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