Progressive scan vs. interlaced video

Interlaced scan

Interlaced scan is one of two methods used for "painting" an image on a television screen (the other being progressive scan). Designed for the analog NTSC television system, interlaced scanning uses two fields to create a frame. One field contains all the odd lines in the image, the other contains all the even lines of the image. A television scans 60 fields every second (30 odd and 30 even). These two sets of 30 fields are combined to create a full frame every 1/30th of a second, resulting in a display of 30 frames per second. Drawbacks to interlaced scanning compared to progressive scanning include flicker, lower resolution and quality issues.

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.

 

progressive scan

Progressive scan (also known as known as 480p) is one of two methods used for "painting" an image on a television screen (the other being interlaced scan), where the lines are drawn in one at a time in sequential order. The entire single frame image is painted every 1/60th of a second, allowing for twice the detail to be sent in the same amount of time used in interlaced systems. Progressive scan is a method used in today's CRTs, computer monitors and high-end television displays. Progressive scanning results in a more detailed image on the screen and is also less susceptible to the flicker commonly associated with interlaced scanning.

Most television displays that are capable of progressive scanning can also perform deinterlacing, which enables interlaced video to be viewed on progressive scan sets. Progressive scan is now the standard of choice for both DVD video and DTV. To use and view progressive scanning technology the source (DVD player, High-definition cable or satellite for example) and the display must both be progressive scan compatible.

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.

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 Used in: Axis network cameras such as AXIS 210 View Full size 640x480   Progressive scan details:

Interlaced scan Used in: Analog CCTV cameras View Full size 704x576 Interlaced scan details:

2CIF (with 'line doubling') Used in: DVRs View Full size 704x576 2CIF details:

4:3 Aspect Ratio Vs. 16:9 Wide screen Wide screen TVs and projectors feature a width-to-height ratio similar to movie theater screens, providing cinema-style entertainment at home. Also enables full-screen viewing of high-definition broadcasts and DVDs produced in 16:9 format .

Progressive Vs. Interlaced Scanning Standard TV use interlaced scanning to produce a picture. Interlaced scanning combines two fields to generate a picture of 525 scan lines (480 of which are displayed). Your television projects an interlaced image by first scanning the 240 odd-numbered lines of one field (in 1/60th of a second), followed by the 240 even-numbered lines of the other field (in 1/60th of a second). So, it takes two fields to build one frame of video. The result or progressive scan is a smoother and sharper picture, with high resolution and no motion artifacts.