Editing Broadcast television systems (section)

==== Viewing technology ====
Analog television signal standards are designed to be displayed on a [[Cathode-ray tube|cathode ray tube]] (CRT), and so the physics of these devices necessarily controls the format of the video signal. The image on a CRT is painted by a moving beam of electrons which hits a [[phosphor]] coating on the front of the tube. This electron beam is steered by a magnetic field generated by powerful [[electromagnet]]s close to the source of the electron beam.

In order to reorient this magnetic steering mechanism, a certain amount of time is required due to the [[inductance]] of the magnets; the greater the change, the greater the time it takes for the electron beam to settle in the new spot.

For this reason, it is necessary to shut off the electron beam (corresponding to a video signal of [[zero luminance]]) during the time it takes to reorient the beam from the end of one line to the beginning of the next (''horizontal retrace'') and from the bottom of the screen to the top (''vertical retrace'' or ''[[vertical blanking interval]]''). The horizontal retrace is accounted for in the time allotted to each scan line, but the vertical retrace is accounted for as ''phantom lines'' which are never displayed but which are included in the number of lines per frame defined for each video system. Since the electron beam must be turned off in any case, the result is gaps in the television signal, which can be used to transmit other information, such as test signals or color identification signals.

The temporal gaps translate into a comb-like [[Spectral density|frequency spectrum]] for the signal, where the teeth are spaced at line frequency and concentrate most of the energy; the space between the teeth can be used to insert a color subcarrier.