With Interlaced scanning, the lines of a frame are displayed in two passes - half in each pass. The NTSC system is composed of 525 lines. However, all 525 lines are not picture information. The "odd" field of picture information is made up of 241.5 lines. The next 21 lines are know as the vertical blanking interval. These lines were intentionally left blank in the original specifications so that the electron beam could rapidly move to the top of the screen. If you increase the screen control on the flyback, on the CRT, you will see this  "retrace" as a saw tooth pattern. The next 241.5 lines are for the "even" fields and again the next 21 lines are left blank for beam retrace. Simple math will tell you that the NTSC system is actually only 483 lines of vertical resolution - not 525. This reduces the bandwidth required to send a picture and was popular with broadcast video. It does result in a bit of flicker on the screen.

Some products claim that they are capable of progressive scan by adding a circuit called a "line doubler" to emulate progressive scanning. This is a simple memory device to combine the "odd" and "even" fields into one frame for progressive output. Unfortunately, it doesn't work as claimed. In video the odd and even fields don't match exactly. This reduces the clarity and sharpness of the picture - especially on live feeds.

With the true Progressive Scan method a whole picture frame is painted at once. It requires the higher bandwidth afforded by DTV, Tivo like devices and DVDs. It results in a much clearer, flicker free picture. This is most notable at the edges of the picture. Just think of it as the SVGA display as compare to the "EGA" display of the early computer monitors. Because the screen format is wide, it can display a wider picture similar to that of a movie theater's screen. Analog TV can only display "cropped" movies. In addition, DTV broadcasts will be in the Dolby surround digital format (5.1) that further enhances the TV viewing experiences.

To understand Progressive Scanning for DVD material, you must first understand how the image is laid on the DVD disk.  DVD standard states that the video image is encoded in 720 lines horizontally and 480 lines vertically.  This image is defined as a "frame", a full snap of a picture from the original film with the full 720x480 lines of image. 

Progressive scan imaging was designed to show both of the fields in one pass, doubling the scan frequency and eliminating the artifact of interlacing.  To do so required re-engineering the display device and the type of input connector used.

In the early days of DVD video, manufacturers and the committee setting the DVD standard known as the DVD Consortium bickered heavily over making progressively scanning players available to the public. Rumors circulated in the industry about brand new players that were destroyed due to pressures from the Hollywood studios and the Motion Pictures Association of America as they fought the ability of the home user to obtain high-resolution video quality and the lobbying groups high-hopes on the DIVX video delivery standard to combat piracy of their property. When the FCC stepped in and mandated DTV and HDTV to replace NTSC - the studio wars ended

What is Progressive scan DVD

In this instance, an explanation into how information is stored on the DVD disk is pertinent to the topic. There are currently two distinct types of video technologies available on the digital video MPEG-2 formats.

Video - Derived from video sources which are captured as interlaced images at 30 frames per second.  This is the source often used for television studio cameras and hand held camcorders. 

Film - Derived from 24 frames per second sources, which must be converted to 30 frames per second, using a technique called 3:2 pulldown, or converting the  24 fps. material into 30 fps.

MPEG-2 Decoding Methods

Video engineers have been studying the process of optimal image display on screen, but the best method used often depends on the source material.

The second artifact is often detected when there is motion in the image, often detected as jagged edges on diagonal lines.

MPEG-2 decoders in the PC uses two different methods for decoding these distinct sources, BOB and WEAVE.  BOB is used to line double video sources, while WEAVE is used to re-create original film frame.  The HTPC is exceptional with the WEAVE method due to it's use of buffering prior to it's output, and it's use of refreshing the image at specific intervals. 

The refreshing or optimally using the vertical frequency for 24fps. sources is done in a reverse telecine method by using WEAVE and displaying the resulting image specific times, the simplest illustrated below:

24fps. shown 3 times = 72Hz
24fps. shown 4 times = 96Hz
24fps. shown 5 times = 120Hz

Bear in mind that not many displays can refresh faster than 60Hz which is typical in most consumer digital TVs.  The higher refresh rate pays off with high-performance displays such as Presentation Monitors and Front Projectors. 

The other benefit of a being able to obtain full original frame from source is the ease of interpolating the sampled image at higher resolution, area where the HTPC excels.  Using the same type of interpolating done in image editing programs such as PhotoShop, the DVD information is upconverted with perfection to a higher resolution video, ideal for displaying to a big screen and digital displays (LCD, DLP, D-ILA) requiring pixel perfect resolution for optimal image presentation.

The advent of the High-Definition TV has given the videophile the minimum requirement for higher-frequency image.  The ability to mimic a PC's 640x480 (480p) as well as the equivalent of 960x540 (1080i) resolutions provides the ability capability of connecting the HTPC to today's Digital TVs.