Interlaced SDTV and HDTV question

So I understand that in interlaced video (from 480i to 1080i) you are simply showing “half” of the image (ie the odd or even scanlines) for 1/60th of a second (approximately) and then showing the other “half” of the image.

But there seems to be some sort of caveat to this…that you don’t actually need to have a vertical resolution of 480 pixels or of 1080 pixels to accomplish this.

How is this possible?

It seems to me that if you’re talking about a CRT with a bunch of holes in it, you have to physically have 1080 holes. In the first “field” you are shooting electrons through the odd numbered holes and in the second “field” you are shooting electrons through the even numbered holes.

If you don’t actually [i]have[i] the right number of holes then how the heck can you get this effect?!

I mean, the whole idea seems to be that you skip over a portion of the screen when you skip a scanline. So for instance if each scanline is like 1mm high, there has to physically be a 1mm gap between the 1st and the 3rd scanline to actually skip. If you don’t physically have 1080 (or 480) actual scanlines of resultion then how does this work?

HELP!

:D

I know this stuff pretty well but, dear God, I have no idea what you’re saying.

http://www.howstuffworks.com/hdtv1.htm :
“Interlaced” or “progressive” refers to the scanning system. In an interlaced format, the screen shows every odd line at one scan of the screen, and then follows that up with the even lines in a second scan. Since there are 30 frames shown per second, the screen shows one half of the frame every sixtieth of a second. For smaller screens, this is less noticeable. As screens get larger, the problem with interlacing is flicker.

Oh well.

Did you even read my question?

I pretty much said exactly what you just reproduced…down to the explanation of showing one half the frame every sixtieth of a second!

:roll:

Yeah, that’s because it’s the core of interlacing - you use the full height resolution. In one frame, you show the even lines, in the other you show the odd lines.
You can’t accurately reproduce a 480i image if you don’t have 480 scanlines. Either you need to distort the image before you broadcast it (which would be a neat trick) or you throw away some data

And no, I didn’t read your question. I was just randomly posting quotes on interlacing, and happened to post it in your thread. What a crazy coincidence!

Well I figured it out:

The concepts involved in television “resolution” for CRT displays are so unfathomably complicated that it simply defies all ability to understand.

http://www.dansdata.com/gz029.htm

What it all comes down to is that analog CRT display simply do not display what you think they are displaying.

I’m so used to thinking about images in terms of a grid of EXACTLY H pixels wide and V pixels high, that I wasn’t prepared to understand that CRT television just aren’t even close to fitting into that paradigm.

In fact, most 1080i and 720p CRT HDTV’s are not able to display 1080 or 720 pixels vertically. They simply downconvert the signal to whatever they can display!

Although in fairness this applies to CRT PC monitors too. You can set your monitor for 1600x1200 but that doesn’t mean you could see the individual pixels at that resolution.

[EDIT: Well thanks for trying to answer my question Gunmetal…sorry to be snarky about it. It’s just so complicated that I feel like I can’t find anything approaching a straight answer to some of the questions I have. Grrr.]

Another interesting article:

http://millimeter.com/mag/video_video_monitors_hdtv/