Broken Secrets

The White Dashes at the Top of a TV Picture

August 20, 2010 at 5:00 am Chad Upton 1 comment

By Chad Upton | Editor

They may appear as rapidly flashing dots, dashes, lines or boxes across the top of your TV screen.

The white lines are more prevalent on HDTVs, although they can show on older standard definition sets too.

They often appear while watching an HD channel that is broadcasting a standard definition signal, which frequently happens during commercial breaks and shows that are not available in high definition.

The lines are supposed to be there, you’re just not supposed to see them. If you have seen them, they will vary in size and shape depending on your TV.

These lines are like barcodes embedded in the picture. Closed captioning, teletext and programming guide information is represented by these white lines. Your TV can interpret them and display the information in a format that you can read.

Although this primarily affects HDTVs, it stems from variances in old Tube TVs (Cathode Ray Tubes). In the early days of television, there were extreme variances in production of television sets — some would cut off more of the picture than others.

Broadcasters overcame this problem by trying to keep all titles and important actions away from the very edges of the screen, in case they were cut off on some TVs. That extra space that you don’t see is called the overscanned image (because of the way that CRTs would paint an image on the screen by scanning side to side sixty times per second).

The overscan area became a good place to hide extra information when closed captioning data was added to TV signals.

HDTV signals do not overscan. Their signals are newer and were designed to encapsulate extra information from the beginning. But, you may still see these lines on an HDTV if the broadcaster is showing content that has the lines.

Most good televisions have the option to adjust overscan, including hdtvs. You’ll have to consult your manual, but this option will allow you to adjust the picture so the white lines are not visible.

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Sources: Wikipedia (Safe area, Overscan)

Entry filed under: Around The House, Demystified, Gadgets and Toys. Tags: boxes, cc, closed captioning, dashes, datacasting, dots, hd, hdtv, lines, overscan, television, tv, underscan, video, white.