Being a physical object, any signal exists in three-dimensional space and time. If the space is defined by the Cartesian coordinates x,y,z, and the time variable is t, then such primary signal can be described by a function of these four variables with a limited domain of definition: $$F\left( {x,y,z,t} \right),x \in \left( { - X/2,X/2} \right),y \in \left( { - Y/2,Y/2} \right),z \in ( - Z/2,Z/2),t \in \left( {0,T} \right),$$ (1.1.1) where X,Y,Z is the size of the spatial (supposedly, rectangular) domain of definition of the signal, T is the duration of the signal.