In analog TV, a 6 MHz analog signal carries intensity and color information for each scan line of the picture. An analog TV signal in the U.S. has 525 scan lines for the image, and each image is refreshed every 30th of a second (half of the scan lines are painted every sixtieth of a second in what is called an interlaced display). The horizontal resolution is approximately 500 dots for a color set.

This level of resolution was amazing 50 years ago, but today it is rather passé. The lowest resolution computer monitor that anyone uses today has 640 x 480 pixels, and most people use a resolution like 800 x 600 or 1024 x 768. People have grown comfortable with the great clarity and solidity of a computer display, and analog TV technology pales by comparison.

Many of the new satellite systems, as well as DVDs, use a digital encoding scheme that provides a much clearer picture. In these systems, the digital information is converted to the analog format to display it on your analog TV. The image looks great compared to a VHS tape, but a great deal of picture quality is lost because of the conversion to analog.

There is now a big push underway to convert all TV sets from analog to digital, so that digital signals drive the TV set directly.

When you read and hear people talking about digital television, what they are talking about is the transmission of pure digital television signals, along with the reception and display of those signals on a digital TV set. The digital signals might be broadcast over the air or transmitted by a cable or satellite system to your home. In your home, a decoder receives the signal and uses it, in digital form, to directly drive your digital TV set.

There is a class of digital television that is getting a lot of press right now. It is called high-definition television, or HDTV. HDTV is high-resolution digital television (DTV) combined with Dolby Digital surround sound (AC-3). HDTV is the highest DTV resolution in the new set of standards. This combination creates a stunning image with stunning sound. HDTV requires new production and transmission equipment at the HDTV stations, as well as new equipment for reception by the consumer. The higher resolution picture is the main selling point for HDTV. Imagine 720 or 1080 lines of resolution compared to the 525 lines people are accustomed to in the United States (or the 625 lines in Europe ) -- it's a huge difference!

Of the 18 DTV formats, six are HDTV formats, five of which are based on progressive scanning and one on interlaced scanning. Of the remaining formats, eight are SDTV (four wide-screen formats with 16:9 aspect ratios, and four conventional formats with 4:3 aspect ratios), and the remaining four are video graphics array (VGA) formats. Stations are free to choose which formats to broadcast.

The formats used in HDTV are:

• 720p - 1280x720 pixels progressive
• 1080i - 1920x1080 pixels interlaced
• 1080p - 1920x1080 pixels progressive

"Interlaced" or "progressive" refers to the scanning system. In an interlaced format, the screen shows every odd line at one scan of the screen, and then follows that up with the even lines in a second scan. Since there are 30 frames shown per second, the screen shows one half of the frame every sixtieth of a second. For smaller screens, this is less noticeable. As screens get larger, the problem with interlacing is flicker.

Progressive scanning shows the whole picture, every line in one showing, every sixtieth of a second. This provides for a much smoother picture, but uses slightly more bandwidth.