The first ever colour broadcast came to USA in 1953. This was based on NTSC - National Television System Committee standard. NTSC is being used by many countries in the American continent and as well as in Asia too, in countries including Japan. NTSC runs on 525 lines/frame. The Phase Alternating Line (PAL) system was introduced in early 1960s and was available in most European countries except France. PAL utilises a wider channel-bandwidth than NTSC, which provides a better picture quality. This system runs on 625 lines/frame. Sequential Couleur Avec Memoire or Sequential Colour with Memory (SECAM) standard was introduced in early 1960s and was implemented in France. SECAM transmits colour information sequentially but uses the same bandwidth as PAL. The standard runs on 625 lines/frame. The reason for using these different standards is based on that the TV receivers require a source of field tinning reference signals to receive the images. At first it was thought that the Mains power supply frequency will be a good source for this field timing. This divided the world in two frequency zones, 50Hz and 60 Hz, which are the ones widely used in different countries. So, the TV standard was divided into 25 frames/second (50 Hz) and 30 frames/second (60 Hz). The designers slightly modified the 60 Hz category and converted it to 59.9 Hz, when they added colour to the signals. The main compatibility between the two categories of TVs remain related to be the field rate and this has not been solved as yet. A solution had to be found to enable the TV to work on any of the standards available in the world. This saw the introduction of a multi-system TV capable of working in a variety of video standards, such as PAL, NTSC and SECAM. This TV will work in almost all the countries in the world, with certain exceptions. Some parts of the world use a very rare video system which requires a special type of TV. These countries are - France Uruguay, Brazil and Paraguay. France broadcasts in SECAM-L, which is the only country doing so, while Brazil, Uruguay and Paraguay broadcast in PAL-M and N, which are not found in any other part of the world. The question often arises whether my NTSC TV can be modified to PAL standard or other way round. The horizontal scan rates for both the standars are nearly identical - 15,734 Hz for NTSC and 15,625 Hz for PAL. Therefore this is not likely to be a problem. If these differed significantly, there would have been large cost issues coming into the design. The problem remains with the vertical scan, with 525 line/60 Hz NTSC and 625 line/50 Hz PAL. But its easier to modify the vertical deflection to accommodate a modest variation in rates. As far as commercial multi-system TVs are concerned, there are not too many seen in U.S. because of lack of demand. These TVs are mainly manufactured for the international markets and are available in abundance in such places. On the other hand, you can view all the standards of video inputs, viz., PAL / SECAM / NTSC, automatically. These are based on auto voltage sensors, switching from one main power of 240 volts, 50 Hz to 100 volts, 60 Hz, or vice-versa, automatically. Most of these TVs allow you to tune with cable or antenna channels in most countries in the world. Different concept is followed in measurement specifications for multi-system TVs. In North America, the measurement is done only on the exposed screen of the TV, while in case of a multi-system TV, the encasing, covering the small edge of the screen, is taken into account. Although an American TV will be of the same size as a multi-system TV, the specifications would differ in the sizes of the two TVs as far as the screen is concerned. The screen measurement in a standard TV is done from the upper left corner to the bottom right corner of the screen. |
