Video Out: Display connections

The five types of HDMI connector. By Jan deno, via Wikipedia Commons.

So you (quite naturally) want to hook up your Mac to an external display. That should be a simple task, but are those ports DVI, Mini DisplayPort, HDMI, or what?

Hooking up most peripherals is fairly simple: establish which connections it requires, now almost universally USB, and connect an appropriate cable between the port on your Mac and the peripheral. You can sometimes be lucky with an external display, and have just the right cable to hand or supplied with your Mac. But given the bewildering number of different output sockets on computers and graphics cards, and the mismatching variety on displays of various types, connection may be delayed pending the correct cable and/or adaptors.

Analogue of old

The early ‘standard’ display connection was VGA (for Video Graphics Array, also sometimes known as SVGA), introduced way back in 1987, and gently evolved since. The VGA family of connections and cables run analogue red, green, blue and synchronisation signals through a standard 15-pin connector commonly known as DB-15, DE-15, D-sub 15, mini sub D15, or similar.

A 15-pin D-sub SVGA port. By Duncan Lithgow, via Wikimedia Commons.
A 15-pin D-sub SVGA port. By Duncan Lithgow, via Wikimedia Commons.

If you have to hook your Mac or iPad up to a video projector at another location, it might still offer this type of connection, and itinerant speakers ensure that they always have a VGA adaptor in their travelling kit. Supported resolutions reflect its age: the maximum normally offered is 1280 x 1024, although some systems can go as high as 2048 x 1536, and even 2560 x 1920. Audio is not included, so should be catered for separately if required. For ultimate versatility you may wish to carry a couple of gender changers to cope with almost anything.

The other analogue standards that may still be encountered on older systems are based on component video, either implemented in a set of three RCA (coaxial-cabled) plugs, or packaged into the SCART connectors that became common on domestic TV equipment before the arrival of high-definition systems. Apple and other suppliers still support these with adaptors, but you are unlikely to encounter them in modern offices and facilities.

Digital with DVI

When computer displays moved from analogue to digital signal input from 1999 onwards, VGA was dropped in favour of the Digital Visual Interface (DVI), although in its early years this was still commonly used with analogue signals (DVI-A). Most recent DVI products support mainly the digital variant, DVI-D.

Unfortunately, in an attempt to make a single ‘standard’ support a wide range of applications, the connections and sub-species of DVI have become very complex. Plugs and sockets have up to 24 connectors arranged in three rows for digital signals, and an idiosyncratic arrangement of 4 connectors around a flattened pin for analogue signals. Most sub-species only use a sub-set of the full range of connectors, and their plugs and sockets can appear quite different as a result.

Different types of DVI port. Wikimedia Commons.
Different types of DVI port. Wikimedia Commons.

For example, DVI-I (integrated) single-link uses two blocks of 9 digital connectors and all 5 analogue ones, whilst DVI-D (digital) dual-link uses all 24 digital connectors. Single-link digital systems support a maximum resolution of around 1920 x 1200 (or 1915 x 1436 to 2098 x 1311 according to aspect ratio). To support higher resolutions, dual-link sends data for alternate pixels over each of two links, and there is no simple limit to the resolution that can then be supported. As with VGA, DVI does not carry audio signals, which need to be connected separately.

Although DVI should offer much better prospects for those who have to connect to unfamiliar projectors or displays, perhaps when working off-site, because of the variants and sub-species it is only too easy to have the wrong cable or adaptor. If you are going to need to connect to DVI, you should get precise details first so that you can equip yourself with the right cable.

Current domestic HDMI

Domestic TV systems have largely moved away from DVI connections to their successor, the High-Definition Multimedia Interface (HDMI), with which there are valuable compatibilities. DVI-HDMI adaptors do not have to perform any signal processing, and the quality and capacity of digital streams should be unaffected by that conversion.

HDMI also incorporates digital audio in a bewildering variety of formats, including several Dolby systems and DTS. Introduced in 2003, HDMI has now reached version 2.0a, and unfortunately supports no less than 5 different plug/socket formats, together with two video protocols to support resolutions of 2560 x 1600 (the maximum for a single-link system) and still greater over dual-link. With the latest 2.0 standard, it copes well with 4K video at 60 frames/sec, up to 32 audio channels, and high dynamic range (HDR) video.

The five types of HDMI connector. By Jan deno, via Wikipedia Commons.
The five types of HDMI connector. By Jan deno, via Wikipedia Commons.

Currently you are most likely to come across Type A and C HDMI connection formats, the latter being a mini version intended for portable devices, although most recently Type E has been introduced for automotive applications. Types B and D are high-capacity dual-link variants of A and C, and remain unusual. Also peculiar to HDMI are specifications for cable types, to support different transmission bandwidths, such as 720p and 1080i (‘standard’ HDMI cable), and 1080p or above (‘high speed’). Given all the possibilities for permutation of specification, version, connector type, and cable, HDMI cables need to be purchased very carefully.

The road to Thunderbolt

DisplayPort is an alternative standard to HDMI aimed more at computer systems rather than domestic and entertainment systems, and has been strongly supported by Apple. It has little in common with DVI or HDMI, though. Unlike the other standards above, it is not only used to connect external displays, but also (using more compact connectors intended for the purpose) to connect internal displays, as in laptops.

Video data can be transmitted over 1, 2, or 4 cable pairs or ‘lanes’, which support bandwidths matching HDMI’s single-link and dual-link variants. Because DisplayPort uses different signals to DVI and HDMI, converters between the different formats, such as DisplayPort to HDMI, require electronics, and in some cases may need to be powered, making them relatively expensive. DisplayPort has optional support for digital audio signals, but is much less catholic in the range of audio than is HDMI.

Apple has popularised a miniaturised DisplayPort connector known as Mini DisplayPort, but other manufacturers may prefer standard-size external connections. There is now a good range of reasonably-priced adaptors to take Mini DisplayPort output and turn it into VGA, DVI-D, and other presentations. These are essential parts of the toolkit of anyone using a modern laptop to drive video projectors and external displays when ‘on the road’. In theory all of these, apart from the obsolescent VGA, should be hot-swappable, but in practice you may need to guess the correct sequence of connection and powering up your Mac and the external display to ensure mutual recognition.

macbkproportsThunderbolt uses the same standard Mini DisplayPort connector to do much more, by adding PCI Express data connections. In its original 1.0 form, Thunderbolt delivers 10 Gbit/sec over each of two channels, plus DisplayPort 1.1a. Newer Macs with DisplayPort 2.0 can aggregate the two data channels to give a single 20 Gbit/sec channel, with DisplayPort 2.0 and its support for 4K video. Thunderbolt 3 is on the way in the next year or so, and should double the data bandwidth to a maximum of 40 Gbit/sec, drive two external 4K displays or a single 5K display at 60 Hz. It will also come with a shorter connector which will probably require another set of adaptors.

Be prepared

Life is not easy for the MacBook or iPad road warrior, who may have but a few minutes to work out the most suitable connection to make with display equipment they have never seen before. A clutch of adaptors, including VGA 15-pin, DVI-D, and possibly Types A and/or C HDMI, should see your presentation at a favourable resolution on most systems. Macs generally cope well with some quite old or quirky display systems, particularly when you are familiar with OS X and the Displays pane. If you can steal a few minutes to set up during a preceding coffee or meal break, then you can stride up to the podium with confidence.

Ports: Macs

Apple has used a wide range of different ports on its many Mac models, but in recent years has tended to follow industry trends. Graphics cards shipped in various builds of Power Mac G5 desktop systems offered combinations of DVI, including single- and dual-link according to supported resolutions, and Apple’s proprietary ADC. The latter can be a problem for sourcing suitable cables.

The first Mac mini featured a DVI port as was common on other models at that time, but Mini DisplayPorts started to appear on the Mac mini and iMacs in early 2009, initially on minis alongside DVI. In mid 2010 DVI ports were replaced by HDMI, together with a Mini DisplayPort, and in 2011 by Thunderbolt. Mac Pro systems have undergone a similar sequence, adding Mini DisplayPort to the earlier dual-link DVI on their standard graphics cards in early 2009, but only the new (2013) Mac Pro has Thunderbolt.

PowerBook G4 models stayed with DVI, the last models requiring dual-link to provide the necessary bandwidth for improving support of higher resolutions. MacBooks and MacBook Pros were initially offered with Mini-DVI, but switched to Mini DisplayPort during 2009. The first MacBook Air model was encumbered with ‘micro-DVI’, which was peculiar to this and an Asus model, but thankfully this has now been wisely replaced by Mini DisplayPort. Laptops switched to full Thunderbolt ports in 2011.

Apple has provided ranges of adaptors for its laptops, in particular, converting to S-video (slightly inferior to component video using just two analogue signal channels, recognised by its unique mini-DIN connector), component video, VGA 15-pin, DVI, dual-link DVI for higher resolutions, and HDMI.

Ports: Displays

Regular CRT monitors from Apple and other vendors almost universally sported VGA 15-pin sockets, although some larger screens required more exotic connectors including RCA component video. When Apple introduced its first flat-screen monitor in 1998, later variants of this 15” Studio Display switched to DVI input before Apple adopted its proprietary ADC (Apple Display Connector) variant of DVI in 2000. ADC connectors are large even compared with DVI, but carry more signals, including both DVI-A and DVI-D, USB, and can supply up to 100 W of power for the display. Being peculiar to Apple, ADC cables have always been expensive and relatively hard to purchase.

More widely-supported DVI-D connections returned in the new range of Apple Cinema Displays launched in the summer of 2004, with the 30” model requiring dual-link DVI to provide sufficient bandwidth for its resolution that peaks at 2560 x 1600. LED Cinema Displays available from 2008 to 2013 come equipped with Mini DisplayPort ports, and of course the 2011 Thunderbolt Display comes with Thunderbolt.

Third-party products aimed at Mac users have tried to track Apple’s succession of different standards. Inevitably ADC proved a non-starter, and in specialist markets such as touch-screens or graphics screen tablets options remain the most generalist, with good old VGA or less ancient DVI. Video projectors can offer a bewildering variety according to the market sector for which they were destined and their age, and you can encounter VGA, component or composite video, S-Video, HDMI, Mini DisplayPort, and almost anything else that has been tried in domestic or professional video equipment.

Updated from the original, which was first published in MacUser volume 27 issue 5, 2011.