I get asked a lot about what the difference is between 720p (Progressive) or 1080i (Interlaced) and which is better when you go to buy a new HDTV. As you can guess, the 1080i has 360 more lines of resolution that make pictures richer and deeper. This may sound like a better deal, but 1080i has some drawbacks too.
A 720p signal consists of 720 lines on your TV screen. Every frame is seen for 1/30th of a second. Visualize seeing 30 different "photographs" zip by every second in a fast paced slide show. This is referred to as "progressive scan" - thus the "p" in 720p.
A 1080i signal consists of 1080 lines. However, all the lines are not displayed on the screen at the same time. Instead they are interlaced (hence the 'i'). This means every other line is displayed for 1/60th of a second. Then the alternate lines are displayed for 1/60th of a second. So, the frame rate is still 30 frames per second, but each frame is split into two fields that your brain puts together subconsciously.
Most of the time interlacing offers better quality. But for fast moving images including sports events (baseball or hockey for example) and intense action movie scenes, the images can pixellate with the 1080i. So, if your a sports buff, I suggest that 720p is the way to go.
But for most consumers, the quality debate between 720p vs 1080i is marginal and should not be a huge deciding factor on which type of HDTV to purchase.
A sidenote: Both 720i and 1080p also exist. But, 720i is not used by broadcasters. The 1080p will likely become more widely used in future years as the technology evolves.
