Model signals and systems: grasp the principal contradiction, simplify complex details.
What signal?
Physical input-output system, such as voltage or current signals are output circuit. Signal is an artificially defined, is what people are most concerned about. Generally, these quantities are a function of time, so the signal is essentially also a function.
Generally an artificial input is controlled physical quantity, the input is the output obtained through the physical system.
What systems are?
The system is to be abstracted into complex things concerned only input and output signals of the black box.
Examples of signals and systems
The following is a heavy spring system, wherein the upper end of the spring is movable. X is a position input signal to move the upper end, the output signal y of the position of the weight. This constitutes a system, obtain the relationship between the input and output based on knowledge of physics associated spring. False input with a certain facility can be obtained by the action of a corresponding output signal of the system.
The following figure shows a two reservoir system connected in series, the input of rainfall r0, the final output is the system of water supply r2. About the middle of some of the physical height of the water level inside the only system variables, not what we are most concerned about input and output.
System in series: a plurality of systems may be composed of a system, a system can be divided into a plurality of internal modules.
Under the figure cited two examples of a mobile phone call. Speech signals need to be converted to an electromagnetic wave sent to the base station, the base station transmits signals to the optical fiber by the base station of the called user, and then sent to the called user equipment via an electromagnetic wave signal, an acoustic signal is ultimately converted.
The system can be regarded as forming a plurality of systems connected in series, it can also be considered a system with a plurality of modules.
Continuous-time and discrete-time signal signal
Input and output signal of the few examples of a function of time. Time-varying physical quantities can be continuous, or can be discrete. Generally the actual physical quantity is continuous, but discrete digital signal processing system to facilitate computer.
CT and DT intermediate signal into a signal
Continuous time (CT) by means of the sampling signal can be converted into a discrete time (DT) signal.
During sampling the signal loss certain accuracy. Therefore DT signal back to the CT signal process on the need to "guess" signal loss. Corresponding to the above-described two processes ADC analog to digital conversion and digital to analog conversion in the DAC circuit.
DT simple recovery method to the CT:
Piecewise linear (piecewise linear)
零阶保持（zero-order hold）
例子：储水的水池和储电的电容