User`s guide

Signal Concepts
3-7
Additional Settings for Discrete-Time Simulations. It is worthwhile to know how the
other solver options available in Simulink affect discrete-time signals. In
particular, you should be aware of the properties of discrete-time signals under
the following settings:
•
Type = Fixed-step, Mode = MultiTasking
•Type = Variable-step (Simulink’s default solver)
•
Type = Fixed-step, Mode = Auto
When the Fixed-step MultiTasking solver is selected, discrete signals in
Simulink most accurately model the prototypical discrete signal described in
“Discrete-Time Signals” on page 3-3. In particular, when these settings are in
effect, discrete signals are undefined between sample times. Simulink
generates an error when operations attempt to reference the undefined region
of a signal, as, for example, when signals with different sample rates are added.
To perform cross-rate operations like the addition of two signals with different
sample rates, you must explicitly convert the two signals to a common sample
rate. There are several blocks provided for precisely this purpose in the Signal
Operations and Multirate Filters libraries. See “Converting Sample Rates and
Frame Rates” on page 3-20 for more information. By requiring explicit rate
conversions for cross-rate operations in discrete mode, Simulink helps you to
identify sample rate conversion issues early in the design process.
When the
Variable-step solver is selected, discrete time signals remain
defined between sample times, just as in the
Fixed-step SingleTasking
setting previously described in “Recommended Settings for Discrete-Time
Simulations”. In this mode, cross-rate operations are allowed by Simulink.
In the
Fixed-step Auto setting, Simulink automatically selects a tasking mode
(single-tasking or multitasking) that is best suited to the model. See “Simulink
Tasking Mode” on page 3-91 for a description of the criteria that Simulink uses
to make this decision. For the typical model containing multiple rates,
Simulink selects the multitasking mode.
Cross-Rate Operations in Variable-Step and Fixed-Step SingleTasking Modes. In
Simulink’s
Variable step and Fixed-step SingleTasking modes, a
discrete-time signal is defined between sample times. Therefore, if you sample
the signal with a rate or phase that is different from the signal’s own rate and
phase, you will still measure meaningful values.