VCD File Format for Exported Simulation Data
R2026bYou can export simulation data to a value change dump (VCD) file using the Simulation Data Inspector. A VCD file records only changes in signal values over time, which reduces file size compared to formats that store every time step. VCD files are supported by digital simulation viewers such as GTKWave and ModelSim. For more information about how to export data from the Simulation Data Inspector to a VCD file, see Export Data to VCD File.
Data Format in VCD File
The format of data in the VCD file depends on the type of signal being exported. Each signal maps to one or more VCD variables, where each variable represents a single data channel in the file.
| Simulink Signal Type | VCD File Format |
|---|---|
| Scalar signal | Single variable with a value recorded at each change |
| Scalar signal with complex data | Two variables that represent the real and imaginary parts |
| Nonscalar signal | Separate variable for each element of the vector, matrix, or array |
| Nonscalar signal with complex data | Separate variable for each element, each split into real and imaginary parts |
| Bus, array of buses | Separate variable for each bus element |
| String | Not supported |
| Message | Not supported |
| Variable-size signal | Separate variable for each element, padded to the maximum size across all time steps |
Signal Hierarchy
The Simulation Data Inspector organizes signals in the VCD file based on the model
hierarchy. Signals that share a common path prefix are grouped together. This structure
helps you locate exported signals in your VCD viewer. For example, a signal with block path
myModel/Controller/PID/Output is stored in the file under three levels
of hierarchy: myModel, Controller, and
PID. The signal name Output appears as a variable
inside the innermost level.
Timing and Value Changes
Unlike file formats that record a value at every time step, the Simulation Data Inspector writes a signal value to a VCD file only when it differs from the previous time step. This change-only approach minimizes file size.
When you export multiple signals, the Simulation Data Inspector merges the time vectors from all signals into a single sorted timeline.
Signal Representation
For a boolean signal, or a signal whose only values are 0 and 1, the
Simulation Data Inspector writes a binary waveform in the VCD file. The Simulation Data
Inspector stores all other numeric signals as analog waveforms.
How the Simulation Data Inspector records NaN and
Inf values depends on the signal type.
For
booleansignals,NaNandInfvalues are written as unknown.For numeric signals,
Infvalues are written as zero, andNaNvalues are skipped, such that the previous value is retained. If the initial value isNaNorInf, the signal starts at zero.
Limitations
Enumeration class names are not preserved in the VCD file. Only the underlying integer index values are written.
The export converts fixed-point values to double, which might reduce precision.
VCD files do not store signal metadata such as units, data types, or interpolation method.
