Difference between Interlaced & progressive scanning

1. Interlaced scanning
Interlaced scan-based images use techniques developed for CRT (Cathode Ray
Tube) TV monitor displays, made up of 576 visible horizontal lines across a
standard TV screen. Interlacing divides these into odd and even lines and then
alternately refreshes them at 30 frames per second. The slight delay between odd
and even line refreshes creates some distortion or 'jaggedness'. This is because only
half the lines keep up with the moving image while the other half waits to be
refreshed.
Interlaced scanning has served the analog camera, television and VHS video world
very well for many years, and is still the most suitable for certain applications.
However, now that display technology is changing with the advent of Liquid
Crystal Display (LCD), Thin Film Transistor (TFT)-based monitors, DVDs and
digital cameras, an alternative method of bringing the image to the screen, known
as progressive scanning, has been created.
There are two dominant interlaced scan systems used in the world today: NTSC
and PAL.
NTSC is based on a 525-line, 60 fields/30 frames-per-second at 60Hz system for
transmission and display of video images. This is an interlaced system in which
each frame is scanned in two fields of 262 lines, which is then combined to display
a frame of video with 525 scan lines. NTSC is the official analog video standard in
the U.S., Canada, Mexico, some parts of Central and South America, Japan,
Taiwan, and Korea.
PAL is the dominant format in the World for analog television broadcasting and
video display and is based on a 625 line, 50 field/25 frames a second, 50HZ
system. The signal is interlaced, like NTSC, into two fields, composed of 312 lines
each. Several distinguishing features are one: A better overall picture than NTSC
because of the increased amount of scan lines. Two: Since color was part of the
standard from the beginning, color consistency between stations and TVs are much
better. In addition, PAL has a frame rate closer to that of film. PAL has 25 frames
per second rate, while film has a frame rate of 24 frames per second. Countries on
the PAL system include the U.K., Germany, Spain, Portugal, Italy, China, India,
most of Africa, and the Middle East.

2. Progressive scanning
Progressive scan differs from interlaced scan in that the image is displayed on a
screen by scanning each line (or row of pixels) in a sequential order rather than an
alternate order, as is done with interlaced scan. In other words, in progressive scan,
the image lines (or pixel rows) are scanned in numerical order (1,2,3) down the
screen from top to bottom, instead of in an alternate order (lines or rows 1,3,5,
etc... followed by lines or rows 2,4,6). By progressively scanning the image onto a
screen every 60th of a second rather than "interlacing" alternate lines every 30th of
a second, a smoother, more detailed, image can be produced on the screen that is
perfectly suited for viewing fine details, such as text, and is also less susceptible to
interlace flicker. The primary intent of progressive scan is to refresh the screen
more often.
Conversion of interlaced to progressive scan:
Converting interlaced to progressive scan requires different processing compared
with line doubling just to increase the number of scan lines per frame for large
screens. To convert to progressive scan and preserve both detail and fullness,
material from two consecutive fields should be combined for "stationary" subject
matter. For "moving" subject matter a needed portion of an even line should be a
blend of the odd lines immediately before and after, and vice versa.
A bit of history:
Up until the late 1980's, flicker on computer screens was very noticeable since
single scan line details made up a much larger portion of screen content. Also with
memory as a limiting factor, consumer PC's only had about 240 scan lines of
picture information which incidentally hid most of the flicker. The "regular VGA"
standard was based on NTSC, exactly twice the scan rate using the same 525 scan
lines per frame and progressive scan, with 480 scan lines holding the picture and
with up to 640 details on a scan line.

3. As larger TV screens were developed, more viewers started noticing the flicker due
to the fading phosphors when the electron beam visited any given spot on the
screen only once every 1/30'th of a second. When you "see the scan lines" you are
really seeing the even gaps between the odd scan lines or vice versa, as the
phosphors fade between refreshes. On small screens in the early days of TV, the
electron beam was thicker than 1/480'th the screen height so these gaps were not as
noticeable.
Progressive scan vs. interlaced video
Today, two different techniques are available to render the video: interlaced scanning and
progressive scanning. Which technique is selected will depend on the application and purpose of
the video system, and particularly whether the system is required to capture moving objects and
to allow viewing of details within a moving image.
Interlaced scanning
Interlaced scan-based images use techniques developed for Cathode Ray Tube (CRT)-based TV
monitor displays, made up of 576 visible horizontal lines across a standard TV screen.
Interlacing divides these into odd and even lines and then alternately refreshes them at 30 frames
per second. The slight delay between odd and even line refreshes creates some distortion or
'jaggedness'. This is because only half the lines keeps up with the moving image while the other
half waits to be refreshed.
The effects of interlacing can be somewhat compensated for by using de-interlacing. De-
interlacing is the process of converting interlaced video into a non-interlaced form, by
eliminating some jaggedness from the video for better viewing. This process is also called line
doubling. Some network video products, such as Axis video servers, integrate a de-interlace filter
which improves image quality in the highest resolution (4CIF). This feature eliminates the
motion blur problems caused by the analog video signal from the analog camera.

4. Interlaced scanning has served the analog camera, television and VHS video world very well for
many years, and is still the most suitable for certain applications. However, now that display
technology is changing with the advent of Liquid Crystal Display (LCD), Thin Film Transistor
(TFT)-based monitors, DVDs and digital cameras, an alternative method of bringing the image
to the screen, known as progressive scanning, has been created.
Progressive scanning
Progressive scanning, as opposed to interlaced, scans the entire picture line
by line every sixteenth of a second. In other words, captured images are not
split into separate fields like in interlaced scanning. Computer monitors do
not need interlace to show the picture on the screen. It puts them on one line
at a time in perfect order i.e. 1, 2, 3, 4, 5, 6, 7 etc. so there is virtually no
"flickering" effect. As such, in a surveillance application, it can be critical in
viewing detail within a moving image such as a person running away.
However, a high quality monitor is required to get the best out of this type of scan.
Example: Capturing moving objects
When a camera captures a moving object, the sharpness of the frozen image will depend on the
technology used. Compare these JPEG images, captured by three different cameras using
progressive scan, 4CIF interlaced scan and 2CIF respectively.
Please note the following:
All image systems produce a clear image of the background
Jagged edges from motion with interlaced scan
Motion blur caused by the lack of resolution in the 2CIF sample
Only progressive scan makes it possible to identify the driver
Interlaced scan 2CIF (with 'line
Progressive scan
doubling')
Used in: Analog CCTV
Used in: Axis network cameras
cameras Used in: DVRs
such as AXIS 210
View Full size 640x480
View Full size 704x576 View Full size 704x576

5. Progressive scan details: Interlaced scan details: 2CIF details:
Note: In these examples, the cameras have been using the same lens. The car has been driving at
20 km/h (15 mph) using cruise control.