High Definition Television (HDTV) in the strictest term describes the TV for displaying images in your front room, local bar or hotel. The definition we are going to use here encompasses the TV itself as well as some of the technology for making the pictures available to us.
Televisions today comes in two formats 4:3 which describes the aspect ratio you are looking at i.e. 4 units across and 3 units down and 16:9 which is also more commonly know as widescreen. In Europe, television has 576 lines of vertical resolution PAL which is fine for small TV screens.
In the past two decades televisions started to increase in size but the resolution stayed the same. This caused a problem with image quality. If you can imagine looking at a large screen TV in your home or watching a football match in your local pub on TV with a vertical depth (top to bottom) of 57 cm. This would approximately make the TV image (576 lines) into one line every centimetere. This is fine if you are looking at the image from a long distance, as the image looks smooth and of reasonable definition, but if you are anyway close, the image is very poor and not very pleasant to watch.
Manufacturers were then required to solve this problem by increasing the quality or adding more lines of resolution into televisions. As TV increased in size so would the resolution of images. There was still one other problem. The way images were captured on most television cameras and video cameras were still at the same old 576 lines of resolution. So even if you were increasing the screen size of the original image that was captured it was still going to have to cover the new screen size and so the quality was not going to improve.
This was not possible with the analogue TV process. Cameras would need to capture better quality images and TV would need to change. This was the advent of the digital television process and then HDTV Digital Television was the solution where images could be captured digitally (as opposed to analogue) on cameras and then transmitted digitally to the higher quality TV sets. The fact that digital signals can be transmitted using a lot less bandwith than analogue signals means more space is available to increase quality or to offer other services from TV stations as they broadcast into the future.
Standards were adopted for the immidiate future in terms of resolution, these were 720P, 1080i and 1080P. An image on analogue televisions that we use at present changes by scanning an image onto the screen and replacing this image at regular intervals. They do this by changing the 576 lines of image approximatlely 50 times a second or at 50Hz, but not every line changes at the same time. Interlacing is used which means that only every second line is changed at a time. This happens so fast we do not notice but some observers say this has an effect on percieved image quality. This is where High Definition comes in. 720P & 1080P means that a new image is captured or displayed on HD cameras or HD TV at a resolution of 720 lines or 1080 lines 25 times a second or whatever frame rate or frequency is selected. This is called “Progresseive†scannining. 1080i means capturing or dispalying by intrelacing the image but at a higher frame rate to avoid lost perception in motion.
At the moment there does not seem to be much of a difference in percieved quality between 720P and 1080i with 1080P being the goal in the medium term. Differences in these standards are catered for in most HDTV sets available for sale but there seems to be a standard war betweeen manufactures much like Betamax vs. VHS in the 1980’s as to which will be the standard of the future.
Other Links European Information Portal Grand Alliance Info(1) Grand Alliance Info(2) University of Washington HDTV Magazine CNET HDTV Info High Definition Blog High Dedfinition Magazine European HDTV forum (now inactive) 3 Dimensional TV research
Books: de Bruin, R. and Smits, J.(1998) Digital Video Broadcasting: Technology, Standards and Regulations, Massachussets: ARTECH HOUSE