Dealing with "nominal analogue blanking"

[zilog jones]

With analogue sources I would normally leave the blanking alone if the destination is going to be, say DVD, but what do I do in other cases (i.e. square pixel formats) in order to maintain correct aspect ratio? Is the picture with the blanking 4:3 or do I crop out the blanking and resize to get 4:3?

[Ghitulescu]

What has the blanking to do with?

[zilog jones]

I'm not sure what your question is... :confused:

I'm talking about the visible blanking with analogue video sources, e.g. if something is captured at 720x576 but only 702x576 in the centre is the actual picture and there are black gaps down the side.

This Wikipedia article with no references is the source of my confusion: http://en.wikipedia.org/wiki/Nominal_analogue_blanking

It gives the impression that when going to a square pixel format that only the actual picture area is 4:3 and one would crop the nominal blanking to get a 4:3 picture (assuming it's not anamorphic or anything).

[Ghitulescu]

It's not blanking, it's the unused part for digital systems.

Blanking is the unseen signal (when the spot is "blanked" during the vertical and line retrace). According to their definition, any black pixel (or a series of them) is a blanking :p.

It's a bit confusing, because it correlates 2 issues that have noting to do one with eachother.

There are no pixels in the analogue world, only lines (scan lines to avoid any confusion). The number of horizontal pixels (called samples) is derivable from the frequency of the signal and its timing, and since it's between 720 and 704, both values are used (full D1 and cropped D1). 720 and 704 are both 16x, a value needed by MPEG.

So, you may have this "blanking" only in digital systems outputting analog from digital. You don't have them in pure analog. You may have black pixels in capturing, but this is only dependent on how the chip selects the timings (capture window).

I've seen that most BBC TV series have the overscan completely blacked (like a black frame around the actual image), this kills 2 birds with one stone.

If you want more, browse the net. The issue is absolute unimportant.

The aspect ratio is irrelevant in connection to this issue, a digital player would output video lines fulfilling the requirements of PAL/NTSC/SECAM. I see a problem only with various AVI that fill up the net with strange resolutions (like 634x436) but not with legal 720/704x576i (x480i).

[zilog jones]

Blanking is the unseen signal (when the spot is "blanked" during the vertical and line retrace). According to their definition, any black pixel (or a series of them) is a blanking :p.
I know it's not part of the horizontal blanking interval, but if it's not blanking what is it? I can't think of a more appropriate term.

So, you may have this "blanking" only in digital systems outputting analog from digital. You don't have them in pure analog.
I do not believe this is true. My video monitor has an "H-V Delay" mode which makes VBI and HBI visible. Although obviously I can't distinguish between this "nominal blanking" and the front and back porch (excluding colour burst) in PAL, there are clearly differences in length of this blanking between different sources, and I'm talking about completely analogue sources. IIRC the front and back porch are supposed to be fixed lengths (within quite small tolerances), so this extra blanking cannot just be a consequence of digital sampling.

The aspect ratio is irrelevant in connection to this issue, a digital player would output video lines fulfilling the requirements of PAL/NTSC/SECAM. I see a problem only with various AVI that fill up the net with strange resolutions (like 634x436) but not with legal 720/704x576i (x480i).

I am talking about a 1:1 pixel aspect ratio environment, where the playback software has no idea what these mystical black lines down the sides of the picture are. Let's just say my final output video is going to be 768x576. My source is 720x576 but only 683x576 is active picture content and the rest is "blanking" - to maintain a 4:3 AR do I leave it as is and resize, or crop out the blanking?

[Wilbert]

I am talking about a 1:1 pixel aspect ratio environment, where the playback software has no idea what these mystical black lines down the sides of the picture are. Let's just say my final output video is going to be 768x576. My source is 720x576 but only 683x576 is active picture content and the rest is "blanking" - to maintain a 4:3 AR do I leave it as is and resize, or crop out the blanking?
The analog capture guide answers this. Whether or not the 683x576 constitutes the 4:3 image depends on your capture window: http://www.doom9.org/capture/capture_window.html. The black bars on the left and the right can be a part of the active picture, but you won't know until you determine your capture window.

[2Bdecided]

The pixels either side of the 702 active pixels in a 720-pixel image certainly are "blanking". They're the tiny part of the 12us "line blanking interval" that is intentionally captured by digital systems (so that slightly mis-timed analogue signals still make it into the digital domain).

It's absolutely correct to treat 702x575 or 704x576 pixels as being the actual 4x3 (or even 16x9) image. In practice, anything within a few pixels of that is fine - just crop away the black and resize to 768x576 or 640x480 or whatever square pixel resolution you're aiming for (if that's what you want to do!). Don't crop or resize vertically if the source is interlaced. Beware of non mod(4) cropping when working with YV12 video.

683 pixels is probably too narrow for it to be really 4x3, unless you captured this yourself with some strange timing on your video card, or unless someone incorrectly resized it at some point. Try it and see.

Cheers,
David.

[zilog jones]

I have an SAA7134 based capture card, which according to other sources (haven't tried the test in the capture guide yet) has a 52.15 µs capture window. I usually capture at 704x576, but if I try 720x576 there's no extra blanking added or anything (picture quality looks more or less identical too).

I think I get what's going on now, thanks.

[Wilbert]

The pixels either side of the 702 active pixels in a 720-pixel image certainly are "blanking". (...)
See comment of zilog jones. That's only the case if your capture window is 53.333 µs and you capture at 720 pixels. Also if you capture at 720x576 (or some other horizontal size), it will always be a scaling of your capture at 704x576 (thus no pixels are added as zilog jones noted).

It's absolutely correct to treat 702x575 or 704x576 pixels as being the actual 4x3 (or even 16x9) image.
No, that's not necessarily the case with analog capture cards. As i said that depends on your capture window of your capture card.

[2Bdecided]

Yes, sorry, of course. I was assuming fixed 13.5MHz sampling, which doesn't usually apply here.

Cheers,
David.