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RMS

Compute root-mean-square value of input or sequence of inputs

Statistics

dspstat3

Description

The RMS block computes the RMS value of each row or column of the input, along vectors of a specified dimension of the input, or of the entire input. The RMS block can also track the RMS value in a sequence of inputs over a period of time. The Running RMS parameter selects between basic operation and running operation.

Basic Operation

When you do not select the Running RMS check box, the block computes the RMS value of each row or column of the input, along vectors of a specified dimension of the input, or of the entire input at each individual sample time, and outputs the array y. Each element in y is the RMS value of the corresponding column, row, vector, or entire input. The output y depends on the setting of the Find the RMS value over parameter. For example, consider a 3-dimensional input signal of size M-by-N-by-P:

• Entire input — The output at each sample time is a scalar that contains the RMS value of the entire input.

• Each row — The output at each sample time consists of an M-by-1-by-P array, where each element contains the RMS value of each vector over the second dimension of the input. For an input that is an M-by-N matrix, the output at each sample time is an M-by-1 column vector.

• Each column — The output at each sample time consists of a 1-by-N-by-P array, where each element contains the RMS value of each vector over the first dimension of the input. For an input that is an M-by-N matrix, the output at each sample time is a 1-by-N row vector.

In this mode, the block treats length-M unoriented vector inputs as M-by-1 column vectors.

• Specified dimension — The output at each sample time depends on Dimension. If Dimension is set to 1, the output is the same as that when you select Each column. If Dimension is set to 2, the output is the same as when you select Each row. If Dimension is set to 3, the output at each sample time is an M-by-N matrix containing the RMS value of each vector over the third dimension of the input.

The RMS value of the jth column of an M-by-N input matrix u is given by

```y = sqrt(sum(u.*conj(u))/size(u,1))	   	% Equivalent MATLAB code
```

Running Operation

When you select the Running RMS check box, the block tracks the RMS value of successive inputs to the block. In this mode, you must also specify a value for the Input processing parameter:

• When you select Elements as channels (sample based), the block outputs an M-by-N array. Each element yij of the output contains the RMS value of the element uij over all inputs since the last reset.

• When you select Columns as channels (frame based), the block outputs an M-by-N matrix. Each element yij of the output contains the RMS value of the jth column over all inputs since the last reset, up to and including element uij of the current input.

Running Operation for Variable-Size Inputs

When your inputs are of variable size, and you select the Running RMS check box, there are two options:

• If you set the Input processing parameter to Elements as channels (sample based), the state is reset.

• If you set the Input processing parameter to Columns as channels (frame based), then there are two cases:

• When the input size difference is in the number of channels (i.e., number of columns), the state is reset.

• When the input size difference is in the length of channels (i.e., number of rows), there is no reset and the running operation is carried out as usual.

Resetting the Running RMS

The block resets the running RMS whenever a reset event is detected at the optional Rst port. The reset sample time must be a positive integer multiple of the input sample time.

When a reset event occurs while the Input processing parameter is set to Elements as channels (sample based), the running RMS for each channel is initialized to the value in the corresponding channel of the current input. Similarly, when the Input processing parameter is set to Columns as channels (frame based), the running RMS for each channel is initialized to the earliest value in each channel of the current input.

You specify the reset event in the Reset port parameter:

• None disables the Rst port.

• Rising edge — Triggers a reset operation when the Rst input does one of the following:

• Rises from a negative value to a positive value or zero

• Rises from zero to a positive value, where the rise is not a continuation of a rise from a negative value to zero (see the following figure)

• Falling edge — Triggers a reset operation when the Rst input does one of the following:

• Falls from a positive value to a negative value or zero

• Falls from zero to a negative value, where the fall is not a continuation of a fall from a positive value to zero (see the following figure)

• Either edge — Triggers a reset operation when the Rst input is a Rising edge or Falling edge, as described earlier

• Non-zero sample — Triggers a reset operation at each sample time that the Rst input is not zero

 Note:   When running simulations in the Simulink® MultiTasking mode, reset signals have a one-sample latency. Therefore, when the block detects a reset event, there is a one-sample delay at the reset port rate before the block applies the reset. For more information on latency and the Simulink tasking modes, see Excess Algorithmic Delay (Tasking Latency) and Scheduling in the Simulink Coder™ User's Guide.

Examples

In the following ex_rms_refex_rms_ref model, the RMS block calculates the running RMS of a 3-by-2 matrix input, u. The Input processing parameter is set to Columns as channels (frame based), so the block processes the input as a two channel signal with a frame size of three. The running RMS is reset at t=2 by an impulse to the block's Rst port.

The operation of the block is shown in the following figure.

Dialog Box

Running RMS

Enables running operation when selected.

Input processing

Specify how the block should process the input when computing the running RMS. You can set this parameter to one of the following options:

• Columns as channels (frame based) — When you select this option, the block treats each column of the input as a separate channel.

• Elements as channels (sample based) — When you select this option, the block treats each element of the input as a separate channel.

This parameter appears only when you select the Running RMS check box.

 Note:   The option Inherit from input (this choice will be removed - see release notes) will be removed in a future release. See Frame-Based Processing in the DSP System Toolbox™ Release Notes for more information.
Reset port

Specify the reset event that causes the block to reset the running RMS. The sample time of the input to the Rst port must be a positive integer multiple of the input sample time. This parameter appears only when you select the Running RMS check box. For more information, see Resetting the Running RMS.

Find the RMS value over

Specify whether to find the RMS value along rows, columns, entire input, or the dimension specified in the Dimension parameter. For more information, see Basic Operation.

Treat sample-based row input as a column

Select to treat sample-based length-M row vector inputs as M-by-1 column vectors. This parameter is only visible when the Find the RMS value over parameter is set to Each column.

 Note:   This check box will be removed in a future release. See Sample-Based Row Vector Processing Changes for more information.
Dimension

Specify the dimension (one-based value) of the input signal, over which the RMS value is computed. The value of this parameter cannot exceed the number of dimensions in the input signal. This parameter is only visible when the Find the RMS value over parameter is set to Specified dimension.

Supported Data Types

• Double-precision floating point

• Single-precision floating point

• Boolean — The block accepts Boolean inputs to the Rst port.