a technical pixel aspect question

[Roland R. kahlenberg]

I assume that they are asking for 1024×576 because that is the default setup with which their broadcast equipment ingests broadcast-ready footage.

720×576, 16:9 is anamorphic and works fine as a distribution aspect ratio. Again, different strokes for different equipment.

I would also suggest that you think about working strictly in square-pixel aspect ratio as not all pugins work well in non-square comps/layers. In fact, with AE’s 3D render engine, you may notice slight abberation’s when trying to finesse non-square layers.

When 16:9 is sent out for broadcast, it is in effect, 1024, where it is anamorphic 16:9 or 1024×576. If it was 720 in both cases, then there wouldn’t be a widescreen effect unless the 576 was reduced. But that just doesn’t make any engineering or aesthetical sense.

Cheers
Roland Kahlenberg
https://www.broadcastGEMs.com – Adobe After Effects project files
https://www.myspace.com/rorkrgbspace

[Michael Duff]

thanks … i’ve only just had a chance to respond to your response… i’m not sure I completely understand the engineering side of this ..i know this is now getting beyond the normal scope of AE..but if someone could clear this up for me I’d be a happy nerd…

This is where I’m getting all my engineering facts from … the FreeTV Australia Operation Practice OP-29 and I’ve uploaded it to http://www.duff.tv/temp/FREETV.pdf

so…

This is what I am lead to believe:
For 4:3 PAL TV there was(is) 720 pixels on the horizontal. Engineers wanted to have a bigger/wider format that would work on the same bandwidth … and therefore a 16:9 pixel aspect was created… ie. still 720pixels wide but an anamorphic image that is then visually corrected at the receiving television set.

See Page15 1.2
“The Active length is defined as a nominal 702 pixels which represents the nominal PAL active line length (a line blanking width of 12usecs)”

And see Page 16 for a diagram.

And then, because we use computers to do much of our video work we decided to work in 1024 so that it ‘looked’ correct on a computer monitor.

So if I work in 720(1.79:1) … and then stretch it out to 1024 to distribute to these people … where did the extra pixels come from? they have been interpolated right? then when it gets sent to air as per my quote above it is getting sent at 720? so it is getting unstretched. and therefore pixels are being taken out? hence a loss in quality because some of those remaining pixels are not original pixels but the interpolated ones?

does this make any sense?

[Roland R. kahlenberg]

Nothing gets thrown away. It’s just an issue with square and non-square pixel aspect ratios. One way to fit 16:9 SD to 4:3 is to shrink it to fit proportionally until the widths are equal. For this to happen you need the full 1024pixels. Anything less would result in distortions or cutoffs.

HTH
Roland Kahlenberg
https://www.broadcastGEMs.com – Adobe After Effects project files
https://www.myspace.com/rorkrgbspace

[Michael Duff]

sorry, i’m just not convinced … i’m not worried about fitting a 16:9 image into a 4:3 screen by means of letterbox or scaling … I’m only talking about true, fullscreen, pan-scan 16:9…. When it is actually sent out over the airways… from the broadcaster … the signal that is sent out (according to that document i quoted above) is just 720pixels… regardless of if it has a 4:3 or 16:9 pixel aspect ratio…

so if we provide a 1024 square pixel image … and it is broadcast at 720 anamorphic, those extra pixels do not exist .. they must be thrown away.

do you see what I’m getting at?

[Roland R. kahlenberg]

OK. Let’s see if this makes sense. If part of the width is thrown away, then it must also be true that part of the height is thrown away. This is the case for letter-boxed broadcast of 16:9 over 4:3.

If you’re talking about pan & scan, then you will not get a letter-boxed view, then there is a strong possibility that pixels were thrown away. And of pixels were indeed thrown away, then it could happen to either or both the width and heigths of the footage.

Take note that the whole idea of pan & scan is to trim 16:9 footage to fit into 4:3. The end-result being no letter-boxing and a loss of the widescreen effect.

If this is the case, then it is truer to say that the 16:9 image has been panned & scanned during some pre-broadcast process (because that is the best way to ensure that nothing major gets panned off) rather than pixels being thrown away at say, from dead centre during broadcast.

In short, you’d still have to provide a full 16:9; 1024×576, non-anamorphic or 720×576 anamorphic so that they can do what it is that they do. If you were to cut anything off before delivery, then their setups will have to be changed for your case, which will likely not happen.

Cheers
Roland Kahlenberg
https://www.broadcastGEMs.com – Adobe After Effects project files
https://www.myspace.com/rorkrgbspace

[Michael Duff]

thanks again for the fast reply … but I don’t think you understand what I’m getting at .. I am in no way referring to letterboxing. I am only talking about true 16:9 that will fill a widescreen TV (and therefore I understand the sides will be cut off on 4:3, but the sides getting cut off is not what I’m talking about). and I am also not talking about actually pan-scanning an image and cropping off the edges in production.

“Let’s see if this makes sense. If part of the width is thrown away, then it must also be true that part of the height is thrown away. This is the case for letter-boxed broadcast of 16:9 over 4:3.”
-No, because I am not talking about letterboxing. I’m only talking about resizing the horizontal. Going from being an anamorphic 720 image to a 1024 square pixel image and back again..

“In short, you’d still have to provide a full 16:9; 1024×576, non-anamorphic or 720×576 anamorphic so that they can do what it is that they do.”

Yes, we could provide either… but what I’m suggesting is that EVERYTHING goes to air at 720 pixels on the horizontal (as per the document i linked to above). So if we work in 720 and then upsize that to 1024 those extra pixels have been interpolated. So we provide the 1024×576 image but it is still actually getting broadcast out of the transmitter at 720×576 anamorphic. So there are now less pixels on the horizontal ie some have been removed in the broadcasting process.

Does that make sense?

[Michael Duff]

ok … this could be much simpler… just for a moment … forget about 16×9 4×3 letterbox pan scan and everything above tv.

You have a photograph in photoshop that is 500×500 pixels. You scale it up 500% and then save it. Then open up that new larger image and reduce its scale by 500%. You have an image the same size as the original, but it is now of a lower quality. Agree?

Essentially that is what we are doing if we work at 720, deliver at 1024 and then broadcast at 720.

does that help?

[Roland R. kahlenberg]

I think it’s best if we agree on three thingies first –

1) the difference between square PAR and non-square PAR
2) 16:9 is broadcast for 4:3, simulcast or otherwise, with a centre-cut
3) 16:9 is also broadcast as pure 16:9

Item 3 above should show why you shouldn’t deliver at 720. Item 2 should show how 1024 gets cut down in size for 4:3 distribution. Item 1 should provide the background info that 720×576 anamorphic is equivalent to 1024×576 square PAR

Lastly, there is a typo in item 3.4.2 in the PDF you’ve uploaded.

Cheers
Roland Kahlenberg
https://www.broadcastGEMs.com – Adobe After Effects project files
https://www.myspace.com/rorkrgbspace

[Michael Duff]

yep ..
1) I agree and understand this
2) yes I a agree
3) yep agreed

“Item 2 should show how 1024 gets cut down in size for 4:3 distribution” – Just to clarify this point .. you mean “cropped” rather than “re-sized” .. .right?

I understand that 720 anamorphic is the same as 1024 square.

But what I’m saying is… the actual PAL signal that is sent out from your broadcaster is 720pixels wide regardless of how you have provided it. So if we work in 720 anamorphic, then upsize to 1024 square, then transmit at 720 anamorphic there has been a redundant re-size and therefore a slight loss of quality …

btw, earlier you said you always work at 1024 … do you also always work at 768×576 for 4:3?

[Roland R. kahlenberg]

[duffbeer911] “”Item 2 should show how 1024 gets cut down in size for 4:3 distribution” – Just to clarify this point .. you mean “cropped” rather than “re-sized” .. .right?”

Correct.

[duffbeer911] “But what I’m saying is… the actual PAL signal that is sent out from your broadcaster is 720pixels wide regardless of how you have provided it. So if we work in 720 anamorphic, then upsize to 1024 square, then transmit at 720 anamorphic there has been a redundant re-size and therefore a slight loss of quality …”

It is 720 wide if it the 16:9 is broadcast as 4:3, center-cut. BUT when the 16:9 is broadcast as pure 16:9, then you will get the full glory of 1024 pixels wide.

The fact that they do in fact broadcast at pure 16:9 should preclude you from wanting to provide anything at 720, unless it’s anamorphic. But from your initial post, they have requested for 1024×576.

[duffbeer911] “btw, earlier you said you always work at 1024 … do you also always work at 768×576 for 4:3?”

Always. Everytime. All the time. Once I’m ready to render, I drop my square PAR comp into an anamorphic comp, as and when required. This workflow ensures that everything I see on my computer monitor is accurate and that all plugins work correctly. Take note that plugins which require pixel information for alignment etc. frequently go bonkers when placed within a non-square PAR layer or comp.

Cheers
Roland Kahlenberg
https://www.broadcastGEMs.com – Adobe After Effects project files
https://www.myspace.com/rorkrgbspace