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We perceive signals with our senses and instruments. Different gauges have been developed to measure different quantities and transform them into signals, using the effects triggered by the variables for the measurement. The measured values are usually processed with computers today. There is a growing demand for making the measured values available in a digital form so that they can be directly processed by computers. The development of measurement devices also follows this trend, and these devices are becoming suitable for the evaluation of measurement results as well.
Through signals we can detect the inputs, outputs, the disturbances, the operation and the control of the system. All state variables that can be detected play an important role in examining the processes in the system.