fundamentals of the signals and system

What is a Signal? Anything which carries information is a signal. e.g. human voice, chirping of birds, smoke signals,  gestures (sign language), fragrances of the flowers. Many of our body functions are regulated by  chemical signals, blind people use sense of touch. Bees communicate by their dancing pattern. Modern high speed signals are: voltage changer in a telephone wire, the electromagnetic field emanating  from a transmitting antenna,variation of light intensity in an optical fiber. Thus we see that there is an almost endless variety of signals and a large number of ways in which signals are carried from  on place to another place.  2.  Signals: The Mathematical Way A signal is a real (or complex) valued function of one or more real variable(s).When the function  depends on a single variable, the signal is said to be one-dimensional and when the function depends  on two or more variables, the signal is said to be multidimensional. Examples of a one dimensional signal:  A speech signal, daily maximum temperature, annual  rainfall at a place An example of a two dimensional signal: An image is a two dimensional signal, vertical and horizontal coordinates representing the two dimensions. Four Dimensions: Our physical world is four dimensional(three spatial and one temporal).