Editing Analog television (section)

==Displaying an image==
[[File:Raster-scan.svg|thumb|400px|[[Raster scan]]ning is performed from left-to-right and top-to-bottom. Once the screen has been scanned, the beam returns to the beginning of the first line.]]
[[File:TV screen close-up.jpg|thumb|right|Close up image of analog color screen]]

A [[cathode-ray tube]] (CRT) television displays an image by scanning a beam of [[electrons]] across the screen in a pattern of horizontal lines known as a [[raster scan|raster]]. At the end of each line, the beam returns to the start of the next line; at the end of the last line, the beam returns to the beginning of the first line at the top of the screen. As it passes each point, the intensity of the beam is varied, varying the [[luminance]] of that point. A [[color television]] system is similar except there are three beams that scan together and an additional signal known as chrominance controls the color of the spot.

When analog television was developed, no affordable technology for storing video signals existed; the luminance signal had to be generated and transmitted at the same time at which it is displayed on the CRT.  It was therefore essential to keep the raster scanning in the camera (or other device for producing the signal) in exact [[synchronization]] with the scanning in the television.

The physics of the CRT require that a finite time interval be allowed for the spot to move back to the start of the next line (''horizontal retrace'') or the start of the screen (''vertical retrace''). The timing of the luminance signal must allow for this.

The human eye has a characteristic called [[phi phenomenon]]. Quickly displaying successive scan images creates the illusion of smooth motion. Flickering of the image can be partially solved using a long persistence phosphor coating on the CRT so that successive images fade slowly. However, slow phosphor has the negative side-effect of causing image smearing and blurring when there is rapid on-screen motion occurring.

The maximum [[frame rate]] depends on the [[Bandwidth (signal processing)|bandwidth]] of the electronics and the transmission system, and the number of horizontal scan lines in the image. A frame rate of 25 or 30 [[hertz]] is a satisfactory compromise, while the process of [[Interlaced video|interlacing]] two [[video field]]s of the picture per [[film frame|frame]] is used to build the image. This process doubles the apparent number of [[video frame]]s per second and further reduces flicker and other defects in transmission.