add video and docs

Author: baggepinnen

docs/src/lib/timefreqresponse.md

@@ -1,10 +1,13 @@
+# Time and Frequency response analysis
 ```@index
 Pages = ["timefreqresponse.md"]
 ```
 
-# Time and Frequency response
 
-Frequency responses are calculated using [`freqresp`](@ref), [`bode`](@ref) and [`nyquist`](@ref).
+
+## Frequency response
+
+Frequency responses are calculated using [`freqresp`](@ref), [`bode`](@ref), [`sigma`](@ref) and [`nyquist`](@ref). Frequency-response plots are obtained using [`bodeplot`](@ref), [`sigmaplot`](@ref), [`nyquistplot`](@ref), [`marginplot`](@ref) and [`nicholsplot`](@ref).
 
 Any [`TransferFunction`](@ref) can be evaluated at a point using
 `F(s)`, `F(omega, true)`, `F(z, false)`
@@ -13,10 +16,31 @@ Any [`TransferFunction`](@ref) can be evaluated at a point using
 - `F(omega,true)` evaluates the discrete-time transfer function `F` at `exp(i*Ts*omega)`
 - `F(z,false)` evaluates the discrete-time transfer function `F` at `z`
 
+A video demonstrating frequency-response analysis in ControlSystems.jl is available below.
+
+```@raw html
+<iframe style="height: 315px; width: 560px" src="https://www.youtube.com/embed/_ZvvRLpCLG0" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
+```
+
+## Time response (simulation)
+
 Simulation with arbitrary inputs is primarily handled by the function [`lsim`](@ref), with [`step`](@ref) and [`impulse`](@ref) serving as convenience functions to simulate responses to particular inputs.
 
+The function [`lsim`](@ref) can take an input vector `u` containing a sampled input trajectory, or an input function taking the state and time as arguments, `u(x,t)`. This function can be used to easily simulate, e.g., ramp responses or saturated state-feedback control etc. See the docstring of [`lsim`](@ref) for more details.
+
 For more extensive nonlinear simulation capabilities, see the notes on ModelingToolkit and DifferentialEquations under [The wider Julia ecosystem for control](@ref).
 
+#### Example:
+The following simulates a step response of a second-order system and plots the result.
+```@example TIMERESP
+using ControlSystemsBase, Plots
+G = tf(1, [1, 1, 1])
+res = step(G, 20) # Simulate 20 seconds step response
+plot(res)
+```
+
+## Docstrings
+
 ```@autodocs
 Modules = [ControlSystems, ControlSystemsBase]
 Pages   = [libpath*"/timeresp.jl", libpath*"/result_types.jl", libpath*"/freqresp.jl", "simulators.jl"]