For 525 line analog TV (NTSC) approximately 480 of the scan lines contain picture content. All of such (480 or so) lines are part of the presentation except that many CRT TV sets are unable to show the first few and last few without severe geometric distortion. Digital 480i and 480p TV signals and DVD's made from the aforementioned analog video utilize exactly 480 of those scan lines for picture content.
Relative to the source material as already recorded, the 480 active scan lines yield 480 lines of resolution.
For the 480i analog video as broadcast (NTSC), each scan line can be subdivided into about 530 equal parts (pixels) of which about 440 contain picture information. The unviewable pixels represent the part of the scan line drawn (more correctly blacked out or blanked) when the beam has to be returned to the left side of the screen (horizontal retrace interval). Formatting information including for black reference (black pedestal) and color synchronizing (color burst) occupy this time. Many more pixels per scan line could be represented using higher video frequencies exceeding NTSC broadcast allotments and the standard is 720 visible pixels for DVD. TV sets may or may not reproduce dots that small and also the first few and last few pixels as displayed on CRT's may take on irregular spacing so as to result in distortion of the extreme left and right edges of the picture.
Video resolution is traditionally measured across the largest circle that fits in the space being referred to. For 4:3 DVD pictures the 720 pixels translate into 540 lines of resolution horizontally. For 16:9 pictures the same 720 pixels give a horizontal resolution of 405. For VHS tape, spots can't be reproduced as narrow, and only about 240 lines of horizontal resolution result from the same analog video scan line format.
Overscan is used on CRT's to, among other things, hide the distorted edges.
Extra information for formatting, closed captions, etc. also occupy bytes between the rows of picture content pixels and between fields/frames of pixels in digital video and these blocks of information are sometimes called horizontal retrace intervals and vertical retrace intervals, respectively. The vertical retrace interval for 480i digital video is not exactly 22-1/2 rows of pixels worth of data (525-480, half of that after the odd field, half after the even field).
DVD and HDTV are not stored/transmitted exactly pixel by pixel. Instead compression is used to save space, for example 5@blue instead of blueblueblueblueblue (oversimplified example). So digital video frames as stored on the DVD or broadcast do vary in size. If a digital video signal is displayed on a CRT, real horizontal and vertical retrace intervals are created and inserted by the TV set. When analog video is displayed on an LCD or other digital display, the scan line is chopped up into however many pixels are needed. It is not out of the question for what would be half of one analog pixel and half of the next to end up as one of the digital pixels so produced. Thus there can be some loss of horizontal resolution relative to the subject matter.
The regular (non-blue-ray) DVD standard allows for 576 scan lines for PAL programs. Other standards could exist too, including HDTV formats, but the limit is the total number of bytes. Much more resolution per frame would result in a very small playing time per disk on regular DVD.
Video hints:
Television and Video Resolution
