720x480 vs 720x486 conundrum

[Lyris]

Hi all,
I'm delivering a broadcast master to a TV station in the US. They want it on NTSC Digital Betacam (which is 720x486). It will be played out across digital cable and satellite (which I believe is 720x480, just like a DVD).

The source is film, and needs delivered at 60i with 3:2 pulldown applied.

My question is, what happens to those extra 6 lines?

The image can't be scaled, because that would destroy the interlacing, so am I right in saying that it will lose 6 lines from the top or bottom of the picture and the image will be distorted slightly?

How is this done in NTSC-land?

[Video Dude]

Don't have an answer for the question, but in the US, digital cable has a SD resolution of 704x480 or 528x480. DirecTV (satellite) broadcasts SD at 480x480. DISH Network (satellite) broadcasts SD at 544x480.

[IanB]

525 line NTSC has active picture content on 2 fields of upto 242.5 lines each. The other 20 lines on each fields is for sync, teletext, etc. A true 4:3 picture needs all 242.5 * 2 lines to make up the full display height.

When digitised 242.5 lines becomes 243 lines, so a full frame has 486 lines (the 2 half lines have no content/are black).

To make the DVD hardware design easier only 480 lines height was selected. The other 6 lines are discarded, but the original Pixel Aspect Ratio, PAR, has been maintained. On display there are 22.5 non-content lines per field instead of 20.

So just add an extra black 6 lines to the bottom of your 480 line source. There is no need to scale as the PAR is the same.

[Mug Funky]

actually, i'd add 3 to the top and 3 to the bottom if your 480 line source is upper field first.

2 at the top and 4 at the bottom if you're in lower field first.

NTSC SDI signals (the digital connections between broadcast gear) are 486, lower field first. NTSC DVD is usually upper field first, and NTSC miniDV or DVCAM is lower-field first, so they need different padding to bump them to 486 lines.

it might help you to ask the people you're delivering to (or, at least ask to talk to someone in the tape room), just so you know how they'd like it padded. there's the possibility that the padding might get chopped off differently, moving the picture up or down.

[TheSkiller]

There was a Thread about this some time ago, here (http://forum.doom9.org/showthread.php?t=152179) it is.
It really seems to vary how these 6 lines are handled both during the creation of a 486 lines NTSC video and the playback of a 480 lines video with say a DVD player.

[Blue_MiSfit]

It's for Closed Captions.

I'd add the matte to the top, since this is usually where caption data would be. This is all assuming you won't be encoding closed caption data into the video image, of course :) If you are, you shouldn't need to ask this question!

Example:

http://www.cpcweb.com/nle/

[Lyris]

No, no captions on tape. Those are being supplied as an external file.
Thanks for the answers, everyone. Good to know that lines are discarded rather than the image being scaled.