What are the main differences between normal TV and HDTV?

The most dramatic change is in resolution, which is measured by number of lines of resolution, which can be thought of as horizontal layers of the image. The more lines of resolution a display contains, the sharper and more realistic the image is. For the sake of comparison, videocassettes have 240 lines of resolution, DVDs 480 lines, and HDTV either 720 or 1080 lines. That improvement of at least 50% yields an incredibly lifelike image.

Another key differentiating factor between SDTV and HDTV is the method of displaying, or “scanning,” the image onto the screen. Standard televisions use a scheme called interlacing which displays two parts of the image (called fields) in rapid succession; they first show the 2nd, 4th, 6th lines, etc., on the screen, then display the 1st, 3rd, 5th, etc., about 1/60th of a second later. Because the two fields are drawn so quickly, the eye sees a single image. This ordinarily works well, but for some fast-moving sporting events, display quality is enhanced by drawing the entire screen, one line at a time. This method of displaying the image is called “progressive scan.” Some HDTV content is displayed using this technique.

What do 720p and 1080i mean?

These number and letter codes describe the combination of resolution and scan format being used to display an image. For example, 720p refers to 720 lines of resolution displayed progressively, while 1080i describes an interlaced image with 1080 lines of resolution. High-definition televisions are able to display either 720p or 1080i, though they are likely to be optimal for one or the other.

What about widescreen? Is it the same thing as HDTV?

Widescreen TVs have a more rectangular profile than standard TVs. The shape of a video image is often expressed in a ratio of width to height. While the ratio for standard TV is four units wide by three units high (written as 4:3), the widescreen ratio is generally 16:9.

HDTV has adopted the widescreen format, but not all widescreen TVs are high-definition. This is often the case with widescreen TVs manufactured in the late 1990s to early 2000s. You need to check the specifications of the TV in question to make sure that it can display 720p or 1080i. Otherwise, it is not truly high-definition.

Where can I get high-definition content to watch?

There are three basic methods to get material for HDTV:

Cable: Many cable systems offer the major networks plus select other channels in high-definition format (for an additional fee). In most cases, they provide a special HDTV cable box to decode the signals. This is different than so-called “digital cable,” which provides a standard 480i image, but offers additional features like on-screen programming guide, live TV pause, and on-demand movies.

Over the air (OTA): Your TV picks up the signals with an antenna, just like in the old days. There are no subscription charges to receive OTA signals, but you may need to buy a separate tuner (called an ATSC tuner) if your TV does not come with one. Hinsdale residents may be able to use an indoor antenna to receive HDTV signals from Chicago. Use AntennaWeb to see what type of antenna is recommended for your street address and what direction to point it.

Satellite: Two providers of satellite TV service, Dish Network and DirectTV, offer some content in high-definition. You will need a special HDTV satellite receiver in order to view this material, for which there will be a subscription charge.

In the future, you’ll be able to watch high-definition DVDs, but for now you’re limited to televised material.

Next time: HDTV Part II.

The next Tech Talk will continue to focus on high-definition television and will include a discussion of types of HDTVs available for purchase, inlcluding the popular flat-panel models.