There are three different addressing schemes for
display device
A display device is an output device for presentation of information in visual or tactile form (the latter used for example in tactile electronic displays for blind people). When the input information that is supplied has an electrical signa ...
s: ''direct'', ''matrix'', and ''raster''. The purpose of each scheme is to set (or maintain) the state of a
pixel
In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a Raster graphics, raster image, or the smallest addressable element in a dot matrix display device. In most digital display devices, p ...
to either black/white or, more commonly, a
grayscale
In digital photography, computer-generated imagery, and colorimetry, a greyscale (more common in Commonwealth English) or grayscale (more common in American English) image is one in which the value of each pixel is a single sample (signal), s ...
level.
Direct addressing
A direct-driven, or directly addressed display runs individual control signals to each pixel. This allows the state, whether on/off or grayscale, to be set and maintained on each pixel. For a screen size of ''m''×''n'' pixels, this scheme would require ''m''×''n'' control signals in grayscale. This is generally considered to be inefficient, and is technically impossible for modern displays -- for example, 1920 x 1080 pixels and an RGB system (3 times as many control signals needed) results in ~6 million control lines.
Matrix addressing
A matrix-driven, or matrix addressed display runs control signals only to the rows (lines) and columns of the display.
(See also:
Display matrix) For a screen size of ''m''×''n'' pixels, this scheme requires ''m''+''n'' control signals in grayscale, or three times as many in RGB.
To address all pixels of such a display in the shortest time, either entire rows or entire columns have to be addressed sequentially. As many images are shown on a
16:9 aspect ratio, the sequential addressing is typically done row-by-row (i. e. line-by-line). In this case, fewer rows than columns have to be refreshed periodically.
Passive matrix addressing Passive matrix addressing is an Display addressing scheme, addressing scheme used in early liquid crystal displays, liquid crystal displays (LCDs). It is a matrix addressing scheme, meaning that only ''m'' + ''n'' control signals are requi ...
is used with the help of persistence of vision of the (usually human) eye so the cell need not be bistable. Persistence of vision is used in simpler, slower changing displays with relatively few picture elements such as clocks. In
active matrix addressing
Active matrix is a type of addressing scheme used in flat panel displays. It is a method of switching individual elements of a flat panel display, known as pixels. Each pixel is attached to a transistor and capacitor that ''actively'' maintain th ...
, some sort of capacitor (external to the cell proper) is used to maintain the state of the cell. When the electro-optical property of cells itself is bistable, passive matrix addressing
without external capacitor can be implemented.
Active matrix displays are able to have higher resolution, contrast, and colors, but passive matrix displays can often be cheaper.
Raster addressing
A raster addressed display (e.g., a
CRT
CRT or Crt most commonly refers to:
* Cathode-ray tube, a display
* Critical race theory, an academic framework of analysis
CRT may also refer to:
Law
* Charitable remainder trust, United States
* Civil Resolution Tribunal, Canada
* Columbia ...
) works by scanning across the entire display in sequence while modulating control signal to activate each pixel as it is scanned. This display uses persistence of the pixel element (e.g.,
phosphor
A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or ...
) to maintain the pixel state until the scan can visit that pixel again. There are only three control signals required for this to work: a horizontal scan control signal, a vertical scan control signal, and an intensity control signal. Timing between these signals is very important, else the image on the screen will show artifacts.
References
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Display technology