I often get asked about the overscan setting in tvtime. This webpage was written to help explain how different television broadcasters use the overscan properties of televisions.
In an NTSC television signal, there are 486 scanlines on which you can put an image, and in digital video there are usually 720 pixels on each scanline. However, on your television not all of this image is visible. The border of the image extends past the image you see on your screen. The part of the image you can't see is the overscan.
|The raw frame||What you end up seeing|
Mostly crap. Ideally, you want to keep image in the overscan from every video signal to allow for post processing filters downstream to not have to deal with the edges of the image. It really depends though what you end up seeing. Usually, you just get some black borders on the left and right sides like this:
Other times you see crap, like check out the bottom scanlines on this country music station:
|Broken bottom scanlines|
The worst around here is The Weather Network which seems to always have a large dosage of random data.
|Crazy random data|
To help broadcasters know when something will be visible on a television, SMPTE defined specific sizes of the action frame called the safe action area and the safe title area. My reference is SMPTE recommended practice RP 27.3-1989 on Specifications for Safe Action and Safe Title Areas Test Pattern for Television Systems.
The safe action area is defined as the area in which "all significant action must take place", and the safe title area is where "all the important information must be confined to ensure visibility on the majority of home television receivers."
The safe action area occupies the center 90% of the screen, giving a 5% border all around. The safe title area occupies the center 80% of the screen, giving a 10% border.
|Safe action and safe title|
For the most part if you stick a safe action and safe title card in front of most stations, it will line up. For example, this TV guide station puts almost their entire content within the safe title area:
|TV guide station with safe title border|
But the safe title area is so small, most news stations these days will put text all the way into the safe action area. That said, it's not difficult to find televisions where the text crawl along the bottom of CNN is being cut off.
|Headline news showing the safe action area|
|A photograph of headline news on my television|
The headline news example does show that info stations have to squeeze as much content as they can into the middle. Look how busy the bottom bar is, yet how much blank space exists in the raw frame itself.