Apple Final Cut Pro 7 - Manual (Page 1901)

720 x 486 Versus 720 x 480
One issue that comes up during post-production is the subtle difference between NTSC
SD formats that use 486 lines per frame (such as Digital Betacam, D-1, and D-5) and
formats that use 480 lines per frame (such as DV, DVCPRO, and DVD). Why is there this
subtle difference? The reason is simple: 480 is divisible by 16, and 486 isn’t. Divisibility by
16 is convenient for certain block-based codecs because each frame is broken into 16 x 16
pixel blocks (known as macroblocks) during compression.
The only time this should be a concern is when you are converting between a 486-line
format like Digital Betacam and a 480-line format like DVD. However, the extra six lines
are typically not visible on an analog television.
Pixel Aspect Ratio
A pixel usually refers to a physical picture element that emanates light on a video display.
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, and another may have a value of 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 BT.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. In both NTSC and PAL video, the frame displayed has an aspect ratio of 4:3, yet
neither 720 x 486 nor 720 x 576 constitutes a 4:3 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. Most SD video devices actually
use 704 or 708 pixels for picture information but stretch these pixels to 720 when recording
to tape.
None of this was obvious in the days of linear editing, when video was simply copied
from one tape to another, because the video equipment always compensated
automatically. However, as people began using computers to work with video, digital
video captured to the computer looked distorted (squashed vertically or stretched
horizontally) because the computer displayed the pixels as squares, without compensating.
1901 Appendix B Video Formats