Technology Inside

What SCART has been represented for the world of analog television, is done today by the HDMI digital connection.

High Definition Multimedia Interface, one cable to deliver high-definition video signals along with numerous high-quality audio channels. A cable with which we have to do for interconnecting digital media sources.

Let's go to see how works the HDMI standard and interconnections.

Invented by a group of Consumer Electronics companies (under the name HDMI.org) , HDMI lets us to use at the best the characteristics of high-definition monitors and digital sources, in a simple and safe way. The ease of connection is ensured by a system of communication between devices, through the same HDMI cable. Monitors and audiovisual sources, talk to each other and exchange information on the format to display, the type of video and audio, the number and type of devices connected, etc.. Besides, HDMI is compatible with the standard DVI, used in computing, which carries only video signals without audio.

In practice is possible a multiple control of several signal sources starting from a single device, with the best quality available, without manual configuration. Even the on / off status of a device can be controlled via HDMI interface. The concept of Plug & Play applied to high-level multimedia .

Among the features of the interface, the copy protection for high-definition signals. System HDCP (High-bandwidth Digital Content Protection) is used to encrypt information between source and monitor, to prevent the illegal recording of contents by users. If a device that uses HDCP is connected to a monitor that does not meet the standard, the signal is automatically demoted to the level of a common DVD.

HDMI cables are quite compact using a connector easy to handle . There are 4 types of male and female type connector, designed for specific needs.

• The most widely used is TYPE A which has 19 contacts and is present on almost all modern digital TV (Dimensions: about 14 X 4,5 mm)
• TYPE B is the extended version with 29 contacts, not widespread, used for high-definition signals (Dimensions: about 21 X 4,5 mm)
• TYPE C has the same 19 contacts of the standard connector Type A, but is smaller, used for portable and compact devices (Dimensions: about 10,5 X 2,5 mm)
• TYPE D is a recent innovation, commonly called Micro HDMI and has dimensions similar to Micro-USB connector, suitable for mobile smartphones, microcomputers, etc.. (Dimensions: about 6,5 X 3 mm)

To carry signals of the order of Gigabit per second, is necessary to keep under control electromagnetic interference. The secret to keep low the size and cost of cable is enclosed in the transmission system of signals, which allows to use common twisted pair cables (the type used in the LAN interface Ethernet) with a light shielding. To limit crosstalk (ie the passage of a portion of the signal between a cable and another) between signals passing through the cable, is used the system TMDS (Transition Minimized Differential Signaling) a method to minimize the transitions between 0 e 1 present in digital streams (binary streams, formed by sequences of zeros and ones).

But how long can a HDMI cable? In general a length of 5 m is the limit to avoid malfunction. There are better quality cables that allow you to get to 10-15 m, but their price goes up significantly.

Alternatively, to cover longer distances, can be used HDMI signal repeater, that amplify and clean up the signals. Be careful to buy a signal repeater HDCP-compliant, that is compatible with the copy protection mechanism, to avoid the bad surprise to get a degraded signal.

As for the USB, are also available HDMI HUBs, devices that multiply the number of available HDMI ports, starting from a primary connection. The hubs are equipped with remote control, to select the various signal sources.

Many audio-amplification systems, have a number of HDMI inputs, to connect several digital streams.

HDMI connections use three independent channels (TMDS CHANNEL 0, 1 and 2) for each color component RGB. Along with the video signals also travel audio channels, until 8, according to current specifications. All signals are digital, uncompressed, for a multimedia experience without compromise. The HDMI protocol supports all major TV standards and can accommodate many configurations of signal. Among the options is important SACD to enjoy superb sound quality. Even computers with HDMI interface can connect easily to HDMI digital displays, in this case the video resolution is set automatically.

In addition to audio-video, are present two connections for information exchange among devices. The CEC (Consumer Electronics Control) allows you to send commands to all devices interconnected, from a single control device. The DDC (Display Data Channel) is a BUS to communicate the technical characteristics of displays, to the various HDMI devices, in order to choose the video resolution and best quality.

The contact HOT PLUG, is used to detect the insertion or the disconnection of a device, to have under control all the active digital units.

From picture, related to a Type A connector, can also be observed GND shielding (Ground), for different groups of contacts.

One of main advantages of HDMI is the constant evolution of its technical specs. From 2002, when was released the version 1.0 of technical specifications, new features were introduced and the resolution increased continuously. Version 1.4, provides a video resolution of well 4096 X 2160 pixel, with data rates up to over 10 Gbit / s.

There is much to write, to fully experience the possibilities of interconnection and control offered by HDMI, from home systems to modern digital theaters. I await your comments and suggestions.