In most developed countries the colour television market is long past the first adopter stage, so television manufacturers are looking to new technologies - such as high definition TV, digital pictures, home cinema and surround sound - as incentives for consumers to buy new televisions.

High definition multimedia interface, or HDMI, is one of an emerging group of standards that provide higher performance, easier use and added features to help make purchasing a new TV a more attractive proposition.

Rather than taking a completely fresh approach, HDMI builds on the standards and techniques that have emerged over the past 10 years, so to understand HDMI it is helpful to review the recent history of the display interface.

A BRIEF HISTORY OF VIDEO CONNECTIONS
Early video interfaces In the early days of video recorders the only video output connection to the TV was through the coaxial aerial socket. Then in 1980 the French introduced the Scart (Syndicat des Constructeurs d'Appareils Radiorecepteurs et Televiseurs) connector.

This was apparently an attempt to limit the sales of imported televisions, because it was illegal to sell a television without a Scart connector, in France but Scart rapidly became a European standard.

The VGA bodge
Since the birth of the home computer, first nineand then 15-pin sub-miniature 'D'-type connectors, now commonly referred to as VGA connectors, have been used as the standard analogue video signal connector for PCs.

However, these connectors were never designed for use with video signals - they were simply industry standard connectors that were relatively cheap and could fit on the end bracket of a video card.

A number of methods for identifying the capabilities of the connected display have been added to the basic VGA connection as part of the general plug and play strategy.

In 1994, DDC (display data channel) was introduced to allow VGA connected displays to send up to 128 bytes of identification data. This was replaced in 1999 by the more capable E-DDC (extended display data channel).

DDC set the standards for EDID (Extended Display Identification Data) and EEDID, which is supported by HDMI.

The first DVI standard
In 1999 digital video interface (DVI) was introduced as the standard interface for the new flat panel displays and was designed for use with computers.

While DVI sought to replace the elderly VGA analogue video standard with a cleaner digital connection, it was hampered by the need to provide backwards compatibility through support for analogue video signals.

If LCD and other flat screen technologies had been able to take over the display market four to five years ago, DVI would probably now be the default video interface.

As it is, the ancient analogue, CRT based technology refuses to die, because it can still provide quality displays at substantially lower prices than LCD. VGA and its extended variants, although pushed well past their original technical limits, have survived with it.

DVI and HDCP The high-definition (HDTV) market has now taken up DVI.

In 2001 Silicon Image announced a system that provided high-bandwidth digital content protection (HDCP), a co-development by Intel and Silicon Image. HDCP encrypts the video output, so that a display with decoding technology is required to view the video.

The Silicon Image PanelLink series of digital video interface chips use this system, with DVI providing the physical connection.

Interface chip design
The key to higher bandwidth interfaces is the design and availability of cheap driver and receiver chips, so it is interface chip technology that sets the pace of interface design.

Transmitting high-definition digital video and surround sound or multi-track digital audio across several metres of cable requires high performance drive and receiver chips.

Although now adopted as an industry standard, HDMI was developed by Silicon Image, the same company that was responsible for DVI.

Silicon Image licenses the HDMI technology to other chip manufacturers as well as manufacturing its own line of PanelLink Cinema chips for HDMI applications.

Convenience for the consumer and protection for media giants
More than just a video interface, HDMI combines uncompressed high-definition video and multi-channel audio in a single digital interface with a bandwidth of up to 5Gbps.

HDMI also capitalises on the interoperability standards created by the Consumer Electronics Association and supports many of the capabilities of the AV.link interoperability protocol popular in Europe, including the control of multiple source devices through a single remote pointed at the digital TV.

From the consumer's point of view, HDMI offers the benefits of a simple single cable interconnection using a standardised connector. It offers the prospect of using identical single cables to link all domestic audio and video systems, along with a common, distributed method for remote control.

As far as the media industry is concerned, HDMI offers copyright protection through HDCP technology, which addresses the issue of protecting high-value content from unauthorised reproduction and distribution.

THE NUTS AND BOLTS
Although the connectors and cables are new, HDMI builds on the protocols and technology developed over the past few years.

The transceiver technology is basically the same as that developed for DVI. HDCP first appeared as an addition to DVI; and the AV.link remote control, the EDID display identity protocol and the surround sound audio features are all pre-existing standards.

TMDS and HDMI cable length
HDMI uses the TMDS (transition minimised differential signalling) channels first seen in DVI. Like DVI, HDMI allows for one or two TMDS links with three TMDS channels per link and a maximum bandwidth per channel of 165MHz.

Video pixel clocks are transmitted over a fourth TMDS, which is used by the PanelLink receiver as a data recovery reference for the other three channels.

HDMI differs from DVI in its support for video and surround audio and for remote control signals. Video and audio signals are multiplexed at the source transmitter, passed across the links as packet data and demultiplexed at the Sink or receiver.

The TMDS technology in PanelLink is designed to support cable lengths of up to 15 metres in copper. Longer connections will be possible using fibre optic cables.

HDMI connectors and cables
HDMI uses a single cable for audio and video. Two connector types are used - the 19-pin type A and the 29-pin type B.

The Type A connector supports a single TMDS link, so it can only carry signals up to 165 megapixels per second.

A Type B connection doubles up to two TMDS links and therefore can support signal bandwidths up to 330 megapixels per second. This is the same scheme as used for DVI, but with different connectors.

It is possible to connect an A device and a B device, or a DVI connection and an HDMI connection with a conversion cable, although the maximum performance of the connected devices may not match.

A CHANGE IN DIRECTION
Since the beginnings of DVI, Silicon Image and its partners in the development of HDMI seem to have altered their approach, probably in response to market pressures.

While the DVI specification clearly states that DVI was intended as a transitional interface - to replace the analogue VGA computer display standard with a dual purpose digital/analogue display interface -the HDMI specification emphasises domestic consumer HDTV applications and doesn't mention computer applications.

In the UK, HDMI implementations are limited to some DVD players for connection to large-format domestic plasma displays.

Although HDMI doesn't seem to be encouraging it, it will be interesting to see if HDMI is taken up by the computer industry and whether it might then enable the longanticipated convergence of computing and home entertainment electronics.

Further reading:
www.hdmi.org
www.siliconimage.com
www.monstercable.com