From Interlaced to Progressive Scanning
With the advent of desktop computers, it was discovered that using a traditional TV for the display of computer images didn’t yield good results, especially with text. This was due to the effects of interlaced scanning. In order to produce a more precise way of displaying images on a computer monitor, progressive scan technology was developed.
What Is Interlaced Scan?
Traditional analog TV broadcasts (along with older cable/satellite boxes, VCRs, and DVDs) are displayed on a TV screen using a technology known as interlaced scanning. There were two main interlaced scan systems in use: NTSC and PAL. NTSC is based on a system of 525-lines, 60 fields, and 30 frames-per-second (fps) at 60Hz. Each frame is split into two fields of 262 lines. The lines are sent alternately and then displayed as an interlaced image. Countries that use NTSC include the U.S., Canada, Mexico, some parts of Central and South America, Japan, Taiwan, and Korea. PAL is based on a system of 625 lines, 50 fields, and 25fps at 50Hz. Like NTSC, the signal is interlaced into two fields composed of 312 lines each. PAL has a frame rate closer to that of film (film content is based on a frame rate of 24fps). Countries that use the PAL system include the U.K., Germany, Spain, Portugal, Italy, China, India, most of Africa, and the Middle East.
What Is Progressive Scan?
Progressive scan differs from interlaced scan in that the image is displayed on a screen by scanning each line (or row of pixels) in sequential order from top to bottom. By progressively scanning the image onto a screen in one sweep (rather than building the image by combining two halves), a smoother, more detailed image can be displayed that is better suited for viewing text and motion. Progressive scanning is also less susceptible to flicker.
Line Doubling
With the advent of high definition LCD TVs and video projectors, the resolution produced by traditional TV, VCR, and DVD sources were not reproduced very well by the interlaced scanning method. To compensate, in addition to progressive scan, TV makers also introduced the concept of line doubling. A TV with line doubling creates “lines between lines”, which combine characteristics of the line above with the line below in order to give the appearance of a higher resolution image. These new lines are then added to the original line structure, and all the lines are then progressively scanned on the TV screen. The drawback of line doubling is that it can result in motion artifacts because the newly created lines also have to move with the action in the image. To smooth out the images, additional video processing is usually employed.
Transferring Film to Video
Although progressive scan and line doubling attempt to address the display flaws of interlaced video images, there is still another problem preventing the accurate display of movies originally shot on film: video frame rate. For PAL-based source devices and TVs, this is not a big issue as the PAL frame rate (25fps) and film frame rate (24fps) are very close, so minimal correction is needed for showing film accurately on a PAL TV screen. However, that is not the case with NTSC as it produces and displays video at 30fps. If you have tried to transfer an 8mm home film movie by videotaping the movie screen using a camcorder, you will notice this issue. Since the frame movement does not match up, this produces noticeable flicker when the film is transferred to video without any adjustment. When a film is transferred to DVD (or videotape) in an NTSC-based system, the differing frame rates of film and video must be reconciled. To eliminate flicker, the film frame rate is “stretched” by a formula that more closely matches the film frame rate to the video frame rate.
Progressive Scan and 3:2 Pulldown
In order to see a film in its most accurate state, it should be displayed at 24 frames-per-second using a projector or TV that can display the frame rate natively. To do this in an NTSC-based system, the source needs to have 3:2 pulldown detection. That way, it can reverse the 3:2 pulldown process to transfer the video from film so that it can be output it in its original 24fps format with progressive scan. This is accomplished by a DVD (or Blu-ray/Ultra HD Blu-ray) player equipped with a special type of MPEG decoder, combined with a deinterlacer that reads the 3:2 pulldown interlaced video signal off of the DVD and extracts the proper film frames from the video frames. The frames are then progressively scanned, artifact corrections are made, and the new video signal is sent out through a progressive scan-enabled component video or HDMI connection to a compatible TV or video projector. If your DVD player has progressive scan without 3:2 pulldown detection, it will still send a smoother image than interlaced video. The player will read the interlaced image of the DVD, process a progressive image of the signal, and pass that on to a TV or video projector within a 30fps system.
What You Need to Access Progressive Scan
Both the source component (DVD player, HD cable, satellite box, antenna, etc.) and the TV or video projector must be progressive scan capable. The source also needs to have a progressive scan-enabled component video output, or a DVI or HDMI output that allows the transfer of progressive scan images. If a video is placed on a DVD in an interlaced form, progressive scan can be applied by the DVD player as one of its playback options. Composite and S-Video connections do not transfer progressive scan video images.
