Distinguishing Characteristics of Signals

• Signals are changing with time. It is said that they have a time-domain representation.
• Signals are also characterized by the range of frequencies that they occupy.
• This is called the spectrum or frequency-domain representation of the signal.
• Signals can be separated easily if they do not overlap in either the time-domain or the frequency-domain.
• The two representations are connected through what is known as a Fourier Transform.
  
    

  Figure 2.2: Time domain (top) and frequency domain (bottom) representation of a signal.