Other HD 101 goodness:
What is ATSC, PSIP, QAM, and 8-VSB?
How to use Dolby TrueHD and DTS-HD with your PS3
Why there are black bars on HDTVs
A test pattern with overscan markers on the four edges.
The easiest way to wrap your head around overscan is to forget about the word for a second and think of cropping an image. If you take a picture and cut off an inch from each edge and blow it up to the original size, then you'd effectively be overscanning. The term comes from the days of CRTs when the scan lines that drew the image literally scanned over the edge of the viewable part of the tube -- PC users were all too familiar with the small print that a 21-inch CRT PC display proclaiming that it was actually 20-inch viewable. The problem with CRTs was that a limitation of the technology was its inability to accurately reproduce images along the edges of the tube. So instead the image was overscanned which resulted in some loss of picture but maintained quality for the center of the image -- the part that matters most.
So if CRTs are dead then why do we still have overscan? This is where the bad news comes in, but the short answer is because it used to be there. The source of the problem is that broadcasters expect the TV to crop the image so they don't mind putting garbage on the edge -- like a misc yellow line on the left or black and white dots show in the images below.
Notice the zoomed part contains white and black dots that are used for the 1st and 10 marker.
Sometimes this garbage serves a purpose. Like in the image above or an analog broadcasts that's converted to digital. Since analog doesn't have a way to send metadata, like rating and closed captions, the data is encoded in the vertical blanking interval and shows up as flashing pixels when the signal is converted to digital and displayed without overscan.
In this zoomed section you can see a faint yellow line on the left hand side. Luckily NBC finally fixed this particular problem.
It isn't just broadcasters who are to blame, the TV manufactures play their part too. You see because some broadcasters don't ensure a clean picture on the edge, the TV manufactures err on the side of caution and crop it just in case. They know that a yellow line on the left side of a brand new HDTV is a great way to get a return. On top of that, talking heads on an HDTV with overscan appear larger, so consumers see this and think, "oh that TV looks better to me." It's really crazy to think that consumers would rather see a zoomed, fuzzy image than a yellow line on the edge, but that's most people for ya.
These are the same screen shot, one is presented with overscan, one isn't. See how much bigger McCain's head is?
It's not all bad
In a perfect world broadcasters would mind their signal and ensure every pixel was worth watching, then TV manufacturers wouldn't feel the need for overscan -- or at least we hope. In the meantime we're just happy that most TVs have a way to turn it off. When an image is displayed properly, it's sometimes referred as 1:1 pixel mapping. This simply means that every pixel in the signal is displayed by a single pixel on the display. Of course each manufacture has its own name for this mode; like Samsung who sometimes calls it Screen Fit, and Pioneer called it Dot by Dot. No matter what it's called, it's rarely on by default -- even when watching 1080p24 from Blu-ray which never has garbage on the edges! -- so you'll have to find the button on the remote called something like format or P.size, depending on the brand. Your best bet is to read the manual -- shocker.
But even if you don't ever find the button or just like it on, modern video processors aren't half bad at scaling the image. Sure it'll never be as good displaying it properly, but to some the occasional garbage on the edge is far worse then the lack of detail -- which they're less likely to notice. But, there are some HDTVs that don't let you turn it off, which is really lame. Some sources like Vudu and HTPCs will let you adjust the overscan at the source, but this only solves half the problem, as seeing the edges of the image is better than nothing, but the image is still being scaled twice.
Vudu overscan adjustment that is on by default.
nVidia overscan and centering adjustment.
Another issue we've seen, although less frequently, is where the HDTV will display the image without overscan, but it still shifts by a line or two and scales the image -- although we admit that we only learned of this by running the Pixel Phase test on Digital Video Essentials HD Basic calibration disc. This is the type of thing that most will never know and unless you test it with a pattern or read it in a review, you'll never be the wiser.