Oops, didn't know I was gonna cause heartache dude
My apologies.
A quote from the net:
Format Description
480i Digital version of current television signals.
480p Also known as "standard definition" - has the same detail as today's television signal but looks sharper
720p The HDTV format used by ABC and probably Fox. This format provides an image just about as good as 1080i, while allowing other 480p signals to be broadcast at the same time.
1080i The most detailed image available from broadcast TV - the HDTV format used by NBC and CBS.
The number refers to the number of lines of vertical resolution . Generally the higher the number, the better the picture. For example A 720p image is much more detailed than a 480p image.
The letter refers to the way the TV makes the picture, either Progressive (p) or Interlaced (i).
A progressive scan means the TV draws line 1 of the image, then draws line 2, then line 3, then line 4, etc. until it reaches the bottom of the screen. Then it starts on the next image. This is what your computer monitor is doing right now.
An interlaced scan means the TV draws line 1 of the image, then draws line 3, then line 5, and every odd numbered line from there. Then it comes back and draws every even numbered line. This is the way regular TVs work.
You may have noticed that the image on your computer looks better than the image on your TV. With today's technology a progressive scan looks sharper than an interlaced scan. So a 480p image will look sharper than a 480i image.
Because a 'p' image looks sharper than an 'i' image, we can reduce the number of lines of resolution and still get a good-looking image. So a 720p image looks almost as good as a 1080i image.
Hope this helps a bit