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HDMI, High-Definition Multimedia Interface, is the name given to a particular High Definition interface. It specifies the signal requirements and the physical connectors. It covers digital RGB video signals and stereo audio as well as some commands and protocols that can be used.
HDMI technology is the global standard for connecting high-definition products: the uncompressed, all-digital interface that delivers both dazzling quality and unmatched ease of use. It has enormous bandwidth capacity – up to 10.2 gigabits per second, more than twice the bandwidth needed to transmit an uncompressed 1080p signal.
- HDMI 1.0 supports 8-channel, 192kHz, uncompressed digital audio and all currently-available compressed formats (such as Dolby Digital and DTS)
- HDMI 1.3 adds additional support for new lossless digital audio formats Dolby® TrueHD and DTS-HD Master Audio™
- bandwidth to spare to accommodate future enhancements and requirements.
- Deep Color (similar to HDR but linear): HDMI 1.3 supports 10-bit, 12-bit and 16-bit (RGB or YCbCr) color depths, up from the 8-bit depths in previous versions of the HDMI specification, for stunning rendering of over one billion colors in unprecedented detail.
- HDMI 1.4 adds reverse audio channels, 3D support, Ethernet support, automotive support. It supports 2880×1440 at 60Hz.
- HDMI 1.4b adds resolutions up to 4K, Audio Return Channel (ARC), 3D, HDMI Ethernet Channel (HEC), and Consumer Electronic Control (CEC) and #HDMI Alternate Mode.
- HDMI 2.0 now includes 4K digital called UHD support. HDCP is a DRM system for 4K signals but HDMI 2.0 may support but doesn't guarantee to support HDCP. HDMI 2.0/Display Port 1.3 supports 2880×1440 at 90Hz.
- HDMI 2.1 adds 8K 60Hz and 4K 120Hz with Dynamic HDR wide color gamuts, supports 48Gb cables. Supports advanced audio signal control and game mode with Variable refresh rate (VRR).
 Control Interface
Meets CEC, Consumer Electronics Control, standards.
- HDMI is a "smart" two-way connection that allows devices to communicate and interact with each other to dramatically improve your home theater experience.
- Devices connected with HDMI have the ability to scan each other’s capabilities and automatically configure certain settings. An HDTV and a DVD player, for example, can auto-negotiate settings like resolution and aspect ratio to correctly match the format of the incoming content to the highest capabilities of the TV.
- Consumer Electronics Control (CEC) provides for integrated, "one-touch" commands across multiple linked components. When enabled by the manufacturer, CEC allows system-wide behaviors like one-touch play or one-touch record, where pressing a single button on your remote launches a series of coordinated commands.
There are now three different cable sizes, standard, mini and micro (as of 1.4) for USB. There is also an Automobile connector which is unique due to the special environmental considerations, temperature, vibration, etc.
DVI, Digital Visual Interface, is an earlier form of HDMI using a different connector. Adapters exist to convert DVI to HDMI. These adapters are generally bi-directional. It is only video. It is missing the following features:
- Audio (up to 8-channels uncompressed)
- Support for YUV Color Space
- CEC (Consumer Electronics Control)
- CEA-861B InfoFrames
- Features beyond 1.0 HDMI are not supported. For example there is no HDCP support.
MHL, Mobile High-Definition Link, is an industry standard for a mobile audio/video interface. It permits connecting mobile phones, tablets, and other portable consumer electronics devices to high-definition televisions (HDTVs) and audio receivers. The newest, 3.0, version offers significant interaction with TV's.
MHL is a reduced pin subset of HDMI, while not pin compatible, can be converted easily to HDMI. Many new TV's are HDMI/MHL compliant and will accept either format. MHL can be on as few at 5 pins and a common adapter plug is HDMI on one end and microUSB on the other. The USB signal is compliant with standard 2.0 USB 5pin standard but contains 5 Volt output up to 2.0A (10 watts) to support external power to the portable device so that streaming will not run down the battery. HDMI has parallel data streams while MHL is serial data so the TV or an external adapter must convert the serial data to parallel for use in the TV. Passive microUSB to HDMI cable adapters are common to this use for compliant TVs. Other cables with more pins are also used. Remote control is supported in this standard.
The latest USB 3.1 USB-C connector also supports MHL using a HDMI to USB-C cable adapter.
 HDMI Alternate Mode
On September 1, 2016 a new HDMI Alternate Mode standard was released for for USB Type-C Connector. This will allow HDMI enabled source devices to utilize a USB Type-C connector to directly connect to HDMI enabled displays with full 1.4b features. A HDMI standard connector is required on the TV end of the cable.