The Amazon High Def Team is asked a lot of questions about
HDMI, so I thought it would be a good idea to explain and explore the cabling format in a three-part series.  In this post, I’ll give
some background information about HDMI along with its association with content
protection.

What is HDMI and how does it work?

HDMI stands for High Definition Multimedia Interface.  It uses a single bi-direction cable with 19
conductors that can carry both high-definition digital video and high-resolution
audio. Since the signal is bi-directional, all devices in the chain can
communicate their current status and capabilities. HDMI devices (TVs, high-def
disc players, etc.) can tell when HDMI cables are attached and detached, when
components are powered on and off, when inputs are switched, and so on.

All HDMI equipped displays are required to have an Extended
Display Identification Data (EDID) chip. 
The EDID includes information about the display such as make, model, and
serial number.  It also includes
important information about the capabilities of the display like audio
capabilities, supported video resolutions, and color depths.  This information exchanged between the
devices makes sure that the source device doesn’t send a signal to the display
that it can’t handle.  For example, if
you have an LCD TV that will accept 720p and 1080i video and will only take two-channel (left and right) audio, your HDMI-connected playback source won’t send
1080p video or multi-channel audio even if it is set to, since the intelligence
built into HDMI knows that your display can’t handle it based on what the EDID
reported.  While this normally works
great, it can cause some issues depending upon how your equipment manufacturer
implemented HDMI.  Additional
authenticating needs to take place when you are viewing content-protected
media.

Content Protection

HDMI uses the High-bandwidth Digital Content Protection
(HDCP) standard for copy protection. 
All HDMI playback devices, including cable and satellite set-top boxes,
upconverting DVD players, and high-def disc players, are required to use
HDCP.  When HDCP-equipped source devices
are first powered on, they use HDMI to “handshake” with receiving devices. All devices, including the source, receiver,
and display, need to authenticate and verify that they are HDCP-authorized by exchanging
a unique key with each other.  Once the
devices are authenticated, the source device starts encrypting and transmitting
the audio and video signal.  The
encryption is random and changes approximately every two seconds when the
source re-authenticates with the receiving device.  If the authentication ever fails, the source
will stop transmitting or reduce the quality of the signal.

HDCP is only used for high-definition copy-protected
content like HD DVD, Blu-ray and high-def pay-per-view. 
It is not used for high-definition over-the-air broadcast signals, or
any standard definition sources like DVD (unless they are upconverted).  It is also not used for non-commercial
content like HD home movies.  Some
monitors equipped with DVI do not support HDCP, so using an HDMI-to-DVI adapter
will not allow you to watch copy-protected content on these monitors. You will get either no picture or a standard-definition
picture.  It’s important to remember that
all devices connected through HDMI must support HDCP if you are watching copy-protected
HD content.

In the next post, I’ll talk about when and why
to use HDMI and some issues to be aware of when you do.  A future post will cover the different
versions of HDMI, the optional features they support, and purchasing
considerations. --Adam

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