HD 101: Overscan and why all TVs do it
By Ben Drawbaugh 
posted May 27th 2010 2:43PM
posted May 27th 2010 2:43PM
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.
I appreciate the attempt to explain this, but as someone who has worked in television for 15 years, this explanation is a bit misleading and incomplete.
The meta data in the video stream (closed captioning, time code, user bits, etc) isn't part of the 1920x1080 frame. They exist in the in whats called the vertical blanking of the signal. If you think of video as a real time constant stream of pixels, the vertical blanking is the time between drawing the last line of the frame and starting the top of the next one. We embed data within this blanking because under correct conditions, the viewer would never see it. And this data existed in analog video well before digital video was developed.
The reason why you occasionally see this on your TV is because of the complexity of numerous interconnected video devices used to bring video from the camera to your TV (Camera -> Truck router -> Switcher -> Truck router -> Sat uplink or fiber transmitter -> network receiver (example: ESPN in Bristol, CT) -> network frame sync -> network master control switcher -> network sat uplink -> cable or sat provider -> their frame syncs -> their master control switcher -> their transmission to your home).
With so many devices in the chain, at many points there is the chance for a timing offset not being set correctly which can result in the frame being shifted either up, down, left or right, which can lead to you seeing the blanking data.
Finally, broadcasters aren't lazy or trying to have crap on the sides of the screen. For good or bad, we work within two picture boundaries: Safe Action (90% of the frame) and safe title (80% of the frame). Like it or not, these boundaries help assure us that what we make will be correctly viewable on all televisions. If I get a video clip for a piece that I'm editing that has crap on the sides, but is outside of safe action, I'm left with only two choices. 1) Scale up the picture, which will make the overall frame softer, or 2) leave it alone which keeps the image quality as high as possible. Personally, I'm okay with trying to keep the picture quality as high as possible. That's what overscan is there for.
@thatian
Thanks for the clarification. Obviously I didn't want to dedicate that much copy to why analog converted to digital has garbage at the top. So it's good that people can find it in the comments if they are interested.
The real problem with overscan to videophiles isn't the missing information on the edges as much as it is the softening of the image from scaling. Personally I don't mind the garbage on the edges so I just leave overscan off all the time.
I know that some broadcasters intend 100% of the image to be seen, at least I've heard people say that's how HD.net does it.
Either way, I'm sure we can both agree that when playing a Blu-ray Disc back there is no point in overscanning.
@BenD Ah, I think I missed something in my original comment. It's not about converting analog video into digital, because the issue remains regardless. Digital video is just a digital model of analog video, much like PCM audio is modeled after analog audio.
In the case of a Blu-ray or PC connection to your HDTV, as far as I know, there isn't a flag within the vertical blanking to tell the TV what the video source is, so a TV can't tell if it's broadcast, DVD, Blu-ray, etc. It treats everything the same in that respect.
I agree with you that TV's scaling up the picture isn't a preferred solution as the picture gets softer. But I guess for me, I'm prioritize that lower than other quality issues we all have to deal with, like video compression and multiple-recompression through the transmission chain, interlacing (ug what a pain), etc.
Anytime video and TVs are concerned, we all have to deal with legacy issues that have been in place since the 50's. Interlaced video, silly frame rates (29.97, 59.94, 23.976), video levels (7.5% brightness is black or in digital terms 16-236 as opposed to 0-255). For whatever reason, each new evolution of video has been built off the existing standards, so unless we all throw everything out from the camera to the TV, there's little we can do about it.
To be honest, I blame all of this on the FCC when they decided that first color television signal had to still be compatible with existing black and white TVs.
Damn, broadcasters and TV set manufactures need to fix this crap. :(
One thing I've always wanted to know and this thread is the best by far to ask it. Why do some of my HD channels fill my 1080p screen while others do the black bands at the top and bottom, making the picture smaller? My HD ABC channel fills the screen, but HD Fox does not. The TV is properly sized for wide screen and set as such. I experience a similar problem with DVD movies which seems to go all over the place size wise in how much of a letterbox they create. The same with Amazon rentals run off my laptop.
@SharonW
It's a long winded answer to your question. Check out this thread over at blu-ray.com - http://forum.blu-ray.com/blu-ray-movies-north-america/5528-black-bars-4-3-16-9-2-35-widescreen-fullscreen-scope-oar-sticky-threads.html
In short, movies, tv, film and everything in between are shot in different aspect ratios. Some fill the screen, some do not. Check out the link and you will get your answer and then some.
@AbSoluTc Wow. Thanks so much for an answer and link. I'm off to read up. Man, I thought everyone was working with the same damn playbook, i.e. same ratios. WTF? How stupid is that?
@SharonW
I'm not sure if the article mentions it, but these different aspect ratios were introduced because cinema wanted to differentiate from TV. Nowadays the director (should) decide what works best for the film. For example for a big epic you want a very wide screen... I think it was Ben Hur that uses 2.7something to 1... you barely see anything but black bars. A comedy might be fine with 16:9 and makes more sense that way.
I do wonder if you haven't noticed it in cinema... some movies are just wider than others.
Finally please avoid TV channels that crop the movies to 16:9 at all costs. It just ruins the vision of the director and director of photography, the shots are carefully composed and are be destroyed by cropping, the feeling that the shot is supposed to convey just doesn't work after cropping. e.g. again epics. You have a wide landscape, like a desert, and somewhere at the edge there is the main character. You see, he is lonely, no one around, etc. But if you crop of the border the desert isn't as vast anymore. Or the italo western... a shot of the actors face, you only see the eyes, it's that close. Crop it and you will see a part of the left and a part of the right eye.
Funny, I just got a new laptop with an Nvidia card, and noticed the edges of the screen were cut off when I hooked it up to my cheapie Proscan LCD. First I thought to change the resolution down a little bit but that created a black border all around, then I found the screen pictured above listed as "adjust HDTV desktop size." Worked pretty well but now I'm going to see if I can find a way to get the TV to go 1:1.
I just bought a Samsung 6 series 120hz.
and I noticed that it has a fit-to-screen option to remove the overscan. I noticed this when I was watching streaming HD netflix and cycled though the pictures options.
this explains why the "just scan" mode on my TV sometimes shows a line of random black and white pixels on Fox
My Philips 47" 47PFL5704D (yeah, it's cheap, so sue me) has an unscaled option on its format menu. What's interesting is that on terrestrial channels, it switches to "wide screen" and doesn't list the unscaled option - this even through a satellite box, so I'm not changing channels, but the moment the signal switches to (I presume) having an image with garbage at the edges, the TV overscans. If it can tell, I'd really rather it put black bars over the garbage or something, but all things considered this seems like a pretty sensible and elegant solution.
Thank you for putting this technically accurate information out there! You don't need my applause, but you have it. Please continue to inform.
Luckily my Panasonic 50PZ70 has the option: Overscan: ON / OFF :-)
I know I'm without hope, but what do I do with my rear projection LCD TV? Why do I have overscan? Couldn't they have made the projected image at least a littler closer to the edges of my screen? Instead, when I connect a computer to it it seems like I'm missing like 100 lines of vertical resolution. What can I do, if anything (short of buying a new TV :) )?
Thanks for the write up. Also a big thanks for providing the link to the earlier post about Dolby TrueHD and DTS-HD with PS3. I kept looking for this post a while ago, when all of the engadget sites moved over to this new layout and design, but was never able to find it. I was pretty sure that I had seen it sometime ago, but I just could not find it again.
Test post as yesterday's comment has not been published. This post suggested the sentence "Notice the zoomed part contains white and black dots that are used for the 1st and 10 marker." Is way off the mark - this is a 16:9 vertical interval flag from a camera!