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What Is 2K and 4K Video?

This is another installment in our series of "What Is...?" articles, designed to offer definitions, history, and context around significant terms and issues in the online video industry. 

Soon, the average consumer will have access to flat- panel "Ultra HD" monitors that make the resolution of the 1080p "True HD" HDTV sitting in their living room seem almost like the quality of old analog television. These new monitors use just one of several 2K and 4K resolutions that consumers and content producers will face over the next few years.

What exactly are 2K and 4K resolutions, and how do they differ from today's HDTV resolutions? More importantly, what approach should the streaming industry take to these new resolutions?

720p and 1080p HDTV

When flat-panel HDTVs first reached critical consumer mass, the format was 720p, or 720 vertical lines of resolution, at a progressive (the "P") rate of either 24 or 30 frames per second (fps) in the United States. While the US based its HD television resolutions on NTSC, Europe used PAL which meant the P in 720p is a constant 25 fps.

After flooding the market with 720p monitors during the analog-to-digital broadcast rollover, consumer electronics manufacturers decided it was time to push consumers towards "True HD" or 1080p, with its 1080 lines of vertical resolution. Ignoring the fact that the vast majority of HD broadcasts are in 480i or 720p, the "True HD" push became one of delivering premium content via Blu-ray players and, to a lesser extent, digital downloads and streaming.

Using 720p and 1080p as baselines, with their respective 1280 x 720 and 1920 x 1080 pixel resolutions, let's look at how the new 2K and 4K formats stack up against HD and True HD monitors. Keep in mind that, as of early March 2013, we don't yet have premium content available for 4K playback, although as mentioned in an article on the 2013 Mobile World Congress, that is beginning to change.

2K Video

We'll start with the simplest format, as a way to describe how the consumer electronics (CE) companies are shifting the definition away from vertical lines of resolution to something more nebulous.

If 1080p has 1080 lines of vertical resolution, one would think that the 2K displays have 2,000 lines of vertical resolution, right? Not so fast: 2K is an incremental bump above 1080p. In fact, 2K doesn't even change the vertical resolution, leaving it at 1080 lines, and only increases the horizontal resolution to a 2048 pixel width.

Run the math and you'll find that the increase from 1920 horizontal pixels to 2048 horizontal pixels is a whopping 3.43 per cent increase. Hardly worth chucking the 1080p "True HD" monitor for a 2K one.

The reason for this confusion lies mainly in the primary purpose of 2K resolutions. To date, 2K has primarily been used for projection within movie theaters, which require a slightly different aspect ratio from consumer HDTVs and computer monitors.

While consumer displays use a 16:9 aspect ratio (or 1.777:1 if converted to the lowest common denominator), the Digital Cinema Initiative (DCI) backed by the major Hollywood studios uses a 17:9 aspect ratio (approximately 1.85:1) that better aligns with several of 35mm and 70mm films formats used to shoot and project movies.

As such, the DCI termed the 2048 x 1080 resolution as 2K, and the name stuck even when it came to consumer applications. It's no wonder that most consumers with decent home theater systems feel like they're getting an equal or better experience to the movie theater: with the exception of a few more horizontal pixels, the image is exactly the same resolution in 1080p and 2K, and the pixel density (measured in PPI or pixels per inch) is quite a bit denser on a consumer 1080p display.

Another way to look at it is overall pixels: 1920 x 1080 equals 2,073,600 pixels, while 2K at 2048 x 1080 equals 2,211,840. The difference is 138,240 pixels, and if the pixels were represented as megapixels (Mpx, the typical way we gauge still camera resolutions) the change between 1080p and 2K would be approximately 0.13 Mpx (2.1 versus 2.2 Mpx).

4K Video

Now that we've looked at 2K, what about 4K? Does it offer resolution benefits beyond 2K and 1080p?

There are three 4K resolutions to consider, each with a different aspect ratio.

Let's consider, first, the DCI version of 4K. The film industry likes to shoot at resolutions at least 3-4 times greater than what it will project, so the DCI came out with a 4K specification at the same time it came out with the 2K display specification.

DCI 4K has 2160 vertical lines of resolution, which is double that of 1080p. So far so good, but what about the horizontal resolution? It clocks in at 4096 pixels. All told, that's 8,847,360 pixels, or 8.84 Mpx.

Running the numbers, that means DCI 4K is 4.26 times the resolution of 1080p and exactly 4 times the resolution of 2K. The difference, as we mentioned above, is the aspect ratios of 1080p (16:9) and DCI 2K / 4K (17:9).

4K Ultra HD (UHD) Video

Ok, so now that we've seen how DCI 4K is four times the pixel count of 1080p, what about the 4K Ultra HD monitors that you'll be able to buy this month from Samsung, Sony, and others for a cool $25,000 at sizes ranging from 55 to 84 inches?

4K UHD does double the vertical resolution, from 1080 to 2160 vertical lines of resolution, but it falls short of the 4,000 pixel horizontal resolution, clocking in at 3840 horizontal pixels. If you've noticed the pattern of doubling the horizontal resolution, you've caught on to the reason that 4K UHD isn't really 4K: 1920 times two is 3840, meaning that consumer displays will still hold the 16:9 aspect ratio.

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