Apple Final Cut Pro 5 - Manual (Page 1708)

Appendix A Video Formats 357
V
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.
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
compensation.
Some video formats use rectangular pixels to reduce the amount of information stored
on tape. For example, DVCPRO HD effectively records 1280 pixels per line (when using
the 720p format), but to save space on tape, the intensity of every 1.5 pixels is averaged
together and only 960 pixels are recorded. These pixels are not representing a square
area, but a wider, rectangular portion of each video line. This results in a 2/3 reduction
in the amount of information recorded on tape.
Video and image editing programs like Final Cut Pro and Photoshop must compensate
for these rectangular pixels so they appear correctly on a computer display. However,
there are several different pixel aspect ratios in use, and there is unfortunately no single
accepted standard in the industry. The exact aspect ratio used may vary slightly from
one software application to another, as well as among different third-party video
interfaces.