Complex Bandpass Decimator
Extract a frequency subband using a onesided (complex) bandpass decimator
Libraries:
DSP System Toolbox /
Filtering /
Multirate Filters
Description
The Complex Bandpass Decimator block extracts a specific subband of frequencies using a onesided, 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 twodimensional 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 onedimensional signal, then the block interprets it as a singlechannel signal.
When you input a variablesize 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 fixedsize signal (frame length does not change during simulation), the frame length can be arbitrary only when you select the Allow arbitrary frame length for fixedsize 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 fixedsize 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 fixedsize input signal.
FixedSize Input Signal
Input Signal  Output Signal Dimensions 

Pby1 column vector, where P is a multiple of the decimation factor D  (P/D)by1 
Pby1 column vector, where P is not a multiple of the decimation factor D 
If you do not select Allow arbitrary frame length for fixedsize input signals, the block errors. 
PbyQ matrix, where P is a multiple of the decimation factor D  (P/D)byQ 
PbyQ matrix, where P is not a multiple of the decimation factor D 
If you do not select Allow arbitrary frame length for fixedsize input signals, the block errors. 
This table gives more details on the dimensions of the output signal when you input a variablesize input signal. When you input a variablesize signal (frame length changes during simulation), the Allow arbitrary frame length for fixedsize 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 variablesize signal of any frame length even if you do not select the Allow arbitrary frame length for fixedsize input signals parameter.
VariableSize Input Signal
Input Signal  Output Signal Dimensions 

PbyQ 

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 fixedsize input signals — Allow arbitrary frame length for fixedsize input signals
off
(default)  on
Since R2023a
Specify whether fixedsize 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 fixedsize input signals and ignores it if the input data varies in size during simulation.
When the input signal is a variablesize 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 fixedsize input signals, if you:
Select the Allow arbitrary frame length for fixedsize 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 variablesize signal. Therefore, to support arbitrary input size, the block must also support variablesize operations, which you can enable by selecting the Allow arbitrary frame length for fixedsize input signals parameter.
Clear the Allow arbitrary frame length for fixedsize 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 recreate 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 

Multidimensional Signals 

VariableSize Signals 

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 complexvalued.
Input signal has a data type of
single
ordouble
.
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 R2018aR2024b: 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 fixedsize signal (frame length does not change during simulation) and you select the Allow arbitrary frame length for fixedsize input signals parameter.
Input signal is a variablesize 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 variablesize signal.
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