Aspect Ratios  -  AR

This is by far the most confusing concept in Videos.  It messes a lot of people up - and it messes a lot of projects up too.  So you need to know it.

First off - there are several types of aspect ratios:

Display AR -

Pixel AR -

AR parameter within the Video File -

 

Display AR Terminology:

16:9  =  enhanced for widescreen TVs, anamorphic widescreen
4:3 = fullscreen, 1:1.33, pan&scan, standard edition, letterboxed widescreen.

When you encode MPEG-2 files for DVD, you use 720x480 for the source file aspect ratio.  But the actual displayed ratio is either 4:3, 16:9, or 2.21:1 (less common).  This is decided by the DVD player based on a parameter that is encoded into the mpeg-2 file. 

In addition, the Source File pixels can be square or oval shaped, which also affects the displayed aspect ratio.  Here are the various combinations of pixel aspect ratios and image aspect ratios - only one of these can be encoded into the mpeg-2 file

this is a screenshot from TMPGenc Express
so the term, "Image", which I have never seen to indicate
aspect ratios of videos . . . must simply be TMPGenc Express term for "Display"

If you have a square pixel (1:1) source file:

If your have a non-square, oval pixel source file:

the actual displayed aspect ratio of the encoded file can vary greatly !!  Therefore you are better off just selecting "square pixels" if this setting is even available in your encoder.  This will give you more constant results. 

Sample aspect ration settings using TMPGenc Express

TMPGenc Express allows you to select the pixel ratio in the "Clip Info" box.  Here are the effects of different settings selected for the source file.  Note the effects as far as the black bars are concerned:


NOTE:  10:11 is the same as the .9 non-square
pixel setting in many Video utilities

TMPGenc Express
720x480 Source Video clip Aspect Ratio Settings
Shown as displayed on a 4:3 Screen

The images show how the source file is interpreted and the resulting aspect ratio if it were to be displayed on a 4:3 screen.  However, the aspect ratio that you select for the final output will be either 4:3 or 16:9, which, as AVIcodec indicates, ends up being 16:9 or 21.1:1, which to add even more confusion, so long as square pixels were selected for the input, ends up being displayed by the DVD player as 4:3 or 16:9 !!!  The easiest way to understand all this, is to ignore the AVIcodec reported aspect ratio, and just realize the following:

 


the two DVD aspect ratios on a 4:3 screen  -  both using 720x480 video

NOTE1:  on a 4:3 screen, if using letterbox (widecreen) the 16:9 video is merely shrunk in size and retains the same aspect ratio, and therefore will have black bars on top and bottom.
If the DVD is played using "Pan and Scan" then it will fill the screen and the sides of the 16:9 are chopped off.

NOTE2:   if these were displayed on a 16:9 widescreen, then the 4:3 would have black bars at the sides

The MPEG-2 standard defines several "layers" which contain both data and variables.  The Sequence layer represents either interlaced or progressive video sequences, (MPEG-1 is strictly progressive).  In addition, MPEG-2 sequence layer contains a variable called "aspect_ratio_information", sometimes called a "flag".  It refers to the overall display aspect ratio (e.g. 4:3, 16:9).  With MPEG-1, the same variable refers to the ratio of the pixels, not the display.

NOTE:  in addition, another important variable is "frame_rate_code", which with MPEG-2 refers to the display rate.  The same variable in MPEG-1 refers to the actual coded frame rate.

So, the mpeg-2 file structure has internal information that flags the DVD player as to whether it should be played as 4:3 or 16:9.  This can actually be edited without re-encoding, through the use of a utility called DVDpatcher.

In BOTH cases, the MPEG-2 file is the same pixel size of 720x480.  So, pixel-wise, that is an aspect ratio of 1.5:1, or simply 1.5  -  and to make it really confusing, here are the various aspect ratios normalized to 1 :      720x480 = 1.5   but 4:3 = 1.33  and  16:9 =   1.78:1

As you can see, none of them are equivalent !!  But today's TV's are either 4:3 or 16:9, and therefore, to fill those screens the DVD player, when given a video clip of 720x480, will NOT play it at the 720x480 aspect ratio of 1.5.  It will either go below that for standard TV, or above that for HDTV.  The DVD player inspects the header of the MPEG-2 file, reads the aspect ratio, and plays it accordingly.