HD

1034 Part XIII Appendixes
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 Express HD 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, nonsquare pixel image dimensions and
compensate on the computer display. Fortunately, major video and graphics applications
such as Photoshop, After Effects, Final Cut Express HD, 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 video tape 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
the next section, “Scanning Method.” For more details about frame rate, see
Appendix B, “Frame Rate and Timecode,” on page 1047.