Shots Across The Bow

What is HD? 1080p or 1080i? Why do I care?

As I wrote the last post, it occurred to me that there are probably a lot of people out there who don't understand what all of the fuss about HD is about, and what all the numbers mean, and I figured that since we're a bit over a year away from the end of television as we knew it, a quick primer couldn't hurt.

First, the quick version. HD produces a sharper, clearer picture with more details, and it does so using a digital signal, so you'll need a new ATSC tuner if you want to keep your old TV. Or you can buy a new TV with an ATSC tuner, and you're all set.

OK, for you non-tech types, that's really all you need to know.

For the rest of us, let's take a look at the old TV so we can understand how the new TV works.

The stronger the beam, the brighter the phosphor glows. The beam starts at the top of the screen and paints a picture all the way across the screen, but just at the very top of the screen. When it reaches the side, it flashes back to the other side, only it drops down a bit and draws another line across the screen. This continues until the beam draws the last line across the bottom of the screen, then it flashes back up to the top of the screen, ready to begin this whole process again.

The sequence above describes the drawing of one single frame of video, and it takes place almost 30 times a second in an old tube type TV. Think of it like an Etch-a-Sketch with three controls instead of two. In addition to left/right and up/down, you also have a knob that controls how hard the scraper touches the screen. The scraper starts in the upper left corner of the screen, and as you wind it across the screen, you vary the pressure with the third knob to create one line of your picture. When you reach the end of the screen, you use the third knob to lift the scraper off of the screen, and then return it to the beginning position. Next, you use the up/down to move the scraper down one line length, and repeat the whole process until you get to the bottom of the screen.

This is how your old, tube type TV's worked. Greatly simplifying things, an electron gun squirts electrons to the screen which is covered with a phosphor, a chemical that glows when it is hit by electrons. Two magnets are used like the buttons of the Etch a Sketch, to push the beam from side to side and up and down. The TV signal is our third knob, and it varies the strength of the electron beam, which affects how brightly the phosphor glows.

When TV first started, a bunch of people, the National Television System Committee, got together and decided we needed a broadcast standard, and they decided a few things. First, they decided that the number of lines across the screen should be 525. They also decided on a frame rate, the number of times a complete image is formed on the screen, should by 30 frames per second, that the aspect ratio, width vs height, should be 4:3, and that the signal should be interlaced, meaning that every other line is skipped on one pass, then the skipped lines are picked up on the second pass. To understand all this, let's pull out our Etch a Sketch again.

The first part of the NTSC standard is that the aspect ratio is 4:3. All that means is that a screen 4 inches wide must be 3 inches tall. If it's 8 inches wide, it has to be 6 inches tall, and so on. In short, an NTSC screen should always be 3/4 as tall as it is wide. So if our Etch a Sketch screen is 12 inches wide, it must be 9 inches tall. Next, we know that our screen is divided into 525 horizontal lines from top to bottom. And finally, we know that our scanning sequence is not a straight top to bottom like we used in our first example. Instead, we'll scan the first, third, fifth, etc lines until we get to the bottom, then we'll scan the second, fourth, sixth etc lines. What this means is we have to scan the screen top to bottom twice for one full frame, which means that our scan rate must be 60 times per minute, or Hz, for a frame rate of 30 frames per minute.

So that's how things started off. We've changed a few things since then. We added color, replaced the electron gun and screen with LCDs or plasmas, but the NTSC standard remained pretty much the same.

Until HD came along.

The new standard for HDTV is called the ATSC standard, for the Advanced Television Systems Committee. Unlike the NTSC which set a single standard, the ATSC left the door open for multiple standards and resolutions, and that's where some of the confusion has crept in. SO lets clear it up a bit.

Let's start with the aspect ratio. The ATSC specifies an aspect ratio of 16:9, instead of the old 4:3. This means that our Etch a Sketch has to be 9 inches tall for every 16 inches in width. Going back to our earlier example, a 12 inch wide Etch a Sketch would now be just a shade under 7 inches tall, instead of 9. Also, the number of horizontal lines changed. the ATSC standard accepts either 720 or 1080 lines of horizontal resolution. And finally, the ATSC accepts either interlaced scanning, like the NTSC, or progressive scanning, where every line is scanned in sequence, rather than the odds then evens in two passes.

OK, so let's put all of this together and see where we stand.

The first thing you should look for is an ATSC tuner. That will give you the ability to pick up your local HD digital TV stations without paying a dime to the cable company. For Knoxville folks, I live about 20 miles from Sharp's Ridge in Kodak, and I can pick up all of the Knoxville stations with an amplified indoor antenna. Reception gets a little spotty at times, so I'll probably invest in an outdoor antenna eventually.

The next thing to look for is 720 vs 1080. Personally, I go for the 1080. There is a visible difference in the resolution, especially when you get up to the larger screen sizes. As for progressive vs interlaced, it's really getting harder to find an interlaced set anymore. Progressive scan isn't much more expensive and it results in a much nicer picture.

We haven't really talked about size much yet. Don't worry; we're about to.

Obviously it depends on where you're going to use the set, and what you're going to use it for, but in general, go big. There's talk of a new, 1440 line standards coming down the pike in the next year or so, but cable, satellite and over the air stations are struggling just trying to pump out a 1080i signal now. They won't switch up to a higher resolution for quite some time, so the only thing the higher resolution will work for is DVD players and the like. Your 1080p will be a good value for years to come.

So go big. I've got a 60" 1080p in my room, and it's just like being at the movies, without the annoying kids, and the sticky floors.

The one other thing you should look for is plenty of connection in the back. The HD standard connection is called an HDMI cable. It carries the video and the audio, and are the best way to connect an HD source, like a cable box, PS-3 or XBox 360, or Blu-ray DVD player, to your TV set. They are also freakishly expensive. If you don't want to pony up for the HDMI cable, you can use component cables. These break up the video signal into three parts and usually come with stereo audio cables as well. For your TV, look for 2 HDMI connections and 2 component connections.

And that's it. If you read this far, I hope you have a better understanding of what all the numbers and symbols mean. If you have any questions, feel free to leave them in the comments. If I don't know the answer, I'll make something up!

Posted by Rich at February 6, 2008 10:58 PM | TrackBack