Adobe 65021048 User Manual page 131

ADOBE PREMIERE PRO CS3 125
User Guide
Interlaced video, noninterlaced video, and progressive scanning
Interlacing is a technique developed for transmitting standard-resolution television signals using limited
bandwidth. In an interlaced system, only half the number of horizontal lines for each frame of video are transmitted
at a time. But because of the speed of transmission, the afterglow inherent in cathode ray tubes, and the persistence
of vision, the viewer perceives each frame in full resolution. All of the analog television standards use interlacing.
Digital television standards include both interlaced and noninterlaced varieties. Typically, interlaced signals are
generated from interlaced scanning while noninterlaced signals are generated from progressive scanning.
Each interlaced video frame consists of two fields. Each field contains half the number of horizontal lines in the
frame; the upper field (or Field 1) contains the odd-numbered lines, and the lower field (or Field 2) contains the even-
numbered lines. An interlaced video monitor displays each frame by first drawing all of the lines in one field and
then drawing all of the lines in the other field. Field order specifies which field is drawn first. In NTSC video, new
fields are drawn to the screen approximately 60 times per second, corresponding to a frame rate of approximately
30 frames per second.
A
B
C
Interlaced scanning of interlaced video fields compared with progressive scanning of noninterlaced video frame.
A. For interlaced video, entire upper field is drawn to screen first, from top to bottom, in one pass. B. Next, entire lower field is drawn to
screen, from top to bottom, in one pass. C. For noninterlaced video, entire frame is drawn to screen, from top to bottom, in one pass.
Noninterlaced video frames aren't separated into fields. A progressive-scan monitor displays a noninterlaced video
frame by drawing all the horizontal lines, from top to bottom, in one pass. Computer monitors are almost all
progressive-scan monitors, and most video displayed on computer monitors is noninterlaced.
The terms progressive and noninterlaced are thus closely related and are often used interchangeably, but progressive
refers to the recording or drawing of the scan lines by a camera or monitor, whereas noninterlaced refers to the fact
that the video data itself isn't separated into fields. For example, it's possible with some modern cameras to use
progressive scanning to record two simultaneous fields per frame of interlaced video.
Create interlaced or non-interlaced clips
Ordinarily, individual interlaced fields aren't apparent to a viewer. However, playing a clip in slow-motion, creating
a freeze frame, or exporting a field as a still image can make a single field, and its missing lines, distinguishable. In
these circumstances, it's sometimes preferable to deinterlace the image—that is, replace pairs of consecutive inter-
laced fields with single non-interlaced frames. These new non-interlaced frames can be generated from the fields in
one or two source frames.
April 1, 2008