Difference between Interlaced & progressive scanning
Interlaced scanningInterlaced scan-based images use techniques developed for CRT (Cathode RayTube) TV monitor displays, made up of 576 visible horizontal lines across astandard TV screen. Interlacing divides these into odd and even lines and thenalternately refreshes them at 30 frames per second. The slight delay between oddand even line refreshes creates some distortion or jaggedness. This is because onlyhalf the lines keep up with the moving image while the other half waits to berefreshed.Interlaced scanning has served the analog camera, television and VHS video worldvery well for many years, and is still the most suitable for certain applications.However, now that display technology is changing with the advent of LiquidCrystal Display (LCD), Thin Film Transistor (TFT)-based monitors, DVDs anddigital cameras, an alternative method of bringing the image to the screen, knownas progressive scanning, has been created.There are two dominant interlaced scan systems used in the world today: NTSCand PAL.NTSC is based on a 525-line, 60 fields/30 frames-per-second at 60Hz system fortransmission and display of video images. This is an interlaced system in whicheach frame is scanned in two fields of 262 lines, which is then combined to displaya frame of video with 525 scan lines. NTSC is the official analog video standard inthe 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 andvideo display and is based on a 625 line, 50 field/25 frames a second, 50HZsystem. The signal is interlaced, like NTSC, into two fields, composed of 312 lineseach. Several distinguishing features are one: A better overall picture than NTSCbecause of the increased amount of scan lines. Two: Since color was part of thestandard from the beginning, color consistency between stations and TVs are muchbetter. In addition, PAL has a frame rate closer to that of film. PAL has 25 framesper second rate, while film has a frame rate of 24 frames per second. Countries onthe PAL system include the U.K., Germany, Spain, Portugal, Italy, China, India,most of Africa, and the Middle East.
Progressive scanningProgressive scan differs from interlaced scan in that the image is displayed on ascreen by scanning each line (or row of pixels) in a sequential order rather than analternate 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 thescreen 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 ascreen every 60th of a second rather than "interlacing" alternate lines every 30th ofa second, a smoother, more detailed, image can be produced on the screen that isperfectly suited for viewing fine details, such as text, and is also less susceptible tointerlace flicker. The primary intent of progressive scan is to refresh the screenmore often.Conversion of interlaced to progressive scan:Converting interlaced to progressive scan requires different processing comparedwith line doubling just to increase the number of scan lines per frame for largescreens. To convert to progressive scan and preserve both detail and fullness,material from two consecutive fields should be combined for "stationary" subjectmatter. For "moving" subject matter a needed portion of an even line should be ablend of the odd lines immediately before and after, and vice versa.A bit of history:Up until the late 1980s, flicker on computer screens was very noticeable sincesingle scan line details made up a much larger portion of screen content. Also withmemory as a limiting factor, consumer PCs only had about 240 scan lines ofpicture 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 scanlines per frame and progressive scan, with 480 scan lines holding the picture andwith up to 640 details on a scan line.
As larger TV screens were developed, more viewers started noticing the flicker dueto the fading phosphors when the electron beam visited any given spot on thescreen only once every 1/30th of a second. When you "see the scan lines" you arereally seeing the even gaps between the odd scan lines or vice versa, as thephosphors fade between refreshes. On small screens in the early days of TV, theelectron beam was thicker than 1/480th the screen height so these gaps were not asnoticeable.Progressive scan vs. interlaced videoToday, two different techniques are available to render the video: interlaced scanning andprogressive scanning. Which technique is selected will depend on the application and purpose ofthe video system, and particularly whether the system is required to capture moving objects andto allow viewing of details within a moving image.Interlaced scanningInterlaced scan-based images use techniques developed for Cathode Ray Tube (CRT)-based TVmonitor 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 framesper second. The slight delay between odd and even line refreshes creates some distortion orjaggedness. This is because only half the lines keeps up with the moving image while the otherhalf 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, byeliminating some jaggedness from the video for better viewing. This process is also called linedoubling. Some network video products, such as Axis video servers, integrate a de-interlace filterwhich improves image quality in the highest resolution (4CIF). This feature eliminates themotion blur problems caused by the analog video signal from the analog camera.
Interlaced scanning has served the analog camera, television and VHS video world very well formany years, and is still the most suitable for certain applications. However, now that displaytechnology 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 imageto the screen, known as progressive scanning, has been created.Progressive scanningProgressive scanning, as opposed to interlaced, scans the entire picture lineby line every sixteenth of a second. In other words, captured images are notsplit into separate fields like in interlaced scanning. Computer monitors donot need interlace to show the picture on the screen. It puts them on one lineat 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 inviewing 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 objectsWhen a camera captures a moving object, the sharpness of the frozen image will depend on thetechnology used. Compare these JPEG images, captured by three different cameras usingprogressive 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
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 at20 km/h (15 mph) using cruise control.