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What is Overscan? Why is it there?

Overscan is extra image area around the four edges of a video image that is not normally seen by the viewer. It exists because [older] television sets...were highly variable in how the video image was framed within the cathode ray tube (CRT).
The cameras in the studio, would have a "mask" on them so that the camera operator knew roughly what the viewer at home would be seeing and made sure that important features (like the tops of heads) remained in this "safe zone". Rather than rewriting what is already pretty well written, here's a link if you need more info on what overscan is and what the history is:

http://en.wikipedia.org/wiki/Overscan


What's Different Today?

Older TVs had 10%+ overscan. Today's newer HDTVs (especially plasmas and LCDs) typically ship with about 2-3% overscan in the default mode. Some new HDTVs also have a 1:1 pixel mapping mode (0% overscan), discussed in the following post:

http://www.digitalhome.ca/forum/showthread.php?p=793236#post793236

Since today's newer TVs have less overscan, the people who produce programming now allow the cameras and production facilities to widen the area of the "canvas", thereby moving items like logos and tickers closer to the edge of the screen.


Why does this cause a problem?

There usually isn't a problem on 4:3 (SD) programming, however, a lot of early HDTVs (CRTs and CRT-based RPTVs) were shipped with overscan typically around 4-6% (RPTVs), but sometimes as high as 10%, or more (Direct View CRTs). What happens on these older TVs is that the logos and tickers are now drifting off the screen, so people see "FO" instead of "FOX", or the top or bottom of a sports/news ticker is not completely visible.


What can you do about this?

If you're the average person, the quick answer is "not much", outside of accepting the fact and understanding why it's happening.

Some (rare) HDTVs have settings in the user menu where you can adjust the vertical and horizontal size of the screen. This user setup menu is the first place to look.

Some (more) HDTVs may have a vertical or horizontal position adjustment, which allows you to move the screen up/down or right/left a few pixels. This allows you to move the screen up a bit if the ticker at the bottom of the screen is unreadable for example, however, you'll then probably have a bit more overscan at the top of the screen...

All HDTVs have a service menu, where a qualified technician may be able to adjust the overscan via parameters like "H-size" or "V-size" or similar. Some HDTVs do not even have these parameters, so no adjustment is possible. Here's a post on service menus, but please heed the warnings:

http://www.digitalhome.ca/forum/showthread.php?t=43201

Some HDTVs (especially RPTVs) have potentiometers which allow you to adjust the size of the image that appears on the screen. One needs to be very careful with these "pots" since taking them out of range can cause damage to the TV - again, leave this to trained technicians and you must have a test pattern to perform proper adjustment. Also, when you adjust these pots on RPTVs, you typically need to completely redo the convergence in manual mode - this can take several hours to perform properly.

If you perform these adjustments, it's best to set the overscan at about 2% all the way around, because if you set it for less, you'll start seeing VBI information, or other stuff on the screen that you shouldn't see - as discussed in the link on 1:1 pixel mapping.


How can you tell how much overscan you have

You can obtain a setup DVD as discussed in the following thread. These DVDs have test patterns which show the amount of overscan (and whether the picture is centered - on each side of the TV) and also of course assist with the setup of the TV, as described in the thread.

http://www.digitalhome.ca/forum/showthread.php?t=76161

On some TVs, HD signals may be different from SD signals when it comes to overscan...


If you have any comments or suggestions for this post, please PM 57.
 

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Discussion Starter · #2 ·
If you're obsessed with the issue and you cannot adjust the TV to your satisfaction, jvincent has mentioned that you could purchase an external (expensive) scaler.
 
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