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Technology Showcase: HDMI Extenders

May 26, 2008 12:00 PM, By Jay Ankeney

Products to extend HDMI's reach without sacrificing signal quality.


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StarTech ST121HDMIT

StarTech ST121HDMIT

In retrospect, the halcyon days of pumping analog video through a single coaxial cable gleam in our memory with the simplicity of a technology's infancy. But now that we have matured into high-definition digital image displays and are faced with the daunting requirements of 1080p/60 becoming the expected standard, just getting that video signal from one place to another has become an engineering challenge by itself.

Back in the era of analog video, a VGA (video graphics array) cable was sufficient for getting a computer-generated image to CRT screens. VGA had been introduced for the IBM PS/2 line of computers in 1987, and although it is now being rapidly superceded by other connectivity approaches, VGA still remains the lowest common denominator that all computer-graphics hardware can support before an application-specific driver takes over. That's why the splash screen that tells you your PC is loading is still in 640×480 VGA resolution.

Then, in April 1999, the Digital Display Working Group (DDWG), spearheaded by Silicon Image and Molex, published a specification for DVI (digital visual interface) in which digital pixels are transmitted as binary data. Capable of carrying both digital and analog signals, Silicon Image's Transition Minimized Differential Signaling (TMDS) was used as the technical basis for the DVI specification in order to ensure backwards compatibility.

DVI, however, did not include audio, and as the last millennium came to an end, it was rapidly becoming apparent that the acceleration of AV communication was going to require a lot more than just pretty pictures. With the proliferation of both digital sources and displays, integrators would also need to enable laptops, computer workstations, projectors, flatpanel screens, and even portable devices to talk to each other as if they originally came from the same family instead of springing from divergent heritages. In addition, the signals that needed to be distributed began to include multiple channels of audio, digital content protection, and metadata — and they still left room for the next data innovation that is sure to come down the pike.

Fortunately, back in 2002, the visionaries at Silicon Image brought together a consortium of industry leaders to come up with the next-generation solution, which would be called high-definition multimedia interface(HDMI). The founders of the HDMI Working Group include Hitachi, Panasonic, Philips, Silicon Image, Sony, Thomson, and Toshiba. By December of 2003, the organization released the HDMI specification version 1.0. The first HDMI products debuted at CES 2004, and there are currently more than 840 consumer-entertainment and PC companies that have adopted the HDMI specification, making HDMI the de facto standard for transmitting high-definition content as well as bridging computer and consumer display technologies.

The growth is predicted to continue. In fact, HDMI-enabled device shipments are expected to grow at an annual rate of 35 percent through 2011, according to In-Stat — a leading provider of actionable research, market analysis, and forecasts of advanced communications services, infrastructure, end-user devices, and semiconductors.

Atlona AT-HDMI250SR

Atlona AT-HDMI250SR

The HDMI specification incorporates the DVI standard, and like its forerunner, it is based on TMDS. That's why it is possible to build cables with an HDMI connector on one end and DVI at the other. HDMI can handle all of the current HDTV formats as uncompressed video, along with eight channels of digital audio over a single wire with 19-pin connectors. To enable advanced control functions such as automatic configuration and one-touch play between a video source such as a DVD player and the display, HDMI incorporates two-way communication. Crucially, it also supports digital rights management security approaches such as High-bandwidth Digital Content Protection (HDCP) and Digital Transmission Content Protection (DTCP), which give Hollywood studios (among others) a bulwark of protection against piracy.

The HDMI spec has been expanded with the evolution of ever-greater capacities in digital communication devices by increasing the capabilities and throughput of what can be transmitted over that single cable. For example, HDMI version 1.3 increased the signal's clock rate to 340MHz, providing support for 1080p/60 at WQXGA (2560×1600) and beyond, along with lossless compressed audio streams such as Dolby TrueHD and DTS-HD Master Audio.

HDMI comes at a cost, however. HDMI adopters pay an annual fee of $10,000 to HDMI Licensing — the agent responsible for licensing the HDMI specification — along with royalty fees for each end-user's licensed product. This has induced the Video Electronics Standards Association (VESA) to develop a competitive standard to HDMI called DisplayPort, with version 1.0 approved in May 2006.

Although the vast number of HDMI-equipped consumer products already in the marketplace will probably ensure its continuing dominance in the consumer display field, the lack of licensing fees for DisplayPort may encourage its adoption by PC manufacturers as a replacement for DVI. However, this has to be taken with a grain of salt because reports have it that the intellectual property owners of some of the technology behind DisplayPort have not yet officially given up their rights. So future licensing fees are not totally out of the question.

In-Stat predicts DVI-enabled shipments will reach 112 million in 2007, but they are expected to decline thereafter due to the entry of the DisplayPort standard in the PC segment. DisplayPort products will be limited in number in 2008, but they will grow to more than 600 million products shipped in 2012.

Still, HDMI is the most prominent digital connectivity standard today. But the usefulness of HDMI depends on maintaining the integrity of its signal, and its very complexity means the HDMI signal risks deterioration over long cable runs.



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