Can someone please explain overscan to me? (1 Viewer)

[clone_tk422]

Hey All,

I have a SAMSUNG CRT Rear Pro HDTV TV and my only gripe about HTPC's is the overscan problem.

Thing is, to this day I still dont understand why overscan exists on digital displays.

As I understand it, if I had bought a plasma or LCD tv I wouldnt have overscan issues.

But the thing I find troubling to comprehend is this...

My TV has a PC input. The PC Input's native display is 1024 x 768. ok fair enough and its analog.

But my HTPC goes through my DVI-D connection

my TV is capable of display up to 1080i HDTV. And when I connect a set top box to it, I get full 1080i and eveything from the STB including the UI displays great on my TV.

So what I dont understand is why with my video card 6600GT which is capable of outputting to 1080i, why do I get overscan?

in my mind, my Set top box is basically a small computer. It has a motherboard, a CPU, an operating system and some of display card which outputs the picture.

So why in the PC world cant this hardware / software be replicated? Why cant modern PC cards and software produce the same perfect digital non overscanned UI like a set top box does?

My video card reports that my TV over DVI has a native resolution of 1024 x 768. But this is incorrect the DVI can take up to 1080i. But I read that because I have a PC plugged into the DVI not a set top box my tv knows this and only lets it display up to 1024 x 768 natively. I can force 1080i but this is where I get overscan.

But again why cant the PC make the TV think a set top box is plugged in and let it display full 1080i??

Anyways I hope I make sense, and I hope someone can please explain to me a bit better because I am annoyed my Media Portal UI is overscanned and the left hand side and top is cut off. I tried playing around with UI calibration but all this does is move the UI around, not shrink it to fit.

Vista Media Centre seems to handle my HDTV a bit better with no overscan on the UI so obviosuly it can be done.. But why is it so difficult fo the software to come out to handle overscan better?

Also because my HDTV is running over DVI it should be fixed pixel.. so no overscan?

Thanks. Basically I just want to know why if a set top box can display proper HDTV and proper UI, why cant a $2000 PC with a $400 video card (which is as much as a whole set top box) do it?

 

[Spragleknas]

Overscan Explained

What is Overscan
Overscan referes to some of the picture being outside the visible area of the TV, essentially cropping the image. Underscan is the opposite, using only part of the available display area.

How is Overscan related to 1:1 pixel mapping?
In a way it IS possible to have a TV provide 1:1 pixel mapping while still overscanning. A TV with a resolution of say 1280x720, might not actually have 1280x720 visible pixels on screen, potentially yielding a scenario where the pixel mapping is perfect, but not all of the display is visible.

Types of Overscan
Digital Zoom
In this case the input is digitally scaled/zoomed so that it becomes larger than the actual display area. If your TV does this and it cannot be disabled you will never achieve 1:1 pixel mapping.

Optical overscan
Optical overscan refers to the TV having less pixels visible than is actually available from the display, or in the case of a rear projection unit, that the image is projected to a larger area than the display surface. In such cases you might get 1:1 with overscan as described above. By giving up on the invisible pixels, this can usually be solved in most recent display drivers, by creating a resolution-within-a-resolution, so that the old resolution is output but only a smaller area is actually available for the operating system(Windows, Linux, Mac OS etc).


Source: http://pixelmapping.wikispaces.com/Overscan+Explained

 

[clone_tk422]

Thanks,

but again, Just wondering why cant a PC replicate functionality of plain old set top boxes..

If you can plug them in to your tv and have a perfect picture and UI with no overscanning, why cant a PC replicate this?

 

[mbuzina]

The plain old setop box is allways connected to a TV by the same means.

PCs are connected in a miriad different ways to different devices (PC monitors have no overscan). Windows is not designed specifically to work on a TV, so it basically ignores overscan and uses the reolution you've got (start menu!).

The problem is not that PCs do strang things, it is the TVs that do that. Your RearPro is set to No Overscan on 1024 x 768, but to do overscan in 1080i (it overscans also for you setop box, but it only uses part of the total resolution for it's UI). In MediaPortal you can use the UI calibration and if you place the corners correctly, your UI should be visible.

Your sentence "Also because my HDTV is running over DVI it should be fixed pixel.. so no overscan?" SHOULD be true, but go ahead and tell that to the TV manufacturers. This depends fully on your TV.

Don't blame your $2000 PC on this, it is a multi-purpose device, no initially designed to be used on TVs, which have no real specification on what they do with the signal they receive.

 

[Spragleknas]

I belive that a STB can never do 1:1 pixelmapping (like you can - depending on TV - do with a PC) for 720P or SD-material. Thre reason is that 720p is 1280x720, while a "HD-ready" TV outputs 1366x768 (= 1 MP), and SD @ Hd can never be 1:1 pixelmapped. (Some LCD's are actually real 720p - but only very few and old).