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Upsample discrete-time models

`sysl = upsample(sys,L)`

`sysl = upsample(sys,L)` resamples the discrete-time
dynamic system model `sys` at a sampling rate that
is `L`-times faster than the sampling time of `sys` (*Ts*_{0}). `L` must
be a positive integer. When `sys` is a TF model, *H*(*z*), `upsample` returns `sysl` as *H*(*z*^{L})
with the sampling time *Ts*_{0} / `L`.

The responses of models `sys` and `sysl` have
the following similarities:

The time responses of

`sys`and`sysl`match at multiples of*Ts*_{0}.The frequency responses of

`sys`and`sysl`match up to the Nyquist frequency*π*/*Ts*_{0}.

Create a transfer function with a sampling time that is 14 times faster than that of the following transfer function:

sys = tf(0.75,[1 10 2],2.25) Transfer function: 0.75 -------------- z^2 + 10 z + 2 Sampling time: 2.25

To create the upsampled transfer function `sys1`,
type the following commands:

L=14; sys1 = upsample(sys,L)

These commands return the result:

Transfer function: 0.75 ------------------ z^28 + 10 z^14 + 2 Sampling time: 0.16071

The sampling time of `sys1` is `0.16071` seconds,
which is 14 times faster than the `2.25` second sampling
time of `sys`.

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