How HDMI Works

HDMI is more than a port on the back of a TV and the often expensive cable that fits inside.

If you've shopped for an HDTV, a PlayStation 3, or an HD-DVD or Blu-ray player, you've probably heard about HDMI. It can seem like just one of many connections on televisions or home-theater receivers. But HDMI is more than a port on the back of a TV (and the often expensive cable that fits inside). It's a set of rules for allowing high-definition electronic devices to communicate.

Before the development of high-definition televisions, most TVs displayed pictures in what is now known as standard definition. The picture was roughly square -- its aspect ratio was 4:3. Its resolution, or the number of dots that make up the picture on the screen, was about 704 x 480 pixels. The picture was interlaced -- each piece of the moving image was really half a picture, but the pictures changed quickly enough that the human brain didn't really notice. Finally, older TVs relied on analog signals, which travel as a constantly varying electrical current.

HDTVs, on the other hand, are digital. They use information in the form of ones and zeros. This information travels through cables as distinct electrical pulses. HDTVs have an aspect ratio of 16:9, so the picture is rectangular. They also have a higher resolution -- current HDTV standards allow for resolutions of up to 1920 x 1080 pixels. HDTV signals can also be progressive, meaning that the each frame of the moving image is a whole picture rather than half of one.

NTSC (standard definition) has an aspect ratio of 4:3. HDTV has a wider aspect ratio of 16:9.

So, compared to standard TVs, HDTVs have a wider screen, more pixels and a faster refresh rate. Often, HDTVs can display more colors than older sets. This means that HDTVs need more data and need it a lot faster than standard-definition TVs do. If an HDTV can receive this information digitally, it also doesn't have to spend time or processing power converting the signal from an analog format.

This leads us to HDMI. Created by a group of electronics manufacturers, the HDMI standard is a set of guidelines for creating high-bandwidth connections between digital devices. With the right setup, HDMI can make a significant difference in a home-theater system. The current standard can carry 1080p high-definition signals, and it supports eight channels of uncompressed audio, enough for a 7.1 surround-sound system. HDMI can cut down on the number of cables required to connect components, and it can even reduce the number of remote controls needed to watch a movie.

But there's a catch. In order to take advantage of everything HDMI has to offer, all of the components of a home theater have to be compatible with them. Some of the features HDMI touts also don't yet exist in the consumer marketplace. In addition, there's a limit to how long an HDMI cable can be, and some users complain that the limit is too short to support convenient setups.

In this article, we'll look at exactly what happens inside an HDMI cable, the standard's features and its pitfalls. We'll also examine whether the newest standard, HDMI 1.3, really renders the earlier standards -- which have been out for only a few years -- completely obsolete.

HDMI SignalsOne 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-to-digital conversion means that the signal is in a pure, undamaged state when it reaches the HDTV set. 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 player, 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.

Photo courtesy HowStuffWorks Shopper

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 ProblemsOne 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).

HDMI ConnectionsThere 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.

HDMI Standards and Revisions

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

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.

1080p: For HDMI Only?One of the big questions surrounding HDTV is whether HDMI is required for 1080p resolution. The answer is maybe. Many devices require HDMI for 1080p output. Others, though, can send 1080p signals over a component video cable.

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.

HDMI FoundersThe following companies are part of the HDMI Founders:

Hitachi Matsushita Electric (Panasonic) Royal Philips Electronics Silicon Image Sony Corporation Thomson and Toshiba Corporation

[source: HDMI]

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.

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 CertificationIn 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.

HDMI Interface

HDMI Cables

Starting around 2003 we saw a rapid adoption of the Digital Visual Interface (DVI) across the digital consumer market. This included DTVs, high definition set -top boxes and computer graphics boards. By the end of that year, well over 500 consumer electronics products featured a DVI connection, with approximately 80% of DTVs shipped to the US using that technology. Later in the year, HDMI also emerged as a digital transmission format, but addressed some specific needs tailored to the consumer electronics market:

1. HDMI could carry both uncompressed high definition video (with support for SDTV and HDTV color spaces) along with all existing multi-channel audio formats and even device control data in a single connector

2. HDMI offered "link intelligence" enabling devices to automatically "discover" each other and recognize resolutions & formats

3. By design, HDMI was intended to be a smaller, more consumer-friendly connection (we would argue this last point due to lack of strain relief and any mechanism to secure the connection in place)

4. HDMI would be fully compatible with DVI

The HDMI Working Group was announced in April of 2002 and included Hitachi, Panasonic, Silicon Image, Sony, Thomson, and Toshiba. The group's charter was to define the next-generation digital interface specification for consumer electronics products.

In excess of six million HDMI-enabled consumer electronics devices were shipped in 2004, up from a mere 250,000 in 2003. There are expectations that there will be 125 million HDMI-enabled devices in the market by 2007. According to IDC and Silicon Image estimates, there will be over 15 million HDMI-enabled digital televisions at the end of 2005, growing to nearly 50 million units at the end of 2007.

Specifications, Versions, and Capabilities

HDMI version 1.0 met the goals of the HDMI Working Group and provided a true one-cable solution for uncompressed HD video and multi-channel audio including Dolby Digital and DTS bit streams (more on format support later).

HDMI 1.1 was a relatively minor update. The primary feature was to add some packets of audio-related content protection information. These packets were required by DVD-Audio in order to permit DVD-Audio content transmission on HDMI. HDMI 1.0 had the audio and video bandwidth and capabilities and HDCP already had the content protection capabilities, but there was some data that the DVD-Audio folks wanted to send to HDMI/HDCP sinks to tell them not to send the DVD-Audio content elsewhere.

As for HDMI 1.2, several companies have requested enhancements to the HDMI spec that are being considered by the HDMI Founders, but these items are, by agreement, not permitted to be discussed publicly until the specification is released. The HDMI Founders designed the HDMI specification to be dynamic. As such, HDMI has plenty of extra bandwidth to accommodate future audio and video requirements, and the Founders are committed to evaluating and updating the specification to accommodate new audio and video formats that may be introduced in the foreseeable future.

Let's Talk Bandwidth

Everyone would agree that if you want the optimal audio and video reproduction quality it is generally best to transfer an uncompressed audio-video stream from the source. Transferring uncompressed audio-video data, however, requires a lot of bandwidth (you're looking at giga bits per second) over a single cable. Interface standards like IEEE 1394 (400/800 M bps) transfer only compressed audio-video data, thereby potentially compromising the picture quality.

A single HDMI link, on the other hand, is capable of transferring up to 24 bits of user data at 165 Mpixels per second, resulting in a massive bandwidth of nearly 4 Gbps . This is enough to support the 1080p resolutions of today's newest high-definition displays while still leaving room to transport up to 8 channels of high-resolution audio with 24 bits of resolution and a sampling frequency up to 192 kHz - all across a single HDMI cable. This is well beyond the maximum specifications of even DVD-Audio, which tops out at 6 channels and a sample rate of 96 kHz. The fact is, the HDMI standard includes extra headroom to allow for future upgrades to audio formats.

TDMS Encoding & Signaling

Like DVI, the HDMI specification is based on TDMS encoding and signaling technology. A TDMS link consists of a single clock channel and three data channels. Eight (8) bits of video data are converted into a 10-bit transition-minimized, DC balanced sequence through the use of an advanced data-encoding algorithm implemented on each of the three data channels. This allows for a very strong transmission, while also minimizing the potential for E M I (electro-magnetic interference) over copper cables. Because of the use of the advanced encoding algorithm, data recovery on the receiving end is very reliable, enabling transmission over fairly long cable runs.

With the exception of the connector itself and the optional control signal, the HDMI physical layer is the same as that for DVI. Compliant DVI sources and displays will be interoperable with HDMI devices through the use of a simple, passive DVI-to-HDMI converter cable or converter. This protects consumers' investment in DVI-enabled CE products.

With a single-link clock frequency of 165 MHz, all existing HDTV video formats can be readily supported, and there is more than sufficient bandwidth to support future video formats such as 1080p at 60Hz (making HDMI ready for the next-generation video formats that have finally started to emerge in 2005). As with DVI, an optional but rarely used second link can be used to double the bandwidth to support any resolutions above UXGA (162 MHz). This second link is only expected to be used in high-resolution PC applications and utilizes a different size connector (we'll expand on this later).

Supported Color Space and Video Formats

HDMI pixel encoding includes support for RGB 4:4:4 as well as digital TV's YCbCr 4:4:4 amd YCbCr 4:2:2 color spaces. The two 4:4:4 encoding formats are both 8-bit per component sampling for 24-bit per pixel delivery. The 4:2:2 encoding format uses up to 12-bits per component for greater color depth.

HDMI can support all existing and planned PC or TV video formats. Several formats were specifically established in order to jump-start compatibility between products and media whose resolutions were different:

SDTV: 720x480i (NTSC), 720x576i (PAL) EDTV: 640x480p (VGA), 720x480p (NTSC progressive), 720x576p (PAL progressive) HDTV: 1280x720p, 1920x1080i (1920x1080p is supported but was not initially defined

when the spec was penned)

All SD formats are available in 4:3 as well as 16:9 aspect ratios while HD formats are available in the 16:9 ratio only.

HDMI Interface Connectors and Compliance Testing

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by Clint DeBoer — last modified May 30, 2007

HDMI and Digital Audio The HDMI spec allows for digital audio and auxiliary data to be carried within the horizontal and vertical blanking intervals of the digital video signal. All basic L-PCMsample rates are supported, including "Basic" 32 kHz, 44.1 kHz and 48 kHz audio as well as "Optional" 88.2 kHz, 96 kHz, 176.4 kHz and 192 kHz audio.

HDMI displays must support all "Basic" audio formats while HDMI sources need to support at least one. Both sources and displays can support any number of the "Optional" audio formats. Audio formats from two-channel 16-bit encoding all the way to eight-channel 24-bit encoding are supported by HDMI. The pixel clock frequency and video format determine the formats that are available for transmission, meaning that SDTV formats can support eight-channel uncompressed audio up to 96 kHz, while HDTV formats can handle the same number of channels at 192 kHz.

Automating the Display's Ability to Adjust Display Settings

Because HDMI includes audio and video InfoFrame Structures, auxiliary data can be carried from the source device to the display, allowing it to adjust properly to the detected signal parameters. While not mandatory, displays that have this capability could automatically adjust for pixel encoding (RGB 4:4:4, YCbCr 4:2:2, etc); aspect ratio (16:9, 4:3, letterbox parameters, etc); overscan/underscan; colorimetry; audio coding type (PCM, AC-3, MPEG-1/2, AAC, DTS, etc); number of audio channels; sample rate; and bit-depth.

In continuing this line of control and data monitoring, HDMI also employs Consumer Electronics Control (CEC), a function based on the AV.link protocol used in European countries. Through the use of additional embedded high-level commands, CEC can have multiple electronics react and configure themselves based on the actions of a single device. For example, a consumer could engage the 'Play' function of a source component which would then turn on and switch to the correct input of a receiver, and also initiate a display device (including tuning it to the correct input.) CEC, if properly implemented across all products, can truly simplify the home theater experience.

HDMI Connectors vs. DVI

HDMI maintains backwards compatibility with DVI 1.0, however it makes significant improvements upon the format:

HDMI is what's known as a "friction-fit" connector, like a Firewire or USB connection, which is smaller and simpler in consumer electronics applications allowing for greater flexibility and easier placement and connectivity

HDMI can be hot-plugged

There are actually two "flavors" of HDMI connectors, similar to the dual-link and analogue/dual-link DVI connectors. In the case of HDMI, however, there was no desire for retaining any analogue interface and the two formats include a single-link and dual-link connector. The single-link, all-digital connector (15mm wide with 19 pins) is used for consumer electronics applications. The dual-link connector (20mm wide with 29 pins) is primarily for computer applications, and also permits compatibility with DVI's dual-link option.

On a purely personal experience note, I'd like to see a new revision of HDMI connector that provided some kind of strain-relief/secure connection that locked the connector to the receptacle with more than just friction. On more than one occasion I watched an HDMI cable being unfairly stressed by a heavyweight cable design - even to the point that it looked as if the connector would pop out of the receptacle at the slightest nudge. It shouldn;t be terribly difficult to make a backwards-compatible revision to the HDMI connector that offered some additional security without sacrificing ease-of-use. Installers everywhere would likely rejoice.

*Illustration courtesy of http://www.pioneerelectronics.com

HDCP (High-bandwidth Digital Content Protection)

Originally developed by Intel for use with the DVI connector to provide a secure link between source and display, HDMI modified and adopted the content protection scheme for use with this

connection (as HDCP 1.10). This secure transfer mechanism has been endorsed by the majority of major motion picture providers and the FCC has, barring any flips and flops, mandated DVI-HDCP or HDMI-HDCP for all DTV receivers sold as "Digital Cable Ready"*. In addition, the DVD Copy Control Association (DVD CCA) and 4C Entity have also approved HDMI-HDCP for Content Scramble System (CSS) and Content Protection for Pre-recorded Media (CPP M) content playback on DVD players.

Using HDMI-HDCP for display interface content protection is popular for three reasons:

HDCP establishes as secure channel that verifies the display device is authorized to receive the secure content

HDCP encrypts at the source and decrypts at the display so that secure content cannot be stripped in transit

HDCP has the ability to identify and "revoke" unauthorized devices to prohibit mass distribution of illegal keys.

The keys which are supplied by HDCP license authority are a 40-element array of 56-bit secret keys and 40-bit binary key selection vector (KSV). Essentially, the source device sends its KSV and 64-bit value to the display device, which responds with its own KSV. The source then simply confirms that the display has not been revoked and the two devices calculate a shared value that will be equal - which is then used to encrypt and decrypt the data on the link. At this point the authentication is established (it is also re-established every two seconds to assure that the link remains secure.)

What makes HDCP rather unique is that it is a "renewable" content protection scheme. An updated list of compromised display devices is sent to al source devices by the licensing authority (through the use of updated System Renewability Messages (SR M) stored on subsequent content medium.) This means that if a display device is compromised, future software that is released can automatically (and rather easily) block the display from being able to decrypt the content.

*This mandate is being phased in over a period of years based on the television screen size.

HDMI Compliance Testing

The HDMI Adopter Agreement mandates that adopters must submit their first product in four HDMI product categories for compliance testing at an HDMI Authorized Testing Center (ATC). The four HDMI categories are: source (e.g. DVD player, STB), sink (e.g. DTV), repeater (e.g. A/V receiver) and cable. ATCs are operated by Silicon Image in Sunnyvale , Calif. and Matsushita in Osaka , Japan . Subsequent products in the above categories may be self-tested by the manufacturer according to the guidelines set forth in the HDMI CTS.