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Interlaced Versus Progressive
As
mentioned, video images are displayed one line at a time, in rapid
succession. There are two ways this is done. With progressive scanning
(indicated by “p” following the line resolution, e.g., 720p), the
images are displayed a line at a time from top to bottom, like on a
regular TV set—line one, followed by line two, followed by line three
and so on through line 720. With interlaced scanning, the images are
scanned sequentially top to bottom, but in two separate
fields—odd-numbered lines (line one, then line three, then line five
and so on, until line 1079), followed by a second field consisting of
even-numbered lines (line two, line four and so on to line 1080).
At first glance, you’d think that 1080i provides better image
quality than 720p, since there are more lines and pixels. But 1080i is
an interlaced format, while 720p is a progressive-scan format. In HDV,
both look about equally good on screen. Progressive scanning does
provide smoother action and slow-motion effects. Some HD cameras use
720p, some 1080i, and some do both.
Frame Rates
Video
images are scanned onto the screen quickly, so our eyes see a whole
image instead of a series of lines moving down the screen. Standard DV
(SD) scans 60 interlaced images per second to match the frame rate of
standard TV sets. (An interlaced rate of 60 really is 30 complete
images per second, one odd-line half-image and one even-line half-image
being required to produce a complete image.) The most common
progressive scan rate is 30 fps, which also puts 30 complete images per
second on the screen.
Some cameras additionally offer a frame rate of 24 fps that matches the
rate at which film movies are displayed and can produce a more
“film-like” effect, especially when motion is involved, as with moving
subjects, pans and zooms.
HDV
The HDV format was introduced in
2003 by a group comprised of Canon, Sharp, Sony and Victor Company of
Japan (JVC) as a more affordable way to record high-definition video.
The video portion uses MPEG-2 video compression, which keeps file sizes
down while maintaining image quality, allowing HDV to be recorded on
standard DV tapes and allowing the same duration of video to be
recorded on a given length of tape as with SD video. The audio portion
of HDV uses MPEG-1 Audio Layer II compression, providing CD-quality
sound.
While standard DV uses intraframe compression (each frame is
compressed individually, sort of like JPEG digital still photos), HDV
uses both intraframe and interframe compression (after each frame is
compressed, all but a few key frames are further compressed). HDV 1080
has one key frame per 15 frames, HDV 720 has one key frame per six
frames. This results in much smaller file sizes and enables the same
amount of HDV video to be recorded on a given amount of tape as SD
video. (Yes, this added compression does reduce image quality, but
HDV’s inherently higher resolution more than offsets that, and HDV
looks much better on-screen than SD.)
Four-Step Program
There are four basic steps to creating a digital video: shooting, capturing, editing and presenting.
- Shooting is always done with a digital camcorder. Capturing—getting the images from camcorder to hard drive—is
generally done by plugging the camcorder into the computer via an
IEEE1394/FireWire connection. Editing—once the video is on the hard drive(s)—is done in the computer using special digital-editing software. The finished video is output for presentation or distribution, most often on DVD or the Internet.
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