3.8 What is the difference between interlaced and progressive video?
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This article is from the DVD Formats FAQ, by jtfrog@usa.net (Jim Taylor) with numerous contributions by others.
3.8 What is the difference between interlaced and progressive video?
There are basically two ways to display video: interlaced scan or
progressive scan. Progressive scan, used in computer monitors and digital
television, displays all the horizontal lines of a picture at one time, as
a single frame. Interlaced scan, used in standard television formats NTSC,
PAL, and SECAM, displays only half of the horizontal lines at a time (the
first field, containing the odd-numbered lines, is displayed, followed by
the second field, containing the even-numbered lines). Interlacing relies
on the persistence of vision characteristic of our eyes (which may only be
psychological, not physical), as well as the phosphor persistence of the TV
tube to blur the fields together into a seemingly single picture. The
advantage of interlaced video is that a high refresh rate (50 or 60 Hz) can
be achieved with only half the amount of data. The disadvantage is that the
horizontal resolution is essentially cut in half because the video must be
filtered to avoid flicker and other artifacts.
It may help to understand the difference by considering how the source
images are captured. A film camera shoots 24 frames per second, while a
video camera alternately scans fields of odd and even lines in 1/60 of a
second intervals. (Unlike projected film, which shows the entire frame in
an instant, most progressive-scan displays trace a series of lines from top
to bottom, but the end result is about the same.)
DVD is specifically designed to be displayed on interlaced-scan displays,
which covers 99.9% of the 1 billion TVs worldwide. However, most DVD
content comes from film, which is inherently progressive. To make film
content work in interlaced form, the video from each film frame is split
into two video fields -240 lines in one field, and 240 lines in the
other- and encoded as separate fields in the MPEG-2 stream. Another
complication is that film runs at 24 frames/second, while TV runs at 30
frames (60 fields) per second for NTSC or 25 frames (50 fields) per second
for PAL and SECAM. For PAL/SECAM display, the simple solution is to show
the film frames at 25/second, which is a 4% speedup, and speed up the audio
to match. For NTSC display, the solution is to spread 24 frames across 60
fields by alternating the display of the first film frame for 2 video
fields and the next film frame for 3 video fields. This is called 2-3
pulldown. The sequence works as shown below, where A-D represent film
frames; A1, A2, B1, etc. represent the separation of each film frame into
two video fields; and 1-5 represent the final video frames.
Film frames: | A | B | C | D |
Video fields: |A1 A2|B1 B2|B1 C2|C1 D2|D1 D2|
Video frames: | 1 | 2 | 3 | 4 | 5 |
For MPEG-2 encoding, repeated fields (B1 and D2) are not actually stored
twice. Instead, a flag is set to tell the decoder to repeat the field. (The
apparently inverted order of C2-C1 and D2-D1 are because of the
requirement that top and bottom fields alternate.) MPEG-2 also has a flag
to indicate when a frame is progressive (that the two fields come from the
same instant in time). For film content, the progressive_frame flag should
be true for every frame. See 3.4 for more MPEG-2 details.
As you can see, there are a couple of problems: 1) some film frames are
shown for a longer period of time than others, causing judder, or
jerkiness, that shows up especially in smooth pans; and 2) if you freeze
the video on the third or fourth video frame when there is motion in the
picture you will see two separate images combined in a flickering mess.
Most DVD players avoid the second problem, although some allow you to
freeze on flicker-frames. (This is what the frame/field still option in the
player's setup menu refers to.)
Most DVD players are hooked up to interlaced TVs, so there's not much that
can be done about artifacts from film conversion. However, see 1.40 for
information about progressive DVD players.
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