How HDMI Works

HDMI Signals

One of the common misperceptions about HDMI is that the digital signal is innately superior to an analog signal. In some people's minds, the lack of analog-digital conversion means that the signal is in a pure, undamaged state when it reaches the HDTV. It's easy to imagine a high-definition, digital signal traveling straight from an HD-DVD player to an HDTV. But signal transmission via HDTV does require an encoding step.

HDMI uses transition minimized differential signaling (TMDS) to move information from one place to another. TMDS is a way of encoding the signal to protect it from degrading as it travels down the length of the cable. Here's what happens:

• The sending device, such as an HD-DVD, encodes the signal to reduce the number of transitions between one (on) and zero (off). Think of each transition as a sharp drop-off -- as the signal travels, this drop-off can begin to wear away, degrading the signal. The encoding step helps protect signal quality by reducing the number of chances for the signal to degrade.
• One of the cables in the twisted pair carries the signal itself. The other carries an inverse copy of the signal.
• The receiving device, such as an HDTV, decodes the signal. It measures the differential, or the difference between the signal and its inverse. It uses this information to compensate for any loss of signal along the way.

HDMI encodes, carries and decodes digital signals to high-definition displays, like HDTVs.

HDMI also has the ability to protect data from piracy. It uses high-bandwidth digital copy protection (HDCP) to accomplish this. HDCP is an authentication protocol. Basically, each home-theater device has identification data and encryption data stored on its extended display identification data (EDID) chip. The source device, such as a Blu-ray player, checks the authentication key of the receiving device, such as an HDTV. If both keys check out, the sending device moves on to the next step. It generates a new key and shares it with the receiving device. In other words, it creates a shared secret. Ideally, this whole process, known as a handshake, takes place almost instantaneously.

HDMI Handshake Problems One common problem people experience with HDMI is handshake failure. Sometimes, this is because the HDTV is an older, DVI-based model without HDCP support. But in other cases, there's an easy fix -- unplug your components and power cycle your system. This may reset the authentication cycle and restore your connection.

The source device encodes its information using the key it generated it. The receiving device decodes it using the same information. If an unauthorized device tries to intercept the data, the source device stops transmitting. It also makes sure that the key hasn't changed and that the system is still secure every few minutes. All HDMI-compatible devices are required to support HDCP, but the companies that manufacture and distribute high-definition content aren't required to enable it. In the United States, this content-protection ability is mandated by the Federal Communications Commission (FCC).

Next, we'll take a look at the HDMI connector and cable and explore how they carry high-definition signals.

HDMI Connections

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.

With a simple adapter, you can plug an HDMI cable into a DVI slot.

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.

HDMI Certification In order to carry the HDMI logo, devices and cables have to pass compliance testing. Manufacturers, known as HDMI adopters, must submit a sample of their product for tests before starting mass production.

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.