Intro to Computer Systems

Chapter 6b: Peripheral Expansion

Display Connections

There are two basic ways of transferring video information from computer to monitor: analogue or digital transmission.

This graphics card includes both an analog VGA (blue) and digital DVI (white) port.
This graphics card includes both an analog VGA (blue) and digital DVI (white) port. Photo: ATI

Digital transmission is the easiest to comprehend: for each pixel in the display, the computer sends a set of data with the necessary colour and intensity information. This also provides the most faithful representation, as the monitor displays exactly what the computer intends. Most LCD-type monitors use a digital connection.

Analogue transmission works not by providing colour and intensity information for each individual pixel, but as a continuously variable 'wave' of information. Such a transmission scheme is used for CRT monitors, as the monitors themselves have no concept of an individual "pixel".

The difference in transmission is similar to that of analogue vs. digital broadcast TV.

Analogue Standards

The common analogue transmission standard is commonly known as VGA (Video Graphics Array), though technically speaking this refers only to its first incarnation in 1987. It is a system that provides a set of timing information, as well as red, green and blue colour intensity information. The timing information is needed so the monitor knows what part of the screen the current data is describing.

The VGA standard has been constantly updated since its inception in 1987 to allow for higher resolutions and better colour information.

This link from Wikipedia summarises all such standards, including those that came before VGA. Don't get too wrapped up in the standard names, it starts to get rediculous (QVGA? WUXGA? WXGA+?)

The VGA port.
A VGA port.

Analogue standards such as VGA are highly susceptible to interference; like a television set, the analogue signal can be contaminated with "ghosting" and other imperfections in the image. Generally speaking, the higher the screen resolution, the more likely it is to be affected by interference.

Digital Standards

The common digital transmission standards for desktop computers are:

A graphics card with two DVI ports (right), a DisplayPort (left) and an HDMI port (centre).
A graphics card with two DVI ports (right), a DisplayPort (left) and an HDMI port (centre).

DVI: The Original Digital Standard

The DVI standard supports both analogue and digital transmission (DVI-I), however it's often only used for the digital signal (DVI-D). DVI ports are often found on LCD monitors to produce high quality images.

A summary of DVI interface pinouts for analog, digital, and dual-link digital output.
A summary of DVI interface pinouts for analog, digital, and dual-link digital output.

Like VGA (and its alphabet soup of variations), there is a limit to how much data can be sent through the cabling. In order to support ultra-high resolution displays, such as 27 and 30 inch computer monitors, there is a special version of DVI called "dual link" DVI: this uses extra signal lines to support the transmission of two interleaved sets of visual information.

HDMI: Digital A/V Connectors

An HDMI interface can support both a DVI digital signal, and a digital audio signal. Designed for high definition audiovisual data, it differs from DVI in that they natively support copyright protection through the High-Bandwidth Digital Copy Protection (HDCP) standard. HDMI is a common connection standard for home theatre equipment, such as Blu-Ray disc players, and large plasma and LCD televisions. There are a number of versions of the standard, some of which can support additional signalling such as ethernet networking and higher-end audio standards.

Wikipedia has a concise summary of what signals are supported on each version of the HDMI specification:
http://en.wikipedia.org/wiki/HDMI#Version_comparison
"The Analog Hole"

HDMI was the first interface where pervasive Digital Rights Management (DRM) made it difficult for end users to make duplicates of protected, copyrighted content. Except for some niche formats like SCMS copy protection on Sony Minidiscs, and the quickly-cracked encryption on DVD discs, consumer audiovisual interfaces were not at a quality considered high enough for rightsholders to be overly concerned about high quality duplicates.

However, the world - and our senses - are inherently analog, so the (protected) digital signals at some stage have to be a raw analog output for faithful representation in the real world of light and sound. The term analog hole describes the circumvention of Digital Rights Management by capturing and duplicating the signal when it was converted into its final analog form, rather than at the digital stage.

DisplayPort

DisplayPort is another high-quality digital video connector which is commonly used on computer equipment. Its feature set is roughly equivalent with HDMI, however it has the benefit of being royalty-free (compared to the expensive licensing required to include HDMI in hardware).

DisplayPort handles higher bit streams than DVI (negating the need for a specialised "dual-link" variant), and like HDMI it can combine other electrical signals such as USB and digital audio on the one cable.