Part I: Intro to Video
Complexity is an unfortunate byproduct of technology. Many substantial innovations we have seen in the last hundred or so years are very hard to understand on the surface. The process of compression is incredibly hard to understand in full unless you feel like studying volumes upon volumes of specifications for codecs (Compressor/Decompressors). Luckily, we don’t have to understand the intricacies of compression to use it handily. Before I continue further about compression, there are some important highlights about video sources that I need to touch on.
In its most simple form: Video is a sequence of still images shown in rapid succession to imply movement. That is the basis on which everything else is built upon. Each individual still image is called a “Frame.” The number of frames displayed in one second is called the “Frame rate.” The frame rate is an important factor. In order for the illusion of movement in video to work, we must show many frames in a single second. Fifteen frames per second is essentially the bare minimum for movement to look smooth. However, there are two standards for video in North America: 24 frames per second for film and 29.97 frames per second for television. To keep optimal visual quality, the frame rate of the video should be maintained. If frames are dropped, the video becomes choppy looking. If we add frames where none existed originally, we have to either double-up on certain frames or try to interpolate in between frames. This adds tremendous amounts of hassle and quirkiness, so it should be avoided when possible.
The width and the height of video are expressed as two numbers with an “x” between them. For example: 640×480 is known as the resolution for standard definition video. That means there are six hundred and forty lines from side to side, and four hundred and eighty lines top to bottom. There are four resolutions in North America that are the most common: 640×480, 720×480, 1280×720, and 1920×1080. As I explained earlier, 640×480 is called “Standard Definition.” Most televisions in North America will accept this resolution. 720×480 is essentially a modification of standard definition used by DVDs to maintain the 16:9 aspect ratio (The long rectangle shape of the image) instead of 640×480’s 4:3 aspect ratio (The more squarish rectangle shape of the image). 1280×720 and 1920×1080 are both known as “High definition” video. Both of these are 16:9 aspect ratio. Here is where video starts to build complexity (And we haven’t even gotten to compression yet!).
In order to use our airwaves more efficiently and to deal with less than stellar television sets, we introduced a method of halving the bandwidth used called “Interlacing.” Introduced in the early years of television, interlacing essentially means that only half of the image changes in each individual frame. In the first frame, all of the odd numbered horizontal lines will change. In the second frame, all of the even numbered horizontal lines will change. That pattern repeats itself over and over to display video that looks smooth to our eyes. The problem with interlacing is that it can cause the video to look odd to us under some circumstances. Because half of the image is always one frame behind the other half, fast moving events can look like they are being torn apart.
It is clearly to our advantage to know if the video we’re using is interlaced or not, so we end up calling different types of video by specific names. For example, 640×480 sources are often interlaced, but we are slowly moving over to “Progressive” video which updates each frame completely. 640×480 video that is interlaced is labeled “480i” while 640×480 video that is progressive is labeled “480p.” Sometimes 480p is called “Enhanced Definition” video. On the Hi-Def side of things, 1280×720 video is always (At least commercially) progressive, so it is known as 720p. 1920×1080 is most often broadcast with interlacing, so it is called 1080i. However, high definition video players like Blu-Ray and the ill-fated HD-DVD can both play 1920×1080 as a progressive signal, so that is called 1080p.
Now that we have the very basics of video, we can move forward into compression, how it works, what it means to us, and how we can use it to our benefit. In the next part of this four-part series I’ll explain the basics of compression and what it means to the average Mac user.
