Moving Standard Deviation
Moving standard deviation
 Library:
DSP System Toolbox / Statistics
Description
The Moving Standard Deviation block computes the moving standard deviation of the input signal along each channel independently over time. The block uses either the sliding window method or the exponential weighting method to compute the moving standard deviation. In the sliding window method, a window of specified length moves over the data sample by sample, and the block computes the standard deviation over the data in the window. In the exponential weighting method, the block computes the exponentially weighted moving variance and takes the square root. For more details on these methods, see Algorithms.
Ports
Input
x
— Data input
column vector  row vector  matrix
Data over which the block computes the moving standard deviation. The block accepts realvalued or complexvalued multichannel inputs, that is, mbyn size inputs, where m ≥ 1 and n ≥ 1. The block also accepts variablesize 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 set Method to
Exponential weighting
and select the Specify forgetting
factor from input port parameter.
Data Types: single
 double
Complex Number Support: Yes
lambda
— Forgetting factor
positive real scalar in the range (0,1]
The forgetting factor determines how much weight past data is given. A forgetting factor of 0.9 gives more weight to the older data than does a forgetting factor of 0.1. A forgetting factor of 1.0 indicates infinite memory – all previous samples are given an equal weight.
Dependencies
This port appears when you set Method to Exponential
weighting
and select the Specify forgetting factor from input
port parameter.
Data Types: single
 double
Output
Port_1
— Moving standard deviation output
column vector  row vector  matrix
The size of the moving standard deviation output matches the size of the input. The block uses either the sliding window method or the exponential weighting method to compute the moving standard deviation, as specified by the Method parameter. For more details, see Algorithms.
Data Types: single
 double
Complex Number Support: Yes
Parameters
If a parameter is listed as tunable, then you can change its value during simulation.
Method
— Moving standard deviation method
Sliding window
(default)  Exponential weighting
Sliding window
— A window of length Window length moves over the input data along each channel. For every sample the window moves by, the block computes the standard deviation over the data in the window.Exponential weighting
— The block computes the exponentially weighted moving standard deviation and takes the square root. The magnitude of the weighting factors decreases exponentially as the age of the data increases, but the magnitude never reaches zero.
For more details on these methods, see Algorithms.
Specify window length
— Flag to specify window length
on (default)  off
When you select this check box, the length of the sliding window is equal to the value you specify in Window length. When you clear this check box, the length of the sliding window is infinite. In this mode, the block computes the standard deviation of the current sample with respect to all the previous samples in the channel.
Dependencies
This parameter appears when you set Method to Sliding
window
.
Window length
— Length of sliding window
4 (default)  positive scalar integer
Specifies the length of the sliding window in samples.
Dependencies
This parameter appears when you set Method to Sliding
window
and select the Specify window length check
box.
Specify forgetting factor from input port
— Flag to specify forgetting factor
off (default)  on
When you select this check box, the forgetting factor is input through the lambda port. When you clear this check box, the forgetting factor is specified on the block dialog through the Forgetting factor parameter.
Dependencies
This parameter appears only when you set Method to
Exponential weighting
.
Forgetting factor
— Exponential weighting factor
0.9 (default)  positive real scalar in the range (0,1]
The forgetting factor determines how much weight past data is given. A forgetting factor of 0.9 gives more weight to the older data than does a forgetting factor of 0.1. A forgetting factor of 1.0 indicates infinite memory – all previous samples are given an equal weight.
Tunable: Yes
Dependencies
This parameter appears when you set Method to
Exponential weighting
and clear the Specify forgetting
factor from input port check box.
Simulate using
— Type of simulation to run
Code generation
(default)  Interpreted execution
Specify the type of simulation to run as one of the following:
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 simulation speed thanInterpreted execution
.Interpreted execution
–– Simulate model using the MATLAB^{®} interpreter. This option shortens startup time but has slower simulation speed thanCode generation
.
Block Characteristics
Data Types 

Multidimensional Signals 

VariableSize Signals 

Algorithms
Sliding Window Method
In the sliding window method, the output at the current sample is the standard deviation of the current sample with respect to the data in the window. 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 standard deviation when the second input sample comes in, the algorithm fills the window with Len – 2 zeros. Len is the length of the window in samples. The data vector, x, is then the two data samples followed by Len – 2 zeros.
When you do not specify the window length, the algorithm chooses an infinite window length. In this mode, the output is the moving standard deviation of the current sample with respect to all the previous samples in the channel.
Consider an example of computing the moving standard deviation 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.
Exponential Weighting Method
In the exponential weighting method, the moving standard deviation is computed recursively using these formulas:
$$\begin{array}{l}{s}_{N,\lambda}=\sqrt{\frac{1}{{v}_{N,\lambda}}{\displaystyle \sum _{k=1}^{N}{\lambda}^{Nk}}{\left[{x}_{k}{\overline{x}}_{N,\lambda}\right]}^{2}}\\ {v}_{N,\lambda}=\frac{2\lambda (1{\lambda}^{N1})}{(1\lambda )(1+\lambda )}\end{array}$$
$${s}_{N,\lambda}$$ — Moving standard deviation of the current data sample with respect to the rest of the data.
$${\left[{x}_{k}{\overline{x}}_{N,\lambda}\right]}^{2}$$ — Difference between each data sample and the average of the data, squared.
$$\sum _{k=1}^{N}{\lambda}^{Nk}}{\left[{x}_{k}{\overline{x}}_{N,\lambda}\right]}^{2$$ — Difference between each data sample and the average of the data, squared and multiplied with the forgetting factor. All the squared terms are added.
$$\frac{1}{{v}_{N,\lambda}}$$ — Weighting factor applied to the sum.
λ — Forgetting factor.
As the age of the data increases, the magnitude of the weighting factor decreases exponentially and never reaches zero. In other words, the recent data has more influence on the current standard deviation than the older data.
The value of the forgetting factor determines the rate of change of the weighting factors. A forgetting factor of 0.9 gives more weight to the older data than does a forgetting factor of 0.1. A forgetting factor of 1.0 indicates infinite memory. All previous samples are given an equal weight.
Consider an example of computing the moving standard deviation using the exponential weighting method. The forgetting factor is 0.9.
Extended Capabilities
C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.
Version History
See Also
Blocks
 Standard Deviation  Moving Average  Moving Maximum  Moving Minimum  Moving Variance  Moving RMS  Median Filter
Objects
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