A CDE Definition
vertical scan frequency
Also called "refresh rate," it is the number of times an entire CRT screen was refreshed, or redrawn, per second. Measured in Hertz, display systems typically ranged from 56 Hz to over 100, but at least 70 was recommended to help prevent eyestrain.
The earliest CRTs were unable to support both the highest resolution along with a high refresh rate; however, later CRTs provided adequate rates at all resolutions. Vertical scan frequency is not an issue with LCD displays because they do not use phosphors that have to be continuously energized. However, the data from the frame buffer is continuously sent to the LCD panel in order to display animation and video. Thus, the flat panel does have a maximum refresh rate to support motion, but not to refresh static images. Contrast with horizontal scan frequency. See interlace.
To illuminate a screen by displaying all odd lines in the frame first and then all even lines. Interlacing uses half frames per second (fields per second) rather than full frames per second.
The interlace method was developed for TV broadcasting because the allotted bandwidth for TV channels in the 1940s was not sufficient to transmit 60 full frames per second. It was decided that interlacing with 60 half frames was visually better than 30 non-interlaced full frames.
Interlace Vs. Progressive Scan (I vs. P)
Interlaced screens display odd lines first: 1-3-5, etc.; then even lines: 2-4-6, etc. Non-interlaced "progressive scan" screens display lines consecutively: 1-2-3-4-5-6, etc.
All old tube TVs (CRTs) were interlaced. Very old CRT computer monitors were interlaced at their highest resolution and progressive at lower resolutions. Today, some digital TV standards are interlaced, such as the high-definition 1080i format, and 1080i material is commonly transmitted by the TV networks. HDTV sets support both interlace and progressive scan methods (see HDTV and DTV). See deinterlace, vertical scan frequency, 4K 3D TV and interlaced GIF.
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