What is the difference between 720p and 720i?

Question

I got a new tv and I am wondering what it means if it says 720p/1080i.

Answer 1

"I" means interlaced.  That is, the picture is drawn with half the screen scanned at every other line, and then  scanned with the lines in between (TV pictures are drawn one line at a time, until the entire screen is depicted, then the next frame is drawn).  In other words, odd numbered (1, 3, 5, etc) lines appear on the screen, then the even numbered lines appear.  This all happens so fast that your eye sees a moving picture and never notices.  This occurs when the data stream is too slow to show all the lines at once, so the rate that each frame appears on the screen is slowed.  You see this as a less sharp picture.

"P" denotes "progressive, meaning that all lines on the screen are presented one after another, no gaps.  This requires twice the data density, as the whole picture is shown at a single moment.  You see this as a crisper picture and enjoy it somewhat more.  Progressive trumps interlaced every time.

Iinterlaced defined early TV, as the frame rate to perceive motion was above 24 frames per second (FPS), but conventional TV transmissions could not reliably transmit this.  So, the interlaced concept became reality, with only half the lines sent per frame, motion became smooth and TV was a reality!

Higher definitions require greatly increased data rates, so interlaced still exists even with higher lines.  1080p is the current definition of High Definition, but it will eventually be replaced by even higher line counts, since TV's keep getting bigger.  480 lines on a TV with a screen 20 inches high is equal to 960 lines on a screen 40 inches high.  On a conventional "tube" type TV, a small screen (19 or 13 inches) always looks sharper than on a larger screen (25 0r 32 inches).  

The best picture quality occurs with progressive, since the movie is effectively shown at twice the frame rate, with the entire picture presented at once.  The picture is crisper and better defined.

In fact, a 720p may look better than a 1080i.

Of course, as mentioned, screen size is critical.  Once home TV,s go beyond 70 or 80 inches, a resolution greater than 1080p will be needed.  Then, we can all go trade in our sets again!

Answer 2

You should be aware though that lots of TVs which support 1080i have fewer than 1080 lines and so scale the 1080 signal down. That's not a huge issue as even scaled down 1080i is far ahead of a regular NTSC signal. It is worth bearing in mind

Answer 3

720i isn't very common. The i means "interlaced" whereas the p means "progressive." Translated, i means every other line in the horizontal scan is drawn on each pass of a screen refresh. Screen refresh is typical 60 or 72 types a second. So p means that every single line is drawn on each pass. The original NTSC tv signal is 480i. This means 240 lines draw 30 times a second and the other 240 lines drawn in the other 30 times a second to complete a 60 frame per second display. All you really need to know is that p is better because there is less flicker, so 1080p would be the best.