HDMI - High-Definition Multimedia Interface

This article takes a thorough look at HDMI - High Definition Multimedia Interface - and explores its advantages over its predecessors as well as giving some technical details about the interface. This article focuses on the HDMI 1.3 specification. Before technical details are given, there is a relatively quick explanation of what HDMI is for those who don't want to read this entire article.

In a few words... A very short description

For those of you who don't want any technical details (well, there are some that 'have' to be included here) here's the short description for you. HDMI is the "to-be standard" for connecting HD consumer equipment for use. It is an all-digital interface that has been made mainly to deliver high definition video and multi-channel audio over a single cable with a small form factor (19-pin mainly) connector. HDMI can deliver uncompressed video and audio from both Consumer Electronics (CE) equipment and PC equipment, allowing consumers to interconnect them. It supports all current video format standards in resolutions ranging from 480p to 1080p and beyond.

The latest specification of HDMI, 1.3, doubled the previous bandwidth available for content to 10.2Gbps (from 4.95Gbps), much more than is necessary for 1080p content with multi-channel audio. It supports RGB and YCbCr colour up to 48bit colour resolutions. All the usual suspect audio formats are supported; Dolby Digital 5.1 - 7.1, DTS, DTS EX etc. Uncompressed (PCM) audio is supported up to 8 channels 192khz, 24bit. HDMI also supports the latest lossless audio compression formats including Dolby TrueHD and DTS HD which are both part of the specifications of Blu-ray Disc and HD DVD.

Since HDMI delivers uncompressed video content, there will be no deterioration of video quality due to "Digital -> Analogue -> Digital" conversions that were necessary using older interfaces to connect a DVD player to a Digital TV set for example. HDMI has also been built with features to bring intelligence to a home entertainment system. DDC (Display Data Channel) feature will make way for "automatic format adjustment" by reading a TV's / Monitor's video display and audio capabilities through HDMI. The source device will adjust the output automatically to best suit the display equipment.

To add even more intelligence to a system, a consumer can get devices that include the optional CEC (Consumer Electronic Control) feature support. Using CEC a consumer links HDMI-connected equipment in a whole new way, allowing one device to communicate with, and control another. For example, "One Touch Play" would allow a user to turn on all the devices necessary for playback with just one touch of a button. Another example of how CEC links devices is to connect a camcorder to a TV and control the camcorder's output using the TV's remote.

Due to the inclusion of HDCP (High-bandwidth Digital Content Protection) - a copy protection feature developed by Intel - HDMI enjoys broad industry support. HDCP will be used to protect HD content from "attacks" that could take place during transmission to copy the data. CE equipment that doesn't include HDCP could suffer from downgraded video resolution on future movie releases for example.

HDMI cables come in a variety of lengths (there is no maximum length specified, just a minimum electrical level to achieve) but typically cables range from 2 - 5 meters in length. To date, there are 3 HDMI connectors. Type A is a the standard 19 pin connector. A 29 pin Type B connector has been specified but it is not in use. HDMI 1.3 also specified a "Type C" mini connector designed for portable devices like camcorders. Type C is much smaller than the Type A connector, but contains the same number of pins.

So that is a brief description of HDMI, but if you still want to know more about it, then read on.

What uses HDMI?

Both Consumer Electronics (CE) devices and PC equipment are available that use HDMI. HDMI can also be found on many common devices such as upscaling DVD players, HD DVD players, Blu-ray Disc players, Camcorders, Set-top boxes, Gaming Consoles (PlayStation 3) & Digital Cameras. By September 2006, over 450 companies that produce CE and PC equipment have become licensed HDMI adopters.

HDMI is backward-compatible with DVI-D equipment through use of a suitable adapter or cable. This means that a DVI source can output to a HDMI monitor or vice versa. For obvious reasons however, the advanced audio support from HDMI will not be available, neither will HDMI's CEC features.

It is important to understand that because of HDMI's rapid adoption and broad industry support, it is technically the de-facto standard for connecting HD Ready equipment. Players for the new HD DVD and Blu-ray optical disc formats (which have been designed specifically for High Definition content) will include HDMI as does most new HDTV sets.

Why HDMI? - What Are The Advantages

Here are the main advantages of using HDMI...
  • All-Digital - HDMI transmits All-Digital data, which means video content will not have to suffer unnecessary digital-to-analog conversions and puts an end to analog interfere problems.
  • Video & Audio Quality - Utilizing its available 10.2Gbps of bandwidth, HDMI can easily transmit uncompressed 1080p video content with multi-channel audio content, ensuring the highest possible playback quality from your display device / sound system.
  • Intelligent Features - HDMI brings intelligence to a system through DDC and CEC. This allows for such advanced features as "automatic format adjustment" and "one touch control".
  • HDCP - In the future, protected digital broadcasts like Pay-Per-View and High Definition movies on next generation discs will require HDCP-enabled equipment to protect the content. While having a new content protection technology doesn't seem too interesting or beneficial to the consumer, it is alarming to know that lacking HDCP in your equipment might leave you with downgraded video quality (standard resolution video) or possibly, in cases, no playback at all.
  • Future-proof - HDMI is an evolving technology and could already, for example, support video content with a higher resolution than 1080p and HDMI 1.3 added support for the xvYCC colour space.
  • Ease-of-use - HDMI can do in one cable what could take several cables otherwise.
There are many many more benefits to using HDMI but these are the advantages that are most important to a consumer.

Technical Details

Below are technical details about HDMI from what drives it to who developed it and the technology it uses.

Supported Video Formats

Whether the video source is a PAL, NTSC or ATSC standard, HDMI will fully support it. The group behind HDMI claims it supports every video format in the consumer electronics industry. Due to the three TMDS channels each capable of 3.4Gbps bandwidth, HDMI supports video resolutions from 480p to 1080p and even higher (1440p) and supports 48bit colour resolutions in the RGB or YCbCr colour space (HDMI 1.3 added support for xvYCC). Combine these capabilities with a maximum refresh rate of 120 Hz. HDMI 1.3 is more than sufficient for UXGA (1600x1200).

So to summarize..
  • Video Formats - HDMI supports any format in the CE industry (PAL, NTSC, ATSC etc.)
  • Highest Resolutions - The highest resolution that the new HD DVD and Blu-ray movies will offer (up to 1080p) is fully supported in HDMI. Due to the extra available bandwidth, even higher resolutions are supported (1440p)
  • Colours in the billions - Up to 48-bit support for RGB and YCbCr makes way for over 1 billion colours. (48-bit support was added in HDMI 1.3)
  • Refresh Rate - Up to 120 Hz

Supported Audio Formats

HDMI 1.3 added support for the Dolby TrueHD and DTS-HD Master Audio lossless compression formats used with HD DVD and Blu-ray Disc. HDMI supports uncompressed audio, up to 8 channels at 192 Khz at up to 24 bit sampling rates. It also supports all compressed formats such as Dolby Digital and DTS audio. It also supports SACD audio and DVD-Audio (both of which are still competing formats for high fidelity audio). HDMI 1.3 added support for the Dolby TrueHD and DTS-HD Master Audio lossless compression formats used with HD DVD and Blu-ray Disc.

So to summarize..
  • Uncompressed (PCM) - PCM audio up to 8 channels at 24-bit sampling rates at a frequency of 192 Khz.
  • Compressed - Supports all the usual compressed formats; Dolby Digital 5.1 - 7.1, DTS etc.
  • SACD - HDMI supports up to 8 channels of one-bit audio used on Super Audio CDs.
  • DVD-Audio - Supports DVD Audio content since HDMI 1.1
  • Lossless - HDMI supports Dolby TrueHD and DTS-HD Master Audio
HDMI 1.3 also added another feature, automatic audio syncing. This works by adjusting the audio processing times to remove lip sync problems. The problem arises due to the fact that video processing generally takes more time than the audio in a DTV.

HDMI Plugs / connectors

There are three HDMI plugs to know about, Type A, Type B and Type C. Only Type A is really in use. It consists of a 19-pin connector. Type B has been defined and has 29 pins but is not currently in use. A new Type C connector is coming for portable devices such as camcorders and like Type A, it has 19 pins. Type A is more than sufficient for 1080p video and multiple channel uncompressed audio. The 19 pins facilitate the three high speed TMDS channels, the TMDS clock channel, DDC channel, CEC Channel, a 5v power supply and hot plug detect feature.

It is important to note that HDMI is backward-compatible with the single-link Digital Visual Interface or DVI-D (technically HDMI Type A is backward-compatible with DVI-D, HDMI Type B is backward compatible with dual-link DVI). A special cable or adapter is needed. For obvious reasons, the audio capabilities of HDMI cannot be used in this way and neither can the intelligent CEC features.

TMDS channels / TMDS Clock channel

TMDS stands for Transition Minimized Differential Signaling. It was developed by Silicon Image and can reliably transfer huge amounts of data through a shielded cable. In an HDMI cable there are three separate TMDS channels, each are capable of transfer rates up to 3.4Gbps giving a total 10.2Gbps. HDMI 1.3 doubled the previous clock speed of 165Mhz to 340Mhz. This brought the available bandwidth from 4.95Gbps to 10.2Gbps. Each channel carries data from one video component (Red, Green, Blue, for example) at up to 48-bit resolution along with control data and digital audio.

The TMDS clock channel provides the pixel clock for timing the data stream enabling reliable data processing in the receiving device across long cable lengths.

DDC channel & EDID chip

DDC stands for Display Data Channel and is defined by the Video Electronic Standards Association (VESA). Using an HDMI cable, the DDC channel is used by a source device (like a DVD player) to determine the audio / visual capabilities of the display device. This is achieved by reading from an EDID ROM chip that is required to be present in all HDMI enabled TVs / Monitors. EDID is defined by VESA. It stands for Extended Display Identification Data and an EDID chip would hold information like..
  • Manufacturer Name
  • Product Type
  • Supported Video Resolutions
  • Supported Audio Formats
  • Colour Depth Capabilities
etc. Because of the DDC Channel and EDID chip, a source device can automatically adjust the video and audio output to the best possible format for the display device, removing the need for the consumer to manually alter settings to get the highest quality output. As a matter of interest, HDCP also utilizes the DDC channel to initiate a "handshake" with all HDCP-enabled devices in a system (we'll see more about that later).

CEC Overview

To add more intelligence to a home entertainment system designed to make it more simple / efficient to use, an optional feature to add to a device is Consumer Electronic Control or CEC. While the addition of CEC features is at the manufacturers discretion, a CEC implementation must be compliant with the HDMI specification. CEC basically connects the devices in a whole new way, making it possible for one device to communicate with, and control another.

For example, a "One Touch Play" feature would allow a consumer at the press of a single button to switch on all devices that are needed to play back content, for example, the source DVD player turns on the display device and surround sound system that are needed for playback. You could think of CEC as sort of turning all remote controls in a CEC-enabled system into a universal remote.

Here are some examples of what CEC features..
  • One Touch Play - explained above
  • System Standby - at press of one button, all system devices switch to standby mode.
  • One Touch Record - At the touch of a record button, whatever is being displayed on the display device can be recorded using a selected device in the system.
  • Deck Control - For example, a display device like a TV can communicate with, and control a device that is outputting content to it so you could control camcorder connected to a TV with the TV remote.
  • Tuner Control - One device can control the tuner of another, like a TV remote changing the channel on a Set-Top Box.
and many more. CEC is part of the HDMI standard allowing you to mix and match different CEC-enabled hardware.

Content Protection - HDCP

Would it be possible for HDMI to have so much support from the motion picture industry if it didn't offer upgraded security over its predecessors? Probably not, which is why HDMI has licensed High-bandwidth Digital Content Protection or HDCP. Created by Intel subsidiary Digital Content Protection , LLC, HDCP is required on HD-capable set-top boxes that use HDMI. It is also used with players that can output HD content like an HD DVD player, Blu-ray disc player or an upscaling DVD player.

Since HDCP is a proprietary technology it requires a license to be implemented on a device. As part of the agreement a licensee has to limit the output capabilities from all non-HDCP video outputs if requested by the source. This means in effect that the quality of video output will be lowered dramatically through a non-HDCP compliant output. This downgrade in quality (outputting standard resolution instead of the sought high definition resolution) is being done to protect high definition content from theft.

HDCP works by encrypting the digital content being sent from the source to the receiver(s) through either HDMI or HDCP-enabled DVI connections. Using HDMI's DDC channel, the source and receivers initiate a "handshake" and validate that each device is an authorized one. During this process, each exchanges a special key called a Key Selection Vector or KSV. Each HDCP-enabled device has 40 unique keys (the KSV is created from the set) and if these keys are not kept confidential, the license agreement is broken.

During this process the Keys and KSV's are generated in a way that ensures each device gets the same 56 bit number which is used in the encryption process. A XOR operation is applied to encrypt each decoded pixel with a 24-bit number. HDCP requires constant updating of keys to ensure security. If a model is considered compromised (hacked) then the KSV is added to a dreaded revocation list. New HD discs would include this list. If a source devices finds a receiver's KSV on the list, it will not send protected HD data to it. The revocation list is protected to ensure that a malicious user couldn't revoke an uncompromised device.

HDCP is used on protected content like Hi-Def Hollywood movies and Pay-per-view transmissions and won't affect non-protected content. No content protection is 100% hack-proof though and several claims have already been made about serious flaws in HDCP. One cryptographer, Niels Ferguson, claims to have broken the HDCP scheme but never published his work citing worries about consequences under the Digital Millennium Copyright Act (DMCA).

HDCP Criticism

HDCP has been subject to much criticism especially now that HD DVD and Blu-ray have arrived and major movies are on the shelves in HD. Firstly, a lot of HDTV owners have discovered that their equipment doesn't feature HDCP and as a result, at some point the quality of any video sent to the receiving equipment may be downgraded. Additionally, even if you have a receiver that features HDCP, in the future for whatever reason (like breach of agreement or hacking) its unique keys may be "revoked", leaving you with a receiver that may be sent a downgraded signal in the name of "content protection".

Who's behind it all?

HDMI was developed by a group of CE and and PC companies including Hitachi, Panasonic, Phillips, Silicon Image Inc. Sony, Thompson & Toshiba. HDCP was developed by Intel Subsidiary Digital Content Protection, LLC. DDC and EDID are both defined by the Video Electronic Standards Association (VESA). Blu-ray Disc was developed by the Blu-ray Disc Association (BDA) led by Sony Corp. HD DVD was developed by a group led by Toshiba.

Industry Support

Here are some of the companies that support HDMI (excluding HDMI founders).
  • Cable Television Laboratories, Inc. - known as CableLabs
  • DirecTV
  • EchoStar Communications Corporation
  • Samsung Electronics
  • Twentieth (20th) Century Fox Film Corporation
  • The Walt Disney Company
  • Universal Studios
  • Warner Bros. Entertainment, Inc.
Additionally, players for the next generation optical disc formats Blu-ray Disc and HD DVD, along with the PlayStation 3 gaming console from Sony Corp. include HDMI output.

HDMI Adoption

A company looking to become an HDMI adopter (to make products like DVD players etc. that include HDMI support) need to sign an Adopter Agreement. HDMI then needs to be licensed and regular fees paid by that company. HDMI Adopters must pay an annual fee of ten thousand dollars (US$10,000) (This figure was dropped from the previous $15,000 after November 1st, 2006 when the annual fee will be dropped, see Recent Developments section). The annual fee is due upon the execution of the Adopter Agreement, and must be paid on the anniversary of this date each year thereafter.

In addition, for each end-user Licensed Product, a fifteen cents (US$0.15) is paid per unit sold. If the Adopter reasonably uses the HDMI logo on the product and promotional materials, then the rate drops to five cents (US$0.05) per unit sold.If the Adopter implements HDCP content protection as set forth in the HDMI Specification, then the royalty rate is further reduced by one cent (US $.01) per unit sold, for a lowest rate of four cents (.04) per unit. Adopters must license HDCP separately from Digital Content Protection, LLC, an Intel subsidiary. (Source: HDMI Official FAQ)

As of September 2006, more than 450 CE and PC companies have become licensed HDMI adopters and this figure is rapidly increasing

HDMI-Enabled Device Shipments

Due to the high rate of adoption and the slow but sure move toward HD content, 60 million HDMI-enabled devices will ship in 2006 alone. By 2009, that number is expected to triple to 180 million devices. HDMI will continue to show up in new products, not just DVD/Blu-ray/HD DVD players or gaming consoles. It is described as an evolving interface and already is ahead of expectations (performance speaking) in key markets.

Recent Developments & Other Interesting Facts

This part of the article may be updated depending on developements that surround HDMI.

Silicon Image presents 340Mhz HDMI transmitter chips - Silicon Image, Inc., introduced two new VastLane HDMI transmitter chips that enable PC manufacturers to drive their digital output to PC monitors and HDTVs from a single transmitter. They offer performance of up to 340MHz or 10.2Gbps.]

HDMI license costs cut by 30 percent - AfterDawn report that HDMI Licensing, LLC, has dropped the annual administration fee for HDMI adopters from $15,000 to $10,000. It is speculated that the drop was aimed at getting more Chinese consumer electronics companies to adopt HDMI.

HDMI 1.3 upgrade published - AfterDawn reported on the HDMI upgrade to 1.3 which saw higher transfer rates from 4.95Gbps to 10.2Gbps by making the clock speed climb to 340Mhz from the previous 165Mhz. It also added support for the emerging xvYCC and lossless audio formats. Additionally, another 19-pin "Type C" connector has been added to the specification, to be used mainly with portable devices (see picture for an idea of the size of the new HDMI mini connector.).

High Definition ICT downgrade delayed - AfterDawn reported about unofficial agreements between Hollywood and some consumer electronics companies including Sony and Microsoft have been discussed to not include the ICT standard until 2010 or possibly even 2012.


This article should have been more than enough to provide you with an explanation of what HDMI is. To talk about HDMI, HDCP or any of the other area's covered here, then please visit our Discussion Forums. If you notice any spelling or factual mistakes, or if you just feel like I did not cover a certain topic properly/enough or have any suggestions at all, please send me a private message (you need to be logged in).

Version History

v1.0- First version published by Dela
Written by: James Delahunty