Untangling display cables

Wednesday, 13 January 2016
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One question we get asked fairly often in the support team is about connecting computers and displays.

Untangling display cables

If you’re upgrading your equipment, planning to make a presentation or simply curious, then this guide is for you.

At the moment, to connect a computer to a display you need a cable. There are several types of cable that do this, and many of them are pretty obscure, so I’m going to concentrate on the most common ones.

VGA - the blue one

VGA connectorVGA is an old standard that mostly hangs on for legacy reasons. It transmits analogue video signals which is OK for CRT monitors but makes no sense for LCD panels, because the extra conversion to digital results in much lower picture quality. For some reason, VGA is still common in projectors and very cheap screens, so I don’t think it’s going away soon. There is no specific limit on resolution (2k is possible!) but I wouldn’t try anything over 1600x1200, as the image quality will be too poor. You also find VGA on the back of servers for compatibility with old KVM systems.

DVI - the white one

dvi cableDVI (Digital Visual Interface) is the most common computer video connector – but it’s not really big enough to drive 4K screens. There are several types, which carry analogue and/or digital video signals. The different types are often not compatible with each other as they have different pins. 

dvi connector types

The main ones you will see are DVI-I (integrated) and DVI-D (digital).

DVI-I

This connector carries both an analogue and a digital video signal. Most graphics cards will have at least one DVI-I output. It is easy to convert between DVI-I and VGA using a passive adaptor.

DVI-D

Many LCD panels use DVI-D. This connector carries a digital signal only, so you can’t use a VGA adaptor with it, and it’s physically missing the analogue pins, so you can’t plug a DVI-I cable into a DVI-D socket. There are two types - single link and dual link. Single link is limited to 1920x1200 at 60Hz, while dual link supports bandwidths of up to 2560x1600 at 60Hz (or 3840x2160 at 30Hz).

HDMI - the TV one

hdmi connectorHDMI is the connector mostly found on the backs of TVs, and it usually also carries audio. It’s really for consumer-oriented devices but shows up on some monitors and laptops. The signal is almost identical to DVI-D, which means it can be converted with a simple passive adaptor. There are lots of versions of HDMI that all use the same connector, with newer versions supporting larger display resolutions. The two important ones are HDMI1.4 which allows 3840x2160 at 30Hz (UHD 4k) and HDMI2.0 which allows 4096x2160 at 60Hz (DCI 4k).

DisplayPort - the new one

displayport cable

DisplayPort is designed to be the universal computer display interface, much as HDMI is designed to be the universal entertainment interface. Like HDMI, it can carry audio and there are versions with added functionality and higher supported resolutions. DP1.2 is the first version that supports 4k at 60Hz. DisplayPort looks pretty similar to HDMI, which can make it difficult when plugging in leads behind computers.

However, the DisplayPort signal is completely different to DVI or HDMI, so in order to convert between them you need an active adaptor. Fortunately, many DisplayPort outputs on graphics cards support ‘dual mode’ functionality. This means that it will notice when a passive DVI adaptor is plugged in and send DVI signals instead of DisplayPort. This is limited to single-link only (ie 1920x1200) and only works at the graphics card end.

MiniDP and Thunderbolt - the multipurpose one

mini displayport cableMini DisplayPort is, as the name suggests, a mini version of DisplayPort. But a lot of MiniDP connectors are actually Thunderbolt connectors. Thunderbolt is an interface that can carry all sorts of data, including the DisplayPort signal. It is mostly found in the Mac world although it is an option on HP workstations and others. Thunderbolt 2 supports DisplayPort 1.2 for 4K displays.

The different kinds of cable do lead to some compatibility issues between older and newer bits of hardware, but with the right knowledge and the right adaptors (and the right cables), pretty much anything is possible.