Interlace, progressive, NTSC, PAL.....What does it all mean?!
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.
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.
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.
