\section{Signals}
\subsection{Classification}
%% TODO

\subsection{Properties}
\begin{table}[h]
  \everymath={\displaystyle}
  \[
    \begin{array}{l l}
      \toprule
      \text{\bfseries Characteristic} & \text{\bfseries Symbol and formula} \\
      \midrule
      \text{Normalized energy} & E_n = \lim_{T\to\infty} \int_T |x|^2 \,dt \\[6pt]
      \midrule
      \text{Normalized power}  & P_n = \lim_{T\to\infty} \frac{1}{T} \int_T |x|^2 \,dt \\[12pt]
      \text{Linear mean}       & X_0 = \frac{1}{T} \int_T x\, dt \\[12pt]
      \text{Mean square}       & X^2 = \frac{1}{T} \int_T x^2 \, dt \\[12pt]
      n\text{-th order mean}   & X^n = \frac{1}{T} \int_T x^n \, dt \\[12pt]
      \text{Rectified value}   & |\bar{X}| = \frac{1}{T} \int_T |x| \,dt \\[12pt]
      \text{Variance}          & \sigma^2 = \frac{1}{T} \int_T \left(x - X_0\right)^2 dt \\[12pt]
                               & = X^2 - X_0 \\[6pt]
      \text{Root mean square}  & X_\text{rms} = \sqrt{X^2} \\
      \bottomrule
    \end{array}
  \]
\end{table}