What is the difference between p & i HDTV?

Question

can someone please explain EASILY the difference between i and p in regards to HDTV's. for example 720p and 1080i.

p is progressive scan, does this mean the picture is displayed all at once?

and i is interlaced meaning one half of the picture is shown a split second later?

am i completely wrong?

Answer 1

You have it right.

A standard def TV is interlace. Look at a 'big screen' and you see horizontal lines caused by fade when the odd rows are drawn, then the even rows are filled in.

Look at your computer monitor - it is a progressive device. It paints the whole screen in 1 pass which provides a much sharper & clearer looking image even if you are only set to 480 lines.

Note: It is much harder to see the difference between progressive and interlace with the new 720 and 1080 formats.  But 1080p is technically the best possible type of display.

Answer 2

The numbers 480, 720, and 1080 refer to the number of lines from top to bottom.  They are broken up into even and odd lines.

We've had 480i as a TV broadcast format for decades.  That's standard definition.

Progressive scan like 720p is a format that transmits all the lines in order from top to bottom.

Interlaced scan like 1080i is a format that transmits the even lines first, then goes back and does the odd lines.  It takes two passes to display the whole image.  The half images consisting of either even or odd lines are called fields.  The two fields together make up a whole frame.

Most HDTVs (LCD, DLPs, and Plasmas) deinterlace the two fields and create a full frame and displays it as one.  It does this with the video electronics in the TV.  The TV just waits a fraction of a second longer to get all the information into a memory chip and then processes it before sending it to display.

The biggest difference between 720p and 1080i, besides the resolution, is that 720p is broadcast at 60 full frames (whole images) per second and 1080i is broadcast at only 60 FIELDS (half images) per second or 30 full frames per second.  720p is smoother as a result of images being refreshed twice as fast as 1080i.  This is because 720 has only 44% the number of pixels as 1080 and they use the same equipment to transmit the broadcast for both of them.  You either get twice as many pixels half as fast (1080i) or half as many pixels twtice as fast (720p).

The reason we have interlaced transmission is because of how the older Cathode Ray Tube (CRT) TV's used to work.  The screen was painted with a material called Phosphor which gave off light when hit with an electron beam.  The electron beam was aimed at the screen with magnetic and electric fields and was moved across the screen like a typewritter (left to right, returned to the left and down, left to right, etc.)  In the early days, by scanning the whole image, the Phosphors at the top were faded already by the time the electron beam worked its way to the bottom of the screen.  They solved that problem by showing half images and relied on peoples physiology to blend the images.

The only time to worry about interlaced is when they use a cheap camera which takes half images representing the even and odd lines where each field (half of a frame) will then represent two unique moments in time.  When the fields are merged and if there were fast moving objects (a car, soccer ball, etc.) in the picture, the combined picture would be jagged around the moving object.  

Jagged pictures from interlaced sources aren't a true problem anymore.  Consider movies shot on film.  The film is progressive by nature - meaning, even if they separated images in half there will still be no jagged edges when the are recombined / deinterlaced by the TV.  Also, more modern video cameras will take the whole image at once and interlace it for broadcast transmission, but your TV will put it back together again (unless it is a CRT).