The High-Definition Multimedia Interface (HDMI) is an industry-supported, uncompressed, all-digital audio/video interface. HDMI provides an interface between any compatible digital audio/video source, such as a set-top box, a DVD player, or an A/V receiver and a compatible digital audio and/or video monitor, such as a digital television (DTV).

HDMI supports 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 data streams, which are passed off to a decoder, and output as uncompressed video data, which can be high-definition. This video data is then encoded into TMDS for transmission digitally over HDMI. HDMI also includes support for 8-channel uncompressed digital audio. Beginning with version 1.2, HDMI now supports up to 8 channels of one-bit audio. One-bit audio is what is used on Super Audio CDs.

The HDMI 1.3 standard was released on 22 June 2006 and as of that date is projected to be available in consumer products by the end of 2006. HDMI 1.3 is necessary in order to output the full bitrate of the new audio formats used on HD DVDs and Blu-ray Discs. . The spec also covers a new mini connector for devices such as camcorders.

The standard Type A HDMI connector has 19 pins, and a higher resolution version called Type B, has been defined, although it is not yet in common use. Type B has 29 pins, allowing it to carry an expanded video channel for use with high-resolution displays. Type-B is designed to support resolutions higher than 1080p.

Type A HDMI is backward-compatible with the single-link Digital Visual Interface (DVI-D) used on modern computer monitors and graphics cards. This means that a DVI 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 both ends, the video quality and resolution may be artificially downgraded by the signal source to prevent the end user from viewing or especially copying restricted content. (While nearly all HDMI connections support HDCP, many DVI connections do not.) Type B HDMI is similarly backward-compatible with dual-link DVI.

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

TMDS channel

• Carries audio, video and auxiliary data.
• Signalling method: According to DVI 1.0 spec. Single-link (Type A HDMI) or dual-link (Type B HDMI).
• Video pixel rate: 25 MHz to 165 MHz (Type A) or to 330 MHz (Type B). Video formats with rates below 25 MHz (e.g. 13.5 MHz for 480i/NTSC) transmitted using a pixel-repetition scheme. Up to 24 bits per pixel can be transferred, regardless of rate.   Supports 1080p60^*.
• Pixel encodings: RGB 4:4:4, YCbCr 4:4:4 (8 bits per component); YCbCr 4:2:2 (12 bits per component)
• Audio sample rates: 32 kHz, 44.1 kHz, 48 kHz, 88.2 kHz, 96kHz, 176.4 kHz, 192 kHz.
• Audio channels: up to 8.

DDC channel

• Allows source to interrogate capabilities of destination device.
• I²C signalling with 100 kHz clock.
• E-EDID data structure according to EIA/CEA-861B and VESA Enhanced EDID (V1.3).

Consumer Electronics Control (CEC) channel (optional)

• Uses the industry standard AV Link protocol
• Used for remote control functions.
• One-wire bidirectional serial bus.
• Defined in HDMI Specification 1.0.

Content Protection

• According to High-bandwith Digital Content Protection (HDCP) Specification 1.10.

Connector

Pin numbers (looking at socket):

Distance limitations

A reported problem with HDMI is the maximum cable length. As with all cables, signal attenuation becomes too high at a certain length. For the standard HDMI copper cables at 28 AWG, some users have found signal performance degrades above a cable length of about 5 meters. For front projection televisions and computer hookups, this can result in lost data and the video device compensating in unacceptable ways.

The HDMI Web site, however, disputes the 5 meter limit. "HDMI technology has been designed to use standard copper cable construction at long lengths. In order to allow cable manufacturers to improve their products through the use of new technologies, HDMI specifies the required performance of a cable but does not specify a maximum cable length. Cable manufacturers are expected to sell reasonably priced copper cables at lengths of up to 15 meters." (from the HDMI FAQ page)

One reported way to increase the distance limit is to increase the thickness of the copper cables, effectively decreasing impedance. 24 AWG wire is considered superior to 28 AWG. Another way is to use fiber optic or dual Cat-5 cables instead of standard copper. Some companies also offer amplifiers and repeaters that can string several HDMI cables together.

See also

• Digital Visual Interface (DVI)
• Unified Display Interface (UDI)
• DisplayPort
• VGA connector

References