Aspect ratios, for fun and profit

Confuse your enemies! Confound your friends!

Some technical terms

The video signal that is being sent down a piece of wire. Includes all visible bits of moving picture and annoying black bars that are being sent down said piece of wire.
Active picture
The bit of the screen that has interesting colourful (usually) moving bits - i.e. without any black bars you might be seeing.
Aspect ratio
The ratio of a picture's width to its height (Width:height.) All pictures are intended to be displayed at a certain ratio pre-chosen for that picture. If they are shown at any other ratio (without the magic described below,) they look squished or stretched.

So, the Raster is the Active picture, sometimes with some extra black bars.

The trouble is that, because of the black bars that can be added, the Active Picture and the Raster can have different aspect ratios...

The Aspect ratio basics

There are two basic aspect ratios in use; 16:9 (aka widescreen,) and 4:3 (aka 12:9.) Normally (see below) the active picture fills the whole raster, and all is simple.

The complexity (and confusion) arises with conversions inbetween the two, and the use of an intermediary format, 14:9. 16:9 can be converted to 14:9 either by cutting off the sides (center cut out) or by squashing it, and adding black bars at the top and bottom (letterbox.) 4:3 can be converted to 16:9 either by adding black bars on each side (pillarbox) or by zooming in, and cutting off some of the top and bottom.

The 14:9 intermediate format is when a combination of letterboxing and center cut out is used on 16:9 material.

Use within the BBC

Theoretically, all the BBC's public service channels now output exclusively in 16:9*. Theoretically, because a lot of material is still in 4:3 format, and the analogue networks (ie BBC One & BBC Two) are still transmitted in 4:3.

So, on analogue terrestrial (ie "old fashioned" BBC One & BBC Two) 4:3 programmes are broadcast in 4:3, and 16:9 programmes are broadcast in 14:9. Or, to be precise, 16:9 programmes are broadcast in a 14:9 format embedded in a 4:3 frame. See where it gets messy yet?

Digital platforms are somewhat complex, but the effect for the end user is that material is broadcast in either 16:9 or 4:3, with codes embedded in the signal to describe the picture. If the user manages to tell their box what shape their television is, the box can format the picture to match. This is where many people get into trouble - which isn't helped by the occasional sending out of the wrong ‘magic code’ with the signal.

* - except when they aren't

Aspect ratio codes

As can be imagined, conversations about aspect ratios can get very confusing. So there is a standard(ish) set of codes to try to describe what you're on about.

Aspect ratio format description codes take the form xxAyyP. In no particular order;

So, in summary, the first part (xxAyy) describes what you're starting with. The last letter (P) lets subsequent users know how the image can be converted to other formats.

* - xx & yy are the first two digits of the aspect ratio code; 16 (for 16:9) 14 (for 14:9) or 12 (for 12:9 = 4:3.)

For example;

The following table contains all the common codes in use in BBC Playout, as well as a few less common ones.

The first column has the code, and the third shows what the raster would look like on a 4:3 monitor.

The fourth and fifth columns show how the material would be re-formatted for a 16:9 or a 4:3 output, based on the protection code.

(image used is test card J for 4:3 active picture, and test card W for 16:9 active picture)

Code Common name Raster Output on 16:9 monitor Output on 4:3 monitor
16F16A 16:9 Full height anamorphic (FHA)
16F16B 16:9 FHA, protect for 14:9
16F16C 16:9 FHA, protect for 4:3
12F12C 4:3
12P16C 16:9 pillarbox
14P16B 14:9 pillarbox in 16:9 raster
14L12B 4:3 semi-letterbox

Common problems (aka ‘Floaters’)

A floater is the attractive name for the situation where a picture is entirely surrounded by borders - it's floating in a sea of black...

16:9 floater

The most common example is when the original material is in 16L12 (i.e. a widescreen picture letterboxed into a 4:3 frame):

This is then (incorrectly) treated as normal 4:3 material (i.e. treated as 12F12C.) This means that a 16:9 conversion will have bars added either side, in addition to the bars at the top and bottom;

This usually happens with old films, that are stored on tape with letterbox bars, and are then replayed with the wrong code set.

4:3 floater

Alternatively, a 12P16 frame (a 4:3 picture with black borders, to make a 16:9 picture);

Is then incorrectly treated as proper 16:9 material (i.e. treated as 16F16A or 16F16B.) So, when it is ouput for 4:3 this results in extra horizontal black bars being added at the top and bottom;

This often happens if a 16:9 programme includes some 4:3 inserts or archive footage.

Just in case you think you now undestand aspect ratios

What have been described here are the aspect ratio codes commonly in use in the BBC. The conversions between formats are done using Aspect Ratio convertors (ARCs.) Programming and operation of ARCs are another equally complex issue.

In addition to this, digital television signals have a similar (but different) set of embedded codes broadcast with them so that viewers' home equipment can automatically format pictures for their sets - and (for extra confusion) these codes are different on the digital terrestrial, satellite and cable platforms.

Finally there are, of course, many other aspect ratios in use besides 12:9, 14:9 and 16:9 (e.g. 22:9) These are rarely encountered within the BBC, and are usually pre-prepared into a standard ratio.