Display addressing scheme

There are three different addressing schemes for display devices: 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.

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: matrix) For a screen size of m×n pixels, this scheme would require m+n control signals. When a row and column are activated, only the pixel at the intersection of the row and column is addressed. In this scheme only a single pixel can have its state set or maintained at any one time, which means that the state of the pixel must persist without a control signal during the time which other pixels are being addressed. This persistence can be accomplished through active matrix addressing, passive matrix addressing, or the persistence of vision of the (usually human) eye. Persistence of vision is used in simpler, slower changing displays such as clocks.

Raster addressing

A raster addressed display (e.g., a CRT) 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.