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What is HDMI.

HDMI or High-Definition Multimedia Interface is an interface standard for connecting different types of digital audio/video sources such as DVD players, HD DVD players, Blu-ray Disc players, satellite receivers, personal computers, video game consoles, etc. to a compatible digital audio/video devices like home cinema systems, video projectors, digital televisions or computer monitors. The main objective in creating HDMI is not only to replace the old analog connection methods (such as radio frequency – RF, SCART or VGA) used in television systems but also to replace the outdated DVI (Digital Visual Interface) standard used in computer systems by expanding its capabilities for data transfer and adding new features for communication between devices. Unlike DVI which is designed primarily for use in computer systems, HDMI can be used not only in PCs, but also in many types of home multimedia devices which is the reason to become an universal interface for transferring audio/video data and therefore it is embedded in almost all modern audio/video devices manufactured in the last few years.

The main advantage of the HDMI interface is the ability for transmission of high-definition video, along with multi-channel digital audio (up to 8 channels of uncompressed digital audio) on a single cable, which eliminates the complexity of cabling that exists in the traditional audio/video systems. HDMI supports CEC (Consumer Electronics Control) functionality which is a set of commands allowing HDMI devices to control each other and giving the users the ability to manage multiple devices with only one remote control handset. This is achieved by two-way communication between devices using one of the HDMI cable wires. It also provides additional features such as DDC which enables the graphic controller to identify the capabilities of the display so it can determine automatically the most appropriate format and resolution for playing video. Because HDMI is a digital interface, it provides the best quality of the video compared with older video standards as the difference is especially noticeable at higher resolutions such as 1080p. Actually HDMI supports all currently available video resolutions (up to 4K x 2K or 3840 x 2160p (Quad HD) at 24/25/30 Hz or 4096 x 2160p at 24 Hz) but there are some limitations depending on which version of the standard is used. Since it was created, HDMI has several versions ( 1.0, 1.1, 1.2/a, 1.3/a/b/c, 1.4/a) that are backward compatible and can use the same cable. The difference between them is the increased bandwidth and features supported in each subsequent version.

For data transfer HDMI uses a special protocol called TMDS (Transition Minimized Differential Signaling) which is also used by the DVI standard. This means that HDMI is backward-compatible with DVI and you can connect various devices supporting both standards simply by using suitable adapters or cables but there are some limitations that must be considered first. For example, HDMI is backward-compatible with single link DVI-D or DVI-I, but not with the DVI-A standard because DVI-A is designed to work only with analog signals that are absent in the HDMI connector. Technically you can send a signal from a single link DVI-D source to an HDMI display as HDMI fully covers the technical specifications of DVI, but some features of HDMI such as audio transport and remote control may not be available. On the other hand connecting HDMI source to a DVI display can be a problem since the proper reproduction of the HDMI signal coming from a typical HDMI-source (such as HD-DVD player or a Blu-ray Disc player) may require HDCP-compliant display. HDCP or High-Bandwidth Digital Copy Protection is an encryption algorithm developed for content protection of DVD Video, DVD Audio, HD DVD and Blu-ray media against unauthorized copying. Therefore it is best when both devices (the source and the display) have HDMI ports.

Cables and cable lenghts.

Basically there are two types of HDMI cables – Standard and High-Speed. Standard (or Category 1-certified) HDMI cables can perform at speeds of 75Mhz or up to 2.25Gbps, providing transfer of video with resolutions of 720p/1080i. High Speed (or Category 2-certified) HDMI cables can provide a transfer speeds of 340Mhz or up to 10.2Gbps, which is the highest bandwidth currently available over an HDMI cable and can successfully handle signals with resolutions of 1080p and higher. Although HDMI does not specify a maximum cable length, the optimum length recommended by most manufacturers is up to 5m. since longer cables increase the attenuation of the signal and as you may know attenuation depends on the technical characteristics and quality of materials used for the production of cables. Of course there are cables with a length of up to 10m. and more but they are more expensive and also there is no guarantee that they will work properly since the normal operation of an HDMI connection depends not only on the cable’s quality but also on the hardware part of the devices that are connected. Fortunately there are solutions able to extend the effective length of the cable to a distance much longer than 10m. Many companies produce the so-called repeaters or active cables that can be implemented as separate external powered devices (amplifiers) or cables with built-in electronic modules that boost the cable’s signal.

Connectors.

Another advantage of HDMI is the smaller connector (compared to the DVI interface) making it suitable for installation especially in mobile devices such as notebook computers where the size of a connector can be critical. There are five HDMI connector types (type A/B/C/D/E) as the most widely used is type A, which has 19 pins and dimensions of 13.9 mm x 4.45 mm for the plug connector and 14 mm x 4.55 mm for the female connector. Type A is electrically compatible with single link DVI-D and can be found in almost any HDMI compliant device except the smallest mobile devices (such as tablet computers, mobile phones or digital camcorders) that use Type C (Mini) and Type D (Micro) connectors. There is also another type of connectors called Type B which has 29 pins and dimensions of 21.2 mm x 4.45 mm. Type B is electrically compatible with dual-link DVI-D and used for cables, allowing data transfer speeds twice as fast as Type A. This makes it suitable for use with very high-resolution displays such as WQUXGA (3,840 x 2,400), but it is still under development and not used in modern audio/video devices.

Type A HDMI connector pinout.

Type B HDMI connector pinout.