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UHDTV: Four Times the Pixels and 100 Times the Hype!

Unraveling overhyphed signal interfacing terms

The roll-out of Ultra-High-Definition TV (UHDTV) continues to build momentum. More models of Ultra HDTVs are coming to market and prices continue to fall. We’re seeing an increase in large 4K display monitors, too. The infrastructure is building for streaming 4K video from the likes of Netflix, Samsung, Amazon, and Comcast.
As is often the case with any hot new technology, the marketing folks get out ahead quickly with plenty of hype. A perfect example is the preponderance of so-called 4K “certification” programs and services, by which a manufacturer (usually of display interfaces) will brand selected products as “4K Certified” or “4K Ready.”
Some manufacturers will even test your 4K media sources and displays to verify that they can indeed pass a 4K signal through their HDMI connections. While this might be a great way to capture more business and assure nervous customers, it’s really not necessary.
To understand why, let’s stop thinking about 4K or UHDTV and concentrate instead on pixel clocks and data rates. The most widely implemented version of HDMI (1.4) rolled out in 2009 and added support for frame-packed 3D to version 1.3. The maximum data rate remained unchanged at 10.2 gigabits per second (Gbps), with about 20 percent of that data rate reserved for overhead (blanking interval, clock, etc.)
It’s already possible to pass Ultra-HD signals through versions 1.3 and 1.4 of HDMI. The standard blanking interval for an Ultra-HD video frame contains 3840×2160 active pixels and 4400×2250 total pixels. The CEA-861 standard calls for changing the size of the blanking interval to maintain a pixel clock rate that is a multiple of 74.25MHz, typically 297 or 594MHz.
HDMI 1.3/1.4 can’t exceed a clock rate of 330MHz, but we can still transport an Ultra-HD signal through that interface, using 8-bit RGB color, at a maximum refresh rate of 30Hz:
4400 horizontal pixels (x) 2250 vertical pixels (x) 30 = 297MHz.
Next, we multiply that product by 10 (8 bits per pixel, plus 2 bits for ANSI overhead) and get 2.97Gbps. Finally, we multiply once more by three for an RGB signal, such as that generated by a computer video card.
2.97 (x) 3 = 8.91Gbps.
This is the total data rate for an Ultra-HD 8-bit RGB display signal, refreshed at 30Hz. And as you can see, the data rate is well below the 10.2Gb/s limit of HDMI 1.3 and 1.4. In this way, an HDMI 1.3/1.4 interface is indeed “4K Certified” or “4K Ready.” It’s just a matter of calculating the numbers, and you don’t need a certification program to do that. You just need to know which version of HDMI your equipment uses.
How about refreshing Ultra-HD images at 60Hz? That’s where things get tricky, because we now wind up with a clock rate of 594MHz—twice that of the 30Hz example, and well past the speed limit of versions 1.3 and 1.4.
This is why HDMI version 2.0 was announced in September 2013, even though it still isn’t widely implemented: Version 2.0 has a much faster maximum clock rate of 600MHz and a maximum data rate of 18Gbps, so an Ultra-HD RGB signal with 8-bit color refreshed @ 60Hz just sneaks under the wire (17.82Gbps).
Of course, Ultra-HD signals that use sub-sampled color (4:2:0 or 4:2:2) run at lower clock rates and are less of a challenge. However, we should always design AV systems to accommodate the highest possible data rates, such as those output by Ultra-HD computer video cards and media players. As of this writing, there are several Ultra-HD laptops offered for sale and every one of them uses an HDMI v1.4 connector, limiting their maximum refresh rate to 30Hz.
There’s no question that 4K/Ultra HD is going to have a significant impact on our industry, but we shouldn’t let marketing hype confuse us when it comes to signal interfacing. The truth is any distribution amplifier or switcher that is equipped with HDMI version 1.3/1.4 can pass and switch Ultra-HD signals as long as those signals don’t exceed the maximum data rate of 10.2Gbps.
It’s just that simple.

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