Complex Bandpass Decimator

Extract a frequency subband using a one-sided (complex) bandpass decimator

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Libraries:
DSP System Toolbox / Filtering / Multirate Filters

Description

The Complex Bandpass Decimator block extracts a specific subband of frequencies using a one-sided, multistage, complex bandpass decimator. The block determines the bandwidth of interest using the specified center frequency, decimator factor, and bandwidth values.

This block supports C/C++ code generation and SIMD code generation. For details, see Code Generation.

Ports

Input

x — Input data

vector | matrix

Specify the input data as a vector or a matrix.

The block treats each column of the input signal as a separate channel. If the input is a two-dimensional signal, the first dimension represents the channel length (or frame size) and the second dimension represents the number of channels. If the input is a one-dimensional signal, then the block interprets it as a single-channel signal.

When you input a variable-size signal (frame length changes during simulation), the frame length of the signal can be arbitrary, that is, the input frame length does not have to be a multiple of the decimation factor. When you input a fixed-size signal (frame length does not change during simulation), the frame length can be arbitrary only when you select the Allow arbitrary frame length for fixed-size input signals parameter.

To determine the decimation factor, click the View Info button.

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

Data Types: single | double
Complex Number Support: Yes

Fc — Center frequency

real scalar

Specify the center frequency of the desired band as a real, finite numeric scalar in the range [–Fs/2, Fs/2], where Fs is the sample rate of the input signal. If you set Sample rate mode to Inherit from input port or Specify on dialog, the block inherits Fs from the input signal or uses the value of the Input sample rate (Hz) parameter, respectively. If you set Sample rate mode to Use normalized frequency (0 to 1), the block sets Fs to 2 (since R2024b).

Dependencies

To enable this port, select the Specify center frequency from input port parameter.

Data Types: single | double
Complex Number Support: Yes

Output

Port_1 — Filtered output

vector | matrix

Output of the complex bandpass decimator, returned as a vector or a matrix. The output contains the subband of frequencies specified by the parameters in the block dialog box.

The output signal dimensions depend on the input signal dimensions, the overall decimation factor of the Complex Bandpass Decimator block, and whether you select the Allow arbitrary frame length for fixed-size input signals parameter. You can determine the overall decimator factor D by clicking the View Info button in the block dialog box.

This table provides more details on the dimensions of the output signal when you input a fixed-size input signal.

Fixed-Size Input Signal

Input Signal	Output Signal Dimensions
P-by-1 column vector, where P is a multiple of the decimation factor D	(P/D)-by-1
P-by-1 column vector, where P is not a multiple of the decimation factor D	ceil(P/D)-by-1 when you select Allow arbitrary frame length for fixed-size input signals. If you do not select Allow arbitrary frame length for fixed-size input signals, the block errors.
P-by-Q matrix, where P is a multiple of the decimation factor D	(P/D)-by-Q
P-by-Q matrix, where P is not a multiple of the decimation factor D	ceil(P/D)-by-Q when you select Allow arbitrary frame length for fixed-size input signals. If you do not select Allow arbitrary frame length for fixed-size input signals, the block errors.

This table gives more details on the dimensions of the output signal when you input a variable-size input signal. When you input a variable-size signal (frame length changes during simulation), the Allow arbitrary frame length for fixed-size input signals parameter is visible in the block dialog box but does not have any impact on the input frame length. You can input a variable-size signal of any frame length even if you do not select the Allow arbitrary frame length for fixed-size input signals parameter.

Variable-Size Input Signal

Input Signal	Output Signal Dimensions
P-by-Q	ceil(P/D)-by-Q

The data type of the output is same as the data type of the input. The output signal is always complex.

Data Types: single | double
Complex Number Support: Yes

Parameters

Filter specification — Filter design parameters

Decimation factor (default) | Bandwidth | Decimation factor and bandwidth

Specify the filter design parameters as one of the following:

• Decimation factor –– Specify the decimation factor through the Decimation factor parameter. The block computes the bandwidth of interest (BW) using the following equation:

  $$BW=Fs/D$$

  where

  • Fs –– Sample rate of input signal

  • D –– Decimation factor

• Bandwidth –– Specify the bandwidth through the Bandwidth parameter. The block computes the decimation factor (D) using the following equation:

  $$D=\text{floor}\left(\frac{Fs}{BW+TW}\right)$$

  where

  • Fs –– Sample rate of input signal

  • BW –– Bandwidth of interest

  • TW –– Transition width specified through the Transition width parameter

• Decimation factor and bandwidth –– Specify the decimation factor and the bandwidth of interest through the Decimation factor and Bandwidth parameters, respectively.

Decimation factor — Decimation factor

2 (default) | positive integer

Specify the factor by which to reduce the bandwidth of the input signal as a positive integer. The frame size (number of rows) of the input signal must be a multiple of the decimation factor.

Dependencies

To enable this parameter, set Filter specification to Decimation factor or Decimation factor and bandwidth.

Bandwidth — Width of frequency band of interest

0.2268 (default) | real positive scalar

Specify the width of the frequency band of interest as a real positive scalar in Hz or in normalized frequency units (since R2024b).

Dependencies

To enable this parameter, set Filter specification to Bandwidth or Decimation factor and bandwidth.

Data Types: single | double

Specify center frequency from input port — Specify source of center frequency

off (default) | on

When you select this check box, the center frequency is input through the Fc port. When you clear this check box, you can specify the center frequency in the block dialog box through the Center frequency parameter.

When you select this check box, the block does not compute the filter response. To view the filter response, clear this check box, specify the center frequency in the block dialog box, and click the View Filter Response button.

Center frequency — Center frequency of desired band

0 (default) | real scalar

Specify the center frequency of the desired band as a real scalar in Hz or in normalized frequency units (since R2024b). The center frequency must be in the range [–Fs/2, Fs/2], where Fs is the sample rate of the input signal. If you set Sample rate mode to Inherit from input port or Specify on dialog, the block inherits Fs from the input signal or uses the value of the Input sample rate (Hz) parameter, respectively. If you set Sample rate mode to Use normalized frequency (0 to 1), the block sets Fs to 2 (since R2024b).

Tunable: Yes

Data Types: single | double

Stopband attenuation (dB) — Stopband attenuation in dB

80 (default) | positive scalar

Specify the stopband attenuation of the filter as a finite positive scalar in dB.

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

Passband ripple (dB) — Passband ripple in dB

1 (default) | positive scalar

Specify the passband ripple of the filter as a positive scalar in dB.

Dependencies

To enable this parameter, set Filter specification to Bandwidth or Decimation factor and bandwidth.

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

Transition width — Transition width

0.0045 (default) | positive scalar

Specify the transition width of the filter as a positive scalar in Hz or in normalized frequency units (since R2024b).

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

Reduce number of complex coefficients — Minimize number of complex coefficients

on (default) | off

Select this parameter to minimize the number of complex coefficients. When you select this parameter, the first stage of the multistage filter is bandpass (with complex coefficients) centered at the specified center frequency. The first stage is followed by a mixing stage that heterodynes the signal to DC. The remaining filter stages, all with real coefficients, follow.

When you clear the parameter, the input signal is first passed through the different stages of the multistage filter. All stages are bandpass (complex coefficients). The signal is then heterodyned to DC if Mix signal to baseband parameter is selected and the frequency offset resulting from the decimation is nonzero.

Mix signal to baseband — Mix signal to baseband

on (default) | off

Select this parameter to mix the signal to baseband. When you select this parameter, the block heterodynes the filtered, decimated signal to DC. This mixing stage runs at the output sample rate of the filter. When you clear this parameter, the block skips the mixing stage.

Dependencies

This parameter applies when you clear the Reduce number of complex coefficients parameter.

Sample rate mode — Mode to specify the input sample rate

Use normalized frequency (0 to 1) (default) | Specify on dialog | Inherit from input port

Since R2024b

Specify the input sample rate using one of these options:

• Use normalized frequency (0 to 1) –– Specify the center frequency, transition width, and bandwidth in normalized frequency units (0 to 1).

• Specify on dialog –– Specify the input sample rate in the block dialog box using the Input sample rate (Hz) parameter.

• Inherit from input port –– The block inherits the sample rate from the input signal. The block calculates the sample rate based on the sample time of the input port.

Input sample rate (Hz) — Input sample rate in Hz

44100 (default) | real positive scalar

Specify the sampling rate of the input signal Fs in Hz as a real positive scalar.

Dependencies

To enable this parameter, set the Sample rate mode parameter to Specify on dialog. (since R2024b)

Data Types: single | double

Allow arbitrary frame length for fixed-size input signals — Allow arbitrary frame length for fixed-size input signals

off (default) | on

Since R2023a

Specify whether fixed-size input signals (whose size does not change during simulation) can have an arbitrary frame length, where the frame length does not have to be a multiple of the decimation factor. The block uses this parameter only for fixed-size input signals and ignores it if the input data varies in size during simulation.

When the input signal is a variable-size signal, the signal can have an arbitrary frame length, that is, the frame length does not have to be a multiple of the decimation factor.

For fixed-size input signals, if you:

• Select the Allow arbitrary frame length for fixed-size input signals parameter, the frame length of the signal does not have to be a multiple of the decimation factor. If the input is not a multiple of the decimation factor, then the output is generally a variable-size signal. Therefore, to support arbitrary input size, the block must also support variable-size operations, which you can enable by selecting the Allow arbitrary frame length for fixed-size input signals parameter.

• Clear the Allow arbitrary frame length for fixed-size input signals parameter, the input frame length must be a multiple of the decimation factor.

View Filter Response — Visualize frequency response

button

Click this button to open the filter visualizer and display the magnitude response of the complex bandpass decimator. The response is based on the parameters you select in the block dialog box. To update the response while the visualizer is running, modify the parameters in the dialog box and click Apply.

You can configure the plot settings and the frequency response measurements from the interface of the visualizer.

On the Scope tab, you can enable the legend, specify to plot both magnitude and phase responses of the filter, modify the plot settings, generate a script to re-create the plot, and even save or share the settings. On the Measurements tab, you can enable data cursors, and display the peak values of the filter response.

For more details on the filter visualizer interface and its tools, see Configure Filter Visualizer.

Dependencies

To enable this button, do not specify any filter specifications from the input port.

View Info — View filter information

button

Display filter information of the Complex Bandpass Decimator block.

This button brings the functionality of the info analysis method into the Simulink^® environment.

Dependencies

To enable this button, do not specify any filter specifications from the input port.

Simulate using — Type of simulation to run

Interpreted execution (default) | Code generation

Specify the type of simulation to run. You can set this parameter to:

• Interpreted execution –– Simulate model using the MATLAB^® interpreter. This option shortens startup time.

• Code generation –– 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 subsequent simulations.

Block Characteristics

Data Types	double | single
Multidimensional Signals	No
Variable-Size Signals	Yes

Algorithms

The complex bandpass decimator is designed by applying a complex frequency shift transformation on a lowpass prototype filter. The lowpass prototype in this case is a multirate, multistage finite impulse response (FIR) filter. The desired frequency shift applies only to the first stage. Subsequent stages scale the desired frequency shift by their respective cumulative decimation factors. For details, see Complex Bandpass Filter Design and Zoom FFT.

Extended Capabilities

C/C++ Code Generation

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

The Complex Bandpass Decimator block supports SIMD code generation using Intel AVX2 code replacement library under these conditions:

• Input signal is complex-valued.

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

The SIMD technology significantly improves the performance of the generated code. For more information, see SIMD Code Generation. To generate SIMD code from this block, see Use Intel AVX2 Code Replacement Library to Generate SIMD Code from Simulink Blocks.

Version History

Introduced in R2018a

R2024b: Support for normalized frequencies

When you set the Sample rate mode parameter to Use normalized frequency (0 to 1), you can specify the center frequency, transition width, and bandwidth in normalized frequency units (0 to 1).

R2024a: Change in the default value of Simulate using parameter

The default value of the Simulate using parameter is now Interpreted execution. With this change, the block uses the MATLAB interpreter for simulation by default.

R2023a: Support for arbitrary input frame length

This block supports input signals with arbitrary frame lengths when the:

• Input signal is a fixed-size signal (frame length does not change during simulation) and you select the Allow arbitrary frame length for fixed-size input signals parameter.

• Input signal is a variable-size signal (frame length changes during simulation).

When this block supports an input signal with an arbitrary frame length, the input frame length does not have to be a multiple of the decimation factor and the output signal is a variable-size signal.