понедельник, 21 декабря 2015 г.

How HDMI Works

How HDMI Works
How HDMI Works

With a simple adapter, you can
With a simple adapter, you can

There are lots of methods you can use to connect home-theater components. For example:

  • Component video carries analog video signals separated into two channels for color and a third for luminance. Component video cables use RCA connectors.
  • S-video transmits analog signals using one cable and a four-pin connector.
  • DVI, or digital visual interface, is a 29-pin connection commonly used with computer monitors. Unlike composite video and s-video, it carries digital signals.

Many HDTV early adopters rely on DVI, since it hit the market before HDMI did. Since DVI and HDMI both use the TMDS protocol, they're compatible. All you need to connect an HDMI cable to a DVI port is a passive adapter.

The DVI and HDMI connectors have some other similarities. Both use a grid of pins to transmit signals from the cable to the device. While DVI has a 29-pin connector, HDMI's type A connector has 19 pins. A DVI connector also uses a pair of built-in screws to anchor it to the device. HDMI plugs don't have this extra support, and some users have expressed concern that this puts unnecessary strain on the device's circuitry. There's also a miniature version of the HDMI connector for use on smaller devices like digital camcorders as well as a 29-pin type B connector, although most consumer devices use type A.

From the HDMI connector's pins, signals travel through twisted pairs of copper cable. Three audio and video channels travel through two pins each, for a total of six pins. The TMDS clock, which allows devices to synchronize the incoming data, travels through one pair of pins. Each of these four total pairs has a shield -- another wire that protects it from interference from its neighbors. The TMDS channels, the clock and the shields make up the bulk of the cable pairs inside the HDMI cable.

The other signals that travel through the HDMI cable need only one pin. One such channel is the consumer electronics channel (CEC). If your devices support it, this channel allows them to send instructions to one another. For example, an HD-DVD player could automatically turn on a home-theater receiver and an HDTV when it started playing a disk. The hot plug detect channel, which uses one pin, senses when you plug in or unplug a device, re-initializing the HDMI link if necessary. The one-pin display data channel (DDC) carries device information and the HDCP encryption information discussed in the previous section. Other channels carry encryption data and electricity to power communication between devices.

The cables themselves come in two categories. Category 1 has a speed of 74.25 MHz. Category 2 has a speeded of 340 MHz. Most consumer cables are the faster category 2 variety.

In addition to the connector and cable, the HDMI standard applies to how TV sets can synchronize sound with video and display color. These capabilities have changed significantly over several revisions to the standard, which we'll compare in the next section.

The first consumer products with HDMI connections hit the market in 2003. Since then, there have been several changes to the HDMI standard. For the most part, these standards have added support for specific types of content or applications. For example, the first revision, HDMI 1.1, added support for DVD Audio.

The most recent major revision -- the jump from version 1.2 to 1.3 -- got a lot of attention. New features included a massive increase in bandwidth, support for 16-bit color and support for the xvYCC color standard, which supports additional colors. A new lip-synch feature also reduced that sound and video would fall out of synchronization during playback, making an otherwise immaculate recording look badly-dubbed. Some reports even claimed that any devices that did not have HDMI 1.3 were obsolete.

In some ways, this was just as confusing as it was impressive. Some of HDMI's new abilities don't exist yet in the consumer marketplace. For example, the increased bandwidth -- from 4.9 Gbps to 10.2 Gbps -- can support a refresh rate of 120 Hz, or 120 frames per second. This is twice as fast as the maximum refresh rate in the current HDTV standard. HDMI 1.3 can support 30-, 36- and 48-bit color options known as deep color, but many media players and recorded video materials don't go beyond 16-bit color. Critics also claim that deep color allows HDTV screens to display colors that most people can't even perceive. In addition, while lip synch and one-touch control abilities can be handy, not all home-theater devices support them.

Fortunately, a lack of 1.3 capability doesn't mean your HDTV is useless. HDMI 1.3 is backwards compatible with previous versions. It's like when color TV debuted. People could watch color TV signals on their black-and-white sets -- the TV still worked, but the picture was still in black and white. If your HDTV has HDMI 1.2 but your new components have HDMI 1.3 capabilities, your TV will still work, but without the expanded 1.3 abilities. Since the bandwidth allotments of previous standards are generally enough for most high-definition applications, your picture should still have a pretty good quality.

Another common concern about HDMI is cable length. Although the HDMI standard requires a minimum operable length of 32 feet (10 meters), some users report significantly shorter operable lengths in practice. This is particularly true when transmitting 1080p signals -- the increased demands on bandwidth speeds up the deterioration of the signal. Fortunately, there are amplifiers and extenders that can decode, re-set and re-encode the signal before sending it on the next leg of its journey.

For people who are concerned about HDMI's potential limitations, there may be another solution on the horizon. DisplayPort is a new high-definition standard that will cover connections inside devices, like within a laptop, and between devices, like from a media player to an HDTV. DisplayPort hasn't hit the market, though, so whether its quality will surpass that of HDTV is still to be determined.

You can find more information on home theater, HDMI and related topics on the next page.

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Original article and pictures take http://electronics.howstuffworks.com/hdmi2.htm site

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