Information about interlaced and progressive scan signals |
Interlaced scan vs. progressive scan image renderingThere are two basic ways in which video images can be displayed, or “rendered” on electronic display screens: interlaced scan and progressive scan. You have probably heard of these two methods of producing an image and may be wondering what the difference is between them, and how each of the technologies works. Likewise, you have probably heard of the resolution standards 480i, 480p, 720p, 1080i and 1080p. The “i” and “p” after the number actually stands for “interlaced” and “progressive” respectively, and since these are the resolutions you will be choosing between when you are thinking of buying a new television system (especially an HD TV), it is important to know what the difference is between, say, a 1080i resolution TV and a 1080p resolution TV to make sure that you choose the kind that is best for you. This Bamboo AV Advice article discusses interlaced scan and progressive scan and describes how each display method produces an image. Interlaced scan and progressive scan are then compared and the advantages and disadvantages of interlaced and progressive scan are summarised. What is an interlaced scan signal?An interlaced scan signal is way of displaying video and is a technique that has been used since the invention of the television, designed specifically for use with the old fashioned, large Cathode Ray Tube (CRT) televisions. Essentially, the interlace technique was invented to improve the picture quality of a video signal on a CRT device without consuming extra bandwidth. NTSC standard definition television broadcasts, VHS tapes and laserdisc signals are all encoded in the interlaced format. It is the way the majority of older televisions and some High Definition sets display an image. How does an interlaced scan signal produce an image?“Scanning” as a technology refers to how the image is “drawn” or “painted” on an electronic screen. The picture on the screen is made up of 480 horizontal lines, which is divided into two sets, odd numbers and even numbers. “Interlaced” means the lines that make up the picture on your TV screen are drawn in an alternating fashion. To display an image, the television sequentially draws all of the odd numbered lines from top to bottom and then proceeds to fill in the even lines. This occurs at a rate of 30 frames per second, meaning the entire process only takes around 1/30 of a second. What is a progressive scan signal?While this interlaced scanning worked well for older, analog televisions, it was not ideal for the new standard of electronic display devices that use a Liquid Crystal Display (LCD). A way to refresh the screen more frequently was desired. As a result, an alternative way of displaying images known as a progressive scan has been designed for those using modern devices such as Thin Film Transistor (TFT) monitors or digital cameras. Progressive scan means the lines that make up the TV picture are displayed all at once in sequence. Progressive scanning was originally called “sequential scanning” and is also referred to as “noninterlaced scanning”. How does a progressive scan signal produce an image?What differentiates a progressive scan from an interlaced scan is that the progressive scans system draws every line on the screen in just one pass, rather than in two passes. Each line is filled in sequential order instead of the alternating odd then even order that the interlaced scan system uses. This results in a faster refresh rate of once every 1/60th of a second. Is interlaced scan or progressive scan more popular?NTSC (National Television System Committee) and PAL (Phase Alternating Line) are the dominant analog video formats that are used throughout the world. While they differ in numerous ways, such as the amount of lines on screen (525 for NTSC, and 625 for PAL), they both use interlaced signals. This fact solidifies interlaced scans as the most popular technology of rendering images. However, the only reason why progressive scan images were not originally used for TV signals is that they required too much bandwidth to send down analog lines. This has now changed thanks to the advent of digital TV (DTV) signals such as HDTV. When buying a television, consumers are offered the choice between units that use progressive scans or interlaced scans, signified by the letter “p” or “i” at the end of the description, `080p or 1080i for example. Since televisions with progressive scan capabilities offer the viewer a better picture, albeit at extra expense, this type of signal is becoming more and more popular. With regards to computers, progressive scan is most definitely the more popular method of rendering images. While the interlaced image rendering method was used in the 1980s, the 1990s saw a growing need for better resolutions. However, when the higher resolutions were implemented in conjunction with an interlaced monitor, large amounts of image flickering would occur. The eyestrain and distraction caused by this flickering of the image led the computing industry to abandon interlaced monitors almost immediately and switch too progressive scan image rendering instead. What are the advantages of interlaced scan signals?
The disadvantages of interlaced scan signals
What are the advantages of progressive scan signals?
The disadvantages of progressive scan signals
It is fairly clear that should you have the budget, a television with progressive scan capabilities is always the preferable choice. It is especially designed for current High Definition sets and offers a far clearer, better quality image than an interlaced scan which was developed over fifty years ago with older CRT televisions in mind. We hope that this Bamboo AV Advice article has been useful to you! |