Quote:
Originally Posted by madshi
Well, do the math yourself:
16/9 = ?
1024/576 = ?
1050/576 = ?
576 * 16 / 9 = ?
It's not a good idea to blindly trust some website if you can do the proper math yourself...  |
It's not quite as simple as that as you aren't considering the aspect ratio of the source...
Standard ITU 601 SD video is conventionally carried in a 720x576 frame. HOWEVER - this frame is slightly wider than 4:3 or 16:9. A 720x576 frame is NOT a 4:3 or 16:9 source. It's wider than 4:3 or 16:9. If you scale directly from a 720x576 ITU 601 source frame to 1024x576 (for 16:9) or 768x576 (for 4:3) square pixels ignoring this fact, then you're going to slightly stretch things...
In fact, for 50Hz video, the central 702x576 portion of the 720x576 frame contains the 4:3 or 16:9 image - with 9 samples either side to ensure no under/overshoots etc. are cropped. (Incidentally - this is why some DVB broadcasts are 704x576 - the nearest multiple to 702x576 compatible with MPEG2/H264 compression - rather than 720x576)
If you simply scale the 720x576 image to 1024x576 (16:9 square pixels) or 768x576 (4:3 square pixels) you're stretching the image slightly, and incorrectly...
If you want to scale 16:9 ITU 601 video to 1024x576 to create a 16:9 frame, then you need to scale the central 702x576 area to 1024x576 square pixels (NOT the 720x576 full frame).
However if you are working with pro-video systems, but need to work square pixels (say in compositing etc.) then you scale the 720x576 frame to 1050x576 square pixels, with the central 1024x576 portion containing the 16:9 frame. It's generally seen as bad practice to crop 720x576 video to 702x576 video during the production chain.
(If you leave the 18 samples outside the 702x576 frame - 26 samples outside the 1024x576 frame - black you can hit issues if you start shrinking pictures - as you'll see the black edges)
1050 x 576 is obviously wider than 16:9 - but explains where the 1050 vs 1024 figures come from.
It's surprising how many companies get this wrong. Adobe did initially... You still see annoying width changes in some broadcast chains where things are going wrong.
http://81-208-25-53.ip.fastwebnet.it...missioning.pdf
http://www.mikeafford.com/blog/2009/...ts-cs4-vs-cs3/
If you scale 720x576 video you either crop 9 samples either side and scale 702x576 to 1024x576, or you scale 720x576 to 1050x576 and then crop 13 samples either side from the 1050x576 frame to get 1024x576.
This all dates back to basics and the analogue TV systems SD digital TV standards are based on. In 576/50i analogue video (aka 625/50 analogue component or analogue composite PAL/SECAM) the active video occupies 52us of a 64us line. If you sample this active video at 13.5MHz (as ITU 601 does for luminance) you end up with 702 samples in 52us. That is 702 samples for a full width 4:3 or 16:9 image. ITU 601 padded this to 720x576 for multiple reasons. This is why a 4:3 or 16:9 ITU frame is contained within the central 702x576 section of a 720x576 ITU 601 frame. Otherwise you'd need to do all sorts of resampling/reclocking when converting between analogue and digital video.
In the late 70s/early 80s when ITU 601 was being ratified (as CCIR Rec 601) digital devices were islands in analogue installations, and needed to be as transparent as possible. As analogue signals are often not very 'clean' - it made a lot of sense to sample a little bit more than the 702x576 active video portion, to avoid cropping edge transients with harmonics etc. which if cropped could induce ringing, and also to cope with devices that had somewhat less-than-ideal line timing (like VTRs of that era). It's similar thinking to having 16-235 rather than 0-255 dynamic range in active video (so that you don't clip undershoots and overshoots which could easily be present on analogue sources)
(NB. I used to be a broadcast video R&D engineer and did a LOT of work with ITU 601 4:3 and 16:9 video back in the day...)