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In Reply to: Because of overscan... posted by C.B. on August 3, 2006 at 07:40:09:
The reason we have overscan is because the originators didn't want people seeing "extra" non-image information. They also wanted to make sure that any recording or other studio equipment wouldn't show up on the viewer's set and because people wanted the screen to be filled -- thus TVs usually have 5 - 10% overscan. The area inside of this overscan is a 2-part area called the "safe zone". From the center out: 90% of the screen is the "safe action" area and 80% of the screen is the "safe title" area. This ensures that any action and/or titles will be seen by the majority of sets in use.The NTSC broadcast signal has 525 lines: it's non-image information (sync, vertical retrace, and closed captioning) that drops the number of actual "image" lines down to 486 analog/480 digital lines.
Follow Ups:
In the early days of TV the picture (raster) size varied noticeably depending on power line voltage and also as the set aged. When the picture shrank to be smaller than the screen, this was objectionable to many TV set owners and the workaround that manufacturers used was overscan.Non-subject matter such as microphone booms should not have been included in any part of the video frame even though the producer expected the picture edges to be seen on so few TV sets due to overscan.
After discounting the 5% strips around all four edges, the safe action area comes out to about 81% of the total video frame. After discounting the 10% strips around the edges, the safe title area comes out to about 64% of the total video frame.
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