation:

  • Slope and Bias scaling

• CoeffMultipliers options are supported only when using a fully parallel architecture. When you select a serial architecture, CoeffMultipliers is hidden from the HDL Block Properties dialog box.

• Programmable coefficients are not supported.

• Frame-based input filters are not supported for:

  • Resettable and enabled subsystems

  • Complex input signals with complex coefficients. You can use either complex input signals and real coefficients, or complex coefficients and real input signals.

  • Sharing and streaming optimizations

Fixed-Point Conversion
Design and simulate fixed-point systems using Fixed-Point Designer™.