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Access data for frequency response data (FRD) object

`[response,freq] = frdata(sys)[response,freq,covresp] = frdata(sys) [response,freq,Ts,covresp] = frdata(sys,'v')[response,freq,Ts] = frdata(sys) `

`[response,freq] = frdata(sys)` returns
the response data and frequency samples of the FRD model `sys`.
For an FRD model with `Ny` outputs and `Nu`
inputs at `Nf` frequencies:

`response`is an`Ny`-by-`Nu`-by-`Nf`multidimensional array where the`(i,j)`entry specifies the response from input`j`to output`i`.`freq`is a column vector of length`Nf`that contains the frequency samples of the FRD model.

See the `frd` reference
page for more information on the data format for FRD response data.

`[response,freq,covresp] = frdata(sys) ` also
returns the covariance `covresp` of the response
data `resp` for `idfrd` model `sys`. The
covariance `covresp` is a 5D-array where `covH(i,j,k,:,:)` contains
the 2-by-2 covariance matrix of the response `resp(i,j,k)`.
The `(1,1)` element is the variance of the real
part, the `(2,2)` element the variance of the imaginary
part and the `(1,2)` and `(2,1)` elements
the covariance between the real and imaginary parts.

For SISO FRD models, the syntax

[response,freq] = frdata(sys,'v')

forces `frdata` to return the response data
as a column vector rather than a 3-dimensional array (see example
below). Similarly

`[response,freq,Ts,covresp] = frdata(sys,'v')` for
an IDFRD model sys returns covresp as a 3-dimensional rather than
a 5-dimensional array.

`[response,freq,Ts] = frdata(sys) `also returns
the sample time `Ts`.

Other properties of `sys` can be accessed with `get` or
by direct structure-like referencing (e.g., `sys.Frequency`).

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