A monitor, sometimes known as a visual display unit (VDU), can be provided as a cathode ray tube (CRT) or as a flat screen device, usually a liquid crystal display (LCD).
A traditional CRT uses red, green and blue (RGB) phosphors to create the image. The earliest, and possibly the best type of monitor, uses a shadow mask tube. This employs three electron guns and phosphor dots arranged in RGB triplets on the front of the screen itself. Lower-cost CRTs use a single gun that operates with an aperture-grille system on the screen.
For working on multimedia projects you should consider using a monitor with integral loudspeakers. Although never as good as stand-alone devices, this saves desk space and reduces cabling. In addition, some monitors have a conventional video input, allowing you to use your monitor for viewing television pictures or other multimedia material.
If you’re feeling extravagant you could use a video projector. These come in LCD and digital light processor (DLP) versions: the LCD models are cheaper but invariably the image contrast begins to fade over time. Projectors are connected in the same way as a normal monitor, although a digital connection is essential for best results. Some models use the DigitalConnect system, which employs a single connector for digital and analogue video circuits, as well as USB data.
All CRTs emit radiation, although a modern device produces very few X-rays, whilst a monitor conforming to TCO-99 generates little in the way of electromagnetic radiation. A screen filter can sometimes be helpful, usually by lessening any discomfort caused by static electricity or any eyestrain caused by optical reflections.
Most modern CRT displays comply with the US Energy Star and TCO-95 standards, incorporating the VESA Display Management System. Such a monitor in sleep mode uses less than 30 watts (W) of power, falling to under 8 W after a further 70 minutes of inactivity.
If you’re really worried about radiation or can’t stand the amount of flicker created by a CRT, you should try an LCD screen. Although more expensive, such devices consume less power and require less space. In addition, a 15-inch LCD gives you the same viewing area as a 17-inch CRT monitor.
To use a monitor, your computer must contain the necessary hardware, usually built into the machine itself or provided as a plug-in video card. Modern video systems include ATI Rage 128, ATI Rage Mobility 128, ATI Rage Pro, ATI Radeon, ATI Mobility Radeon, Nvidia GeForce2 MX, GeForce3 MX and GeForce4 Titanium, as well as several other variations.
All of these provide graphics acceleration, supports enhanced three-dimensional (3D) graphics and work with QuickTime, QuickDraw 3D or OpenGL. Some accelerators also accommodate Glide technology, as developed by 3Dfx, which is useful for the fast graphics employed in some computer games. Others also act as a decoder for playing DVD Video discs.
Some older PowerPC Apple machines have a designated 32-bit or 64-bit PCI slot that accepts a suitable video card. In G3 models this slot runs at 66 MHz, twice the speed of the standard PCI slots.
Other machines have a 128-bit Accelerated Graphics Port (AGP) card fitted in a matching AGP slot. A double-speed card or
2× card of this type runs at 133 MHz, a
4× card runs at 266 MHz and an
8× card runs at 533 MHz. Such cards must be used in a slot of sufficient speed rating, although those designed for PCs can also be fitted in AGP-equipped Macs. However, Apple’s Cube needs a smaller AGP card, complete with extra pins for the ADC power feed.
An external monitor has to be connected to your computer by means of some kind of monitor interface. This appears on the back of your machine as a monitor socket, which should be connected directly to the monitor via a suitable monitor cable.
There are several different types of monitor interface, each appearing on a different kind of connector. The following list describes the common interfaces found in modern computers. Your computer’s video hardware should match interface in your chosen monitor. If not, you must get an alternative monitor or fit a suitable monitor adaptor, if available.
Some Apple computers have a TwinView video card, providing both ADC and SVGA outputs for a wide range of display devices. Later machines have separate ADC and DVI outputs, allowing the use of two digital monitors at once.
Although some aspects of a monitor are purely subjective, other parameters can be important in terms of safety and comfort. For example, the lines on the screen should never jitter and you shouldn’t be aware of any flicker, although you may only notice the latter when viewing the screen indirectly.
The great majority of displays provide adequate brightness. This shouldn’t be set too high as it can cause eye discomfort. Brightness is often measured in candelas per square metre (cd/m2). There should also be a good, but not excessive, contrast between light and dark shades.
Last but not least, the clarity of the image should be considered. A computer creates images in the form of thousands of square picture elements, more commonly known as pixels. With a good display these tiny blocks should be clearly visible.
If you’re seriously worried about flicker or the radiation from a CRT, you can use a liquid crystal display (LCD) screen or a portable machine with a built-in LCD screen. Most LCD displays employ thin-film transistor (TFT) technology, also known as active matrix technology.
Most cathode ray displays have a range of controls for modifying the size, position and shape of the image. Some of these can be tricky to set up, especially since some adjustments may require you to alter several settings to get the desired results. Typically, you can adjust the height, width, tilt, as well as trapezoidal shape and pincushion parameters in both the horizontal and vertical directions. Unfortunately, these controls don’t usually have any effect on a monitor’s flicker or jitter.
A rapid vertical scanning rate (frame frequency), also known as the refresh rate, can reduce the subjective effect of CRT flicker. A rate of 60 Hz or less can give noticeable flicker but rates of 66.7, 70, 75, 80, 85, 88, 89, 95, 100, 115, 117, 119, 120, 130, 133 or 160 Hz give progressively better results. If you’re particularly sensitive (perhaps if you’re short-sighted or find your eyes become tired after a short period on the machine) you should get a monitor with a faster refresh rate.
The horizontal scanning rate (line frequency) is related directly to the vertical rate and the number of lines used to create an image on the screen. A modern multi-scan monitor accepts signals with a horizontal rate of between 30 and 121 kHz and a refresh rate of between 50 and 160 Hz. This wide tolerance ensures that the monitor is reliably controlled by the computer. Older monitors, including many older models of Apple origin, can’t accept signals at higher rates.
The quality of a monitor is set by the number of pixels it can clearly display within a given area. Depending on the type of CRT, this is determined by the spacing of phosphor dots or the aperture grill. This figure, usually given in millimetres (mm), is known as the monitor’s dot pitch (dp). For example, a monitor with a dp rating of
.31 dp will have its dots or grill spaced by 0.31 mm. A typical modern monitor has a dp of
.28, although a better rating is
Some of the finer points concerning monitor characteristics are covered in the following sections.
A monitor’s pixel count, also known as resolution, shows how many pixels make up the image. For example, a monitor using 832 pixels vertically and 624 pixels horizontally is said to provide a display of
832 × 624 pixels. Although the pixel count is often related to the screen size, this isn’t always the case. For example, a 20-inch monitor doesn’t show any more detail than a 14-inch display with the same pixel count, even though the image is bigger.
A monitor’s pixel density, sometimes confused with resolution or colour depth (see below), is given in dots per inch (dpi) or pixels per inch (ppi). Modern Macs operate at 100 dpi, whilst older machines use 72 dpi, although 36, 40, 64, 69, 75, 76, 77, 80, 82, 84, 88, 90, 92, 95, 110 and 120 dpi are also employed. Although the image on a 72 dpi monitor is the same size as the printed result, other pixel densities give a smaller screen image, causing difficulties with printed material. Although a higher density lets you see more on a small screen, a 110 or 120 dpi display can produce small text that’s difficult to read.
The aspect ratio of a monitor defines the relative proportions of its height and width. Unfortunately, comparing monitors of different ratios isn’t easy. The following table shows typical ratios and the multiplication factor that needs to be applied to a display’s height to obtain its width:-
The following list shows standard pixel counts and commonly related monitors, not all of which may be fully supported by particular video hardware:-
|240 ||3:4||Pocket |
|320 ||4:3||Early ||VGA|
|512 ||3:2||Classic |
|1024 ||4:3||2||15″ ||SVGA/|
|1152 ||4:3||•||19″ ||+|
|1160 ||4:3||19″ ||+|
|1260 ||7:5||21″ ||+|
|1280 ||3:2||15″ ||WXGA|
|1280 ||4:3||17″ ||+|
|1280 ||5:4||3||17″ ||SVGA/|
|1440 ||16:||*#||17″ ||WSXGA|
|1600 ||16:||*||20″ ||+|
|1600 ||4:3||4||19″ ||SVGA/|
|1680 ||16:||4*||20″ ||+|
|1920 ||16:||4||23″ ||+|
|1920 ||16:||4*||23″ ||WUXGA|
|2560 ||16:||4*||30″ |
1 Also considered optimum for 17″ monitor
2 Also considered optimum for 20″ monitor
3 Highest acceptable pixel count for 17″ monitor
4 Text is too small on a 17″ monitor, higher pixel counts not supported by some video cards
• Sometimes specified as 1152 × 870
* 16:10 is close to 16:9 aspect of TV broadcasting, whilst 1920 × 1200 pixels used for High Definition (HD) TV
# Wide-screen iMac can also emulate 1152 × 720, 1024 × 640 and 800 × 500 (all 16:10)
+ Non-SVGA, but accepted by most multi-scan SVGA monitors
640 × 480image with a black band at the edges to avoid distortion. However, with a suitable video card and third-party software the image increases to
704 × 512pixels.
Unlike older monitors, a modern multi-scan monitor, often controlled directly by the computer, accepts video signals over a wide range of pixel counts. And although modern portable computers, such as Apple’s iBook and PowerBook models, have a ‘native’ mode, they use scaling to accommodate other pixel counts. For example, the iBook has a
1024 × 768 display, but also works at
800 × 600 or
640 × 480, whilst the PowerBook G4, with its
1152 × 768 display, also operates at
896 × 600,
720 × 480,
1024 × 768,
800 × 600 or
640 × 480. However, such scaling often results in a ‘lumpy’ appearance.
The number of shades of greyscale or colour that you can see on your screen is determined by the the colour depth of your video interface, sometimes known as its colour resolution. This is set by the number of bits that represent each pixel on your monitor. This number is often given as a measure in bits per pixel (bpp). The most common depths are shown below:-
The 256 shades provided by 8-bits are adequate for some purposes, including the Internet, but the results aren’t very exciting. 8-bit colours can be expanded by using each of the 256 values to select a shade from a larger palette of colours. For example, the 18-bit palette used on a PC gives access to 262,144 colours whilst a 24-bit palette gives 16,777,216 shades. Unfortunately, any such palette still only lets you use 256 colours at any one time. So, for desktop publishing (DTP), where you need all possible shades at once, you should have true 24-bit or 32-bit video performance.
The maximum colour depth and pixel count is limited by the amount of video memory, also known as video RAM, that’s in your video hardware. This table shows the colour depths available with various amounts of RAM and pixel counts:-
|Pixel ||256 ||512 ||768 ||1 ||2 ||4 |
You’ve probably noticed that colours shown on various monitors are often different. This is due to variations in the performance of the three phosphors or light-emitting devices used in each display.
Special software, with an instrument such as a colorimeter or spectrophotometer, can be used to calibrate the colour output of a screen. Fortunately, if you aren’t involved in desktop publishing (DTP), you can use ColorSync, as built into the Mac OS, for a ‘rough and ready’ calibration.
For the technically inclined, there are two considerations: white point and gamma correction.
This figure specifies the colour of the screen when displaying ‘pure’ white and is measured as an absolute temperature in kelvin (K). As you may have observed, any object that burns at a fairly low heat has an orange or yellow colour, whilst something burning at a higher temperature is slightly blue.
Most monitors have a white point of 9300 K, which is slightly on the blue side, resulting in a ‘cold’ image. A lower white point of between 5000 and 6500 K is better. The latter, also recommended by the Commission Internationale de l’Eclairage (CIE), is specified as the white point for Apple’s LCD screens.
Devices that are used to obtain images, such as scanners and digital cameras, produce an electrical voltage that increases linearly with light intensity. Unfortunately, CRT devices generate light according to a power law, which means that the dimmer parts of a picture appear darker than in reality.
Gamma correction fixes this problem by modifying the signal, making it non-linear in the opposite direction to the non-linearity of the display. It does this by lightening the darker areas of the image and toning down bright areas.
The correction is given as a number between
1 indicates no correction, whilst a higher number gives a darker image. The Mac OS employs
1.8 but Windows uses
2.2, which means that images made on a Mac look dark on a PC.
Modern PCs commonly support one or more of the analogue SVGA, VGA, UXGA, SXGA, XGA or MCGA standards. All three XGA standards utilise interlaced scanning, also known as interleaved scanning. Unfortunately, this means that these interfaces can’t be used with a Macintosh computer, since the Mac OS is designed for non-interlaced (NI) scanning.
Super VGA is an improved, but non-standardised form of VGA (see below) and is also known as VESA (Video Electronics Standards Association) or as VESA VBE (VGA BIOS Extension).
SVGA supports screen sizes of
640 × 480,
800 × 600 and
1024 × 768 pixels, although
1600 × 1200 and
1920 × 1080 are also used. On a PC it commonly provides between 256 and 32,768 colours from a 24-bit palette. The usual scanning rate is 70 Hz, although higher rates are often used.
This older standard operates in several modes, as shown below, although the text modes are rarely used in modern systems. The graphics modes typically offer a choice of 2, 16 or 256 colours from a 18-bit palette or a 24-bit palette. Scanning is often at 60 Hz, although higher rates are used.
|Text||-||40 ||50 |
|Text||-||80 ||50 |
|Graphics||320 ||240 ||Low|
|Graphics||640 ||480 ||Standard|
|Graphics||800 ||600 ||High|
VGA can also emulate the outdated CGA modes and MDA text modes. A VGA or SVGA monitor is required on a PC if you want to use Windows 95.
Also known as XGA-2, the SXGA standard was introduced with the OS/2 system for PCs. It’s compatible with XGA (see below) and typically provides a display of
1280 × 1024 pixels with a 16-bit or 24-bit palette. Ultra XGA (UXGA) is a further extension of this standard.
XGA originated in the PS/2 variety of the PC. It accommodates some of the VGA modes, including DCT mode, and also emulates CGA and EGA. The screen size is
1024 × 768 pixels, usually with 256 colours. Scanning is typically at 72 Hz, although higher rates are often used.
This was used in some PS/1 and PS/2 models of the PC, providing several VGA modes as well as DCT mode (see VGA above). It also emulates the outdated MDA, CGA and older EGA modes.
©Ray White 2004.