Why different analog TV standards exist?Keywords - TV analog,PAL,NTSC,SECAM,standards |
Not all the world's TV receivers work in the same way. Different countries use different types of broadcast TV system, most of which are to varying extents incompatible with each other. Unfortunately, video recordings retain many of the characteristics of the original signal of which they are a recording. In general, recordings are more likely to be compatible than recieving equipment, but only with their own "family".
Why Did This Happen?
In order to work, TV receivers require a source of field timing reference signals. These are signals that tell the TV receiver to be ready to receive the next picture in the stream of images.
Early set designers decided to use the power supply frequency as this source for two good reasons:
- Older types of power supply would get rolling hum bars on the TV picture if the mains supply and power source were not at exactly the same frequency.
- TV studios would have had enormous problems with flicker on their cameras when making programmes.
Later the 60Hz camp made a small adjustment and changed the field rate to 59.94Hz when they added color to the signals. The issue of field frequency remained sufficently 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.
Compounding The Problem
Beyond the initial divide between 50 and 60Hz based systems, further sub-divisions have appeared within both camps since the inception of Color broadcasting. The majority of 60Hz based countries use a technique known as NTSC originally developed in the United States by a committee called the National Television Standards Committee. NTSC works perfectly in a video or closed ciruit environment but can exhibit problems of varying hue when used in a broadcast environment.
This hue change problem is caused by shifts in the color sub-carrier phase of the signal. A modified version of NTSC soon appeared which differed mainly in that the sub-carrier phase was reversed on each second line; this is known as PAL, standing for Phase Alternate Lines. PAL has been adopted by a few 60Hz countries, most notably Brazil.
Amongst the countries based on 50Hz systems, PAL has been the most widely adopted. PAL is not the only color system in widespread use with 50Hz; the French designed a system of their own known as SECAM, standing for SEquential Couleur Avec Memoire. SECAM was widely adopted in Eastern Block countries.
In general, since the field and scan rates are identical, you can expect to get a monochrome picture from a PAL video recording replayed on SECAM equipment, and vice versa. Transmission frequencies and encoding differences make equipment incompatible from a broadcast viewpoint. Transcoders between PAL and SECAM, while often difficult to find, are reasonably cheap.
In Europe, a few Direct Satelite Broadcasting services use a system called D-MAC. It's use is not wide-spread at present and it is transcoded to PAL or SECAM to permit video recording of it's signals. It includes features for 16:9 (widescreen) aspect ratio transmissions and an eventual migration path to Europe's proposed HDTV standard. There are other MAC-based standards in use around the world including B-MAC in Australia and B-MAC60 on some private networks in the USA. There is also a second European varient called D2-MAC which supports additional audio channels making transmitted signals incompatible, but not baseband signals.
It Gets Worse
In addition to the incompatibilities of 50 and 60Hz systems, and the different color systems, there is a further barrier to compatibility. For reasons of number of stations and terrain, TV pictures can be transmitted in any of three frequency ranges, VHF, UHF and Microwave. Equipment designed to receive signals in only one of these bands cannot receive transmissions in any of the other bands.
Further, there are differences between the encoding of the sound between countries using the same frequency bands. Within 50Hz PAL UHF transmissions, audio signals can be at 5.5Mhz offset (system G), or at 6MHz offset (system I). Similar differences exist between the Middle Eastern versions of SECAM (MESECAM) and the Eastern Bloc (OIRT) version.
And That's Not All
In addition to standard combinations of scan rate, color system and transmission frequencies, there are further complications when it comes to additional features like stereo sound, sub-titling and other information services. Fortunately, such differences do not effect the basic operation of equipment conforming to the same broadcast standard, but they can restrict the use of various features.
In the cases of both stereo sound and additional textural information carried in the top few lines of the picture, there are three competing systems of varying technical merit. The oldest still operational of the stereo sound systems is the American MTS system based on NTSC transmissions, only slightly more recent is the twin channel FM-FM system used in Germany, Austrua, Australia, the Netherlands and Switzerland. The most recent system, NICAM 728, was designed by the BBC in the late1980s using digital audio technology.
The oldest of the subtitling systems is the BBC and IBA designed TeleText system which has been is use in the UK since the mid 1970s. It is also the most widespread and the most flexible of the systems in widespread use. The US closed captioning mechanism came about through political pressure from the Deaf organisations in the USA and has not been developed beyond the simple job of producing subtitles for the Deaf. The French developed a subtitling and information system called Antiope which has not found favor elsewhere, largely due to the existing widespread use of the BBC developed TeleText system. A few US stations have now adopted the BBC-style Teletext and a few manufacturers, most notably Zenith, fit the decoders to their sets.
For more information, read "Analog TV Systems: A Comparison" and "Broadcasting System Details", additional InfoSource article on the subject of international video broadcast standards.
