The following artical appeared in the July 1936 edition of Television and Short-Wave World magazine.
- By R. Barthelemy
|The Eiffel Tower transmitter uses a different system of synchronising signal from the accepted practice in other countries. In this article, translated from "L'Onde Electrique ", R. Barthelemy explains the reasons for its adoption. Experimenters in this country should note the differences which preclude the use of a "universal" television receiver for French and other continental transmissions.
THE question has often been raised why in the case of the television transmissions from the Eiffel Tower the same type of synchronising' system was not used as in other countries. The different conclusions to which the French engineers have come originate in the fact that they have considered in the first place the simplicity of the receiver, while others have been more concerned with the convenient planning of the transmitter. The French point of view now seems to receive American approval, for it is reported that, in recent tests, the R.C.A. adopted the transmission of pictures in negative form, i.e., the dark parts corresponding to the highest carrier intensity.
A first condition which was found necessary from the start in 1929, consisted in the creation of a synchronising signal the "sense " of which is that of the blacks of the picture. On observing the mean of scenes to be transmitted, it will be clear that in an amplifier constructed to cover the whole of the frequency band in use the impulses due to white surfaces can take considerable amplitudes, but that, if the low frequencies are well transmitted, the inverse amplitudes due to the dark parts do not produce such marked impulses.
|Fig.1 - General shape of modulation curve
|Fig.2 - Waveform of the British and German transmissions
It would obviously be possible with a special scene to produce impulses towards the dark parts, but such a case does not exist in practice; as a whole, the modulation curve takes in general the shape represented in Fig. 1, where in the ordinary case of capacity couplings the total of positive parts and negative parts is nil in the interval between picture scans.
It is evident that if the synchronising signals are mingled with those of modulation the chances of spreading will be notably diminished, and subsequent filtering will be facilitated by placing the amplitudes S of the "tops" in the most uniform region, that is to say in the direction of the dark parts of the picture. Experience confirms this hypothesis.
In most transmissions the line synchronising signal is obtained by modulation of the carrier wave to 100 per cent, i.e., by the cancellation of the current in the aerial. Theoretically the waveform of this signal is rectangular and lasts five-hundredths of the duration of the line. In practice, however, it is fairly rounded and is at a tangent to the axis of the t's.
According to what we know of the essential link in synchronisation in the receiver, the thyratron, there will be serious difficulty in obtaining an exact discharge point, which perhaps explains the fact that it is little used outside France, being replaced by a group of three or four valves.
|Fig.3 - Waveform in the French transmissions
In order to maintain the necessary exactitude a very short
and pointed synchronising signal is made lasting for 1/100 of the line, and
in order that this point might be perfectly transmitted with the maximum energy,
the direction of increase in the current of the aerial is chosen to transmit
the top; this implies that the "whites" will diminish the high-frequency
intensity, while the "blacks" with the synchronising signals will
increase it. Fig. 3 shows the shape of the aerial current curve (i.e., is the
carrier current in the absence of modulation).
The following advantages were found in this method :
|On the Reception Side.
|The positive nature of the synchronising signals is superior to that of image signals, for at the moment of transmission of the"top" the energy is almost quadrupled in the aerial. In general, the picture on reception is synchronised before being" visible, which would be impossible with other systems.
|Separation of the synchronising signals from the rest of the modulation is extremely simple and is obtained by a bias adjustment of the thyratron.
|The exactitude of the discharge of the thyratron is governed by the duration of the voltage impulse which is of the same order as the image point. The thyratron is therefore controlled almost rigidly and its degree of freedom cannot influence the quality of the picture.
|Protection against interference and the flickerings which it produces in line deviation is very much increased, as much by the raised level of energy of the t1 top fi as by its brevity.
If to these advantages there is added that given by the time signal of the picture obtained by the suppres- sion of a line signal it will be realised that with four valves and two thyratrons (1 octode + 1 HF + 2 LF) the construction of a .complete television receiver may be achieved, and we regard this extreme simplicity as the essential basis of television development. Another advantage of the "sense" chosen is that the control grid of the cathode-ray tube may be directly modulated from the detector and a positive picture obtained; it is thus possible to apply to this grid the D.C. component which creates the background colour, and this without any additional device.
|It may be objected a priori that the transmitting valves are badly utilised for it seems that they must be capable of handling the maximum strength of the synchronising signals, whereas normal modulation only uses a quarter of their capacity. In reality this objection only holds good from the point of view of insula- tion values and the length of the straight parts of the characteristics, for, from the energy point of view the duration of the power peak must be remembered. This duration is from 1 to 2 per cent. of the total time, so that if one admits a double overload, the average strength and in consequence the heating is only increased by about five per cent.
It is therefore unnecessary to choose valves of four times the normal power output and it is sufficient to see that the response and feed voltage remains correct for the large amplitudes of the synchronising signals.
To summarise: the direction and the form of the signals which has been adopted offer indisputable ad-vantages, verified by experience as far as receivers are concerned; also it does not entail any marked additional difficulty in transmission. The only objection which might be raised is that a receiver imported from Berlin or from London cannot, without modification, function in Paris.
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