TV formats reflect number of scan lines

By Andy Walker

What's the difference between HDTV formats 720p and 1080i?

What it comes down to is scan lines. And to understand scan lines I have to backtrack into history.

The standard North American TV signal is what is called NTSC or 480i. It consists of 480 horizontal lines of visible picture.

These lines of picture are drawn on the screen from top to bottom: Odd lines first, even lines next.

So lines 1,3,5 draw down the screen in 1/60th of a second. Then, in the next 1/60th of a second, lines 2,4,6 and so on are drawn.

Your brain sews these together. This is called an "interlaced" picture.

There is also a standard called 480p. The p is short for progressive. This means that all 480 lines are drawn all at once. This is double the resolution of 480i.

And, as it turns out, 480p is the standard used by DVDs. That's why a DVD picture appears crisper than a standard (480i) TV picture.

In the HD world, 720p is a high-definition format where 720 lines of a picture are drawn all at once - or progressively.

1080i is also a high-definition standard where 540 odd lines draw first. Then, in the next fraction of a second, the 540 even lines draw. And, once again, your eye sews the picture together.

Side by side, you'd be hard-pressed to tell the difference. There is also such thing as 1080p. That's where all 1080 lines are drawn at once.

1080i and 720p are high-definition formats used by broadcasters, while 1080p is the high-definition format used by the new high-definition DVD discs: HD DVD and Blu-ray formats.