Apple Final Cut Express 4 - Manual (Page 1066)

1066 Part XIII Appendixes
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, while 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, 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.
Some video formats use rectangular pixels to reduce the amount of information
stored on tape. These pixels are not representing a square area, but a wider, rectangular
portion of each video line. This results in a 4-to-3 reduction in the amount of
information recorded on tape.
Video and image editing programs like Final Cut Express 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.