Interlaced and progressive rendering and encoding
Introduction
There are two ways that video files can be dispayed or 'rendered' on a TV or monitor. One method is interlaced rendering and the other is progressive rendering. They are different and have an impact on the quality of the image you see. If you have ever looked at a range of TVs or computer monitors, you may have seen 480p, 480i, 720p, 720i, 1080p and 1080i in the technical information. The i and the p stand for 'interlaced' and 'progressive' and describe how the equipment takes an image and displays it. There are also two ways that a video can be shot, using interlaced and progressive encoding. We will deiscuss these concepts below.
Interlaced displays
This method is the one that has been around for a long time. It was used for CRT TVs and some of the earlier HD TVs and computer monitors. The screen is made up of 480 horizontal lines. They are split up into two groups, the odd numbered lines and the even numbered lines. When an image is drawn on your screen, all of the odd number of lines are drawn, and then all of the even numbered lines are drawn, and then the odd ones again, and then the even ones, and so on. In fact, both the odd and even lines are drawn and redrawn very quickly, 30 times in one second. It takes just 1/30th of a second to draw (scan) a picture on the screen. An image is drawn in two passes using interlaced scanning. The lines are 'interlaced' with each other. By drawing the images on the TV or monitor in this way, what we see is a smooth video.
Progressive displays
Interlaced scanning works well with analogue devices and early digital ones, but is not so good for the latest hardware that uses LCD, where you need to refresh the screen at faster rates. A progressive scan means that the lines that make up an image are drawn one after the other, starting at line 1, then line 2, then line 3 and so on until the end, when it returns and redraws the image on the screen. An image is drawn in just one pass using progressive scanning, which means that a scan can take place in half the time interlaced scanning does, in 1/60th of a second.
Comparing interlaced displays with progressive scanning displays
NTSC and PAL used interlaced scanning and are still highly popular. The interlaced technology is established and less complex so devices that use interlaced scanning tends to be cheaper than devices that use progressive scanning. They also use half the bandwidth that progressive scanning devices use so are more popular with broadcasters. The quality of the image is not always good with interlaced scanning during high-motion videos, because the odd lines are drawn at a slightly different time to the even ones. This can lead to a slight blurring of the image and a distracting flickering effect called 'interline twitter'. Progressive scanning devices don't suffer from blurring, flickering or interline twittering and produce higher quality images because they are refreshed at twice the rate of interlaced scanning. Because they use twice the bandwidth of interlaced scanning, however, broadcasters don't usually use it.
Progressive encoding versus Interlaced encoding
When you shoot a video at 24 FPS, you can shoot it using progressive or interlaced encoding. Progressive footage means that you capture each frame at the same instant in time. Interlaced footage (as used with NTSC DV cameras, for example) means recording all of the odd lines first in one instant of time, and then all of the even lines in the next instant of time. A single frame is actually made up of two shots, taken at very slightly different times, which can lead to a blurring of an image in fast motion scenes.
Encoded in one format but displaying it on a TV or monitor that uses the other format?
This is possible! The software in a progressive TV or monitor detects if a broadcast is interlaced and then converts it so it can be displayed. Equally, an interlaced TV or monitor can display a progressive broadcast.