Multiplexed display
Encyclopedia
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
, that is, driven one at a time, but the electronics and the persistence of vision
Persistence of vision
Persistence of vision is the phenomenon of the eye by which an afterimage is thought to persist for approximately one twenty-fifth of a second on the retina....
combine to make the viewer believe the entire display is continuously active.
A multiplexed display has several advantages compared to a non-multiplexed display:
- Fewer wires (often, far fewer wires) are needed
- Simpler driving electronics can be used
- And both lead to reduced cost
- Reduced power consumption
Multiplexed displays can be divided into two broad categories:
- Character-oriented displays
- Pixel-oriented displays
Character-oriented displays
Most character-oriented displays (such as seven-segment displaySeven-segment display
A seven-segment display , or seven-segment indicator, is a form of electronic display device for displaying decimal numerals that is an alternative to the more complex dot-matrix displays...
s, fourteen-segment display
Fourteen-segment display
A fourteen-segment display is a type of display based on 14 segments that can be turned on or off to produce letters and numerals. It is an expansion of the more common seven-segment display, having an additional four diagonal and two vertical segments with the middle horizontal segment...
s, and sixteen-segment displays) display an entire character at one time. The various segments of each character are connected in a two-dimensional matrix and will only illuminate if both the "row" and "column" lines of the matrix are at the correct electrical potential. The light-emitting element normally takes the form of a diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
so electricity will only flow in one direction, keeping the individual "row" and "column" lines of the matrix electrically isolated from each other. For liquid crystal display
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
s, the intersection of the row and column is not conductive at all.
In the example of the VCR display shown above, the illuminated elements are the plates of many individual triode
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
s sharing the same vacuum enclosure. The grids of the triodes are arranged so that only one digit is illuminated at a time. All of the similar plates in all of the digits (for example, all of the lower-left plates in all of the digits) are connected in parallel. One by one, the microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
driving the display enables a digit by placing a positive voltage on that digit's grid and then placing a positive voltage on the appropriate plates. Electrons flow through that digit's grid and strike those plates that are at a positive potential.
If the display had been built with every segment being individually connected, the display would have required 49 wires just for the digits, with more wires being needed for all of the other indicators that can be illuminated. By multiplexing the display, only seven "digit selector" lines and seven "segment selector" lines are needed. The extra indicators (in our example, "VCR", "Hi-Fi", "STEREO
Stereophonic sound
The term Stereophonic, commonly called stereo, sound refers to any method of sound reproduction in which an attempt is made to create an illusion of directionality and audible perspective...
", "SAP
Second audio program
Second audio program , also known as secondary audio programming, is an auxiliary audio channel for analog television that can be broadcast or transmitted both over the air and by cable TV.-Usage:...
", etc.) are arranged as if they were segments of an additional digit or two or extra segments of existing digits and are scanned using the same multiplexed strategy as the real digits.
A few character-oriented displays drive only one segment at a time. The display on the Hewlett-Packard
Hewlett-Packard
Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...
HP-35
HP-35
The HP-35 was Hewlett-Packard's first pocket calculator and the world's first scientific pocket calculator . Like some of HP's desktop calculators it used reverse Polish notation. Introduced at US$395, the HP-35 was available from 1972 to 1975.Market studies at the time had shown no market for...
was an example of this. The calculator took advantage of an effect where very brief pulses of light are perceived as brighter than a longer pulse of light with the same time-integral of intensity.
Pixel-oriented displays
By comparison, in dot matrix displayDot matrix display
A dot matrix display is a display device used to display information on machines, clocks, railway departure indicators and many other devices requiring a simple display device of limited resolution...
s, individual 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....
s are located at the intersections of the matrix's "row" and "column" lines and each pixel can be individually controlled.
Here, the savings in wiring becomes far more dramatic. For a typical 1024x768 (XGA) computer screen, 786,433 wires would be needed for non-multiplexed control. That many wires would be completely impractical. But by arranging the pixels into a multiplexed matrix, only 1792 wires are needed; a completely practical situation.
Pixel-oriented displays may drive a single pixel at a time or an entire row or column of pixels simultaneously. Active-matrix liquid crystal display
Active-matrix liquid crystal display
An active matrix liquid crystal display is a type of flat panel display, currently the overwhelming choice of notebook computer manufacturers, due to low weight, very good image quality, wide color gamut and response time...
s provide a storage element at each pixel so that the pixel continues to display the correct state even when not being actively driven.
"Break up"
Because most multiplexed displays do not present the entire display simultaneously, they are subject to "break up" if the observer's point of regard is in motion. For example, if the observer were to rapidly swing their vision across a multiplexed display, they might see a jumble of individual digits rather than a coherent display. This effect can also sometimes be provoked by chewing hard candyCandy
Candy, specifically sugar candy, is a confection made from a concentrated solution of sugar in water, to which flavorings and colorants are added...
; this causes vibration
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
of the user's eyes, leading to the break-up of the display.
The multiplexed nature of a display can also be revealed by observing it through a mechanical stroboscope
Stroboscope
A stroboscope, also known as a strobe, is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. The principle is used for the study of rotating, reciprocating, oscillating or vibrating objects...
, for example, a spinning slotted wheel.