Aspect ratio

From MythTV Official Wiki
Revision as of 13:22, 12 October 2012 by Wagnerrp (Talk | contribs)

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to: navigation, search
Wikipedia-logo-en.png
Wikipedia has an article on:
Aspect ratio is the ratio of the width of an image to its height.

What's this whole Aspect Ratio thing, anyway?

The two most common aspect ratios heard mentioned are 4:3 (1.33:1, or traditional TV) and 16:9 (1.77:1, "widescreen" TV). Film releases are usually somewhere between 1.85:1 and 2.35:1, with Cinerama coming in at 3:1 or wider.

Sample Displays

16:9 Widescreen TV showing 2.35:1 Cinemascope Image

AR235.jpg

16:9 Widescreen TV showing 4:3 Picture

AR43.jpg

When working with aspect ratios and television, it's important to remember that there are more than one of them involved: the AR of the original program, that of the broadcast, and that of your set.

The display resolution of your set is a related, but slightly different issue in displaying SD and HD signals. The topic of Aspect Ratio, which is the ratio of picture width to height, is one of the most complicated issues a MythTV user 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 (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 that AVI files do not), then you must guess (or MythTV must, and it may well guess wrong).

Aspect Ratio Modes

With a few rare exceptions, most programs produced originally for television are created in either 4:3 or 16:9 format. How that master tape is aired on a network, however, may vary. For example, The West Wing is mastered in 16:9 and broadcast that way on NBC-HD, but on the standard definition network, the show is broadcast in letterbox format: the 16:9 image is vertically centered between a pair of horizontal black bars.

If you're watching an HD channel on an SD television set, you'll get it in letter box. If you're watching the HD channel on a widescreen set, you'll get the picture full-screen, the way the director intended.

It's when you combine the two that problems crop up.

Most widescreen sets have an Aspect control, and different sets can do different mappings from non-HD signals to the wide screen (and most have different names for each, of course):

  • Normal: places the 4:3 picture horizontally centered on the screen between 2 vertical black bars.
  • Full: linearly stretches the 4:3 picture horizontally to fill the entire 16:9 screen. Unless the original signal was a squeezed 16:9 original (as from a recent camcorder), this is almost certainly the wrong choice for normal television viewing, since it makes everyone look fat (this is the source of my term "Fat-Head Syndrome", and yet it's the most common setting among 'civilians'. It's even seen quite a bit on sales floors, where the people really ought to be expected to know better. It's just that people seem to *really* hate seeing those black bars on the sides. (Some sets have multiple tuners, or inputs, and can stack 3 additional 4:3 pictures on one side to fill the space.)

AR43stretch.jpg

  • Stretch: this mode, not found on all sets (some Mitsubishis are known to have it) stretches only the last 1/5 of the picture on each side, non-linearly, to fill the sidebars, leaving the center of the picture unstretched. This is actually a better compromise in practice than you might expect.
  • Zoom: This mode uses the 4:3 picture to fill the screen horizontally (all other modes fille it vertically), and zooms in on the center stripe of the picture. It clips about 5% of most broadcast letter boxed pictures at the top and bottom, but is a reasonable choice for watching letterboxed SD broadcasts on a 16:9 set.

These modes all apply to what happens when the set it presented with a 4:3 program signal; in general, most widescreen sets presented with an HD signal merely display it to fill the screen.

Configuration

Xorg

  • The Aspect ratio can be set using the DisplaySize in xorg.conf using Height Wide (mm) eg DisplaySize 656 369 (16:9 ratio- Widescreen)
  • With Xorg 7.2, MythTV detects Xinerama as being active at all times and so will ignore the display sizes when calculating the aspect ratio for displaying video. To fix this, use the following SQL command:
UPDATE settings SET data = '1.7777' where value = 'XineramaMonitorAspectRatio' AND hostname='yourhost';

Set the value to 1.7777 for a 16:9 monitor, to 1.3333 for 4:3, etc.

MythTV

MythTV determines the aspect ratio from the video stream. That value can be overridden using the following setting:

  Setup -> TV -> Playback -> General

Mythbuntu

The menu containing the option to override the aspect ratio is:
Utilities/Setup > Setup > TV Settings > Playback