DVB-T UK, 16:9, 1.77 or 1.85?

[kipico]

ello

on playback of a capped 16:9 DVB mpeg in BSPlayer the picture is 640x360, giving a AR of 1.77'

but i constantly read that 16:9 should be 1.85

in the past ive always resized my AVI's at 640x352, AR of 1.85, out of habit.
but have recently experimented in using 624x352, AR of 1.77

which AR should be used?
(my instinct says 624x352, but im open to correction )

[ronnylov]

16/9=1.7777777778

What is the resolution of the DVB stream?
For PAL there are different formats:
720x576
704x576
544x576
528x576
480x576
352x576
352x288

I may have forgotten some format but I think those are what I have seen in PAL DVB. But the actual picture area is smaller than the video resolution. For instance the PAL 720x576 format specifies an active picture area of 702x576 and the PAL 704x576 format does also specify 702x576 as active picture area.

See following link:
http://www.uwasa.fi/~f76998/video/conversion/#conversion_table

The 16:9 picture on the TV is made from the active picture area. Let's say the active picture area (according to the table in the link) is 702x576 and is sent as 16:9 anamorphic widescreen. That means the 702x576 active picture are is stretched to fill the 16:9 TV screen (or actually overfill it because some of the active picture area is hidden by overscan in the TV).

So what you want to do in this case is resizing the 702x576 to 16:9 avi file with 1:1 pixel aspect ratio. Then you should do the following:

Crop the 720x576 to 702x576 by cropping the sides (no resizing, just shave off pixels). Then you resize this cropped 702x576 to anything with height 9/16 of the width, for instance 640x360. But the height 360 is not optimal for encoding (should be mod 16). Then crop the height of this 640x360 picture to 640x352. Alternatively resize 702x576 directly to 624x352 (with a minor aspect ratio error which does not matter).

If you prefer cropping the width intead of the height you can resize 702x576 to 654x368 and then crop the sides to make it 640x368.

But where comes the 1.85:1 aspect ratio from?
This is the original aspect ratio of some movies. When they are converted to 16:9 anamorphic format on DVB or DVD it is either slightly letterboxed or slightly cropped on the sides to fit in the 16:9 picture of the TV (and to fit in the active picture are of the video resolution). Some movies are originally 2.35:1 and even more letterboxed and/or cropped into the 16:9 picture.

But no matter of what the original aspect ratio was it has to be fit in the 16:9 format (or 4:3 if sent in 4:3 format). Just think of it as an active picture area in which the original picture is fitted into a 16:9 format.

Don't mix up active picture area and the real picture area. The real picture may not fill up all of the active picture are, sometimes there are black borders on top and bottom (letterboxing) as well as on the sides also after the cropping into the active area. And sometimes the real picture is larger than the active area but this is cropped by the playback device and will not be shown if you capture the signal on the video output of your DVB receiver. You may still want it in your avi file so then you can use the pixel aspect ratio to calculate how to resize.

The table I linked to seems to only have the pixel aspect ratio of a 4:3 picture inside of the active picture area in the video resolution. Look at the example of 720x576 PAL DVD. It says the pixel aspect ratio is 128/117 (x/y). The pixels are not square pixels, they are 1.094 times wider than high when shown on the 4:3 TV. 702x576 pixels in 4:3 format correponds to 768x576 in square pixel resolution. But if the whole 720x576 resolution of the video is filled with picture that you want to resize to square pixels then you get 788x576 in 4:3 format (if the video was sent in 4:3 format).

But now you have a video in 16:9 format and in this example 720x576 resolution. This means that the active picture area of 702x576 should get a 16:9 picture. If we keep the height 576 and resize to square pixels and 16:9 format then we get 1024x576 resolution. This means each of the 702x576 pixels is 1.459 times wider than high when viewed on the 16:9 TV.

Let's say you want to crop your source before the resizing. Maybe you crop a 720x576 16:9 video to something like 696x430 to get rid of all borders of a letterboxed 2.35:1 movie. You still have pixels that are 1.459 times wider than high. The 696x430 cropped but not resized video would correspond to 1015x430 in square pixels resolution. But you may want it 640 pixels wide which would make the height 271 pixels with the same width/heigth ratio. But 272 is a better value for compression. OK, with 272 pixels height the width would be 642 pixels. So to make it simple you can resize the original cropped 696x430 to 642x272 and then crop the width to 640x272 and you have a 2.35:1 avi with a correct aspect ratio in square pixel format.

I think you know what I mean. Sometimes I just writes too much to answer a simple question...

[kipico]

thanx for the detailed reply
so...
would this avisynth script be ok then?
im trying to keep 16/9, witout too much AR error, hence resize... then crop
(720x576 > 624X352)

###

LoadPlugin("C:\Program Files\AviSynth 2.5\plugins\MPEG2Dec3dg.dll")

LoadPlugin("C:\Program Files\AviSynth 2.5\plugins\UnDot.dll")


MPEG2Source("E:\20050518_195303.d2v")

Lanczos4Resize(640,360)

Crop(8,4,-8,-4)

Undot()

###

[ronnylov]

See script below. You should crop to the correct active picture area in the source. The active picture area (which contains fullscreen 16:9) in a 720x576 source is 702x576. Then resize to a 16:9 square pixel format and then crop to a mod 16 resolution for optimal encoding.
#-----------------------------------------------------------------
# (720x576 > 702x576 > 640x360 > 624X352)
LoadPlugin("C:\Program Files\AviSynth 2.5\plugins\MPEG2Dec3dg.dll")
LoadPlugin("C:\Program Files\AviSynth 2.5\plugins\UnDot.dll")

MPEG2Source("E:\20050518_195303.d2v")
Crop(18,0,702,576) # crop to get the active picture area 702x576
#Deinterlace here if needed before the resizing
Lanczos4Resize(640,360) # resize to 16:9 square pixels
Crop(8,4,-8,-4) # to crop into 624x352

Undot() # Noise reduction
#-------------------------------------------------------------------

If you want to resize directly and then crop make sure to use the correct pixel aspect ratio when resizing. 720x576 use 1.459:1 pixel aspect ratio and corresponds to 1050x576 square pixels which can be resized to a smaller resolution.

To resize 720x576 directly and then crop (should give the same end result):
#--------------------------------------------------------------------
# (720x576 > 656x360 > 624X352)
LoadPlugin("C:\Program Files\AviSynth 2.5\plugins\MPEG2Dec3dg.dll")
LoadPlugin("C:\Program Files\AviSynth 2.5\plugins\UnDot.dll")

MPEG2Source("E:\20050518_195303.d2v")
#Deinterlace here if needed before the resizing
Lanczos4Resize(656,360) # resize to square pixels
Crop(16,4,-16,-4) # crop into 624x352

Undot() # Noise reduction
#--------------------------------------------------------------------

Both scripts above assumes that the source is progressive. If the source is interlaced you should deinterlace before the resizing.

[ronnylov]

Alternatively you can resize 720x576 directly to 642x352 (which is very close to correct aspect ratio when resizing from 720x576) and then crop the sides to get 624x352. This method may be preferred if you want less cropping and still keep the correct aspect ratio:

# (720x576 > 642x352 > 624x352)
LoadPlugin("C:\Program Files\AviSynth 2.5\plugins\MPEG2Dec3dg.dll")
LoadPlugin("C:\Program Files\AviSynth 2.5\plugins\UnDot.dll")

MPEG2Source("E:\20050518_195303.d2v")
#Deinterlace here if needed before the resizing
Lanczos4Resize(642,352) # resize from 720x576 16:9 to square pixels
Crop(9,0,624,352) # crop into 624x352

Undot() # Noise reduction

[kipico]

thanx ronnylov
if i could buy you a beer, then i'd buy you two!

that's given me three options to test with, and see the results are like

your assistance is greatly appreciated and will hopefully aid others too

il try out the 3 scripts tomorow and report back with my findings for others benefit maybe

thanx again ronnylov

[ronnylov]

Thank's for the beer offer! I like writing avisynth scripts and digital video is my hobby so I don't mind help others.

The scripts above is only valid for a 720x576 source in 16:9 format but the principle is the same for other resolutions and formats. Find the active picture area and crop the source to that and then resize to a 16:9 format (or 4:3 format if the source is 4:3) with 1:1 pixels, then crop to your desired resolution.