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(source: Electronics World, Jul. 1963)
By PATRICK HALLIDAY
DURING 1963 firm decisions are expected in several European countries on color television services which so far have not advanced beyond the experimental stage. Britain, for example, is committed to a 625-line u.h.f. color service but final technical characteristics are still unknown.
The main reason for the hesitation is the growing belief that a compatible color-TV system developed in France--"Secam" or "sequential and memory"--may prove more suitable than the adapted NTSC systems which have been used experimentally for several years by the BBC and other European broadcasting organizations.
This alternative color system, "Secam," already has many supporters and is currently under evaluation at the BBC high-power u.h.f. (450-mc.) transmitter in London, built in readiness for the new British black-and-white 625-line service scheduled to start in April 1964. "Secam" was originated by Henri de France and is now being actively developed by the Compagnie Francaise de Television in cooperation with other British and European companies. It differs in a number of important respects from the NTSC color system.
The "Secam" system transmits much the same basic information as NTSC: a luminance signal and two color-difference ( "chroma ") signals. But whereas in the NTSC system all this information is being transmitted simultaneously, with "Secam" only one chroma signal is being radiated at any given moment.
What happens is that during one line-period the first chroma signal is radiated on the subcarrier; during the next line-period the other chroma signal goes out, and so on in sequence. Since only one chroma signal is going out at a time the subcarrier can be frequency-modulated and the complex NTSC technique of two simultaneous signals in quadrature on a suppressed carrier is avoided.
At the receiver, a "memory" device--consisting of an electrical delay--is introduced to hold the information received from the first chroma transmission until the second chroma signal begins to arrive. Both chroma signals are then applied simultaneously to the matrixing and hence to the display tube--which can be a Shadowmask--over two line-periods. The usual form of memory device in "Secam" receivers is a quartz block in which signals are stored briefly by reflection from face to face. Such delay components are fairly expensive--about $11--but the total receiver costs are roughly the same or a little less than for NTSC receivers.
Since each chroma signal is sent out for half the total time, the vertical definition of the color is only half that of the NTSC system. However, because of the imperfection of human vision, there is little point in providing color definition equal to that of the luminance signal. In both NTSC and "Secam" systems the color definition in the horizontal direction is reduced to allow the information to be sent on the subcarrier. In "Secam" the color definition is similarly reduced in the vertical direction, permitting the chroma signals to be sent sequentially, but having little observable effect on the picture.
When first mooted several years ago, "Secam" demonstrations were watched by BBC engineers who then came to the conclusion that the new system would not supersede NTSC. Recent improvements, however, have made them much less certain and many now support the full-evaluation testing of "Secam" taking place in London and elsewhere. "Secam" has also been tested by the Swiss broadcasting authorities.
What are the practical advantages claimed for this color system? These are substantial both at the transmitting end and for the viewer.
The "Secam" waveform is considered to be much easier to handle, particularly for network distribution and for recording on magnetic tape. Again, since the strict phase conditions imposed on NTSC color signals no longer apply, the transmitted signal is much less affected by multipath propagation--that is by "ghost" signals. Since color TV is to be radiated in Britain on u.h.f. rather than v.h.f., this could be an especially important consideration.
Because there is no synchronous demodulation, "Secam" color receivers do not require a reference oscillator and--very important to the viewer--do not need "hue" or "saturation" controls. It is claimed that the unnatural colors which can arise from inaccurate setting of these controls on NTSC cannot occur on "Secam." Receiver servicing, with emphasis on accurate phase conditions, is also held to be much more straightforward.
An early problem with "Secam" was the rather noticeable pattern on black-and-white pictures arising from the unsuppressed subcarrier. The adoption of frequency-modulation of the subcarrier and other modifications have reduced this patterning.
The chief engineer of one of Britain's commercial TV broadcasting companies which has been working on " Secam" for more than 18 months, has expressed a number of findings distinctly favorable to "Secam" although he states frankly, "No one can express an objective opinion on this matter until after the high-power tests have been carried out." On the other hand, those who would like to see Britain adopt the NTSC system point out that experience in the United States and Japan has shown it to be a basically sound and practical system and that, unlike "Secam," most of the development work on receiver circuits and components has already been carried out.
Most Western European countries are anxious that the same color system be adopted in each country in order to facilitate the exchange of color programs. Some are also investigating a German Telefunken color system which combines features from both the basic NTSC and "Secam" ideas.
This year will see much color testing and talking in Europe. But the decision cannot now be long delayed.
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