WORLD TV STANDARDS

A number of TV standards now co-exist worldwide. This article takes a look at how and why they originated, as well as a comparison. It also offers detail listings of standards prevalent in various countries.
Television broadcast commenced approximately 50 years ago. The knowledge gained over the years has helped evolve better standards. As a result of this, the US which saw the birth of wide spread commercial television broadcasts, evolved the first system which predictably, is also the most primitive. Subsequent television systems have learnt from earlier mistakes.

Before we consider different television systems, we need to take a look at the basics of television transmissions. A television transmission consist of a series of rapidly changing pictures which convey to the viewer, an illusion of continuous motion. The pictures need to flash at a rate of more than 16 pictures per second, to fool the eye into seeing continuous motion. Each of these rapidly changing pictures is termed as a "frame". Typically a television transmission consists of either 25 or 30 frames per second. This is Each picture consists of several closely spaced lines. The lines are scanned (written) from left to right and from the top of the screen to the bottom of the screen. Typically a TV picture consists of 525 or 625 lines. In view of the large number of lines, if all lines were written one after the other on the screen, the picture would begin to fade at the top of the screen by the time the last few lines at the bottom of the screen are written. To avoid this, the first frame carries only the odd numbered lines e.g. line numbers 1, 3, 5 etc.
The next frame carries only even numbered lines e.g. line numbers 2, 4, 6 etc. In this manner, successive frames carry the odd and even numbered lines. This provides a uniform intensity to the picture, and is called "interlacing".

TIMING :
TV receivers require a source to time the rapid succession of frames on the screen. Designers decided to use the Mains power supply frequency as this source for two good reasons. The first was that with the older type of power supply, (non SMPS) you would get rolling hum bars on the TV picture if the mains supply and power source were not at exactly the same frequency. The second was that the TV studio lights or for that matter all florescent, non incandescent lights flicker at the mains frequency. Since this flicker is much higher than 16 times per second the eye does not detect it. However this flicker could evolve into an extremely pronounced low frequency flicker on TV screens due to a "beat" frequency generated between the light flicker and the mains frequency.This would have made programmes unviewable particularly in the early days of development of TV receivers.
There are two Mains power frequencies widely used around the World, 50Hz and 60Hz. This immediately divided the worlds TV systems into two distinct camps, the 25 frames per second camp (50Hz) and the 30 frames per second camp (60Hz). Later the 60Hz camp made a small adjustment and changed the field rate to 59.94Hz when they added colour to the signals. The issue of field frequency remained sufficiently deep rooted in both TV standards that the vested interest remained long after the original technical justification had gone.
The biggest compatibility problems between TV standards remain related to the field rate; these are also the hardest problems to solve.