Electronics Manufacturing – M528

What’s inside Your Television Monitor

The emergence of the ‘television set’ since the late 1930s has spawned a large amount of developing technologies over the last 7 decades.

Starting initially with a diversion about the current main display techniques, it is hoped in conclusion to relate this to serial and economic trends now and in the future.

The main display technique used since the advent of television is the C.R.T. (cathode ray tube). This consists of a glass tube with a phosphor coating backing to the display surface. Electrons are “fired” at high-speed in a methodical line by line format onto the phosphor surface to produce an optical image visible from the projected image surface or ‘television screen’. This type of television display is typical of the analogue broadcast system although ’set-top’ boxes containing digital to analogue signal conversion are available that convert digital broadcasts signals to analogue display format.

There are several newer types of display technologies now in current use. The main ones being LCD (liquid crystal display) and plasma display technologies.

Plasma Display Technology

The plasma display panel, as it is known, operates as follows. It consists of many tiny cells constructed between two panels of glass. Each cell contains a mixture of noble gases, namely xenon and neon. The gas in the cells is electrically turned into a plasma which then excites phosphors to emit light. This type of display is used often for televisions greater than 32”, and is becoming increasingly popular in home entertainment.

Long electrodes are also sandwiched between the glass plates in front of and behind the cells. The address electrodes sit behind the cells, along the rear glass plate. The transparent display electrodes, which are surrounded by insulating dielectric material and covered by a magnesium oxide protective layer, are mounted in front of the cell, along the front glass plate. Control circuitry charges the electrodes that cross paths at a cell, creating a voltage difference between the front and back and causing the gas to ionise and form a plasma; as the gas ions rush to the electrodes and collide, the photons are emitted.

It would now be wise to describe the operation of an LCD display before contrasting various benefits of differing technologies.

LCD Technology

LCD technology uses and exploits the properties of polarised light. Two thin polarised panels sandwich a thin liquid crystal gel that is divided into individual pixels. An X/Y grid of wires allows each pixel in the array to be activated individually. When an LCD pixel darkens, it polarises at 90 degrees to the polarising screens.

This cross polarising blocks light from passing through the LCD screen where that pixel has darkened. The pixel darkens proportionally to the voltage applied to it; for bright detail a low voltage is applied to the pixel, for dark areas a higher voltage is applied. LCDs are not completely opaque to light, even for the darkest pixels.

Utilising LCD Technology

There are some problems with LCD technology, namely ‘ghosting’ with LCD television displays. Since with CRT displays most of the light is emitted in a very short period of time (1 ms) compared with what is known as the refresh period (20 ms for 50 fps video). In LCD displays, each pixel emits light of a set intensity for a full period of 20ms plus the time it takes to switch to the next state, typically 12-25ms. This second period, known as the response time, can be shortened by the panel design (for black to white transitions), and by using the technique of overdriving (for black to grey, and grey to grey transitions), however this can still only go down to as short a time as the refresh period. This fine for watching film based-material where the refresh period is so long (typically 1/24th of a second or 41.6ms) and jitter is so strong on moving objects, that film producers almost always try to keep objects of interest immobile in the film’s frame.

Video material shot at 50-60 frames a second actually tries to capture the motion. When the eye of a viewer tracks a moving object in video, it doesn’t jump to its next predicted position on the screen with each refresh cycle, but it moves smoothly; thus the TV must display the moving object in the ‘ correct places’ for as long as possible and erase it afterwards as quickly as possible.

Although ghosting was a problem when LCD TVs were newer, the manufacturers have been able to shorten reponse-time to 4ms on most computer monitors and about 8ms for most TVs.

There are two new techniques to solve this problem. Firstly, the back light of the LCD can be ‘fired’ for a period shorter than the refresh period preferably when the pixel brightness has settled to its intended brightness. This does reintroduce the flicker problem of CRTs because the eye is able to sense flicker at the typical 50-60Hz refresh rates.

Another method is to double the refresh rate of the LCD panel and reconstruct the intermediate frames using various motion compensation techniques, as tested on the more expensive ‘100Hz’ CRT televisions in Europe.

An approach maybe to allow the viewer to be able to switch on the 2 techniques when viewing the video material or off when viewing film based material.

One third type of popular television display is the digital light processing technology. DLP along with LCD are the current display technologies behind rear-projection television, having supplanted CRT projectors. These rear projection technologies compete against plasma flat-panel displays in the HD TV market.

Digital Micro-mirror Device

With DLP projectors, the image is created on a microscopic scale by a matrix of small mirrors laid out on a semi-conductor chip, known as a digital micro-mirror device. Each mirror represents one pixel in the projected image. The number of mirrors corresponds directly to the resolution of the projected image. 800 x 600, 1024 x768, 1280 x 720 and 1920 x 10880 (HDTV) matrices are common DMD sizes. These mirrors can be repositioned rapidly to reflect light either through the system lens or onto a heatsink (otherwise known as a light dump). The rapid switching of the mirrors between effectively ‘on’ or ‘off’ enables the DMD to vary the intensity of the light being transmitted out through the lens, creating varying shades of grey in addition to white (full on) and black (fully ‘off’).

There are two basic methods by which DLP projector systems create a colour image, those utilised by single chip DLP projectors and those utilised by three chip projection systems.

In single chip projection systems the colours are produced by inserting spinning colour wheel between the lamp and the DMD. The colour wheel is usually divided into four sections; the primary colours of red, green and blue, and extra clear section to boost brightness. An effect of the clear section is that it reduces colour saturation so it may be optionally disabled, whilst a clear section is emitted from the spinning wheel altogether in some models. Optionally other colours are sometimes included such as yellow. The DMD chip is synchronised with rotating motion of the colour wheel so that each colour component of the DMD display is synchronised with that colour segment in the spinning colour wheel. Provided this synchronisation is carried out at a sufficiently high speed, a full colour image is displayed via the lens. In early systems this image was equal to one rotation per frame, but later models spin the wheel at two to three times the frame rate so that better performance may be achieved. In some more recent model systems, the white bulb and colour wheel have been replaced by a package consisting of super bright LEDs of the three primary colours. Owing to the fact that LEDs can be switched of and on very quickly this type of system allows for even high rates of sequential single colour image projection. Bulb life is also longer and the light intensity emitted from the bulb is more consistent with bulb life expectancy, than with earlier technology for DLP.

A problem with the single chip projection system is the ‘rainbow effect’. This occurs most visibly when bright white objects appear on dark backgrounds. When the observer’s eye passes across the image, the viewer perceives a rainbow distribution of colours. This occurs mainly due to the field sequential colour display technique and is not exhibited by three chip projectors. It is considered that the origins of the rainbow effect are to do with the concept of flicker fusion threshold.

By using a colour wheel where the colours are divided up into an Archimedian spiral can reduce the effect considerably. With segmented wheels, the DMD must go black while a transition is made from one colour to another. This not only interferes with the persistence of the vision and thereby accentuated the rainbow effect, but it also means the more segments there are, the darker the display will be all else being normal. The spiral wheel can greatly reduce these problems. There are no or few problems with the LED packs used also because of the high switching rates which can be achieved. There is also a greater degree of individual control over light and therefore colour intensity with the LED system as well.

With three chip DLP projectors, a prism is used to split light into the three primary colours each of which is routed out to a separate DMD device and then recombined out through a lens. Three chips DLP projectors can resolve greater resolution of shade and colour than one chip projectors, because each colour has a longer time to be modulated within each video frame.

Texas Instruments is the main manufacturer of DLP technology, which is used by many licencees who market products of TIs chip sets.

DLP is rapidly becoming a major stake-holder in rear projection TV and has, so far, achieved a 10% market share. DLP chips, at the moment, represent 5% of TIs total sales.

Data Encryption Flag and Television Broadcast

There have been recent talks to introduce a mandatory data encryption flag in the packets of data broadcast digitally. This if introduced would have significant effects in the future direction of home entertainment and possibly wider reaching effects for the future of cinematic entertainment in general.

If a tariff could be assigned to each blockbuster movie viewed at home, it could render most cinema complexes unprofitable to run and cause not only a decline in the future of cinema based film entertainment but also a decline in related community business, such as restaurants and night-time entertainment in general.

Furthermore, whilst human beings essentially evolved as social beings, encouraging a decline in community social life even if unintended, would further undermine the social structure of society, which although controversial, has partially been due to an over eager emphasis on technological development over the last decades of history.

Therefore it is worth considering how we risk changing human progress by small changes in legislation affecting a vast industry.

The knock on effects of such changes, do not just rest with social change but change to industry which is world-wide now in electronics.