Traditional CRT televisions display their picture in a 4:3 format with 480 visible lines of detail. The latest widescreen HD TVs, the most widely sold versions today, have more visible lines and a completely different format. They are, in fact so different and so superior that you shouldn't think of buying anything else.

The older type of 4:3 screen has a squarer format, with four units of width to every three units of height. Widescreen TVs have a 16:9 format, the same as movies and more recently made HDTV shows. They are, therefore, the future since all programmes will eventually be made with this format. However, until this stage is reached, comprises have to be made.

Anyone watching a movie on a traditional TV screen will know that the widescreen format of the film results in black lines at the top and bottom of the screen - known as the letterbox effect. If you have a widescreen TV, the movie will fill the screen completely but any programmes made with the 4:3 format will cause black bars at either side of the screen.

Although this is a transitional problem that will decrease and eventually virtually disappear, the waste of screen space is a considerable source of annoyance to many widescreen TV owners. One solution is to stretch the image to fill the screen, although this results in a distorted picture. Alternative actions are to crop or zoom the image, filling the screen but losing some of the picture.

You can experiment to find the best results but usually the best option is to accept the situation and know that it will improve as more programmes are made in widescreen format.

HDTV refers to high definition format and gives a superior picture quality. The original standard definition TV (SDTV) displayed 480 lines while enhanced definition TV (EDTV) can display HDTV signals but has the same number of lines and so can't achieve high definition quality. High definition TVs can handle standard and high definition signals and have either 720 or 1080 lines, giving a much clearer and more detailed picture. As an illustration, SDTV provides 150,000 to 300,000 pixels, EDTV manages 300,000 to 400,000 while the pixel count for HDTV is between 900,000 and 2.1 million.

TV descriptions often show the number of visible lines followed by a letter (720i or 1080p). These indicate how the lines are scanned onto a TV picture - 'i' means interlaced, indicating the odd lines are scanned first and then the even lines are filled in, while 'p' is progressive and all lines are scanned consecutively. The latter should deliver a smoother image that stays sharper during motion.

A criticism of HD TVs is that watching standard definition channels can result in very poor quality pictures. This is because they show a bigger and sharper picture, revealing the flaws and relative softness of SD content. As with screen formats, this is a transitional problem that should reduce as more high definition programmes are made.