This article is devoted to survey of the various methods, an endeavour is made to gauge the trend of development.
The principal methods which have been proposed for achieving television and which have proved of sufficient value to find practical application have been briefly described in this series of articles, and it remains to consider the most probable trend of future development. Whatever methods are employed for transmission and reception, it is clear that very high frequencies are involved - many hundreds of times higher than we are accustomed to deal with in the highest quality sound transmission. So far as the purely electrical side of the problem is concerned, this need cause no special difficulty. It is quite possible, although not easy, to construct amplifiers with flat frequency characteristics up to at least 300,000 cycles.
The use of such high modulation frequencies necessitates the operation of the transmitter in the ultra-short waveband for two reasons because modulation at much lower wavelengths is impossible, and because the frequency band occupied by the transmitter would be prohibitively great in any other band. Comparatively little is known about the properties of ultra-short waves, and the technique of transmission and reception is only in its early stages. Even if the purely television apparatus had reached such a stage that it could be operated by the unskilled and would give a picture sufficiently free from distortion to be of entertainment value, reliable television reception would hardly be a possibility on account of the lack of development in ultra-short wave apparatus.
The purely reception difficulties, of course, are not of great importance, and a determined attack by designers and experimenters would soon remove them. It is safe to say that were there sufficient incentive, the major problems of ultra-short wave working could be overcome within a year. Incentive is lacking at the moment, however, on account of the rarity of transmitters, and it is probable that development will proceed only slowly until the greater difficulties of television are overcome or until a regular service of high quality sound broadcasting takes place.
The introduction of drastic contrast in make-up is an important feature of successful presentation in the television studio of to-day.
The greatest hope of good reception would appear to lie with the superheterodyne using a very high intermediate frequency, probably one corresponding to a wavelength no higher than 30 metres. Such a receiver could be made very stable in operation, and easy to control, while the high intermediate frequency would permit the passage of the high modulation frequencies which are so essential to television.
The problems of transmission and reception in themselves offer no special difficulty, therefore, and there is every sign that they will be solved by the application of known methods. The same can hardly be said of the television equipment, however, and so many different systems vie with one another that it is even difficult to pick out the one most likely to become the method of the future.
The system in widest use and upon which most work is being carried out is one which may be conveniently termed variable intensity scanning, in distinction to variable speed scanning. Variable intensity scanning can be carried out in a variety of ways, of which the chief are the Nipkow disc, the mirror drum and the cathode ray tube. Its particular beauty, however, is that one is not confined at the receiver to the same type of apparatus as that used at the transmitter.
A drum may be used at the transmitter, but it is quite feasible to use disc, drum, or cathode ray tube at the receiver, according to taste. The greatest objection to this system, however, is that any developments in transmission which offer an improved performance through a variation in the number of scanning lines or pictures a second, throw all receivers out of date and necessitate extensive alteration if any intelligible picture is to be received.
At the present time, this objection is not of great importance on account of the small number of receivers in use. It is easy to see, however, that were a television service of this type to be established which achieved any great measure of popularity it would probably represent the end of development. It should be pointed out, of course, that these remarks do not apply in their entirety to receivers employing cathode-ray tubes, for slight modifications to these would render them capable of reception from improved transmitters.
For reasons of this nature, therefore, the variable speed scanning system is particularly attractive, since the receiving equipment consists of little more than the cathode ray tube, and synchronising apparatus appears to be unnecessary. Such a system is more nearly comparable to sound broadcasting, in which development can take place at either transmitter or receiver and without either being rendered useless. For some reason this scheme has attracted less attention than others, and it is not so highly developed. On account of the simplification to the receiving apparatus which it offers, however, it is to be hoped that work on it will continue, and that the final system of television will be either this or some other of equal simplicity.
In this series of articles, the transmissions have usually been assumed to take place with the aid of cinema films rather than by the televising of an actual scene. This has been done for the sake of simplicity, since the difficulties of obtaining adequate illumination at the transmitting end do not occur in the same degree. In cases where transmission of an actual subject, as distinct from a film, is required the difficulties of transmission are greatly enhanced, but it is of course by no means impossible.
It will thus be apparent that one can hardly expect television broadcasting of a quality comparable with that of sound transmissions to take place in the near future. That such broadcasting will take place on the ultra-short wavelengths appears certain, and the difficulties here are certainly not insurmountable. The chief problems lie in the television apparatus, and while there is no doubt that a solution will eventually be found, perhaps by methods now unknown, it will be a long time before anything approaching finality is reached.