Choosing a monitor can be tricky, especially if you’re worried about flicker or
radiation. If you can, try a new monitor for a period of time, before buying it.
The Liquid Crystal Display (LCD) displays in a PowerBook are particularly
favourable since they don’t produce any flicker or radiation!
Most colour cathode ray tube (CRT) displays use red, green and blue (RGB) phosphors. The earliest and best types employ a shadow mask tube with three ‘guns’ — unfortunately they also produce the most radiation! A screen filter can reduce the problem and also minimises any discomfort caused by static electricity or reflective glare.
Image Quality
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Scan Rates
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A rapid vertical scan rate (frame rate) produces subjectively less flicker — some people notice this more than others! A rate of 60 Hz or less gives noticeable flicker but 66.7 Hz is better. If you’re particularly sensitive (or find your eyes becoming tired after a short period on the machine) you should use a 70 or 75 Hz monitor. The effect of flicker is more pronounced when the screen is viewed from the corner of your eye.
Dot Pitch
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On a colour monitor the quality is determined by the density of the coloured phosphor dots. The spacing of these dots (and hence the density) is given by the dot pitch (dp), measured in millimetres (mm). For example, a .31 dp monitor has a dot spacing of
0.31 mm. Typical modern monitors have a .28 dp.
· A horizontal line two-thirds of the way down the screen isn’t due to a fault.
Your monitor has a Sony Trinitron tube — it’s perfectly normal!
Screen Size and Pixel Count
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A monitor’s pixel count (often wrongly called resolution) indicates the number of pixels used horizontally and vertically. Although often related to the physical screen size this needn’t be the case! For example, a 20 inch monitor can have the same resolution as a 14 inch model — the 20 inch image looks bigger but you don’t see any more detail!
Most monitors have a true resolution of 72 pixels or dots per inch (dpi) — other values such as 36, 40, 64, 69, 75, 76, 77, 80, 82, 84, 90, 92, 95 or 120 dpi will give a different scale. For example, an 80 dpi monitor gives a smaller image than a 72 dpi screen.
For the image size to match printed work you should use a 72 dpi monitor. This is the resolution for a 17 inch monitor of 832 x 624 pixels or a 20 inch at 1024 x 768. However if you used 1024 x 768 on the 17 inch screen you would see a lot more, but at a smaller size!
Video Graphics Array (VGA) and Super VGA (SVGA) monitors are used with many computers, including PCs.
The original VGA models offered a choice of 2 or 16 colours from a palette of 262,144 shades. In low resolution mode the image only contains 320 x 240 pixels, but in standard mode this increases to 640 x 480.
SVGA provides 640 x 480, 800 x 600 or 1024 x 768 pixels with a choice of 256 colours.
Video Interfaces
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To use a particular display to the best of its ability you must have suitable hardware inside your Mac. Many models include an inbuilt video interface — if there’s a video socket on the back of the Mac there’s usually circuitry inside! If not, it needs a video card (with a video connector on the back of it) that plugs into a NuBus, PCI or other expansion slot.
The number of shades of colour or greyscale that can be displayed is set by the size of video RAM (VRAM) that’s in the operational video card or motherboard. Most domestic Macs use 8-bit video, offering 256 shades of colour. Although this is adequate for many purposes it isn’t good enough for desktop publishing (DTP) — this requires millions of colours, necessitating a 24 - or 32-bit video interface.
√π See below for details about VRAM
The following list shows horizontal and vertical sizes (in pixels) of various monitors, not all of which may be fully supported by the Mac or video card hardware:-
Early Macs, such as the Plus or SE, only support a small one-bit monochrome display. Since this requires only 21 K of memory they can store the image within the main RAM. Other Macs come with a separate VRAM, usually between 256 K and 4 M in size.
VRAM is supplied as SIMMs (usually 256 K or 512 K) or DIMMs (usually 1 M or 2 M). More recent machines employ dynamic RAM (DRAM) DIMMs of the EDO variety. These have offset pins to prevent you inserting them into the wrong type of machine.
The following table shows how much VRAM you’ll need to show colours to 4, 8, 16 or 24-bit accuracy on various sizes of monitors:-
Size VRAM n 256 K 512 K 768 K 1 M 2 M
in pixels
640 x 480 4 8 16 24 24
832 x 624 4 8 8 16 24
1024 x 768 - 4 8 8 16
The available number of colour or greyscale shades is given by the following table:-
Bits Colours Notes
4 16
8 256 Usually selected from a palette of 16,777,216
16 65,536 'Thousands' of colours
24 16,777,216 'Millions' of colours
· 32-bit video devices are actually 24-bit devices that usually employ the extra bits for
colour planes and translucency.
Video Connections
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The video connections on a Mac don’t carry digital circuits — they use analogue signals to represent the intensity of the red, green and blue (RGB) components in the image. In addition there’s usually a separate composite synchronisation (synch) signal that keeps the scanning electronics of the monitor in step with the Mac.
Most Apple Macs and monitors are fitted with a 15 way D socket (DB15) and are joined using a plug-to-plug cable. Some video cards use special D connectors that include integral co-axial contacts. In some cases the monitor is connected to a cable via separate BNC co-axial plugs for red, green, blue and synch signals. Extra links may be needed inside the plug at Mac end to persuades the computer to produce the correct signals.
VGA and SVGA Connections
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Some recent machines, including those that meet the PowerPC Platform (PPCP) standard, are fitted with a 15 way high-density D socket (HD15) as used on an SVGA monitor. This type of 15 way connector can be identified by its three rows of contacts. To connect a VGA or SVGA monitor to a Mac with a DB15 you must use an adaptor cable wired as follows:-
Mac Video Connector VGA/SVGA Monitor
Circuit DB15 pin HD15 pin
Red 2 ___________________ 1
Green 5 ___________________ 2
Blue 9 ___________________ 3
Red Ground 1 ___________________ 6
Green Ground 6 ___________________ 7
Blue Ground 13 ___________________ 8
Horiz Sync 15 ___________________ 13
Vert Sync 12 ___________________ 14
Horiz Sync Ground 14 ___________________ 10
Composite/ Vert Sync Ground 11 ___________________ 4
You should also fit a link in the DB15 plug between pin 7 (ground) to pin 10 (video selection) to instruct the video card to produce an SVGA signal. If you don’t feel like wiring up such a cable you can buy an in-line adaptor from your Apple dealer — just plug it into the back of the Mac and connect it to the monitor with a standard SVGA cable!
SCART Connections
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Some audio-visual equipment uses a SCART connector wired as shown below. If your video card supports PAL or NTSC formats you can use an adaptor cable to connect the Mac’s monitor port to a SCART-equipped display.
Pin Sound Circuit Video Circuit Other Circuit
1 Right Out
2 Right In
3 Left Out
4 Audio Ground
5 Blue Ground
6 Left In
7 Blue In
8 Source Switching
9 Green Ground
10 Data Bus
11 Green In
12 Data Bus
13 Red Ground
14 Data Bus Ground
15 Red In
16 Blanking
17 Composite Ground
18 Blanking Ground
19 Composite Out
20 Composite/Sync In
21 Ground/Screen
√π See the Movies chapter for about television formats
TTL Monitors
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Some monitors are designed for older computers that produce a Transistor Transistor Logic (TTL) RGB output instead of an analogue signal. A monitor operating with TTL signals can only represent eight colours (each colour can only be on or off) and requires a 5 volt input — sadly, the Mac only produces a 1 volt output!
There are two ways to use a TTL monitor with a Mac:-
• Fit a video interface card onto the Mac’s motherboard and connect it directly to
the monitor.
• Install a video card that supports a TTL monitor.
Although TTL monitors are often cheaply available the results won’t be as good as a modern display — but if that’s all you can afford it’s better than nothing!
One TTL system, used in early IBM machines, is the Colour Graphics Adaptor (CGA). It creates 16 shades by using an intensity control circuit to switch the standard shades to bright or dim. It employs a 9 way D connector (DB9) wired as follows:-
Pin Circuit
1 and 2 Ground
3 Red
4 Green
5 Blue
6 Intensity
7 Monochrome
8 Horiz Sync
9 Vert Sync
Monochrome outputs, often designated as MONO, provide a video signal only on pin 7, usually for a 720 x 348 pixel image. A colour output, usually labelled RGB, supplies signals on pins 3, 4 and 5 to give a 640 x 200 pixel image — or 320 x 200 for grey on black. If the video source can’t provide the required intensity output you’ll be limited to 8 shades.
CGA was replaced by the Enhanced Graphics Adaptor (EGA), giving 640 x 350 pixels, and Super EGA, providing 640 x 480 pixels. Both use three intensity circuits, known as secondary colour circuits, to give 64 shades. Both systems employ a 9 way D connector for compatibility with CGA:-
