Revision as of 00:30, 1 February 2006

(Because one editor wasn't careful about capitalization (me, that is :-), there's even more discussion of this thorny problem available. --Baylink)

What's this whole Aspect Ratio thing, anyway?

The topic of Aspect Ratio, which is the ratio of picture width to height, is one of the most complicated issues the Myth Boxer is likely to face, and this is mostly because there are things no one talks about.

There are several things you could be talking about when aspect ratio comes up.

You might mean

• the aspect ratio of the physical display
• the aspect ratio of the signal, or
• the aspect ratio of the *image* within the signal.

Let's look at these in turn.

Display Aspect

There are three measurements that interact to determine the display device aspect ratio:

• the physical dimension of the panel, or the optical image that a projector throws on the wall
• the number of physical pixels

and, the result of those two

• the pixel aspect ratio.

This last one is the kicker, because people tend to assume that it's square (an aspect ratio of 1.0), and it almost never is.

The standard size for 'D-1' digital video {Fix Me: what's the actual controlling standard called again?} is 720x480, though this is a 3:2 (or 1.5:1) aspect ratio, the physical dimensions of the monitor are commonly the television standard 4:3 (or 1.33:1); the difference comes because the pixels aren't square.

A common size for a computer output is 640x480; in *this* case, the pixels are square, since the 4:3 (1.33:1) ratio of the pixel counts matches the physical dimensions of the screen.

A related issue here is that many devices (including LCD panels and projectors, though they may have to hardware-scale to accomplish the task) can present images in different resolutions, and the pixel aspect ratios may be different between these settings; this can confuse some programs, including MythTV, in certain circumstances.

So, once we know what the physical aspect ratio of the display device is, we turn next to:

Signal Aspect

By the 'signal', I mean to specifically describe the size of what's being transmitted/recorded/played back, to distinguish for the case when, for example, a 4:3 NTSC picture carries a letterboxed 16:9 image, as with a network broadcast of The West Wing.

The important thing to realize is that you *cannot infer the aspect ratio of the signal from the pixel counts*. As we noted above, the aspect ratio of the *pixels* themselves is not a fixed quantity, so unless you know or the file format carries signal aspect ratio information in it (which MPEG files do, and I believe {Fix Me} that AVI files do not), then you must guess (or MythTV must, and it may well guess wrong).