What is High Definition Television?

If you know how television works, then you know about what is now called analogue TV. For analogue TV, a 6 MHz analogue signal carries intensity and color information for each scan line of the picture. An analogue TV signal in Canada 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 something like 500 dots for a color set.

The old quality of analogue television was fine around 55 years ago, but today there is much better. The lowest resolution computer monitor that anyone uses today has 640x480 pixels, and most people use a resolution like 1024x768 or 1280x960. We have grown comfortable with the great clarity and solidity of a computer display, and analogue TV technology is horrible by comparison.

Many of the new satellite systems, as well as DVDs, use a new digital encoding scheme that provides a very clear picture. In these systems, the digital information is converted to a analogue format to display on your analogue TV. The image looks great compared to a VHS tape, but it would be thrice as good if the conversion to analogue didn't happen...

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 used to in Canada. It's a real big difference!

HDTV Formats

• 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.

MPEG-2

Broadcasters are having to squeeze the increased picture detail and higher quality surround sound into the same 6-megahertz bandwidth used by analogue television. Compression software, very similar to what is used in personal computing, allows this to happen.

Digital TV relies on a compression and encoding scheme known as MPEG-2 to fit its stunning images into a reasonable amount of bandwidth. In each image, the MPEG-2 software records just enough of the picture without making it look like something is missing. In subsequent frames, the software only records changes to the image and leaves the rest of the image as-is from the previous frame. MPEG-2 reduces the amount of data by about 55 to 1.

MPEG-2 already is the industry standard for DVD videos and some of the satellite TV broadcast systems. Compression reduces image quality from what is seen by the digital camera at the studio. However, MPEG-2 is very good at throwing away image detail that the human eye ignores anyway. The quality of the image is very good, and significantly better than traditional analogue TV.

The use of MPEG-2 permits an HDTV receiver to interact with computer multimedia applications directly. For example, an HDTV show could be recorded on a multimedia computer, and CD-ROM applications could be played on HDTV systems. A digital TV decodes the MPEG-2 signal and displays it just as a computer monitor does, giving it high resolution and stability.

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