Apple Final Cut Pro 7 - Manual (Page 1902)

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 1.33 pixels is averaged
together (a process known as subsampling) 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 4-to-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.
These days, the biggest challenge comes when exchanging graphics between applications
that use different pixel aspect ratios, or when using an application that does not support
rectangular pixels with one that does. The key to a simple workflow is to use applications
that can work at the native, non-square pixel image dimensions and compensate on the
computer display. Fortunately, major video and graphics applications such as Photoshop,
After Effects, Final Cut Pro, and DVD Studio Pro can work with graphics and video at native
resolutions. This way, you are always working with the exact pixel dimensions that you
will eventually output to videotape or DVD.
Frame Rate
The frame rate of any motion picture, whether film or video, defines how often pictures
are taken per second. The higher the frame rate, the more accurately you capture moments
in time and reduce flicker during playback. To achieve double the perceived frame rate
(flicker), film projectors actually double or triple the shutter speed, even though the same
frame is repeated two or three times, respectively. This is because a faster flicker creates
more convincing motion. Video uses a similar, although more complicated, technique
called interlacing. For more information about interlacing, see
“Scanning Method.” For
more details about frame rate, see “Frame Rate and Timecode.”
Scanning Method
A video frame is made of horizontal lines that are scanned from one side of a display to
the other. Progressive video scanning happens when each line of a video frame is scanned,
one after another. Interlaced scanning fills the entire frame with only half the lines, which
requires half the time, thus doubling the perceived frame rate and reducing flicker.
1902 Appendix B Video Formats