What Is HDTV?

First lets take a look at analog television vs. digital television. Television works by displaying a series of images, one after another, fast enough so that it seems like the content of the images is moving.

Each image is broken up into horizontal lines and each line is made up of dots (pixels). Analog televisions transmit these lines through the air using a 6MHz signal. In the United States, there are 525 lines that make up each image and 30 images per second.

To draw just one image, the television draws every other line and then makes a second pass and draws in the alternate lines it did not draw on the first pass. This is called interlacing. You may be thinking, why draw every other line and go back and draw the ones missed? Why not draw every line from top to bottom on every pass? Welcome to progressive scanning.

Progressive scanning is a newer technique used to draw every line from top to bottom on your screen in one pass. This allows the images drawn per second to be increased to 60 instead of the standard 30. That means you shouldn't see any flickering. Most computer monitors use progressive scanning for a solid image.

Now that you understand the difference between interlaced and progressive scan images, you know why progressive scan dvd players cost a bit more. Let's get into a bit more detail on how the signal comes through.

With each line of each image being transmitted, there are also horizontal and vertical sync signals so that your television knows where to place each line. These three signals together make a composite video signal. This composite signal can be transmitted over the air and received directly by your TV antennae.

Digital television follows a similar patter, but instead of transmitting the color and intensity of each dot (pixel) in each line of the image to display, the line is encoded in a digital format and sent to you. The FCC gave each TV broadcaster a frequency to use for digital broadcasts in addition to the frequency they have been using for analog broadcasts. So now each TV broadcaster can send up to 19.39 Mega-bits-per-second of data for their broadcasts. (19.39 Mbps is considere their bandwidth)

The broadcasters can then divide this bandwidth up and send different channels through to you in a digital format, or keep the full bandwidth available for a single channel with very high quality. (Images with a lot of movement require more bandwidth since the images change so drastically in comparison to an image of a person talking where only the pixels around the mouth and face would change. Likewise, progressive scan images take more bandwidth than interlaced.).

Let's take it a step further. Remeber how we said there are 525 lines that make up each image that is drawn on your television. By increasing that number, we'll get a clearer picture. Broadcasters now have the option and sending images to you in three different formats:

480 - Interlaced (480i) or progressive (480p) - 704 x 480 pixels - Standard Definition

720 - Progressive (720p) - 1280 x 720 pixels - High Definition

1080 - Interlaced (1080i) or progressive (1080p) - 1920 x 1080 pixels - High Definition

The High Definition (HD) formats of these signals have a different aspect ratio, meaning the picture is wider than your normal television set and thus HDTV is born.