newb - pixel aspect ratio question ?

[Bansaw]

I'm just trying to read up about pixel aspect ratio.

NTSC Widescreen has a pixel aspect ratio of 1.2121
but its also called 16:9

How can it be called 16:9 if 9 * 1.2121 is 10.9089 ?

How do you reconcile 16:9 and 1.2121 ?

[aedipuss]

16/9 is a display aspect ratio, not a pixel aspect ratio. par refers to the shape of an individual pixel. dar refers to the shape of the output video as a whole.

[Cornucopia]

And PAR (pixel ar) = W / H * DAR (display ar).
W/H is sometimes (I say incorrectly) called "Storage AR" or SAR.
(These W & H dimensional #s are actually stored separately, never as a single AR, and it is just some hobbyists' laziness that reduces this to an AR that then gets confused with everything else).

Scott

[jagabo]

And PAR (pixel ar) = W / H * DAR (display ar).
W/H is sometimes (I say incorrectly) called "Storage AR" or SAR.
(These W & H dimensional #s are actually stored separately, never as a single AR, and it is just some hobbyists' laziness that reduces this to an AR that then gets confused with everything else).

SAR is no more confusing that PAR and DAR? They are all ratios: PAR = pixel width/ pixel height, and DAR is display width / display height.

What's confusing is the change in terminology between MPEG 2 and MEPG 4 part 10:

MPEG 2: DAR = SAR * PAR (storage AR * pixel AR)
MPEG 4: DAR = FAR * SAR (frame AR * sample AR)

[Cornucopia]

It's not confusing to me (at ALL).
It's confusing to newbies, yet it doesn't have to be if it didn't get bandied about so freely. You know as well as I do that there is no "5:4" AR setting saved anywhere in an MPEG2 file that is 720x480 - there's just the "720" and the "480" raw dimensions themselves.

Once newbies get an understanding of the relationship between the 3 types of entities, lots of problems go away. Keeping H & W vs. SAR nomenclature can go far in removing another "AR" to be confused with.

Scott

edit: I do agree that MPEG should have gotten their act together. I've also seen (somewhere) in the specs PAR=Picture Aspec Ratio (=FAR, DAR), as well as other things. Doesn't surprise me that people get confused.

[intracube]

Unlike most computer systems, standard definition video doesn't usually use square pixels. That is, when playing back a 16:9 DVD on your TV, if you could zoom in and see the individual pixels, they wouldn't be square.

High def video is easy to understand. 1920x1080 and 1280x720 resolutions are exactly 16:9 (square pixels).

Standard def 'PAL' (digital) typically uses 720x576* pixels for both 4:3 and 16:9 material. The (4:3 or 16:9) video taken by a camera gets distorted to fit into the 720x576* frame. It then gets undistorted on playback. Sometimes lower resolutions like 544x576 are used for 4:3/16:9 satellite broadcasts, etc.

*depending where you get your info from, you might notice slight errors in your calculations. This might be down to different standards - sometimes only a 702x576 region within the 720x576 frame is used for the actual picture content. Unless you really want to know the details, you can skip over this point.

SD video has a lot of caveats; differences in resolution between 'NTSC' and 'PAL', 'NTSC' is sometimes 720x480 or more rarely 720x486. Video may be padded, cropped, scaled slightly depending on the particulars.