HD or "High Definition" TV is the latest development in digital television, offering higher quality pictures and sound, than ever before. Pictures and sound are converted into digital format and compressed, so that signals require less space or "bandwidth" than traditional analogue TV signals.
This in turn, means that more TV channels can be transmitted, along with new features such as on-screen listings, interactivity, audio description, etc.. Digital TV signals can be received by a standard TV aerial, a satellite dish or a cable connection, but in any case, must be decoded by a set-top box or a TV set with a decoder built in.
All About HD Digital TV
HD or "High Definition", refers specifically to the total number of picture elements or "pixels", of which an image – or a screen on which that image is displayed – is composed. Traditional, standard definition pictures in the U.K. have a resolution of 720 x 576 pixels, or "576i" for short. HDTV pictures on the other hand, have a resolution of 1,280 x 720 pixels (720p) or 1,920 x 1080 (1080i, or 1080p).
The "i" and the "p" refer respectively to "interlaced" and "progressive" scanning, or the process by which an image is drawn or scanned onto the screen. Interlaced scanning is the older, less sophisticated method, whereby the odd-numbered lines are scanned first in one pass, followed by the even-numbered lines in a second. Progressive scanning, on the other hand, scans the lines sequentially in a single pass, so that there is no appreciable "flicker" in the picture displayed.
Another important feature of HDTV is the so-called "aspect ratio", or the ratio of the width of an image, or screen, to its height. Standard definition TV pictures have an aspect ratio of 4:3, which accounts for the almost square box-like appearance of traditional, CRT ("Cathode Ray Tube") TV sets. HDTV sets, by contrast, have a true, widescreen aspect ratio of 16:9, so that movies for example, can be viewed exactly as the director originally intended.
The most important feature of HDTV however, is the resolution. The higher the resolution, or number of pixels, the higher the clarity and overall quality of the picture, all other things being equal. This means that with up to five times more pixels than standard definition TV pictures, HDTV pictures are significantly clearer, sharper and richer in colour.
Obviously, to enjoy HDTV you need a source of HD broadcasting in the first place. Nowadays Sky, Virgin Media, Freesat and Freeview offer HD channels. You'll also need a suitable set-top box – or a TV set with a decoder built in – and a TV set designated "HD Ready" or "Full HD". Even then, this doesn't mean that you'll be able to watch everything in HD; you'll be able to watch it, but it may not be broadcast in HD just yet.