Digital Visual Interface CablesHDMI/DVI Cables

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Frequently Asked Questions & Definitions

   What makes digital better than analog?
   What is TMDS?
   What is HDCP?
   What is CEC?

      Acronyms & Definitions

CEC -  Consumer Electronics Control   . . More Info
DDC -  Display Data Channel
DDWG -  Digital Display Working Group
DFP   -  Digital Flat Panel
DTS -  Digital Theater Sound
DTV -  Digital Television
DVI  -  Digital Visual Interface   . . More Info
EDID -  Extended Display Identification Data
EIA -  Electronics Industry Alliance
HDCP  -  High-bandwidth Digital Content Protection   . . More Info
HDMI -  High Definition Multimedia Interface   . . More Info
HDTV -  High-Definition Television
OSD -  On-Screen Display
P&D -  Plug and Display (M1)
PAL -  Phase Alternating Line
PnP -  Plug and Play
SDTV -  Satellite Digital Television
SVGA -  Super Video Graphics Array (VGA extensions)   . . More Info
SXGA -  Super Extended Graphics Array (1280 x 1024)   . . More Info
TMDS  -  Transition-Minimized Differential Signaling   . . More Info
UXGA -  Ultra Extended Graphics Array (1600 x 1200)   . . More Info
VBE -  VESA Bios Extension   . . More Info
VESA -  Video Electronics Standards Association
VGA   -  Video Graphics Array   . . More Info
XGA -  Extended Graphics Array   . . More Info

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       Why is digital better than the traditional analog transmission technology?

DVI and HDMI are better than the traditional analog transmission technology because analog is more sensitive to the phase changes of cables.  This is also the reason DVI and HDMI are better than component video. 

The only drawback of digital transmission technology is that the data being transferred can only maintain full fidelity for about 5 meters with DVI and about 15 meters for HDMI.  Cable runs can be increased with the use of fiber optic technologies, amplifiers, and repeaters, however, their incredible expense makes the idea commonly unreasonable.

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      Transition-Minimized Differential Signaling (TMDS)

DVI makes use of Transition-Minimized Differential Signaling (TMDS) wherein eight bits of video data are converted to a 10-bit transition-minimized, DC-balanced value.  This value is then serialized and sent to the receiver where the data is de-serialized and converted back to eight bits. 

The three TMDS signals needed for RGB make up one TMDS link.  A DVI connector can carry up to two TMDS links providing for better resolutions and timing requirements.  Single TMDS link DVI cables can support resolutions and timings that use a video clock rate of about 25-165 MHz. 

The following resolutions and timings are supported by a single link DVI cable:

SXGA resolution (1280 x 1024) with 85-Hz refresh rate
UXGA resolution (1600 x 1200) with 60-Hz refresh rate
SDTV resolutions of 480i, 480p, 576i and 576p
HDTV resolutions of 720p and 1080i

Video clock rates of 165-330 MHz on the other hand, are supported only by dual TMDS link DVI cables.  With dual link DVI cables, however, each TMDS link operates at only one-half the frequency of single link DVI.  So the clock and bandwidth is shared by these two links.

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      High-bandwidth Digital Content Protection (HDCP)

Along with the introduction of DVI technology came the need to prevent the digital video data from being pirated, or copied without authorization.  In order to address this issue High-bandwidth Digital Content Protection (HDCP) was derived.  HDCP, developed by the Intel Corporation, implements a key encryption encoded into the DVI signal which, without the proper HDCP decoding mechanism, produces either extremely low resolutions or static.  To avoid this, both source and display devices must have DVI connections that support software key HDCP decoding.

HDCP, also used by the new HDMI technology, was strongly endorsed by the entertainment industry, for obvious reasons.  What makes this technique of key encryption possible is its renewability.  The Digital Content Protection LLC, an organization responsible for licensing of the HDCP technology, monitors the secret keys used to encrypt the data.  If at any time they believe a set of secret keys has been compromised by an unauthorized source, those keys are placed on a revocation list and new keys are provided to devices authorized by license. 

HDCP also uses a method of authentication to ensure that the receiving device is licensed to receive HDCP encrypted data.  This authentication process repeats itself every few seconds in order to be certain that an illegal device has not been connected after the initial authentication.  If and when this authentication fails, the source device will immediately end all encrypted transmissions.

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       Consumer Electronics Control (CEC)

Consumer Electronics Control (CEC) is an optional protocol used in HDMI systems that allows HDMI devices to pass control functions along to all devices within the system.  CEC was designed to operate at low speeds with minimal processing and memory overhead.

The following is a set of all the features offered with CEC.  Devices may use all or only some of these features, depending upon functionality.

  • One Touch Play – A device may be played and become the active source by the press of a single button.
  • System Standby – Enables devices to be set on standby by the press of a single button.
  • Preset Transfer – A device’s presets can be auto-configured to match those of the TV.
  • One Touch Record – Allows recordings to be made by the press of a single button.
  • Timer Programming – Any device can program a timer recording on a recording device.
  • System Information – Allows devices to auto-configure their language and country settings.
  • Deck Control – Allows a device to control and interrogate a playback device.
  • Tuner Control – Allows a device to control the tuner of another device.
  • Vendor Specific Commands – Allows vendor-defined commands to be used.
  • OSD Display – Allows a device to display text using the on-screen display of the TV.
  • Device Menu Control – Allows a device to control the menu of another device.
  • Routing Control – Enables control of CEC Switches for streaming of a new source device.
  • Remote Control Pass Through – Allows Remote Control commands to be passed along to other devices within the system.
  • Device OSD Name Transfer – Devices may request the preferred OSD name of other devices within the system.

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