Video Basics - Television Video, Digital Video, Resolutions, Framerates, and Aspect Ratios

Television Video

Obviously, if you're reading this, you know at least something about video. You know that it's basically a bunch of pictures changing really fast to give the illusion that something's moving. I won't bore you all with a history lesson about the origins of motion pictures, but first we need to understand the origins of what we today consider "Video".

In the beginning, there was Analog. And it was good. And the Government came down upon the masses and said unto them, "Hark, man has created a great technology called television. For the good of the consumer we must standardize this miraculous medium." Thus NTSC was born.

NTSC stands for National Television Standards Committee, which was the body that created the standard which was used in US televisions almost exclusively until a few years ago (there is now also ATSC, or more commonly known as HDTV/DTV). NTSC defined several basic things about how a television received and displayed data. First off, NTSC said how wide a carrier signal to use to store a television signal, as well as other things such as number of scanlines (the vertical resolution, or the number of horizontal lines on the screen). NTSC also defined how often the television screen was to be redrawn. They set it at 29.97 Frames per Second. Don't ask why - it's a long story (and a pretty stupid one). For most purposes people just say 30 frames per second, but in actuality it's 29.97. Please note this doesn't take into account interlacing and fields, topics which will be discussed shortly. NTSC also defined the basic resolution of television video (although that's not entirely accurate, as analog video doesn't really have any fixed resolution, but for the purpose of this guide we will say it has such a thing). NTSC defines the resolution of TV at 720 pixels wide and 525 scanlines. Yes, you heard me, 525.

"But what about 480 scanlines? That's what I've always heard! My video capture card captures in 720x480 and it's the entire picture!" Well that's not entirely accurate. There are 525 actually scanlines, but you can't see the top and bottom chunks - they are called "overscan" because they aren't visible on the screen. So for all intensive purposes we can say that the theoretical visual portion of an NTSC picture is 720x480. Now that's not entirely accurate either, because most Televisions block out the edges and even more of the top and bottom. But when viewing video on a computer screen (which doesn't have this issue since your monitor resolution is probably higher than 720x480) you can see all of those pixels (but you still don't see the extra overscan most of the time). For those interested, that extra overscan area of the video is where Macrovision copy-protection is stored.

For those of you in Europe or some other parts of the world, you may use PAL or SECAM. Please accept my condolences. =) PAL/SECAM uses a practical resolution of 720x576 (625 scanlines counting overscan) and has 25 Frames per Second. So while you may get a higher resolution picture, it's not updated as often. For the purpose of these guides, I will be focusing on NTSC video because it's what the majority of the readers of this article probably use.

Digital Video

Unlike conventional Analog video - Digital Video is stored/transmitted very differently. Analog video essentially turns the picture into a waveform and transmits it in terms of an electrical/radio wave (OK that's a GROSS simplification but it's enough for the purposes of this article). Digital Video on the other hand, stores the video in 0's and 1's, storing a color value for each pixel of each frame. There are several ways to store color data, the two main ones being RGB and YUV. The differences between these two are explained later.

Framerates

As discussed previously, the framerate of a video is the number of times the entire screen is refreshed with a new picture. In NTSC land this is 29.97, in PAL land it's 25, in conventional film it's 24, and in IMax film it's 48.

Aspect Ratios

Now some of you may be wondering why a TV displays video in 720x480, while your monitor displays pictures at 640x480, 800x600, etc. The reason is because your monitor has square pixels. A TV does not. Televisions have pixels which are taller than they are wide. This means that you need more pixels in the horizontal direction to make up for it. The wonders of this will be detailed later when you have to actually deal with editing footage.

Suffice to say that when things are displayed, regular TVs have a 4:3 aspect ratio and widescreen TVs have a 16:9 aspect ratio. Film aspect ratios include 1.66:1 (used mostly for animated features like Disney movies), 1.85:1 or 1.78:1 (commonly known as 16:9 or Academy Flat ratio), and 2.35:1 (called cinescope, or scope for short, which is usually used in action movies). How these film aspect ratios are adapted to TVs is another thing you will learn about in practice - particularly the magic that is anamorphic footage.

 ErMaC and AbsoluteDestiny (May 2004)