I want my DTV!

With the switchover to Digital television coming in February 17, 2009, I thought I would give a little information on what a digital signal is, and what it means to get 'Digital'.

First off, lets look at the current signal, analog. The analog TV standard has been in use in the United States for about 50 years. To review quickly, here are the basics of analog television transmission:

• A video camera takes a picture of a scene. It does this at a frame rate of 30 frames per second.

• The camera rasterizes the scene. That is, the camera turns the picture into rows of individual dots called pixels. Each pixel is assigned a color and intensity.

• The rows of pixels are combined with synchronization signals, called horizontal sync and vertical sync signals, so that the electronics inside a TV set will know how to display the rows of pixels.

This final signal, containing the color and intensity of each pixel in a set of rows, along with horizontal and vertical sync signals, is called a composite video signal. Sound is completely separate. When you plug your vcr in to the back of the TV, you plug in the yellow cable, or composite video cable. The sound cable is white (and with stereo sound there is an additional red one). When a composite video signal is broadcast over the airwaves by a TV station, it happens on a specific frequency. In the United States, we know these frequencies as VHF channels 2 through 13 and UHF channels 14 through 83.

If you currently have an analog TV, and it works fine with broadcast tv, vcr's, dvd players, and so on, an obvious question would be, "What's wrong with analog TV?"

The main problem is resolution.

• The resolution of the TV controls the crispness and detail in the picture you see.
• The resolution is determined by the number of pixels on the screen.
• An analog TV set can display 525 horizontal lines of resolution every thirtieth of a second. In reality, however, an analog TV displays half of those lines in a sixtieth of a second, and then displays the other half in the next sixtieth, so the whole frame is updated every thirtieth of a second. This process is called interlacing.

But now we have all become conditioned by computer monitors to be comfortable with much better resolution. The lowest-resolution computer monitor displays 640x480 pixels. Because of the interlacing, the effective resolution of a TV screen is perhaps 512x400 pixels.

So the worst computer monitors you can buy have more resolution than the best analog TV set; and the best computer monitors are able to display up to 10 times more pixels than that TV set. There is simply no comparison between a computer monitor and an analog TV in terms of detail, crispness, image stability and color. If you look at a computer monitor all day at work, and then go home and look at a TV set, the TV set can look very fuzzy.

The first idea that is new to digital TV is the digital signal.

Analog TV started as a broadcast medium. TV stations set up antennas and broadcast radio signals to individual communities. You can put a pair of rabbit ears on your TV and pick up channels 2 through 83 for free. What you receive, as described earlier, is a single, analog composite video signal and a separate sound signal.

Digital TV has started as a free broadcast medium as well. Each broadcaster has one digital TV channel, but one channel can carry multiple sub-channels if the broadcaster chooses that option. Here's how it works:

On its digital channel, each broadcaster sends a 19.39-megabit-per-second (Mbps) stream of digital data. Broadcasters have the ability to use this stream in several different ways. For example:

• A broadcaster can send a single program at 19.39 Mbps.

• A broadcaster can divide the channel into several different streams (perhaps four streams of 4.85 Mbps each). These streams are called sub-channels. For example, if the digital TV channel is channel 53, then 53.1, 53.2 and 53.3 could be three sub-channels on that channel. Each sub-channel can carry a different program.

Finally, the HD formats of digital TV have a different aspect ratio than analog TVs. An analog TV has a 4:3 aspect ratio, meaning that the screen is 4 units wide and 3 units high. For example, a "25-inch diagonal" analog TV is 15 inches high and 20 inches wide. The HD format for digital TV has a 16:9 aspect ratio, as shown below:

See how the digital image on the left can show more of the screen, and in a better resoultion?

In my next post, I will give some information on Do I Need to Buy an HDTV set, and what to look for if you decide to make a purchase.