Observation of frequencies
All operations carried out on and with signals depend on the observation of how the signals were determined, measured and transmitted earlier.
Observing the formation and spreading of sound waves, vibrations and echo distances shaped ancient techniques and provoked thoughts. The direction of sun rays demonstrated the relationship between time and different phenomena; later the rotation angle of the analogue clock hand on the clock face became the tool of time measurement. The angle of rotation inspired also the representation of a vector by its absolute value and phase angle on the complex plain.