Apple Final Cut Express HD - Manual (Page 1033)

Appendix A Video Formats 1033
XIII
Pixel Aspect Ratio
A pixel usually refers to a physical picture element on a video display that emanates
light. But a pixel is also a term for a sample of light intensity—a piece of data for storing
luma or chroma values. When stored on tape or on hard disk, the intensity of a pixel
has no inherent shape, height, or width; it is merely a data value. For example, one pixel
may have a value of 255, while another may be 150. The value of each pixel determines
the intensity of a corresponding point on a video display. In an ideal world, all pixels
would be captured and displayed as squares (equal height and width), but this is not
always the case.
The ITU-R 601 specification makes it possible to transmit either NTSC or PAL
information in a single signal. To achieve this goal, both NTSC and PAL video lines are
sampled 720 times. This results in either a 720 x 486 frame (NTSC) or a 720 x 576 frame
(PAL). In both NTSC and PAL, the frame displayed has an aspect ratio 4:3, and yet
neither 720 x 486 or 720 x 576 creates a 4:3 aspect ratio! The solution to this problem is
to display the pixels (the samples of light intensity) taller-than-wide, or wider-than-tall,
so that they fit into a 4:3 frame. This results in the concept of “rectangular pixels”—
pixels that must be stretched or squeezed to fit in the 4:3 frame. To further confuse
matters, most standard definition video devices actually use 704 or 708 pixels for
picture information, not all 720.
720 x 486 Versus 720 x 480
Another issue that comes up is the subtle difference between NTSC SD formats that use
486 lines per frame (Digital Betacam, D-1, D-5) and formats that use 480 lines per frame
(DV, DVCPRO, DVD). Why is there this subtle difference? The reason is simple: 480 is
divisible by 16, and 486 isn’t. Divisibility by 16 is important for any MPEG-like compression
codec, because each frame is broken into 16 x 16 pixel blocks (known as macroblocks)
during compression. All of the video formats that use 480 lines per frame are DV or MPEG-
based compressions, all of which use similar approaches to compression.
The only time this should be a concern is if you are converting between a 486-line
format like Digital Betacam and a 480-line format like DVD. However, the absence of 6
lines is hardly noticeable on an analog television, especially at the top and bottom,
which is usually masked anyway.