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| Cables and Connectors |
The
Tech Plus Guide to Monitor Cables
Analogue
– VGA/SVGA The
adoption of LCD technology in computer screens has led to the development
of the DVI standard (digital video interface). Early LCD screens
converted the analogue signal from the computer into a digital signal
within the LCD display. Significant improvements in image quality
have resulted from the now widespread availability of graphics cards
with digital outputs thereby avoiding any signal loss from a conversion
to analogue in the graphics card and then back to digital at the
screen. DVI-D single link
DVI-D
dual link DVI-I The monitor cable itself is very different for digital signal transmission compared to analogue. The three coax cores for the RGB signal are replaced by foil shielded twisted pairs one for each colour plus a 4th twisted pair for "clock". In dual link cables two such twisted pairs are used for each colour, the graphics card driving alternate pixels down each pair to achive higher bandwith The digital signal is very delicate and effective transmission over 5 metres cannot be guaranteed, which means that analogue SVGA cables will continue to be used in many applications e.g. where display units such as plasma screens or projectors are sited over 10m, from the signal.
If both graphics card and screen are fitted with DVI-I or DVI-D connectors and the maximum screen resolution is 1280 x 1024 then a DVI-D single or dual link cable should be used with a recommended maximum length of 5m. Longer distances up to 10m may work but this cannot be guaranteed and will depend on the make of card and screen. DVI-I combined digital analogue cables should not normally be used and may well not physically fit if either device is fitted with a DVI-D connector (as it would be unable to accept the 4 analogue pins). Dual link cables are only necessary for screens with resolutions of 1600 x 1280 and higher, provided the graphics card has dual link capability. If
one device has a DVI-I connector and the other a traditional SVGA
HD15 connector then use a DVI-A to HD15 cable to carry an analogue
signal. |
The standard
connector for the VGA/SVGA monitor for the past 15 years has been
the 15 pin High density D-Type connector. A female HD15 connector
being present on both the graphics card and monitor with male HD15
connectors on the monitor cable. The first 3 pins of the connector
carry the RGB signal to the 3 guns of the CRT, with other pins dealing
with grounds, HZ and vertical sync and clock. The typical monitor
SVGA cable contains 3 foam insulated coaxes for the RGB signal with
6 individual 24AWG copper wires for the other data. Ferrite cores
are usually fitted to suppress interference.
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18 of the 24 pins are used to carry a digital only signal at resolutions
up to 1280 x 1024. The 4 analogue pins are not present. DVI-D cables
are supplied as standard with most NEC LCD screens.
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All 24 pins are used to carry an enhanced digital signal for
This
connector can support both digital and analogue signals and is used
on graphics cards and monitors to provide both options without having
to incorporate both DVI-D and HD15 SVGA connectors.
DVI-A
monitor cables carrying only the analogue signal, are used where
one end is to mate with a DVI-I connector and the other end with
a standard HD15 connector. NEC screens fitted with only DVI-I inputs
come with such a cable as standard, for connection to analogue only
graphics cards.