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Progressive and Interlaced Scanning Explained
by Keith Taylor
Progressive scanning is the simplest and most logical way to scan a TV screen. The very first TV systems were progressive scan, but this needs a very high video frame rate to create a smooth image. The cost of transmission and storage increases dramatically with increasing frame rate, so it needs to be kept as low as possible without creating a jerky image. Video frame rates are set at either 25 fps or 30 fps, depending upon where you live in the world, but we will use the 25 fps standard for this article. Film uses a world standard frame rate of 24 fps, which provides an adequately smooth image for this medium.
Video tends to be more demanding than film when it comes to frame rate and even though the standard video rates are slightly higher than film, itís not quite high enough to give a universally acceptable result when using progressive scanning. Fortunately we have a nice little trick called interlaced scanning, which gives us a much smoother image at the standard frame rate. The way this is achieved differs for analogue and digital TV systems, and there is an extra benefit for analogue systems.
NTSC is the American standard for colour TV signal encoding and was developed in the early 50s. It uses 525 horizontally scanned lines and a frame rate of 30fps. The PAL system uses 625 lines and a frame rate of 25fps and was developed in the late 50s. Both use interlaced scanning. The PAL system is a much-improved version of NTSC, in that it has built in colour correction (the Phase Alternate Line system) and the extra 100 lines gives a significant increase in vertical resolution. The loss of 5 frames per second is insignificant, due to the use of interlaced scanning.
It must be stressed that PAL and NTSC are ageing analogue systems and are entirely different from the modern digital systems. However, you will see modern equipment, DVDsí and stock footage quoted as being NTSC or PAL. This is a misnomer, and only refers to the resolution and frame rate now used by the parts of world which historically used PAL or NTSC. There is no phase alternate line system in digital video, as the methods used to encode the signals are now entirely different.
Interlacing is better explained with the analogue system, so we will look at that first.
The electron beam in the cathode ray tube of an analogue TV set starts at the top left corner of the screen, horizontally scans the required number of lines, and then shoots back up to the top left to repeat the same process ad infinitum. In a progressive 625line/25fps system the beam would scan 625 lines, 25 times per second. The screen is said to be refreshed at 25HZ (25 times per second), however progressive scanning is not used in analogue broadcast TV. In the interlaced system the beam scans the whole screen with 312.5 lines, 50 times a second. It has scanned only half the number of lines, so the spaces in-between the lines are twice that of a progressive scan. The first set of 312.5 lines fills the screen and then the second set fits in-between the first set, thereby producing the complete set of 625 lines 25 times per second, but we are still creating a screen full of lines 50 times per second. The screen is only at half the vertical resolution at any one time, but the eye doesnít notice this and perceives it as a full frame refresh at 50HZ. The only side effect of this is that the image jumps up and down, by one line height, at 25HZ. But this is only noticeable with captions.
The images are smoother because the camera scans the image in the same way and the camera is capturing a full screen, albeit with half the number of lines, 50 times per second. This gives an effect close to that of 50fps progressive scan.
Interlacing also solves another big problem with analogue TV: that of flicker. In fact this was the main reason for interlacing being introduced. A light needs to flash at least 50 times a second to give the appearance of a constant light source, so scanning the screen at 25HZ creates an unacceptable amount of flicker. Interlacing refreshes the screen at 50HZ, so the flicker is equivalent to a light flashing at 50HZ.
The problem of frame rate also exists in digital systems. But we can get an image with a similar smoothness to a 50fps refresh rate, with only 25fps, by sampling the pixels with the same interlaced scanning sequence as the analogue system. Some cameras offer a 25HZ progressive setting which results in the image being refreshed at only 25HZ. This gives a jerky image which some believe is similar to the jerkiness in film. However, as pointed out before, film appears less jerky than video, due to an overall reduced stability and softness in the image.
Ultimately the reason for using interlaced scanning is to reduce bandwidth or, in other words, the quantity of information which needs to be transmitted and stored. This equates directly to cost. The ideal video system would be progressively scanned at 50fps or greater, but interlacing provides an invaluable solution to the need for a high scan rate and a small bandwidth.