009: NTSC primer no. 1 (all pixels are not created equal).
Carol recently emailed me with a video resolution question, so I thought it might be a good time to discuss that topic on the blog. The analog signal standard for North American televisions, and many North American video cameras, is NTSC. An NTSC image has a resolution of 720 pixels wide by 480 pixels high* and an aspect ratio of 1.33:1**.
Divide 720 by 480 and you get 1.5. Given that pixel count, how can an NTSC image have an aspect ratio of 1.33:1?
The pixels aren't square.
"Skinny" NTSC pixels displayed as square pixels -- notice how the image looks a bit wide?
The same image, properly displayed as 640 x 480 square pixels.
(Click either of the examples*** for the full-resolution version.)
As a reasonable human being, I find it absurd that somebody created a broadcast standard based on pixels that are not square -- it is an unnecessary complication. But NTSC is littered with plenty of other bad decisions, some of which I'll discuss in later posts.
So, anyway, what do these non-square pixels mean to you? If you're working on a project that starts in NTSC and stays in NTSC (through delivery on DVD, for example), there's not much to worry about. Software editing systems (like Final Cut Pro) compensate for the screwy NTSC pixels, so your images look fine on the computer monitor (which, sensibly, has square pixels).
If, however, you're exporting NTSC video to another format (like a QuickTime video for the web), you'll need to change the video resolution on output to something with the proper 1.33:1 aspect ratio. For a "full resolution" video, you'd output to 640 by 480 pixels. I guess that's technically only full vertical resolution. But, in the video world, for some reason, it's all about the vertical resolution (often referred to as "lines"). So, it's best to keep your image 480 pixels tall and adjust the horizontal pixels as appropriate. You can make smaller video for computers at 480 x 360 pixels, 320 x 240 pixels, etc.
I think that's enough math for today -- more NTSC madness next week.
*A "full" NTSC image (available, for example, from a DigiBeta tape) is actually 720 by 486 pixels. Most modern NTSC video mediums (including MiniDV and DVD) throw out six vertical pixels for the sake of simpler mathematics.
**This 1.33:1 aspect ratio applies to a classic "standard definition" image. An "enhanced definition" image has an aspect ratio of 1.78:1 -- more on that topic at a later date.
***That sexy guy in the space suit is Chuck, by the way.