The High-Definition Multimedia Interface (HDMI) is an all-digital audio/video interface capable of transmitting uncompressed streams. HDMI is compatible with High-bandwidth Digital Contect Protection (HDCP) Digital Rights Management technology. HDMI provides an interface between any compatible digital audio/video source, such as a set-top box, a DVD player, a PC, a video game system, or an AV receiver and a compatible digital audio and/or video monitor, such as a digital television (DTV).

It is a modern replacement for older analogue standards such as RF - Coax, SCART, Composite Video, Component Video, VGA, DVI-A, and RCA connectors, and the consumer electronics replacement for older digital standards such as DVI (DVI-D & DVI-I). In the computer world, HDMI is already found on many peripherals and a few newer video cards, with adoption rapidly increasing.

General notes

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 concurrent version of the specification uses the same cables, but increases the throughtput and/or capabilities of what can be transmitted over the cable. For example, previously, the maximum pixel clock rate of the interface was 165MHz, sufficient for supporting 1080p at 60Hz or WUXGA (1920x1200), but HDMI 1.3 increased that to 340MHz, providing support beyond the highest resolution of computer monitors available today.

HDMI also includes support for 8-channel uncompressed digital audio at 192kHz 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 SuperAudio CD. With version 1.3, HDMI now also supports very high bitrate lossless compressed streams such as Dolby TrueHD and DTS-H D Master Audio.

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 use. Type B has 29 pins, allowing it to carry an expanded video channel for use with very high-resolution future displays. Type B is designed to support resolutions higher than WQSXGA (3200x2048).

Type A 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 Contect Protection (HDCP) on the display, the signal source may prevent the end user from viewing or especially copying certain restricted content. (While all HDMI displays currently support HDCP, most DVI PC-style displays do not.) Type B HDMI is similarly backward-compatible with dual-link DVI.

The HDMI Founders include consumer electronics manufacturers Hitachi, Matsushita Electronic Industry,  Philip, 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.

HDMI and High Definition Optical Media Players

In 2006, two competing high definition optical disc formats were released: Blue-ray Disc and HD DVD. These formats support higher fidelity audio than the DVD format. These audio formats include Dolby Digital Plus, Dolby TrueHD and DTS-HD Master Audio. Not all of these are mandated by the Blue-ray Disc and HD-DVD formats. High definition players provide a number of ways to transmit this audio. Currently, the best fidelity is available when the player is set to output LPCM over HDMI when using one of the higher fidelity formats. This requires a preprocessor or audio/video receiver capable of handling multi-channel LPCM over HDMI. While this has been allowed by the HDMI spec since 1.0, not all devices supporting HDMI 1.1 support this feature. In the future, it is likely that most devices claiming HDMI 1.1 as a feature will support at least 5.1 LPCM over HDMI.

HDMI 1.3 will provide for sending TrueHD and DTS-HD over bitstream rather than LPCM. This would allow a preprocessor or audio/video receiver with the necessary decoder to decode the data. It is unclear how this will be useful, as all current players are decoding the audio stream - this is required for advanced content (interactive audio). The players would either have to forgo mixing of interactive audio, or encode the mixed audio to one of these formats before sending it over HDMI.

Specifications

Each channel in HDMI can be purposed to carry audio, video, multimedia, or device-controlling signals, or a combination of these signals.

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 120Hz 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).

Content protection

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