Manipulating Complex-Valued Data
Supported Operations for Complex Data
System Identification Toolbox estimation algorithms support
complex data. For example, the following estimation commands estimate
complex models from complex data: ar, armax, arx, bj, covf, ivar, iv4, oe, pem, spa,
and n4sid.
Model transformation routines, such as freqresp and zpkdata,
work for complex-valued models. However, they do not provide pole-zero
confidence regions. For complex models, the parameter variance-covariance
information refers to the complex-valued parameters and the accuracy
of the real and imaginary is not computed separately.
The display commands compare and plot also
work with complex-valued data and models, but only show the absolute
values of the signals. To plot the real and imaginary parts of the
data separately, use plot(real(data)) and plot(imag(data)),
respectively.
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Processing Complex iddata Signals at the Command Line
If the iddata object data contains
complex values, you can use the following commands to process the
complex data and create a new iddata object.
| Command | Description |
|---|
| abs(data) | Absolute value of complex signals in iddata object. |
| angle(data) | Phase angle (in radians) of each complex signals in iddata object. |
| complex(data) | For time-domain data, this command makes the iddata object
complex—even when the imaginary parts are zero. For frequency-domain
data that only stores the values for nonnegative frequencies, such
that realdata(data)=1, it adds signal values for
negative frequencies using complex conjugation. |
| imag(data) | Selects the imaginary parts of each signal in iddata object. |
| isreal(data) | 1 when data (time-domain
or frequency-domain) contains only real input and output signals,
and returns 0 when data (time-domain
or frequency-domain) contains complex signals. |
| real(data) | Real part of complex signals in iddata object. |
| realdata(data) | Returns a value of 1 when data is
a real-valued, time-domain signal, and returns 0 otherwise. |
For example, suppose that you create a frequency-domain iddata object Datf by
applying fft to a real-valued
time-domain signal to take the Fourier transform of the signal. The
following is true for Datf:
isreal(Datf) = 0
realdata(Datf) = 1
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 | Transforming Between Time- and Frequency-Domain Data | | Linear Model Identification |  |
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