Median Filter

R2026b

Median filter

• 

Median Filter block

To add a block to a model, double-click the canvas and start typing the block name. Then, select the block from the list.

Libraries:
DSP System Toolbox / Filtering / Filter Designs
DSP System Toolbox / Statistics

Description

The Median Filter block computes the moving median of the input signal along each channel independently over time. The block uses the sliding window method to compute the moving median. In this method, a window of specified length moves over each channel sample by sample, and the block computes the median of the data in the window. This block performs median filtering on the input data over time.

You can change the window length during simulation by selecting the Make window length tunable parameter. Specify the tunable window length from the block dialog box by updating the Window length parameter or through the input port WL by selecting the Specify window length from input port parameter. For more details, see Algorithms. (since R2026b)

Ports

Input

x — Data input

column vector | row vector | matrix

Data over which the block computes moving median. The block accepts real-valued or complex-valued multichannel inputs, that is, m-by-n size inputs, where m ≥ 1, and n ≥ 1. The block also accepts variable-size inputs. During simulation, you can change the size of each input channel. However, the number of channels cannot change.

This port is unnamed until you enable the WL input port.

Data Types: single | double
Complex Number Support: Yes

WL — Tunable window length input

positive integer

Since R2026b

Specify the tunable window length as a positive integer. The block uses this value as the sliding window length for computing the moving median.

When you select Auto-adjust invalid window length, the block adjusts invalid window length values from the input port and issues a warning. For more information on what values are invalid, see the Auto-adjust invalid window length parameter description.

When you clear the Auto-adjust invalid window length parameter, the block throws an error for invalid window length values.

Dependencies

This port appears when you select Make window length tunable and Specify window length from input port.

Data Types: single | double

Output

Port_1 — Moving median output

column vector | row vector | matrix

The size of the moving median output matches the size of the input. The block uses the sliding window method to compute the moving median. For more details, see Algorithms.

Data Types: single | double
Complex Number Support: Yes

Parameters

Make window length tunable — Option to enable tunable window length

off (default) | on

Since R2026b

Select this parameter to make the window length tunable, that is, you can change the window length during simulation through the block dialog box by updating the Window length parameter or through the input port WL by selecting the Specify window length from input port parameter.

When you clear this parameter, the window length is fixed and you cannot change it during simulation. Specify the window length through the Window length parameter.

Specify window length from input port — Source of tunable window length

off (default) | on

Since R2026b

When you select this parameter, the block reads the tunable window length from the input port, WL. When you clear this parameter, the block reads the tunable window length from the Window length parameter.

Dependencies

This parameter appears when you select Make window length tunable.

Window length — Length of sliding window

5 (default) | positive integer

Specify the length of the sliding window in samples as a positive integer.

When you select the Make window length tunable parameter, the window length you specify is tunable and must be less than or equal to the value you specify in the Maximum window length parameter. When you clear the Make window length tunable parameter, the window length is fixed. (since R2026b)

Dependencies

If you select Make window length tunable, you must clear the Specify window length from input port parameter.

Maximum window length — Maximum value of tunable window length

20 (default) | positive integer

Since R2026b

Specify the maximum value of the tunable window length as a positive integer. The window length you specify during simulation cannot exceed this value.

Dependencies

This parameter appears when you select Make window length tunable.

Auto-adjust invalid window length — Automatically adjust invalid window length values

on (default) | off

Since R2026b

When you select this parameter, the block adjusts invalid window length values from the input port and issues a warning.

• If the tunable window length value is greater than Maximum window length, the block uses Maximum window length.

• If the tunable window length value is less than 1, the block uses 1.

• If the tunable window length value is not an integer, the block uses the floored value.

When you clear this parameter, the block throws an error for invalid window length values.

Dependencies

This parameter appears when you select Make window length tunable and Specify window length from 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

Sliding Window Method

In the sliding window method, the output for each input sample is the median of the current sample and the Len−1 previous samples. Len is the length of the window in samples. To compute the first Len−1 outputs, when the window does not have enough data yet, the algorithm fills the window with zeros. As an example, to compute the median value when the second input sample comes in, the algorithm fills the window with Len−2 zeros. The data vector, x, is then the two data samples followed by Len−2 zeros. This object performs median filtering on the input data over time.

Consider an example of computing the moving median of a streaming input data using the sliding window method. The algorithm uses a window length of 4. With each input sample that comes in, the window of length 4 moves along the data.

Tunable Window Length Algorithm

When you specify the window length to be tunable, the median filter algorithm maintains an internal buffer of size MaxWL−1 to store past input samples, where MaxWL is the maximum window length. At each time step, the median filter algorithm performs these operations:

1. Concatenates the buffer with the current input to form an extended data sequence.

2. Extracts the most recent window length samples from the extended data sequence.

3. Computes the moving median over the extracted samples using a sliding window and discards the endpoints.

4. Stores the most recent MaxWL−1 samples in the buffer for the next time step.

This approach allows you to change the window length at each time step while the algorithm maintains continuity of the output signal.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2016b

R2026b: Tunable window length

You can now change the window length during simulation by selecting the Make window length tunable parameter. Specify the tunable window length from the block dialog box by updating the Window length parameter or through the WL input port by selecting Specify window length from input port. The Maximum window length parameter sets the upper bound for the tunable window length. Select Auto-adjust invalid window length to control whether invalid window length values are automatically adjusted with a warning or cause an error.

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.