HDMI Interface - A Beginner's Guide

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

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.

References
HDMI.org website: http://www.hdmi.org/
Silicon Image, Inc website: http://www.siliconimage.com
HDMI: Gaining Momentum by Raj Karamchedu, Senior Product Marketing Manager, Consumer Electronics Products
HDMI: High-Definition Multimedia Interface by Stevan Eidson, Brett Gaines, Paul Wolf of Silicon Image, Inc. Sunnyvale , CA
Pioneer Electronics website: Http://www.pioneerelectronics.com