HDTV/UHDTV: Are You Getting the Complete Picture?
Late 2012 saw the introduction of 4K UHD TVs, and at the 2014 CES in January, a slew of new 4K models were announced with dramatically lower price points compared with last year’s very expensive first-generation sets. These also have smaller screen sizes designed to appeal to a wider audience beyond well-heeled videophiles.
The marketing hype for these new TVs drives home the idea that a 4K set is a must-have, with claims of vastly improved detail (4 times the pixels!!!) compared with 1080p models.
Here’s the kicker, though: The vast majority of viewers watching 1080p flat panel TVs aren’t even seeing the full 1920 x 1080 resolution that they paid for.
Overscan: HDTV’s enemy
A main reason why most viewers aren’t seeing full HD resolution on their TVs is that the out-of-the-box default settings for virtually all 1080p sets include 2.5% overscan, where the original image gets slightly enlarged to trim off occasional visual oddities that appear at the screen edges. The most common of these is when HD and SD content is combined, with flickering lines or dashes appearing at the top of the screen where the SD image is placed, or when improperly formatted SD content is displayed (local TV advertising, for example).
While a little overscan might not seem like much, it has the effect of destroying the set’s HD picture, reducing its resolution by half (yes, by half) or even more.
You can see this yourself using any of the available Blu-ray test discs that feature 1080p resolution charts. For example, on Joe Kane’s most excellent Digital Video Essentials HD Basics Blu-ray disc, there are a number of digitally-generated 1080p resolution charts that show the destructive effect of even a teensy bit of overscan.
Go to the Overscan chart (below) in the 1080p patterns section first. Look to see how much of the image is cropped as you cycle through your TV’s various Picture Size modes. Look for a mode that displays the full chart with no cropping. (These are often labeled with names such as Just Scan, Dot by Dot, 1:1, etc.) Note: Some TVs don’t give you access to an un-cropped 1:1 mode by pressing the remote control’s Picture Size button, so you’ll have to hunt around in the set’s onscreen menu to find a secondary Picture Size option. Sneaky bastards.
Once you’ve found the 1:1 mode for your set, call up one of the 1080p resolution charts, such as this one:
Note both the blocks at the screen corners and the one at center-screen. These each contain a group of three horizontal and vertical alternating-pixel black/white line patterns. The lines in the top two patterns are single-pixel width, the middle ones two pixels wide, and the bottom ones are three pixels wide. If your TV is set for a true 1:1 mode (no overscan), then you should be able to see the lines in the single-pixel pattern (you’ll probably have to get up close to the screen to see them). Here’s what they’re supposed to look like:
Now, cycle through the TV’s other Picture Size modes, and you’ll see the destructive effect that even a small amount of overscan can have, with aliasing and crushed detail showing up in the resolution blocks.
Here’s another test. Call up the single-pixel 1080p resolution chart, which shows six strips of alternating pixels (2 sets of 1×1, 2×2 and 3×3) against a gray background. With the TV set for 1:1, you should be able to discern the 1×1 alternating black and white pixels (again, you’ll probably have to get close to the TV).
Cycle through the set’s other Picture Size modes and you’ll see that detail goes right out the window when any amount of overscan is introduced—the 1×1 strips turn to gray mush. Depending on the level of overscan (and the quality of the TV’s video processing), it’s possible that the 2×2 strips will also turn to gray mush, and in some cases even the 3×3 ones. The image below shows a zoomed-in portion of the pattern with the 1×1 dot structure enlarged to illustrate what the 1×1 pixel strips should look like with a true 1:1 Picture Size mode selected.
Size matters: screen size and viewing distance
Now try this: Select the set’s 1:1 mode and return to where you normally sit. Can you see the 1×1 pixel strips clearly, or do they go gray and blend in with the gray background? If the case is the latter, your TV’s screen isn’t properly sized for your seating distance.
The # 1 mistake that TV shoppers make is buying a too-small HDTV. In order to clearly discern full 1080p detail, you need to be seated, at most, 3 times the picture height (PH) away from the TV. That number is derived from actual research into visual acuity and perceived resolution.
Say you have a 55-inch set. That’s a pretty good size, right? The PH for a 55-inch TV is 27 inches, and 3 times that comes out to 6.75 feet. Sit back any further than 6.75 feet, and your ability to properly resolve full 1080p HD detail sinks like a rock.
Let’s now call up DVE’s 1080p vertical resolution pattern:
Because of aliasing caused by re-sizing the original 1920×1080 pixel image to fit our page, you’ll note that the single pixel width lines (Full) are grayed out in the image above. Below is a cropped image of the same test pattern showing the lines:
Set your 1080p TV to its 1:1 mode, and have a look from your normal seating position. Can you discern the single-pixel lines (labeled Full), or do they appear grayish? Now look at the half-resolution (2 pixels-thick) lines. Are these visible as lines, or do they look grayish? If the answer is the latter, then congratulations—you’re watching a slightly-better-than-SD 540p-resolution image, which is decidedly not HD (true HD is 720p resolution or higher). If you can’t even make out the 1/3 lines, well, then you’re seeing a soft picture that doesn’t even qualify as true standard definition (360 lines, versus SD’s 480 lines).
Sizing up for 4K/Ultra HD
So, now it’s possible for us to buy 4K/Ultra HD sets with 4 times as many pixels as 1080p ones! But before you go shopping for one, you had better be prepared to pony up some really big bucks for a really big TV.
That’s because the optimum viewing distance for 4K/UHD TV requires that you either get a really big screen, or re-arrange your living room to get up-close and really personal with the new set.
With a 4K TV, you can toss the 3 x PH optimum viewing distance calculation we used for 1080p right out the window, because it’s now down to 1.5 x PH! You will be able to see a detail improvement only up to 2 times picture height.
Let’s say you want to swap out your 55-inch 1080p set with a 55-inch 4K model. If you want to see full 4K UHD resolution, you’d now better be parked no more than 3.5 feet away from the set. Sitting back further than that will diminish your ability to properly resolve a 4K-resolution image.
Let’s also say that your viewing distance is about 5 feet away from your current 55-inch 1080p TV. You’d like to swap it out with a 4K model, but don’t want to re-arrange your living room (or are not allowed to).
For a 5-foot viewing distance, a very pricey 85-inch 4K set is what’s needed (42-inch PH x 1.5), if you want to be able to discern true 3840 x 2160-pixel 4K/UHD resolution. Going instead with a 60- or even a 70-inch set simply won’t be enough.
Want to sit about 7 feet away from your new 4K TV? Now you’re in second mortgage territory, as you’ll need to get a budget-busting 110-inch model. (Here’s another, less expensive, option: buy a 4K front projector/screen combo.)
Of course, with 4K TVs, the same caveat about overscan we discussed for 1080p sets holds true: If the TV isn’t in 1:1 display mode, it’s no longer showing 4K.
While I have not yet done a full-on review of a 4K TV here at home (I hope to get my hands on one of the newer 2014 models soon), I was able to spend a good chunk of quality time with video expert Joe Kane at a SMPTE conference in Hollywood last fall. Joe, who regularly conducts 4K demos for Hollywood cinematographers, was holding court in the 4K demo room and displaying actual 4K native content and test patterns on Samsung’s 85-inch S9 series UHDTV. His demonstration convinced me that the 1.5 x PH rule for 4K is entirely valid.
Taking it all in
Here’s my advice for early 4K/UHDTV adopters: If you want to see real 4K resolution when actual 4K content becomes available, then prepare to shop for a set with a much, much bigger screen than you might already be considering (or, plan to sit a lot closer to the screen than you have been with your 1080p set). Also, steer clear of your TV’s detail-killing overscan function by using the tests described above to make sure it’s set to the 1:1 Picture Size mode—a rule that holds true for HDTV s (both 720p or 1080p models) and 4K UHDTVs.
For more about where the 1080p and 4K UHD (2160p) PH calculations mentioned above came from (and the research methodology that produced them), download the BT.1769 document from the ITU’s (International Telecommunications Union, a branch of the United Nations and the mother ship when it comes to researching and setting large screen display standards, among other things) website. While you’re there, you might also want to download the BT.2020 document, which covers all things 4K (and even 8K). Unlike some other standards-setting organizations where you must be a member or pay a fee to obtain standards documents, the ITU documents are free.
Special thanks to Joe Kane of Joe Kane Productions for the images, and for pointing me toward the BT.1769 document.
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