High-Definition Multimedia Interface (HDMI)

The High-Definition Multimedia Interface (HDMI) is a licensable audio/video connector interface for transmitting uncompressed, encrypted digital streams. HDMI connects DRM-enforcing digital audio/video sources, such as a set-top box, a Blu-ray Disc player, a PC running Windows Vista, a video game console, or an AV receiver, to a compatible digital audio device and/or video monitor, such as a digital television (DTV). HDMI began to appear in 2006 on prosumer HDTV camcorders and high-end digital still cameras.

It represents the DRM alternative to consumer analog standards such as RF (coaxial cable), composite video, S-Video, SCART, component video and VGA, and digital standards such as DVI (DVI-D and DVI-I).


General notes
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HDMI supports any TV or PC video format, including standard, enhanced, or high-definition video, plus multi-channel digital audio on a single cable. It is independent of the various DTV standards such as ATSC, and DVB (-T,-S,-C), as these are encapsulations of the MPEG movie data streams, which are passed off to a decoder, and output as uncompressed video data on HDMI. HDMI encodes the video data into TMDS for transmission digitally over HDMI.

Devices are manufactured to adhere to various versions of the specification, where each version is given a number, such as 1.0 or 1.3. Each subsequent version of the specification uses the same cables, but increases the throughput and/or capabilities of what can be transmitted over the cable. For example, previously, the maximum pixel clock rate of the interface was 165 MHz, sufficient for supporting 1080p at 60 Hz or WUXGA (1920x1200), but HDMI 1.3 increased that to 340 MHz, providing support for WQXGA (2560x1600) and beyond across a single digital link. See also: HDMI Versions.

HDMI also includes support for 8-channel uncompressed digital audio at 192 kHz sample rate with 24 bits/sample as well as any compressed stream such as Dolby Digital, or DTS. HDMI supports up to 8 channels of one-bit audio, such as that used on Super Audio CDs at rates up to 4x that used by Super Audio CD. With version 1.3, HDMI now also supports lossless compressed streams such as Dolby TrueHD and DTS-HD Master Audio.

HDMI is backward-compatible with the single-link Digital Visual Interface carrying digital video (DVI-D or DVI-I, but not DVI-A) used on modern computer monitors and graphics cards. This means that a DVI-D source can drive an HDMI monitor, or vice versa, by means of a suitable adapter or cable, but the audio and remote control features of HDMI will not be available. Additionally, without support for High-bandwidth Digital Content Protection (HDCP) on the display, the signal source may prevent the end user from viewing or recording certain restricted content.

In the U.S., HDCP-support is a standard feature on digital TVs with built-in digital (ATSC) tuners, (it does not feature on the cheapest digital TVs, as they lack HDMI altogether). Among the PC-display industry, where computer displays rarely contain built-in tuners, HDCP support is absent from many models. For example, the first LCD monitors with HDMI connectors did not support HDCP, and few compact-LCD monitors (17" or smaller) support HDCP.

The HDMI Founders include consumer electronics manufacturers Hitachi, Matsushita Electric Industrial (Panasonic/National/Quasar), Philips, Sony, Thomson (RCA), Toshiba, and Silicon Image. Digital Content Protection, LLC (a subsidiary of Intel) is providing HDCP for HDMI. In addition, HDMI has the support of major motion picture producers Fox, Universal, Warner Bros., and Disney, and system operators DirecTV and EchoStar (Dish Network) as well as CableLabs and Samsung.

Specifications
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HDMI defines the protocol and electrical specifications for the signaling, as well as the pin-out, electrical and mechanical requirements of the cable and connectors.

Connectors
The HDMI Specification has expanded to include three connectors, each intended for different markets.

The standard Type A HDMI connector has 19 pins, with bandwidth to support all SDTV, EDTV and HDTV modes and more. The plug outside dimensions are 13.9 mm wide by 4.45 mm high. Type A is electrically compatible with single-link DVI-D.

A higher resolution version called Type B is defined in HDMI 1.0. Type B has 29 pins (21.2 mm wide), allowing it to carry an expanded video channel for use with very high-resolution future displays, such as WQSXGA (3200x2048). Type B is electrically compatible with dual-link DVI-D, but is not in general use.

The Type C mini-connector is intended for portable devices. It is smaller than Type A (10.42 mm by 2.42 mm) but has the same 19-pin configuration.

Cable
The HDMI cable can be used to carry video, audio, and/or device-controlling signals (CEC). Adaptor cables, from Type A to Type C, are available.

TMDS channel
The Transition Minimized Differential Signaling (TMDS) channel:

-Carries video, audio, and auxiliary data via one of three modes called the Video Data Period, the Data Island Period, and the Control Period. During the Video Data Period, the pixels of an active video line are transmitted. During the Data Island period (which occurs during the horizontal and vertical blanking intervals), audio and auxiliary data are transmitted within a series of packets. The Control Period occurs between Video and Data Island periods.
-Signaling method: Formerly according to DVI 1.0 spec. Single-link (Type A HDMI) or dual-link (Type B HDMI).
-Video pixel rate: 25 MHz to 340 MHz (Type A, as of 1.3) or to 680 MHz (Type B). Video formats with rates below 25 MHz (e.g. 13.5 MHz for 480i/NTSC) transmitted using a pixel-repetition scheme. From 24 to 48 bits per pixel can be transferred, regardless of rate. Supports 1080p at rates up to 120 Hz and WQSXGA.
-Pixel encodings: RGB 4:4:4, YCbCr 4:4:4 (8–16 bits per component); YCbCr 4:2:2 (12 bits per component)
-Audio sample rates: 32 kHz, 44.1 kHz, 48 kHz, 88.2 kHz, 96 kHz, 176.4 kHz, 192 kHz.
-Audio channels: up to 8.
-Audio streams: any IEC61937-compliant stream, including high bitrate (lossless) streams (Dolby TrueHD, DTS-HD Master Audio).

Consumer Electronics Control channel
The Consumer Electronics Control (CEC) channel is optional to implement, but wiring is mandatory. The channel:

-Uses the industry standard AV Link protocol.
-Used for remote control functions.
-One-wire bidirectional serial bus.
-Defined in HDMI Specification 1.0, updated in HDMI 1.2a, and again in 1.3a (added timer and audio commands).
This feature is used in two ways:

-To allow the user to command and control multiple CEC-enabled boxes with one remote control, and
-To allow individual CEC-enabled boxes to command and control each other, without user intervention.
An example of the latter is to allow the DVD player, when the drawer closes with a disk, to command the TV and the intervening A/V Receiver (all with CEC) to power-up, select the appropriate HDMI ports, and auto-negotiate the proper video mode and audio mode. No remote control command is needed. Similarly, this type of equipment can be programmed to return to sleep mode when the movie ends, perhaps by checking the real-time clock. For example, if it is later than 11:00 p.m., and the user does not specifically command the systems with the remote control, then the systems all turn off at the command from the DVD player.

Alternative names for CEC are Anynet (Samsung), BRAVIA Theatre Sync (Sony), Regza Link (Toshiba), RIHD (Onkyo) and Viera Link/EZ-Sync (Panasonic/JVC).

Content protection
-According to High-bandwidth Digital Content Protection (HDCP) Specification 1.2.
-Beginning with HDMI CTS 1.3a, any system which implements HDCP must do so in a fully-compliant manner. HDCP compliance is itself part of the requirements for HDMI compliance.
-The HDMI repeater bit, technically the HDCP repeater bit, controls the authentication and switching/distribution of an HDMI signal.

Versions
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Devices are manufactured to adhere to various versions of the specification, where each version is given a revision number. Each subsequent version of the specification uses the same cables, but increases the throughput and capabilities of what can be transmitted over that cable. The need for a new HDMI cable if you already have one really depends on the cable (which also has a HDMI rating). The main thing to consider is if any current cable would be able to handle the increased bandwidth—for example the 10.2 Gbit/s that comes with version 1.3. Cable compliance testing is included in the HDMI Compliance Test Specification (see TESTID 5-3), with "Category 1" and "Category 2" defined in the HDMI Specification 1.3a (Section 4.2.6).

A product listed as having an HDMI version does not necessarily mean that it will have all of the features listed under the version classification, indeed some of the features are optional. For example in HDMI v1.3 it is optional to support the xvYCC wide color standard. This means if you have bought a camcorder that supports the wide color space (which for example is branded by Sony as "x.v.Color") you have to specifically check that the display supports both HDMI v1.3 and the xvYCC wide color standard.

HDMI 1.0
Released December 2002.

-Single-cable digital audio/video connection with a maximum bitrate of 4.9 Gbit/s. Supports up to 165 Mpixel/s video (1080p60 Hz or UXGA) and 8-channel/192 kHz/24-bit audio.

HDMI 1.1
Released May 2004.

-Added support for DVD Audio.

HDMI 1.2
Released August 2005.

-Added support for One Bit Audio, used on Super Audio CDs, up to 8 channels.
-Availability of HDMI Type A connector for PC sources.
-Ability for PC sources to use native RGB color-space while retaining the option to support the YCbCr CE color space.
-Requirement for HDMI 1.2 and later displays to support low-voltage sources.

HDMI 1.2a
Released December 2005.

-Fully specifies Consumer Electronic Control (CEC) features, command sets, and CEC compliance tests.

HDMI 1.3
Released 22 June 2006.

-Increases single-link bandwidth to 340 MHz (10.2 Gbit/s)
-Optionally supports 30-bit, 36-bit, and 48-bit xvYCC with Deep Color or over one billion colors, up from 24-bit sRGB or YCbCr in previous versions.
-Incorporates automatic audio syncing (Audio video sync) capability.
-Optionally supports output of Dolby TrueHD and DTS-HD Master Audio streams for external decoding by AV receivers. TrueHD and DTS-HD are lossless audio codec formats used on HD DVDs and Blu-ray Discs. If the disc player can decode these streams into uncompressed audio, then HDMI 1.3 is not necessary, as all versions of HDMI can transport uncompressed audio.
-Availability of a new mini connector for devices such as camcorders.

HDMI 1.3a
Released 10 November 2006.

-Cable and Sink modifications for Type C
-Source termination recommendation
-Removed undershoot and maximum rise/fall time limits.
-CEC capacitance limits changed
-RGB video quantization range clarification
-CEC commands for timer control brought back in an altered form, audio control commands added.
-Concurrently released compliance test specification included.

HDMI 1.3b
Testing specification released 26 March 2007.

Cable length
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The HDMI specification does not define a maximum cable length. As with all cables, signal attenuation becomes too high at a certain length. Instead, HDMI specifies a minimum performance standard. Any cable meeting that specification is compliant. Different construction quality and materials will enable cables of different lengths. In addition, higher performance requirements must be met to support video formats with higher resolutions and/or frame rates than the standard HDTV formats.

The signal attenuation and intersymbol interference caused by the cables can be compensated by using Adaptive Equalization.

HDMI 1.3 defined two categories of cables: Category 1 (standard or HDTV) and Category 2 (high-speed or greater than HDTV) to reduce the confusion about which cables support which video formats. Using 28 AWG, a cable of about 5 metres (~16 ft) can be manufactured easily and inexpensively to Category 1 specifications. Higher-quality construction (24 AWG, tighter construction tolerances, etc.) can reach lengths of 12 to 15 metres (~39 to 49 ft). In addition, active cables (fiber optic or dual Cat-5 cables instead of standard copper) can be used to extend HDMI to 100 metres or more. Some companies also offer amplifiers, equalizers and repeaters that can string several standard (non-active) HDMI cables together.

HDMI and high-definition optical media players
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Both introduced in 2006, Blu-ray Disc and HD DVD offer new high-fidelity audio features that require HDMI for best results. Dolby Digital Plus (DD+), Dolby TrueHD and DTS-HD Master Audio use bitrates exceeding TOSLINK's capacity. HDMI 1.3 can transport DD+, TrueHD, and DTS-HD bitstreams in compressed form. This capability would allow a preprocessor or audio/video receiver with the necessary decoder to decode the data itself, but has limited usefulness for HD DVD and Blu-ray.

HD DVD and Blu-ray permit "interactive audio", where the disc-content tells the player to mix multiple audio sources together, before final output. Consequently, most players will handle audio-decoding internally, and simply output LPCM audio all the time. Multichannel LPCM can be transported over an HDMI 1.1 (or higher) connection. As long as the audio/video receiver (or preprocessor) supports multi-channel LPCM audio over HDMI, and supports HDCP, the audio reproduction is equal in resolution to HDMI 1.3. However, many of the cheapest AV receivers do not support audio over HDMI and are often labeled as "HDMI passthrough" devices.

Note that all of the features of an HDMI version may not be implemented in products adhering to that version since certain features of HDMI, such as Deep Color and xvYCC support, are optional.

Criticism
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Among manufacturers, the HDMI specification has been criticized as lacking in functional usefulness. The public specification devotes many pages to the lower-level protocol layers (physical, electrical, logical); there is inadequate documentation for the system framework. HDMI-peripherals include audio/video sources, audio-only receivers, audio-video receivers, video-only receivers, repeaters (which have more downstream ports than upstream ports), and switchers (which have more upstream ports than downstream ports). The specification stops short of offering examples of system behavior involving multiple HDMI-devices, leaving implementation to the product engineer's interpretation. Even between devices which use chips from Silicon Image (a promoter and supplier of HDMI IP and silicon), interoperability is not assured. The industry is working to improve through plugfest events (i.e. manufacturer conferences) and more comprehensive design-validation services.

Another criticism of HDMI is that the connectors are not as robust as previous display connectors. Currently most devices with HDMI capability are utilizing surface-mount connectors rather than through-hole or reinforced connectors, making them more susceptible to damage from exterior forces. Tripping over a cable plugged into an HDMI port can easily cause damage to that port.

Closed captioning problems
According to the HDMI Specification, all video timings carried across the link for standard video modes (such as 720p, 1080i, etc.) must have horizontal and video timings matching those defined in the CEA-861D Specification. Since those definitions allow only for the visual portion of the frame (or field, for interlaced video modes), there is no line transmitted for closed captions. Line 21 is not part of the transmitted data as it is in analog modes. For HDMI it is but one of the non-data lines in the vertical blanking interval.

Although an HDMI display is allowed to define a 'native mode' for video, which could expand the active line count to encompass Line 21, most MPEG decoders cannot format a digital video stream to include extra lines—they send only vertical blanking. Even if it were possible, the closed captioning character codes would have to be encoded in some way into the pixel values in Line 21. This would then require the receiver logic in the display to decode those codes and construct the captions.

It is possible, although not standardized, that some measure of content in text form can be transmitted from Source to Sink using CEC commands, or using InfoFrame packets. Again, as there is no standardized format for such data it would likely work only between a source and sink system from the same manufacturer. Such uniqueness goes against the standardization mission of HDMI, which is focused in part on interoperability.

Of course, it is possible that a future enhancement of the HDMI Specification may encompass closed caption transport.

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