HDMI (High-Definition Multimedia Interface) 

HDMI (High-Definition Multimedia Interface) is a means of transferring digital audio and video signals from a source device to a television. HDMI cables allow consumers to connect devices such as DVD players, Blu-ray players, cameras, tablets, and gaming consoles to their televisions. HDMI allows for the transmission of high-definition digital video, multichannel surround sound audio, and advanced control data between these devices and high-definition televisions. HDMI also helps to simplify the cabling setup of home entertainment systems by reducing the number of cables needed to connect various devices to one's television. Since its debut in 2003, HDMI technology has undergone a number of key upgrades that improved and expanded its capabilities. Today more than 1,600 of the world's largest and most successful manufacturers of consumer electronics have adopted HDMI technology for use in their products. Because of this, HDMI provides virtually unrestricted connectivity among a wide variety of peripheral audiovisual devices and high-definition televisions.

Background

In significant part, the development of HDMI technology was a direct result of the emergence of high-definition television. Prior to the advent of high-definition television, the picture seen on most televisions was displayed in standard definition. Standard-definition television offered a generally square picture presented in a 4:3 aspect ratio with a resolution—the number of pixels (dots) that combined to create the picture—of around 704 x 480 pixels. In addition, most standard-definition televisions ran on analog signals that traveled as a constantly varying electrical current.

The introduction of high-definition television in the late twentieth century dramatically changed the way television technology worked. Unlike standard-definition television, high-definition television was broadcast in a wider 16:9 aspect ratio and a resolution of 1920 x 1080 pixels. More importantly, it also relied on digital signals that allowed for the transmission of data in the form of ones and zeros through distinct electrical pulses.

These changes meant that to operate normally, high-definition televisions had to be able to receive more data at a much faster rate than their standard-definition predecessors. This presented a particular problem when it came to the many peripheral devices, such as DVD players and gaming consoles, that many consumers connected to their televisions. At the time, most of these devices were connected to televisions with analog cables that could not deliver the high-quality picture that high-definition televisions were capable of displaying. To ensure the best possible picture quality from peripheral devices, a new type of connection was required. Eventually, this need led to the development of HDMI technology.

In the early 2000s, a group of electronics manufacturers joined forces to create a set of guidelines for establishing high-bandwidth connections between high-definition televisions and other digital devices. This set of guidelines, known as the HDMI standard, laid the groundwork for HDMI technology. The first HDMI cables and HDMI-enabled consumer electronic products subsequently debuted at the Custom Electronic Design and Installation Association (CEDIA) Expo in September 2003. With that, a new era of digital connectivity was launched.

Overview

HDMI technology has a number of key features for which it is known. Most notably, HDMI uses a method of coding called transition-minimized differential signaling (TMDS) to transmit data between devices. TMDS helps to prevent the digital signal from degrading as it travels from one end of the HDMI cable to the other. The TMDS encoding process works through a pair of wires that are twisted together inside the HDMI cable. While one of these wires carries the main signal, the other carries an inverse copy of that signal. Eventually, this pair of signals reaches the receiving device, which then measures the difference between the two and uses the resulting information to compensate for any degradation that may have happened within the cable.

Another unique feature of HDMI technology is a method of piracy prevention called high-bandwidth digital copy protection (HDCP). Essentially, HDCP is a type of authentication protocol designed to ensure that data is received only by authorized devices. Source devices such as Blu-ray players and receiving devices such as televisions all have special authentication keys that are stored on their extended display identification data (EDID) chips. Using the HDMI cable, the source device can check the receiving device's authentication key. If that key and the source device's keys match up, the source device creates a new key and shares it with the receiving device so that the two can work in tandem. Referred to as the "handshake," this whole process normally takes place in just a fraction of a second.

From a hardware standpoint, HDMI technology is relatively simple. HDMI connectors use a grid of nineteen pins that physically connect the cable to source and receiving devices. Larger and smaller HDMI connectors are available for different types of devices. In standard-sized HDMI cables, a trio of audio and video channels are transmitted through two pins each. The TMDS clock, which helps connected devices synchronize the data they send back and forth to one another, is sent through another pair of pins. Several other signals, including the consumer electronics channel (CEC), the hot plug detect channel, and the display data channel (DDC), also travel through the cable on one pin apiece. All of these channels are protected by the shield, a special wire that prevents interference among different channels.

Over the years, several updated versions of HDMI technology have been released. The original version was known as HDMI 1.0. Subsequent updates generally added new capabilities and other improvements. HDMI 1.1, for example, added the ability to transfer Dolby Digital and other surround sound signals. HDMI 1.2 allowed for the transfer of Super Audio CD (SACD) signals. HDMI 1.3 and 1.3a included critical support updates for then-emergent Blu-ray and HD-DVD technologies. HDMI 1.4 added enhancements such as an Ethernet channel, an audio return channel, and 3-D capabilities. HDMI 2.0 offered expanded 4K resolution compatibility, independent video streaming, dynamic synchronization of audio and video streams, and other features. HDMI 2.0a, which was released in 2015, added support for High Dynamic Range (HDR) technologies such as Dolby Vision.

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