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Technology Showcase: HDMI Distribution

Jun 4, 2009 12:00 PM, By Bennett Liles

With broad support in consumer electronics, HDMI is moving on up.

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Throughout its history, HDMI has been a rapidly evolving technology, and it poses a moving target to those trying to keep up with the expanded capabilities afforded by top-quality peripheral gear. Since its first commercial appearance in 2003, the same year Silicon Image opened the first authorized testing center for HDMI products, the technology has been adopted by more than 800 computer and consumer electronics companies. That makes HDMI one of the fastest techno-movers of the decade, and it’s now crossing the threshold in surpassing DVI in total sales. However, there is still a substantial market for both, and this is reflected in the wide variety of consumer and professional video products incorporating a combination of the two.

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Each new HDMI version has used virtually the same hardware while increasing the bandwidth and capabilities of the signals carried. From version 1.0 to 1.3, we’ve seen clock rates increase to 340MHz from 165MHz and video pixel rates up to 680MHz on dual-link connections. Eight-channel audio is also carried at sample rates up to 192kHz, and IEC61937-compliant compressed audio streams, including Dolby Digital and DTS, are now supported.

As of June 2006, HDMI 1.3 has provided support for 1440p (2560x1600), deep color with bit depths up to 48 bits per color, and automatic lip-sync correction. It delivers all this and more through the smaller Type C connector, but it took us a while to get there. For users, there is no difference between HDMI 1.3, 1.3a, and 1.3b. These only outline testing and manufacturing issues and do not vary in features or functionality. At the time the standard was first announced, there was no technical infrastructure to support it and no products to deliver its promises to the public. Commercially, the technology has only hit full stride within the past two years.

Cable is king

The typical new HDMI transmission is like a busy expressway with different signals going at different speeds in both directions. The quality of the copper cable carrying these signals has become critical to the point that one that had been carrying the earlier HDMI traffic from an older player might not have any video signal from a new source device. The limitations of lesser cabling with the newer HDMI signals have been learned by all too many home-theater buffs who permanently installed older and cheaper cable inside the walls of their new homes only to upgrade the players and monitors and find that they don’t see each other. Reclocking and equalization are features that have become vital to maximizing the transmission distance and video clarity on quality HDMI distribution devices. As previously noted, it’s no longer a one-way street from player to display. EDID signals that allow the display to identify itself and allow the signal source to configure its output to suit that display must also have a clear path.

At the bit rates necessary to feed high-def video to modern displays, the capacitance in copper-pair cable results in insertion loss. This eventually causes the receiving display trouble in telling ones from zeros in the digital stream, and it doesn’t take long for the picture to develop sparkles or flashes, or to disappear completely. Imperfections in the cable can also cause skew from differences in the arrival time of the various color signals. Heavier-gauge copper conductors and active signal equalization can minimize these effects and maximize the distance over which a quality HDMI signal can be carried. These and other factors make it more challenging to select quality gear in the HDMI realm, and it pays to shop around. Let’s have a look at what the current market in HDMI distribution has to offer. The products included here use HDMI connections on the inputs and carry audio within those same connectors to multiple outputs.

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