Analog signals are a representation of time varying quantities in a continuous signal. Basically, a time variance is presented in a manner in which some sort of information is passed using various types of methods, including electrical, mechanical, hydraulic, or pneumatic systems. Unlike digital signals, which use a numeric method of transmitting information, analog signals use small fluctuations in the signal itself to pass information.
These signals act essentially like simulations of a continuous time varying quantity. They duplicate the features of the actual quantity by presenting a different one. In other words, they use one method of recording information and transfer it to a different format that, in turn, presents the information in that medium.
Each analog signal uses a property of the final medium to convey the information for the signal. For example, a thermometer will use the heat of a particular object to determine its temperature. The heat is then transferred to mercury, which changes its position to display the temperature information on the gauge.
The most common analog form of transmission occurs electrically. In order for this to happen, a voltage must be sent at a specific frequency. The flow of this electrical charge is known as current. By controlling the frequency of the current, information can be transmitted to another medium and presented on that medium. For example, magnetic tape on a cassette conveys information to the stereo, which transmits it as electrical signals of specific frequencies, which in turn tell the speakers what noise to make.
Analog signals have a great advantage over digital signals in that they have a much higher density that can present more refined information. Essentially, there is the potential for the signal resolution to be infinite. In addition, the process to create this type of signal is achieved much more simply. By merely adjusting the time quantities, information can be presented.
Disadvantages of the system include the tendency to create unwanted variations in the information transmission such as noise, which can occur in random patterns. When a signal is copied and potentially re-copied, each subsequent version exhibits more of the random patterns, making information transmission harder and ultimately causes signal loss.
In order to avoid these disadvantages, or at least mitigate their effects, the concept of modulation can be used. The base signal is modified in some way to help retain the information as it is transmitted. An example of this is when the amplitude of a waveform is altered in what is known as amplitude modulation. Other options for retaining an electric signal over different generations are by using increased shielding or different cable types twisted together.