Interlacing is a technique developed for
transmitting 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. Because of the speed of transmission,
the afterglow of displays, 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, whereas
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.
View full size graphic
Interlaced scanning of interlaced video fields compared with
progressive scanning of noninterlaced video frame.
- A.
- For interlaced video, entire upper field (odd-numbered
lines) is drawn to screen first, from top to bottom, in one pass.
- B.
- Next,
entire lower field (even-numbered lines) is drawn to screen, from
top to bottom, in one pass.
- C.
- For noninterlaced
video, entire frame (all lines in counting order) 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
scanning 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.
Chris Pirazzi provides technical details of fields and interlacing
on his Lurker's Guide to Video website.
Trish and Chris Meyer provide a variety of materials about interlacing,
field order, field dominance, field rendering, and separating fields:
