SD TV resolution and what mediainfo says about it

[grl]

This one has been bugging me for a while. My google-foo failed me, so hopefully somebody here knows.

Digital SD TV is 720 x 576 and media info reports this correctly, but it also say its is 16/9 which is just wrong. 16/9 is a ratio of 1.77777777 (recurring) and 720/576 is 1.25. Of course 1920/1080 for HD is 16/9.

So what magic is happening to make this actually work. I don't detect any stretching of images in SD or truncations compared to the HD equivalent and there are no black bars, so how does it look right?

On the path through my toolchain I see 720x576 after ProjectX and after mplex. AutoGK in my case is set to 720 fixed width and the results come out as 720x400 which is a ratio of 1.8 which is probably close enough to 16/9 for me not to notice in playback.

[tonymy01]

You made a rookie mistake. You don't divide 720/576 to determine the aspect ratio. This will be the ratio for *square* pixels, but they aren't square for 16:9 material.
Most HD here is now 1440x1080 also, and that is still 16:9 (again, due to pixels not having to be square).

edit: Look what happens when you play 16:9 SD material on a square pixel device (that doesn't understand the WSS/aspect ratio flags), everything looks tall and skinny. But stretch out those horizontal pixels, and it will look fine, thus the SD widescreen TV was born. Remember, TVs didn't have pixels in the past, they swept an electron beam across the display, one horizontal line at a time, and so it was very easy to stretch or shrink based on the flags in the material for early analogue widescreen TVs. Now it is all done with internal screen scaling tricks in screens with pixels. Same thing when you play 16:9 flagged material on a PC monitor with square pixels, the PC software will do the trickery to stretch it out depending on the flags.

edit2: In the olden days, they called this "anamorphic" as this was a way of getting widescreen material onto 4:3 35mm film negatives. They used a special lense on the movie cameras and this lens horizontally squeezed the image in, and then another lens at playback time would stretch it back out. So when DVDs first started becoming "widescreen" they often called it "anamorphic" for a similar reason, that when played native res, the image would be stretched wrong, but when the flags were taken into consideration, the aspect would be corrected by the player or by the screen (letterboxing corrects it on a player that is intended to send it to a 4:3 screen for example.. letterboxing removes one horizontal line in every 4 to essentially vertically compress these 16:9 source DVDs/etc, losing a bit of resolution, but enabling black bars top and bottom to get the image back to the correct aspect for a 4:3 screen).

[netmask]

Welcome to the confusing world of DAR (display aspect ratio) and PAR (pixel aspect ratio), and how they relate to frame size. In short, DAR is the 4:3, 16:9, etc. ratio that is usually called just "aspect ratio", while PAR describes the shape of the pixels themselves.

Australian Digital TV transmits a 16:9 frame regardless of aspect ratio of the program material and the viewing TV. So 4:3 material is pillaboxed within a 16:9 frame with black bars either side. On a widescreen TV you will see the black bars, on a 4:3 CRT TV you won't. AutoGK detects black bars and cuts them off and then resizes if necessary - have a read of the log file.

If you have a LCD/Plasma with a screen resolution of 1366 x 768 you can still view 16:9 or 2.35:1 material albeit not as clear as a true HD full 1920 x 1080 screen and depending on how far back from the screen you sit you may not be able to pick the difference.

http://help.adobe.com/en_US/AfterEffect ... 7f3aa.html

[tonymy01]

On a widescreen TV you will see the black bars, on a 4:3 CRT TV you won't.

Not entirely correct, you have to 4:3 Centre Cut/4:3 Pan&Scan in most STBs to be able to slice off those hard coded black pillarboxes. If you don't, and have letterboxing setup, you will end up with "a postage stamp" which is where the black bars the broadcaster sticks in, combined with the letterboxing black bars to turn the 16:9 image back to 4:3, end up making the image look like it is surrounded by black on a 4:3 TV until you toggle the aspect to Centre Cut.

[grl]

Thanks for the pointers.

A couple of points after googling in the right direction and some test recordings on HD.

The ABC HD and SBS HD channels have square pixels. 1280x720 is 16/9

All digital TV is supposed to have square pixels for those following SMPTE standards and practices. I am not sure why Australia chose 1440x1080 as we are part of SMPTE.

There is mention of compatibility with SD imaging systems which might explain the use of rectangular pixels for digital SD and our use of 720x576.

[IanB]

1280x720 is commonly used for 720P and also transmits 50 full progressive frames per second and happens to have square pixels.

1080i at full resolution can be transmitted at upto 1920x1080, but to conserve bandwidth cunning and wiley broadcasters can crib a bit and use a lesser horizontal resolution, typically 1440x1080. This reduces the source content to 75% of the original data size. This compromise sacrifices a small amount of horizontal clarity for greatly reduced mpeg artefacting (which can still be quite bad sometimes ). This format transmits 50 half height fields per second, i.e 1440x540, resulting in 25 interlaced 1440x1080 frames per second.

Do the maths it's all about how hard to compress :-
1280 * 720 = 921600
1440 * 540 = 777600
1920 * 540 = 1036800
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