DC Blocker

Block DC component

Library

Signal Operations

dspsigops

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Description

The DC Blocker block removes the DC component of the input signal. This block supports SIMD code generation. For details, see Code Generation.

Dialog Box

The DC Blocker dialog box changes based on how the DC offset is estimated. The dialog box for the IIR method is shown below.

The dialog box for the FIR method is shown below.

The dialog box for the CIC method is shown below.

The dialog box for the Subtract mean method is shown below.

Algorithm for estimating DC offset

Specify the algorithm used for estimating the DC offset. Select from the following:

• IIR uses a recursive estimate based on a narrow, lowpass elliptic filter. This algorithm typically uses less memory than FIR and is more efficient.

• FIR uses a nonrecursive, moving-average estimate. This algorithm typically uses more memory than IIR and is less efficient.

• CIC uses a lowpass filter that does not employ any multipliers. If the algorithm is CIC, then fixed-point data must be input to the DC Blocker.

• Subtract mean computes the means of the columns of the input matrix and subtracts the means from the input. This method does not retain state between inputs. For example, if the input is [1 2 3 4; 3 4 5 6], then the DC Blocker block in Subtract mean mode outputs [-1 -1 -1 -1; 1 1 1 1].

Normalized bandwidth of lowpass IIR or CIC filter

Specify the normalized filter bandwidth as a real scalar greater than 0 and less than 1. The DC Blocker uses this parameter only when the estimation algorithm is set to IIR or CIC.

Order of lowpass IIR elliptic filter

Specify the filter order as an integer greater than 3. The DC Blocker uses this parameter only when the estimation algorithm is set to IIR.

Number of past input samples for FIR algorithm

Specify, as a positive integer, the number of samples to use when the estimation algorithm is set to FIR.

View Filter Response

Opens the fvtool and displays the magnitude response of the DC Blocker. The response is based on the block parameters. Changes made to these parameters update fvtool.

To update the magnitude response while fvtool is running, modify the block parameters and click Apply.

Simulate using

Select the simulation type from the following:

• Code generation (default)

• Interpreted execution

Examples

Use DC Blocker to Remove DC Component of Signal

This example shows how to use the DC Blocker to remove the DC component of a signal.

Load the DC Blocker example by typing ex_dc_blocker in the MATLAB command prompt.

The spectral output from the DC Blocker is displayed in Spectrum with Blocking, while the spectrum of the input signal is displayed in Spectrum without Blocking.

The two sine wave sources are set to use 1000 samples per frame because the Subtract mean estimation algorithm requires a statistically significant number of samples to calculate a valid mean.

In the model, run the simulation. The spectrum of the input signal shows tones at 150 Hz and 250 Hz and a significant (0 dBW) DC component.

Using the default IIR setting for the DC Blocker estimation algorithm, the tones at 150 Hz and 250 Hz are unaffected while the DC component has been attenuated by 30 dB.

Select the DC Blocker block by double-clicking on it and change the algorithm type from IIR to Subtract mean. Rerun the simulation. The spectral output from the DC Blocker shows that the Subtract mean estimation method results in a DC component of less than −100 dBW.

Try all three estimation methods. Modify the IIR and FIR parameters to illustrate the performance of the DC Blocker using the various estimation techniques.

DC Blocker with Fixed Point Data

This example shows how to use the DC Blocker to remove a DC offset from fixed point data.

Load the DC Blocker example by typing ex_dcblock_cicmode in the MATLAB command prompt.

In the model:

• 64-QAM data passes through an AWGN channel.

• A DC offset of 1 is added to the signal .

• The Double -> Fixed block converts the data to 16-bit fixed point.

• The fixed-point data passes through the DC Blocker, which has the CIC algorithm selected, to remove the DC offset.

• The Fixed -> Double block converts the data back to floating point.

Constellation diagrams and spectrum analyzers are used to show the improvements from the DC Blocker.

Run the simulation. The first constellation diagram, Noisy Constellation, shows a 64-QAM signal with white noise.

Observe the constellation diagram of the signal after the DC offset of 1 has been applied. The signal, represented by the yellow data points, has shifted one unit to the right.

Look at the spectrum of the noisy signal with the DC offset. Notice that the signal has a peak at 0 Hz.

Observe the noisy constellation after the DC offset is removed. The signal has shifted back to the left so that the data clusters are aligned with their corresponding reference points.

Observe the spectrum of the noisy signal after the DC Blocker removes the offset. The spectral peak at 0 Hz has been removed.

To visualize the efficiency of the DC Blocker under different conditions, try changing the DC offset or the Normalized bandwidth of lowpass IIR or CIC filter parameter.

Algorithms

This block implements the algorithm, inputs, and outputs described on the dsp.DCBlocker reference page. The object properties correspond to the block parameters.

Supported Data Types

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

References

[1] Nezami, M., “Performance Assessment of Baseband Algorithms for Direct Conversion Tactical Software Defined Receivers: I/Q Imbalance Correction, Image Rejection, DC Removal, and Channelization”, MILCOM, 2002.

Extended Capabilities

C/C++ Code Generation

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

The DC Blocker block supports SIMD code generation using Intel AVX2 technology when the 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.

HDL Architecture

This block has a single, default HDL architecture.

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).

Complex Data Support

This block supports code generation for complex signals.

Fixed-Point Conversion

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

If the input is fixed point, it must be signed integer or signed fixed point with power-of-two slope and zero bias.