The Secret Life of HDMI - Part 1
Thu 13 January 2011 |
Technical articles |
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HDMI has revolutionised the connectivity of many home cinema devices. So what does that innocent little 19 pin connector do that’s so special and are all the different standards for?
In this series or articles we aim to dispel some of the myths concerning the HDMI connection, explore some of the lesser-known facts and figures about the HDMI interface and look at how and why it came about.
Not so humble beginnings
Digital TV recording first appeared in homes in early 1998 with the arrival of TiVO in North America. Laser disc and DVD were also possible sources of digital home video but at that time none offered a digital video output. In fac ta digital output would have been practically useless anyway as there were few if any digital TV sets around.
Plasma TV sets were the first widely available digital sets. A fixed array of pixels (852 across by 480 high) could each emit a mixture of red, green and blue light i.e. the component colours of a TV picture. Fujitsu and Philips launched the first models in 1997 and while they gained a good deal of PR, their price (approx. £10,000 / $15,000) meant there potential market was pretty small.
Over the next 5 years Plasma prices plummeted and cheap LCD TV sets appeared in smaller screen sizes. With the availability of affordable digital TVs and widely available digital media (on DVD) the obvious question became “they’re both digital, why can’t I connect them digitally”?
Making films and TV shows is expensive and the creators of the content aim to make a great deal of money out of their work (after costs are covered of course). Having lost control of analogue content when VHS recorders arrived, Hollywood and the TV companies were keen to keep the distribution of new digital content firmly under their own control.
While digital connections between a source and display offered a potentially better picture quality it also meant theoretically perfect copies could be made. Clearly this was not what the content providers wanted and so a secure system had to be found before they would agree to a digital interface for consumer products.
SDI (Serial Digital Interface)
Some digital video connections did exist. In the recording and mixing world a single coaxial interface called SDI (Serial Digital Interface) was used for moving video. This was both simple and reliable. However, SDI provided no copywrite protection at all. After-market suppliers did offer enterprising modifications to DVD players and early Plasma screens, adding an SDI connection but these never found big favour, probably due to the complexity of the work involved an the invalidation of warranties that the modifications entailed.
Enter DVI (Digital Video Interface)
Originally intended for computer monitors, the DVI interface is a highly capable system. Offering “Full HD” resolution (1920 x 1080 pixels) it was perfect for the computer industry that were already working with higher definition sources (XGA etc.). Sadly though, just like SDI, DVI in it’s earliest forms didn’t offer any copywrite protection either.
HDCP (High Definition Content Protection)
The solution to content protection came from chip giant Intel with their HDCP system. This delivered a very secure system for digital signals moving over a DVI connection. Both source and display had to have the correct hardware and internal software or the system wouldn't work. The all important protection “keys” that secure the content are unique to every chip manufactured with only the Digital Content Protection body (a subsidiary of Intel) holding the master key.
In later forms DVI was modified to carry Audio too. “DVI-HDTV” offers full HD plus very high quality digital audio over a single DVI cable connected each end with a DVI connector. However, wide-spread use of DVI-HDTV never happened before it was quickly overtaken by the more consumer friendly HDMI…
Next week we look at the capabilities of HDMI in its simplest form and compare its data rate with that of other high speed systems such as USB2, IEE1394 (Firewire) and Ethernet.
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