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. This feature eliminates the motion 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 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
   

Progressive scan   Progressive scan details:

Interlaced scan Used in: Analog CCTV cameras     Interlaced scan details:

2CIF (with 'line doubling') Used in: DVRs     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.