Difference

Compute element-to-element difference along specified dimension of input

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Libraries:
DSP System Toolbox / Math Functions / Math Operations

Description

The Difference block computes the difference between adjacent elements in rows, columns, or a specified dimension of the input array u. You can configure the block to compute the difference only within the current input, or across consecutive inputs (running difference).

Examples

Compute Difference of a Matrix

Simulink^® model example that shows how to calculate the rowwise and columnwise difference of a matrix.

• Open Model

Ports

Input

Port_1 — Input signal

vector | matrix | N-D array

Input signal, specified as a vector, matrix, or an N-D array.

Data Types: single | double | int8 | int16 | int32 | uint8 | uint16 | uint32 | fixed point
Complex Number Support: Yes

Output

Port_1 — Output signal

vector | matrix | N-D array

The difference between adjacent rows, columns, or a specified dimension of the input, returned as a vector, matrix, or an N-D array.

The output is the same data type and complexity as the input, but the dimension that the difference was calculated on is one less. The length of the other dimensions is unchanged.

For more details on how the output is computed, see the Difference along parameter.

Data Types: single | double | int8 | int16 | int32 | uint8 | uint16 | uint32 | fixed point
Complex Number Support: Yes

Parameters

Main Tab

Running difference — Running difference

No (default) | Yes

Specify whether the block computes a running difference.

• No –– The block computes the difference between adjacent elements in the specified dimension of the current input. In this mode, the block can compute the difference along the columns, rows, or a specified dimension of the input depending on the Difference along parameter.

• Yes –– The block computes the running difference along the columns of the input. See Running Operation for more information.

Difference along — Difference dimension

Columns (default) | Rows | Specified dimension

Specify whether the block computes the difference along the columns, rows, or a specified dimension of the input.

• Columns –– The block computes differences between adjacent elements in each column of the input. Equivalent MATLAB^® code is given by:

  y = diff(u)				

  For M-by-N inputs, the output is an (M – 1)-by-N matrix whose jth column has these elements:

  $$y_{i,j}=u_i{}_{+1}{,}_j-u_{i,j}1\le i\le (M-1)$$

• Rows –– The block computes differences between adjacent elements in each row of the input. Equivalent MATLAB code is given by:

  y = diff(u,[],2)			

  The output is an M-by-(N-1) matrix whose ith row has the following elements:

  $$y_{i,j}=u_{i,j+1}-u_{i,j}1\le j\le (N-1)$$

• Specified dimension –– The behavior of the block is an extension of the row-wise differencing described earlier. The block computes differences between adjacent elements along the dimension you specify in the Dimension parameter. Equivalent MATLAB code is given by:

  y = diff(u,[],d)  

  where d is the dimension.

  The output is an array whose length in the specified dimension is one less than that of the input, and whose lengths in other dimensions are unchanged. For example, consider an M-by-N-by-P-by-R input array with elements u(i,j,k,l) and assume that the value of the Dimension parameter is 3. The output of the block is an M-by-N-by-(P–1)-by-R array with the following elements:

  $$y_{i,j,k,l}=u_{i,j,k+1,l}-u_{i,j,k,l}1\le k\le (P-1)$$

Dimension — One-based dimension

1 (default) | 2 | 3

Specify the one-based dimension along which to compute element-to-element differences.

Dependencies

To enable this parameter, select Specified dimension for the Difference along parameter.

Data Types Tab

Rounding mode — Method of rounding operation

Floor (default) | Ceiling | Convergent | Nearest | Round | Simplest | Zero

Specify the rounding mode for fixed-point operations as one of the following:

• Floor

• Ceiling

• Convergent

• Nearest

• Round

• Simplest

• Zero

For more details, see Rounding Modes.

Saturate on integer overflow — Method of overflow action

off (default) | on

When you select this parameter, the block saturates the result of its fixed-point operation. When you clear this parameter, the block wraps the result of its fixed-point operation. For details on saturate and wrap, see overflow mode for fixed-point operations.

Accumulator — Accumulator data type

Inherit: Inherit via internal rule (default) | Inherit: Same as input | fixdt([],16,0)

Accumulator specifies the data type of the output of an accumulation operation in the Difference block.

• Inherit: Inherit via internal rule — The block inherits the accumulator data type based on an internal rule. For more information on this rule, see Inherit via Internal Rule.

• Inherit: Same as input — The block specifies the accumulator data type to be the same as the input data type.

• fixdt([],16,0) — The block specifies an autosigned, binary-point, scaled, fixed-point data type with a word length of 16 bits and a fraction length of 0.

Alternatively, you can set the Accumulator data type by using the Data Type Assistant. To use the assistant, click the Show data type assistant button.

For more information on the data type assistant, see Specify Data Types Using Data Type Assistant (Simulink).

For illustrations on how to use the accumulator data type in this block, see Fixed-Point Data Types.

Output — Output data type

Inherit: Same as accumulator (default) | Inherit: Same as input | fixdt([],16,0)

Output specifies the data type of the output of the Difference block.

• Inherit: Same as input — The block specifies the output data type to be the same as the input data type.

• Inherit: Same as accumulator — The block specifies the output data type to be the same as the accumulator data type.

• fixdt([],16,0) — The block specifies an autosigned, binary-point, scaled, fixed-point data type with a word length of 16 bits and a fraction length of 0.

Alternatively, you can set the Output data type by using the Data Type Assistant. To use the assistant, click the Show data type assistant button.

For more information, see Control Data Types of Signals (Simulink).

For more information on the output data type, see Fixed-Point Data Types.

Output Minimum — Minimum value that block can output

[] (default) | scalar

Specify the minimum value the block can output. Simulink uses this minimum value to perform:

• Simulation range checking. See Specify Signal Ranges (Simulink).

• Automatic scaling of fixed-point data types.

Output Maximum — Maximum value that block can output

[] (default) | scalar

Specify the maximum value the block can output. Simulink uses this maximum value to perform:

• Simulation range checking. See Specify Signal Ranges (Simulink).

• Automatic scaling of fixed-point data types.

Lock data type settings against changes by the fixed-point tools — Prevent fixed-point tools from overriding data types

off (default) | on

Select this parameter to prevent the fixed-point tools from overriding the data types you specify in the block dialog box.

Note

Floating-point inheritance takes precedence over the data type settings defined on this pane. When inputs are floating point, the block ignores these settings, and all internal data types are floating point.

Block Characteristics

Data Types	double | fixed point | integer | single
Direct Feedthrough	no
Multidimensional Signals	no
Variable-Size Signals	yes
Zero-Crossing Detection	no

More About

Running Operation

When you set the Running difference parameter to Yes, the block computes the running difference along the columns of the input.

For an M-by-N input matrix, the output is an M-by-N matrix whose jth column has the following elements:

$$y_{i,j}=u_{i+1,j}-u_{i,j}2\le i\le (M-1)$$

In the Running difference mode, the first element of the output for each column is the first input element minus the last input element of the previous frame. For the first frame, the block subtracts zero from the first input element.

$$y_{1,j}(t)=u_{1,j}(t)-u_{M,j}(t-T_f)$$

Fixed-Point Data Types

This diagram shows the data types used within the Difference block for fixed-point signals.

You can set the accumulator and output data types in the block dialog as discussed in Parameters.

Extended Capabilities

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

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

Version History

Introduced before R2006a