Intro to Computer Systems

Chapter 6b: Peripheral Expansion

Display Technologies

The resolution of a display system refers to how many discrete light elements, known as pixels, it can show in each direction.

A pixel is a discrete picture element on the screen. The screen resolution is expressed in terms of how many pixels make up the screen area.
A pixel is a discrete picture element on the screen. The screen resolution is expressed in terms of how many pixels make up the screen area.

For example, a computer monitor running at "1024x768 resolution" means that the screen is made up of a grid of 1024 pixels across, and 768 pixels deep. For light, each colour pixel is made up of three subpixels; one each for red, green, and blue, the primary light colours. Such a process is called RGB (red, green, blue) colour.

Monitors

The display system of a computer is its primary means of interacting with its user. Unlike the series of lights and switches common to machines of old, the modern keyboard-mouse-monitor method of instant user input and output is an effective and user-friendly means to operate a computer system.

CRT Monitors

Legacy computer monitors were built using Cathode Ray Tubes (CRTs), like old televisions. The way these tubes display pictures influenced how the analog video standards such as VGA encoded their images.

A CRT monitor.
A CRT monitor. Photo: Samsung

A CRT monitor works by focusing a beam of energy onto the surface of a glass screen that is treated with a phosphor. When the energy hits the phosphor, it lights up. The energy beam is moved from one part of the screen to another through the use of a deflection coil, a component that uses an electrical field to direct the energy stream.

CRT monitors are cheap and offer excellent reproduction of colour; however this is often at the expense of power consumption and a slight fuzziness to the image. They may also be responsible for eyestrain if used for prolonged periods.

The CRT phosphor (on the left) does not have a fixed relationship between colour elements and pixels. This is unlike the LCD monitor display system (on the right).
The CRT phosphor (on the left) does not have a fixed relationship between colour elements and pixels. This is unlike the LCD monitor display system (on the right).

There is no fixed relationship between the arrangement of the phosphor coating and the pixels of the screen; this lets CRT monitors view many kinds of resolutions, all at equivalent picture quality.

LCD Monitors

Liquid crystal display (LCD) monitors are the modern standard, as they are far more power-efficient and friendlier to the eyes than CRT systems. Because they are electrically simpler, they also take up far less space.

A flat panel LCD monitor.
A flat panel LCD monitor. Photo: Samsung

The LCD panel works by having a grid of pixels that, under the influence of an electrical charge, can become transparent or opaque. A light source is placed behind this grid and images can be created by switching the individual pixels "on" and "off" by supplying electrical charges.

Because the screen display is explicitly split up into a grid of individual pixels, LCD monitors have a "native resolution" that they look best at. If the resolution is changed, it doesn't fit neatly into this grid and can cause dramatic degradation to the quality of the picture.

LCD Panel Technologies

Over the last 30 or so years of LCD display panels being used in end-user equipment, there has been a lot of research and development effort in improving the technology to various needs, such as colour accuracy, response time, and manufacturing cost. This has led to the development of several different technologies in building the LCD matrix itself, each with their own benefits and drawbacks.

Contemporary LCD panels are constructed with a variant of one of three technologies:

A comprehensive guide to these technology variants is available at:
http://www.tftcentral.co.uk/articles/panel_technologies.htm

TN - Twisted Nematic

TN panels are the construction type that is most popular with cost-conscious electronics markets such as notebook computers and small-screened appliances for which the display panel is a secondary concern. This is due to their low cost of manufacture, and excellent response time to moving images.

However, these come at a disadvantage of images having poor viewing angles that distort the image either through darkening or washing-out of the image. This effect is due to how the panel crystal works: it allows the backlight illumination to be transmitted through the panel by twisting the plane of the light such that it is in line with a polariser, letting the light through. In the off state, the light is not in plane with the polariser, which prevents the light from being visible.

A photo of a TN panel at various angles, showing the distortions at wide viewing angles. Photo: notebookcheck.de
A photo of a TN panel at various angles, showing the distortions at wide viewing angles. Photo: notebookcheck.de

VA (MVA, PVA, etc.) - Vertical Alignment

A further development on LCD technology to address the viewing angle issue was Vertical Alignment. In order to improve the viewing angles, it split each LCD subpixel into a number of regions oriented in different directions, such that at any given viewing angle at least some parts of each pixel would faithfully represent the true colour.

A close-up photo of a TN panel (left) and a VA panel (right). Note the regions of the VA panel's subpixels. Photo: digitalversus.com   A close-up photo of a TN panel (left) and a VA panel (right). Note the regions of the VA panel's subpixels. Photo: digitalversus.com
A close-up photo of a TN panel (left) and a VA panel (right). Note the regions of the MVA panel's subpixels. Photo: digitalversus.com

However, much like the propagation delay issue in integrated circuits, this meant that each subpixel was not fully 'stable' in colour until all regions of the subpixel were at the correct colour, and this meant the improved viewing angles came at the expense of slower response times.

IPS - In Plane Switching

A second solution to the issues of the TN panels came in the In Plane Switching (IPS) technology. This is generally regarded to offer the highest quality image, with stable colour and very wide viewing angles - however, again, at the expense of response time. For these reasons, most all professional-quality monitors use some form of IPS technology in their panels.

An IPS panel works by moving the liquid crystal in parallel to the panel (rather than perpendicular in the case of TN and VA panels). This ensures that in an IPS panel, almost all light is directed in a uniform direction - and it is this which provides the colour stability, accuracy and viewing angles associated with the technology.

Organic LED Panels (OLED, AMOLED)

An Organic LED (OLED) panel is fundamentally different to an LCD panel. In an LCD panel, it is the backlight that is the light source, and the LCD matrix filters out the light to various degrees to vary the light intensity. In an OLED panel, the OLED matrix itself is the source of light: the pixels are made up of organic materials which illuminate when given an electric current.

The oft-cited benefit of this approach is that OLED panels have pure blacks, and the improvement in image contrast that comes from it. LCD panels are not usually perfect in filtering light so some gets through in black areas, whereas a black in an OLED panel is a region that is completely free of light emission.

Another advantage is that it is easier to manage the power consumption of the display; a dark image takes less power to display than a bright one.

Some mobile phones have a "power saving" option where the screen is inverted. This takes advantage of this property of OLED panels.

PenTile Matrix

One display design that is often conflated with OLED panels is the PenTile matrix. There is nothing specific about OLEDs that mandate a PenTile matrix; however, in the marketplace many OLED panels (notably Samsung's AMOLED range) utilise the matrix to achieve its pixel density.

PenTile is a technique where not all pixels have their own unique red, green and blue subpixels, and some are instead shared among pixels. These matrices of subpixels are laid out and controlled with the properties of the human eye in mind, such that images can be displayed without the need for unique colour subpixels for each pixel.

A comparison of a conventional RGB matrix (left) and PenTile matrix (right). Source: digitaltrends.com
A comparison of a conventional RGB matrix (left) and PenTile matrix (right). Source: digitaltrends.com

There are a few variants of the PenTile matrix, discerned by the colour arrangement: RGBG (red-green-blue-green) or RGBW (red-green-blue-white). The primary advantages of this approach (five subpixels per two pixels, rather than 6 in a conventional RGB panel) come from the resultant need for fewer subpixels to arrive at any particular "effective resolution": this makes the panels both cheaper to make, and reduces power consumption.

The difference between an RGB (left) and PenTile (right) screen, when displaying line art. Source: theverge.com
The difference between an RGB (left) and PenTile (right) screen, when displaying line art. Source: theverge.com

However, in order for the PenTile effect to work, it must be at a pixel density where the individual pixels cannot be discerned. Early PenTile panels were not so dense, and the fuzzy, cross-hatched appearance of the subpixels was rather obvious, especially on line art which included straight white lines. On newer generation, more pixel-dense PenTile panels, the effect is less intrusive and often invisible unless looking closely at the screen.