DVD is the core of the home theater revolution. With its outstanding image quality and versatile hardware, the DVD phenomenon is unprecedented in home entertainment. One of the main contributions of DVD is the employment of the progressive scan option for television viewing. Progressive scan pushes the limits of standard resolution and provides a more film-like image display that is more pleasing for viewing DVDs on a television display. The following is an overview of progressive scan and what it means for the home theater experience.
Interlaced Scan - The Foundation Of Traditional Video Display
Before we get into the nature of progressive scan, it is important to understand the way traditional television images are displayed on a television screen. Analog television signals, such as those from your local TV station, cable company, or VCR are displayed on a television screen using a technology known as Interlaced Scan. Basically, there are two dominant interlaced scan systems in use in the world today: NTSC and PAL.
NTSC is based on a 525-line, 60 fields/30 frames-per-second at 60Hz system 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. NTSC is the official analog video standard in the U.S., Canada, Mexico, some parts of Central and South America, Japan, Taiwan, and Korea.
PAL is the dominant format in the World for analog television broadcasting and video display (sorry U.S.) and 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. Several distinguishing features are one: A better overall picture than NTSC because of the increased amount of scan lines. Two: Since color was part of the standard from the beginning, color consistency between stations and TVs are much better. In addition, PAL has a frame rate closer to that of film. PAL has a 25 frames per second rate, while film has a frame rate of 24 frames per second. Countries on the PAL system include the U.K., Germany, Spain, Portugal, Italy, China, India, most of Africa, and the Middle East.
The remainder of this article discusses the issue of progressive scan as it relates to the NTSC video system, which has special considerations with regards to the progressive scan issue.
The Development Progressive Scan
With the advent of home and office desktop computers, it was discovered that using a traditional television for the display of computer images did not yield good results, especially with text. This was due to the effect of interlaced scan. In order to produce a more pleasing and precise way of displaying images on a computer, progressive scan was developed.
Progressive scan differs from 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 every 60th of a second rather than "interlacing" alternate lines every 30th of a second, a smoother, more detailed, image can be produced on the screen that is perfectly suited for viewing fine details, such as text, and is also less susceptible to interlace flicker.
Seeing this technology as way to improve the way we view images on a television screen, progressive scan has now been applied to the display of DVD and certain types of HDTV images.
Continue on to Page 2: Progressive Scan Applications