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For designers of VDUs the flat screen display is a kind of philosopher's stone. As a theoretical possibility it is too desirable to be completely forgotten even while as a commercially practical proposition it seems to become ever more remote.
The vast majority of large screen displays use cathode ray tubes which have been the dominant technology for the last 20 years. The CRT, as witnessed by its success, has a number of inherent advantages as a display device in digital systems, as well as various handicaps.
All CRTs use a system of electrodes known as an electron gun which produces a beam of high velocity electrons which is projected at a phosphor coated glass screen. The electron beam is focused to a point on the screen either electrostatically, using internal grid or control electrodes, or magnetically using an electromagnet in the shape of a yoke around the neck of the tube. The movement of the beam is controlled by internal deflection plates or by external magnetic deflection coils. Because the beam is negatively charged it can be repelled or attracted by applying positive or negative voltage to the deflection coils or plates.
The whole assembly is housed in an evacuated bulb of glass or metal with a glass face-plate. Metal bulbs are strongly bonded to the glass screen and insulated. Because of the relatively high external air pressure and glassware needs to be very thick: as much as0.5 in for a 21 in television screen. This means a CRT can weigh as much as 20 lb or 9 kg.
The length of a CRT tube (front to back) is dictated by the deflection angle of the electron beam and the length of the gun itself with its focusing/deflection apparatus. Standard CRT guns are 6-8 in long while a high resolution gun measures 14 in. Deflection angle contributes to tube length by requiring an increase in length proportional to the co-tangent of the maximum deflection angle. A standard 110° deflection tube with a 21 in screen needs to be 15 in long. A high resolution 42°, 5 in tube is 19 in long.
Stringent pre-delivery checks on this microcomputer show how the volume of the CRT dictates the size of the VDU.
The large volume of the CRT has tended to dictate the overall size of the VDU with most other components being packed into the spaces remaining at the side of the tube. Another problem is that the deflection coils or plates need voltages of 12-15 kV to drive them. A replacement for the CRT has been sought for many years, with research aimed at producing a flat screen display which can be driven from the low voltages supplied by integrated transistor circuits.
Several devices are currently marketed although none of them is yet competitive with the CRT for large numbers of characters. VDUs with more than 2000 characters cost about 25 c/character. The cheapest flat screen display systems presently available cost about $4/character. These prices include drive, control and interface circuits. The point about CRTs is that cost/character falls as the display gets bigger while flat screens get more expensive with each additional character. This is largely due to the problem of addressing.
Looking at overall trends it is clear that the CRT will continue to be used for some time. It has been extensively developed, it is very fast and it has the particular advantage that the information can be applied serially over one line, and thus share the electronics required for addressing. Flat screen systems either have individual drive electronics for each character (or each element) or some multiplexing is possible by using a matrix of individual points. These points can be turned on by the sum of the voltages in two drivers which are addressed in the X and Y directions in the plane of the screen. Thus M x N points can be addressed by M + N drivers. For the matrix system to work it is necessary for the elements to have a threshold voltage below which they are not activated. The X and Y drive circuits are designed so that one by itself is below the threshold, but the sum of the two drivers above it.
Another consideration is the desirability of inherent memory. This means that each element should stay on until it is switched off. A display without inherent memory (like a CRT) has to be refreshed many times a second to prevent flicker. For a display composed of a matrix of 500 x 500 points this means that 250,000 points need to be refreshed at least 30 times a second.
So, the hypothetical flat screen would have the following characteristics. First of all it is flat (a couple of inches thick at the most). Secondly it doesn't weigh much (a few pounds for a 21 in screen), so it can be used in portable instruments, car dashboards, pocket computers, wrist-watch televisions or just hung on the wall. Thirdly it operates off low voltage power sources compatible with integrated circuits. Fourthly it should have a threshold turn-on voltage and finally it should have inherent memory to eliminate high speed refreshing. There are other desirable qualities: grey scale or colour for example; CRT standard resolution; use of an established technology, but the ones already listed are quite enough to disqualify most likely contenders.
All displays can be classified as light emitters or light controllers. Light emitters, of which the CRT is one, do not require external light sources to be visible. Plasma panels and electro-luminescent devices come under this heading. Light controllers modulate incident or reflected light to produce an image. They have the advantage over light emitters that the stronger the ambient light the more visible the display becomes. Light controllers as a rule need very little power and operating voltages are ic compatible. Examples include liquid crystal displays, electrophoretic systems, electrochromics and of course, plain old hard copy.
Matrix addressed light emitters are mostly solid state devices in which the colour is dependent on the light emitting substance used. Electroluminescent displays generate a sufficiently broad spectrum around the yellow region for filters to isolate up to three colours. LEDs are available in red, yellow, green (and perhaps blue). Gas discharge/plasma displays are normally amber, but a green display can be obtained by coating the front glass with an appropriate phosphor to absorb ultra violet energy from the discharge and release secondary photon emission. Red displays can be obtained by filtering orange displays.
Plasma panels consist of a screen incorporating a honeycomb of cells. These are filled with a neon-based gas to give an array of tiny neon lamps. Transparent metal strips are evaporated on to the front of the screen in parallel, while a similar pattern of strips is plated to the back at right angles to the first set. The intersection of two strips forms a pair of electrodes for the neon cell between them. The gas radiates a number of yellow and red spectral lines when it is electrically excited. Excitation may be AC or DC although DC is most widely used.
Commercially successful DC panels appeared with the development of a method of depositing the thick film series resistors associated with each cell to the required accuracy to take advantage of the inherent memory in the cells. A cell is addressed by activating the two electrodes common to the cell with the 'striking' voltage while a 'maintain' voltage is applied to all cells via the thick film resistors. Erasure is accomplished by supplying erase pulses to the common electrodes. Maintain voltage is 170 V and a strike pulse of 50 V is required. Character writing time is about 2 ms and dissipation is 60 mA/character.
An example of the DC plasma display - Burroughs Self-Scan.
A version of the DC plasma panel called the Self-Scan has been patented by Burroughs which provides up to 480 5 x 7 dot matrix characters. IEE also sells a similar panel known as the Argus. The panels use a technique called preferential glow transfer to reduce the number of drive circuits needed to maintain the display. The cathode wires are orthogonally associated with two sets of anode wires. Cathode ionisation takes place in a part of the cell where it is not visible to the viewer and is then selectively transferred to a visible part of the cell. A three phase clock is used to transfer the glow from column to column.
The problem with DC plasma panels has been the difficulty of producing a unit with a large number of characters at a low enough price. It still costs less to drive a DC than an AC panel, however, and DC has greater colour capability. AC plasma panels were first developed at Illinois University (the Digivue project) and at Owens-Corning Corp. The panels use a high frequency drive of 50-60 kHz. The pulses are able to energise a cell capacitively so the electrodes can be outside the glass. Inherent memory is achieved by driving the panel with a sustaining square wave upon which is imposed the strike pulse needed to light a single cell.
An offshoot of Owens-Corning called Photonics has developed the world's largest AC plasma panel. It is a 24 in diagonal unit that can display over 21,000 characters. The panel, which is used by the US Defence Department has a 1024 x 1024 addressable matrix, a resolution of 60 pixel/in and an overall thickness of 0.5 in. The bad news is that the panel costs $45,000, five-off.
The second major type of light emitting display, the electroluminescent panel, consists of a phosphor coating over a flat screen which is energised by an electric field applied at the junction of the phosphor layer and a transparent conducting film. Electroluminescence can be formed using AC, DC or pulsed drive voltages.
During manufacture the phosphor is formed for emission at a certain electrical charge. Using a higher voltage will raise the tolerance of the phosphor. For this reason application of a continuous d.c. voltage is unsuitable, but application of a pulsed voltage in excess of the original forming voltage gives a higher brightness without changing the characteristics of the material. Alphanumeric displays have been made to give up to 25 rows of 80 characters in a 7 x 5 format. The technique is very robust; grey scale can be shown and colours can be produced with filters. There is no inherent memory however, no threshold to permit multiplexing of the drive circuit and the drive voltage is 100-150 V.
The most significant of the light controlling displays is the LCD because it operates on a very low voltage and characters get cheaper as the display gets larger. Liquid crystal is an anisotropic substance which means light is affected differently by the direction in which it passes through the crystal due to its rod-like molecular structure. A typical LCD is a sandwich of crystal between two sheets of glass on which are deposited transparent electrodes. The most popular crystal is the twisted nemotic variety which has the effect of twisting polarised light through 90° as it passes through the sandwich. If the sandwich is placed between cross-polarisers the orientation of the polarised light is compatible at the two surfaces and light passes through. If an electrical field is created across a portion of the liquid crystal the light twisting feature is destroyed and the portion of crystal appears darker.
The power requirement of an LCD is about 1 mA with drive voltages between 2 and 50 V depending on the speed of operation required. The voltage must be AC and a frequency range of 25 Hz to 30 Hz is appropriate. Turn-off time is 100-500 ms and turn-on time is 20 ms. Threshold is achieved through the use of chemical additions to the liquid crystal, and addressing is by X-Y matrix. The displays have the advantage of good contrast in sunlight, low power drivers and low cost/element. The display has no inherent memory however and can only operate reliably in the -10°C to +60°C temperature range before changes in the viscosity of the liquid affect performance. There are also problems associated with producing sheets of glass flat enough to maintain a uniform thickness throughout the crystal sandwich.
Experimental LCD panels for portable TVs have been demonstrated by Matsushita, Seiko Denki Co and Hitachi. The Matsushita panel is a 1.4 x 1.9 in screen with 240 x 240 pixels while the Hitachi device is 3 in diagonal with 240 x 380 pixels. In the US Westinghouse and Hughes have developed LCD panels, the former using thin film cadmium-selenide transistors for drive circuitry. The display, which measures 6 x 6 in was developed by T P Brody who has now established his own company called Panelvision which is marketing a high resolution, low power LCD In the UK a group led by W E Spear at Dundee University is developing an LCD panel using amorphous silicon transistors although a similar project was abandoned by Thomson CSF in France because of the poor frequency response of the substance.
Long before plasma, LCD and electroluminescent panels were thought of, way back in the 1950s, the first practical flat screen display was being devised by William Aiken of Kaiser Electronics and Aircraft Corp in the US. The device, now known as the Aiken tube was a version of the CRT with an electronic gun mounted to the side of the screen rather than behind it. The concept was developed further in the 60s by a gentleman called Tony Krause of 20th Century Electronics who eventually ended up working for Clive Sinclair in the UK in a project to develop a tube for Sinclair's pocket TV.
Flat screen display (FTV1) from the side and above.
The Sinclair tube, which is expected to be available in sample quantities by the third quarter of this year is a surprisingly simple device. It measures 6 x 2 x 0.75 in and is assembled from two sheets of glass: a flat front plate and a vacuum-formed backing plate. The phosphor screen is coated on the inside of the rear plate so that it is viewed through the front plate. The electron gun is mounted to one side, its axis parallel with the screen. Two sets of electrostatic deflection plates provide X-Y scanning. The beam of electrons is 'bent' toward the screen by a third set of deflection plates formed by the metal backing of the phosphor screen itself and a transparent plate evaporated on to the front screen. This means that the screen is viewed from the same side the electrons strike which makes it three times brighter than an ordinary screen with 0.25-1/10 of the power. The front screen incorporates a Fresnel lens which magnifies the picture, and a correcting modulation applied to the vertical deflection plates helps eliminate the trapezoid distortion created by the eccentric position of the electron gun. Connections to the electron gun and deflection assembly are screen printed to the face-plate, reducing the component count considerably.
Sinclair has obtained financial backing for a large manufacturing plant although the company is not saying where the money comes from. One obvious market for the tubes which are expected to sell for less than £10 is France where the PTT is planning to install 30 million VDUs in homes instead of telephone directories. The sets will be supplied by Thomson CSF, Matra and CIT-Alcatel, but where they get the screens from is another matter.
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