Apple Final Cut Pro 6 - Manual (Page 1867)

384 Part V 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.
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.