Each output unit of computer system transmits information from the CPU and converts the electrical pulses to an appropriate output form. A printer, for example, is an output device that transmits output from the CPU and converts it to a printed form. There are many other output devices that can perform the sane electrical transmission and conversions to produce other output forms. The Visual Display Unit (VDU) is the most common form of output device. Mostly, it is based on the Cathode Ray Tube (CRT). But these have markedly increased in quality and decreased in price. Standards over, colour monitors developed over the years include the Colour Graphics Adaptor (CGA), Enhanced Graphics Adaptor (EGA) and Vedeo Graphics Array (VGA). These have become widespread. Alternatives to CRT including light emitting diodes, liquid crystals, and gas plasma discharge panels, are coining into use, particularly for portable computers.