I've decided to put together a simple guide to understanding HDTV. This is intended primarily to help me solidify things in my own mind. Please take it as such. If it helps you too then great.
The BIG Question: Do I need an HDTV?
The answer is probably no. More precisely, "No. Not right now."
Ask yourself the following questions: 1) Is my current TV dying or in some way unsatisfactory? 2) Do I have an Xbox 360, or Playstation 3 (The Wii is not high-def)? 3) Do I have an HD-DVD player or Blu-ray player? 4) Do I care about having marginally better picture quality on a handful of television stations?
If you answered yes to most of the above questions then you are probably one of the rare people that can benefit from an HDTV.
What is HDTV? What does it mean for me?
The answer to this question depends on you having an idea of how TV works. Bear with me. I'll try and keep my epeen in my pants and keep this simple.
The Basics: Moving Pictures
I suppose we really have to start at the beginning with the humble frame. When you were a kid you probably had or made one of those flipbooks. The kind that was made up of lots of pages with slightly different pictures on each page that you then flipped through to see a crude animation. Thats pretty much how TV works.
For this discussion I'm going to use the term frame. A frame is very like one of the pages in a flipbook.
The Basics: SDTV or The Old-Fashioned Way
Long ago it was decided that in America TV would be made by taking a picture (a frame) 30 times a second. 30 frames per second was deemed to be enough that the human eye and brain would perceive continuous motion. These frames could then be packaged and transmitted to homes everywhere.
The trick is taking this package of frames and showing it to viewers. In an old-fashioned TV (referred to as an Analog SDTV) this is done by firing what is essentially a ray gun the back side of a screen. The ray gun is aimed by magnets. It starts, lets say, in the upper left-hand corner and moves across in a straight line (a scanline). When it finishes a line it moves back to the left edge and down one line and starts over. When it gets to the bottom it goes back to the top and starts again. As it's firing and being aimed it's also changing intensity. All together this should re-create a frame.
So, in America, the normal television picture (Known as NTSC)provides for a frame made of 525 scanlines roughly 30 times per second. Of these 525 scanlines only 484 contain actual picture, the rest contain other goodies like closed captions and instructions to tell the gun to get back to the top, etc.
What's important to understand here is that pictures are made up of scanlines. Each one of these scanlines is an analog thing... like drawing a line on paper with a pencil and varying how hard you press as you move across.
The other thing that's important to understand here is something called interlacing. What this means is that instead of drawing all 484 scanlines at once and then starting over we actually cheat and draw half of them, start over, and draw the other half. We draw each half in half the time (roughly 1/60th of a second).
000000000000000 111111111111111 000000000000000 111111111111111
(This is an interlaced picture. On even 60ths of a second we draw all of the 0s, on odd 60ths of a second we draw 1s)
This may seem confusing but there are a lot of good reasons for it, mostly having to do with how our eyes work. Most importantly, If we drew all 484 lines every 1/30th of a second you would see the changes as a flickering (Most people see flicker at anything slower than 1/60th of a second... European TVs used to use 1/50th. Some people are more sensitive and still see flickering at even higher speeds.)
The downside of interlacing is that pictures can seem dimmer and scenes with fast-moving subjects will appear to smudge as there is too much difference between scanlines:
0 _______xXXXx________ 1 ______________XXXXX_ 0 _______XXXXX________ 1 ______________^XXX^_ __ = background xX^ = A football in motion, for example (subject)
Remember as you look at the above picture that the first 1/60th of a second we're drawing all of the 0 lines and the second we're drawing all of the 1 lines. What this looks like (thanks to something called persistence of vision) is that the football stretches as it flies down the field. Most of us are so used to this by now that we don't even notice it.
All of this is to explain normal TV (now referred to as Standard-Definition TV, or SDTV). This is also called 480p.
For computer geeks reading this: scanlines are pixels' ancestors.
(??? Am I conflating 480i and NTSC? They may not be technically the same thing)
Moving on: Deinterlacing and Progressive Scan
(I got off on the wrong foot here and need to do some serious rework)
The first step in to the HD world (We're not there yet, though!) is something called Progressive Scan (The "p" in 480p). After going to all those lengths to explain interlacing, we're not going to do it any more. We're, instead, going to draw ALL 484 lines 60 times a second.
There are two types of non-interlaced pictures. The most obvious is to display 60 different frames of 480 lines a second. This is referred to as Progressive Scan. The second way is to use some trickery and turn 30 frames worth of picture in to 60 frames. This second approach is
If you think about this for a sec you'll see a problem: there's not that much information coming in. We're only getting enough information (from a traditional TV signal, anyway) to draw 242 lines every 60th of a second (or, really, 484 lines every 30th of a second.) Where do we get the other 242 lines per 60th of a second that we need?
The answer is deinterlacing. Honestly, I'm not going to get in to the details of deinterlacing here. Suffice it to say that It's only useful to make old-fashioned interlaced TV signals look decent on newer TVs. In the future I suspect that interlacing will fade away or only be used as stopgaps between bumps in picture quality.
The thing about deinterlacing
There are a couple of important things to get here: 1) in the brave new world of TVs interlacing largely goes away. 2)
MORE LINES! or 720p on up
Woo! We're now in the land of HD. 720p is the beginning of that brave new world. Think back a little. I said that drawing 484 lines every 60th of a second is called 480p (I hope I said that.) As you can imagine, 720p means that we'll be drawing, instead, 720 lines every 60th of a second.
If you imagine, for a second, that your old SDTV and your new HDTV are the same height then will realize that drawing more lines in the same space means skinnier lines (the resolution has improved.) This means you'll be able to make out more little details (like moles, or hairs, or whatever little details you're in to).