High Definition TV (HDTV) transmits pictures in 720p (720 lines of 1280 dots) or 1080i (1080 lines of 1920 dots). Old TVs are “Standard Definition” (SD) and support 480i (480 lines of 720 dots). Broadcast and Cable TV transmit both SD and HDTV signals. DVD is SD but Blu-Ray is HD.
The “i” in 1080i stands for “interlaced”. This is a method of picture transmission designed for the old fat tube type (CRT) TV sets of the 1950s. Today all new sets are flat panel LCD or Plasma (with a few expensive OLED sets), and an interlaced signal makes no sense. So although the TV signal may be 1080i, the TV set is 1080p where the “p” stands for Progressive but just means the set operates normally like a computer monitor. A 1080i signal can be displayed on a 1080p set with a simple internal conversion.
There is very little technical different between a computer monitor and a flat panel TV set. You can hook any modern TV up to your computer, and you can display TV on your computer screen. However, by convention computer monitors typically have slightly different resolutions than the TV standard. For example, since the IBM monitors released in 1987, the closest computer resolution to 720 lines has been 768 lines. So a lot of computer monitors today have resolutions of 768 rows of 1280 dots, and when they display 720p TV signals in full screen there are 24 rows at the bottom and 24 rows at the top of the screen that remain blank. Alternately, some monitors have 768 rows but maintain the TV aspect ratio by adding proportionately more dots per line, producing a resolution of 1366x768.
The next higher resolution for screens is 1080p (1920x1080), but some computer monitors add 120 additional lines to produce 1920x1200.
Although HDTV is “widescreen” and all TV shows are produced today for this screen size, movies are often released for an even wider theater screen. When these movies are released on Blu-Ray, they preserve the original theatrical shape by reducing the number of rows in the picture. Although the TV may have 1080 rows, the picture will have a resolution of 1920x800. On a 1920x1200 computer monitor, this means that there will be black bars above and below the movie with 200 unused rows in each bar.
Warning: Some TV vendors will advertise that their sets support 1080p. The fine print is that they will display a 1080 signal, but the TV set is actually a native 1366x768 monitor. Read the specifications, and if you want a real 1080p set make sure that the actual native screen resolution is 1920x1080 (or 1920x1200).
De-interlace and Pulldown
Movies are shot at 24 frames per second. TV sets show their picture at 30 frames per second. So a movie has to be “stretched out” to take the existing 24 frames and add 6 additional frames to each second. This could be done electronically today with computer technology. Back in the 1950s, however, it was done using the peculiarities of picture tube TVs and the “interlaced” signal.
You can still get files and DVDs with Standard Definition “interlaced” data. If you display it on a flat panel TV set directly, you will see “jagged lines”. When there is movement in the picture, every other line from the 480 line picture may be slightly left or right of the previous line. Modern electronics can smooth this out if you turn on an option to deinterlace the data. This is also sometimes know by the unhelpful technical term of “inverse telecine”.