Addressing scheme
Encyclopedia
There are three different addressing schemes for display device
s: direct, matrix, and raster. The purpose of each scheme is to set (or maintain) the state of a pixel
to either black/white or, more commonly, a gray-scale level.
) For a screen size of m×n pixels, this scheme would require m+n control signals. In active matrix addressing, some sort of capacitor (external to the cell proper) is used to maintain the state of the cell. Passive matrix addressing
is used when cell itself has a bistability, so no external capacitor is needed. In some situations passive matrix addressing 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 such as clocks.
) 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
) 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.
Display device
A display device is an output device for presentation of information in visual or tactile form...
s: direct, matrix, and raster. The purpose of each scheme is to set (or maintain) the state of a pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
to either black/white or, more commonly, a gray-scale level.
Direct addressing
A direct 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. This is generally considered to be an inefficient use of I/O and physical space.Matrix addressing
A matrix addressed display runs control signals only to the rows and columns of the display. (See also: matrixMatrix (mathematics)
In mathematics, a matrix is a rectangular array of numbers, symbols, or expressions. The individual items in a matrix are called its elements or entries. An example of a matrix with six elements isMatrices of the same size can be added or subtracted element by element...
) For a screen size of m×n pixels, this scheme would require m+n control signals. In active matrix addressing, some sort of capacitor (external to the cell proper) is used to maintain the state of the cell. Passive matrix addressing
Passive matrix addressing
Passive matrix addressing is an addressing scheme used in earlier LCD displays, and may be used in future LCD displays. This is a matrix addressing scheme meaning that only m + n control signals are required to address a m × n display...
is used when cell itself has a bistability, so no external capacitor is needed. In some situations passive matrix addressing 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 such as clocks.
Raster addressing
A raster addressed display (e.g., a CRTCathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
) 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
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
) 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.