A Guide to Monitor Cables
Analogue - VGA/SVGA
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.
Digital - DVI
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.
A new standard connector has been developed for the Digital Video interface
comprising a rectangular block of 24 pins for the digital signal plus 4 pins either
side of an earth pin for transmission of a conventional analogue signal where
required.
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. 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.
Four different versions of this connector are used:
DVI-D single link
18 of the 24 pins are used to carry a standard digital only signal. The 4 analogue
pins are not present.
DVI-D dual link
All 24 pins are used to carry an enhanced digital signal for
High resolutions e.g. above 1600 x 1280. Again no analogue pins are present.
DVI-I
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
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.
Choosing a DVI Cable
If both graphics card and screen are fitted with DVI-I or DVI-D connectors
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).
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.
If a Matrox twin or quad head graphics card is used (G550/G200MMS) fitted with
Molex LFH60 pin connectors then a special "splitter cable" is used to
connect the LFH60 connector to two LCD digital monitors. We can not supply this
cable.
On the left you will notice 3 rows of 8 pins each; these 24 pins are the only
pins required to transmit the three digital channels and one clock signal. The
crosshair arrangement on the right is actually a total of 5 pins that can transmit
an analogue video signal.
This is where the specification divides itself in two; the DVI-D connector
features only the 24 pins necessary for purely digital operation while a DVI-I
connector features both the 24 digital pins and the 5 analogue pins. Officially
there is no such thing as a DVI-A analogue connector with only the 5 analogue
pins although some literature may indicate otherwise. By far, the vast majority
of graphics cards with DVI support feature DVI-I connectors.
The idea behind the universal nature of this connector is that it could eventually
replace the 15 pin VGA connector we are all used to as it can support both analogue
and digital monitors.
What to do about scaling?
A major problem when dealing with digital flat panels (the primary market for
the DVI spec) is that they have a fixed "native" resolution that they
can properly display at. Since there are a fixed number of pixels on the screen
itself, attempting to display a higher than native resolution on the screen is
impossible.
It is quite often however that a lower resolution will be displayed on the
screen; case in point would be the Apple 22" Cinema Display monitor with
a native resolution of 1600 x 1024. Playing a game at that resolution would be
silly not to mention that most games do not even support such odd ratio resolutions,
and thus you would have to play at 1024 x 768 or 1280 x 1024. The problem with
this is that the image must now be scaled to properly be displayed on the screen.
It used to be that scaling was not even considered an important matter and
was left ignored but as digital flat panels increased in popularity it became
something that manufacturers worried about. The DVI specification places the duty
of properly scaling and filtering non-native resolutions where it should lie,
on the monitor manufacturer shoulders. So any monitor that is fully DVI compliant
should handle all scaling/filtering itself and obtaining a relatively nice scaling
algorithm is not too difficult meaning there should not be much difference between
monitors in this respect, although we are sure there will inevitably be some.
DVI Information
The DVI specification offers the benefits of digital video while maintaining
compatibility with analogue monitors.
The digital connection uses 24 pins, plus support for the VESA-DCC (DESA Display
Data Channel) and EDID (Extended Display Identification Data) services. The analogue
connection uses six of these pins as well as five others around a plus shaped
key. A DVI-I integrated socket has a plus shaped hole to accommodate the analogue
connection. A DVI-D digital only socket does not. Also the pins and sockets use
pins which are flattened and twisted to create a Low Force Helix (LFH) contact.
Supports display resolutions of UXGA (1600 x 1200), HDTV (1920 x 1080) and
QXGA (2048 x 1536)
|
