Progressive Scan - What You Need to Know

Progressive Scan - The foundation of video processing

Interlaced Scan vs Progressive Scan
Interlaced Scan vs Progressive Scan. Images provided by Samsung

With its introduction in the mid-1990's, DVD become the core of the home theater revolution. With its vastly improved image quality over VHS and analog TV, DVD marked a huge advance in home entertainment. One of the main contributions of DVD was the employment of the progressive scan technique to improve TV viewing quality.

Interlaced Scan - The Foundation Of Traditional Video Display

Before we get into what progressive scan is and its importance in improving the TV viewing experience, it is important to understand the way traditional analog video images were displayed on a TV screen.

Analog TV signals, such as those from a local station, cable company, or VCR was displayed on a TV screen using a technology known as Interlaced Scan. There were two main interlaced scan systems in use: NTSC and PAL.

  • NTSC is based on a system of 525-lines, 60 fields/30 frames-per-second at 60Hz 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. Countries with an NTSC foundation are the U.S., Canada, Mexico, some parts of Central and South America, Japan, Taiwan, and Korea.
  • PAL 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. In addition, PAL has a frame rate closer to that of film. PAL has a frame rate of 25 per second, while film content is based on a frame rate of 24 frames per second. Countries that have a PAL system foundation include the U.K., Germany, Spain, Portugal, Italy, China, India, most of Africa, and the Middle East.

    What Progressive Scan Is

    With the advent of home and office desktop computers, it was discovered that using a traditional TV for the display of computer images did not yield good results, especially with text. This was due to the effect of interlaced scan technology. In order to produce a more pleasing and precise way of displaying images on a computer, progressive scan technology was developed.

    Progressive scan differs from an 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 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 fine details, such as text and motion is also less susceptible to interlace flicker.

    Seeing this technology as a way to improve the way we view images on a video screen, progressive scan technology was next applied to DVD.

    Line Doubling

    With the advent of large screen high definition Plasma, 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.

    Although there are many ways this can be applied, at its core, a TV with line doubling capability 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 television screen.

    However, the drawback with line doubling is that motion artifacts can result, as the newly created lines also have to move with the action in the image. To smooth out the images, additional video processing is usually required.

    3:2 Pulldown - 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 that prevents the accurate display of movies originally shot on film to be viewed properly on a TV. For PAL-based source devices and TVs, this is not a big issue as the PAL frame rate and film frame rate is very close, so minimal correction is needed for showing film accurately on a PAL TV screen. However, that is not the case with NTSC.

    The problem with NTSC is that films are generally shot at 24 frames per second and NTSC video is produced and displayed at 30 frames per second.

    This means that when a film is transferred to DVD (or videotape) in an NTSC-based system, the differing frame rates of film and video must be addressed. If you have ever tried to transfer an 8 or 16mm home movie by videotaping the movie screen as the movie is being shown, you will understand this issue. Since the movie frames are projected at 24 frames per second, and the camcorder is taping at 30 frames per second, the film images will show a severe flicker effect when you play your videotape back. The reason for this is that the frames on the screen are moving at a slower rate than the video frames in the camera, and since the frame movement does not match up, this produces the severe flicker effect when the film is transferred to video without any adjustment.

    In order eliminate flicker, when a film is transferred professionally to video (whether DVD, VHS, or another format), the film frame rate is "stretched" by a formula that more closely matches the film frame rate to the video frame rate.

    However, the question remains as to how to display this accurately on a TV.

    Progressive Scan and 3:2 Pulldown

    In order to see a film in its most correct state, it should be shown at 24 frames per second on either a projection or TV screen.

    In order to do this as accurately as possible in an NTSC-based system, the source, such as a DVD player needs to have 3:2 pulldown detection, reverse the 3:2 pulldown process that was used to put the video onto DVD, and output it in its original 24 frames per second format, while still being compatible with a 30 frames per second video display system..

    This is accomplished by a DVD player that is equipped with a special type of MPEG decoder, combined with what is referred to as a deinterlacer that reads the 3:2 pulldown interlaced video signal off the DVD and extracts the proper film frames from the video frames, progressively scans those frames, makes any artifact corrections, and then transfers this new video signal through a progressive scan-enabled component video (Y, Pb, Pr) or HDMI connection.

    If your DVD player has progressive scan without 3:2 pulldown detection, it will still produce a smoother image than traditional interlaced video, as the progressive scan DVD player will read the interlaced image of the DVD and process a progressive image of the signal and pass that on to a TV or video projector.

    However, if the DVD player has the addition of 3:2 pulldown detection, not only will your video display a smoother progressively scanned image, but you will experience the DVD film in as close a state as possible to what you would see coming from an actual film projector, except that it is still in the video domain.

    Progressive Scan and HDTV

    In addition to DVD, progressive scan is applied to DTV, HDTV, Blu-ray Disc, and TV broadcasting as well.

    For example, standard definition DTV is broadcast in 480p (the same characteristics as progressive scan DVD - 480 lines or pixel rows progressively scanned) and HDTV is broadcast at either 720p (720p lines or pixel rows progressively scanned) or 1080i (1,080 lines or pixel rows that are alternately scanned fields made up of 540 lines each). In order to receive these signals, you need an HDTV with either a built-in HDTV tuner or an external HD tuner, HD Cable, or Satellite box.

    What You Need To Access Progressive Scan

    In order to access progressive scan, both the source component, such as a DVD player, HD cable, or satellite box, and the TV, video display, or video projector need to be progressive scan capable (which all are if purchased 2009 or later), and the source device (DVD/Blu-ray Disc player, Cable/Satellite Box), needs to have a progressive scan-enabled component video output, or a  DVI (Digital Video Interface) or HDMI (High Definition Multi-media Interface) output that allows the transfer of standard and high-definition progressive scan images to a similarly equipped television.

    It is important to point out that standard Composite and S-Video connections do not transfer progressive scan video images. Also, If you hook up a progressive scan output to a non-progressive scan TV input, you will not get an image (this really only applies to most CRT TVs - all LCD, Plasma, and OLED TVs are progressive scan compatible).

    In order to view progressive scan with reverse 3:2 pulldown, either the DVD player or TV needs to have 3:2 pulldown detection (not a problem with anything purchased 2009 or later). The preference would be for the DVD player have the 3:2 pulldown detection and actually perform the reverse pulldown function, with a progressive scan capable television displaying the image as fed from the DVD player. There are menu options in both a progressive scan DVD player and a progressive scan capable (HDTV) television that will assist you setting up a progressive scan capable DVD player and television or video projector.

    The Bottom Line

    Progressive Scan is one of the technical foundations of improving the TV and home theater viewing experience. Since it was first implemented, things have evolved. DVD now coexists with Blu-ray, and HDTV is transitioning to 4K Ultra HD TV, and with that progressive scan has not only become a part of how images are displayed on a screen, but also provided an additional foundation for further video processing techniques, such as video upscaling.