Put simply, HDMI is a new connection type that carries both digital audio and digital video (at resolutions up to 1080p) in a single cable. It can be found on HDTV's, some high-definition cable and satellite boxes, upscaling DVD players, the new HD disc formats HD-DVD and Blu-ray, and it is slowly making its way to surround sound receivers and processors. If you are buying a new HDTV, you should make sure the set includes an HDMI input. If your existing HDTV has a DVI input, you can use an adapter or a cable with HDMI on one end and DVI on the other to feed HDMI video data to the TV (you'll need to make sure your set'd DVI input has HDCP copy protection protocols included, though).
HDMI's video format is borrowed straight from DVI, which was developed originally by the computer industry for use with LCD monitors and then brought over to the consumer electronics world for use with HDTV's. When DVI was adopted by HDTV manufacturers, it was paired up with a new security protocol called HDCP (high definition copy protection) – a system for verifying that the source and display were both secure before allowing video data to pass, so that the raw digital video data being passed between devices wasn't stripped off and used to make high quality bootlegs of movies. Not all DVI displays and source components use HDCP, but HDMI made it an integral part of the standard. HDMI also expanded the allowable video data formats from 8-bit RGB only (the format used by DVI) to 8-bit RGB as well as 8-, 10-, or 12-bit YCrCb (YCrCb being the format in which video is stored on DVD's). Because of the ties between the two formats, however, it is still possible to connect HDMI and DVI sources with nothing more than an adapter or a cable terminated for HDMI at one end and DVI at the other. One more twist? Well, HD-DVD and Blu-ray discs both have a bit of data encoded into them called the Image Constraint Token (the ICT, for short). Any disc which has the ICT set to "on" is restricted to outputting full-resolution video over the HDMI output only – component video outputs (which can carry analog video data at resolutions up to 1080i) are restricted to a maximum 540p in that case, effectively reducing the output video resolution to 25% of the original resolution. As of mid-2006, no studios are setting the ICT on their HD-DVD and Blu-ray discs and there are no indications that any studios plan to change that policy.
In most cases, using HDMI to carry the audio data is purely a convenience. Cable and satellite receivers' digital audio signals are either PCM stereo or Dolby Digital, both of which can be carried just as well by a coaxial or optical digital audio cable (which is also readily compatible with any surround processor manufactured since the late 1990's). Likewise, DVD-Video discs will output PCM stereo, Dolby Digital, or DTS, any of which can be carried by a coaxial or optical digital audio cable. The cases where HDMI starts being handy for audio are DVD-Audio (if you have version 1.1 of HDMI at both the source player and the receiver and the player supports DVD-Audio), SACD (if you have version 1.1 or 1.2 of HDMI at both ends and the player supports SACD), and the new HD disc formats (if you have version 1.1 or higher of HDMI at both ends). In all three cases, HDMI represents a digital signal path that takes the place of six or eight analog audio cables, but HDMI v1.1 inputs were slow to appear on receivers and surround sound processors. The arrival of HDMI v1.3 (which was published in June 2006) helped speed that adoption, as many manufacturers held off on adding HDMI across the line for fear of releasing a product that was out of date as soon as it reached store shelves.
That's the short answer – HDMI in a nutshell. If you want to find out more detail, though, keep reading. The rest of this FAQ digs a good bit deeper into HDMI and the issues it raises for home theater systems.
Last updated on May 5, 2011 by Alex Robles
