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Has the 405 line system been fully exploited?

The problem of the development of television as a general medium of entertainment, has been the subject of intense discussion in technical circles recently. The proposals outlined in the Report of the Television Committee have been the main reasons for this renewed interest in television. It is felt, however, that certain ideas that have been expressed by various persons, neglect quite a few important technical considerations, while on the other hand important factors which, although not exactly within the province of technical designers, are still of importance in the development of a television entertainment service, have been neglected.

Possibly the most controversial point in the Report, and the point that has raised the most heated discussion, is the proposal to retain the present definition standard of 405 lines, interlaced. The proposal has been attacked in general upon the grounds that this is an inadequate standard of definition, and that it is wrong to commit the country to this standard for some years to come. This attitude is clearly expressed by R W Hallows, who regards the proposal as 'flogging the almost dead horse of 405 line television'.  ^{[1] Television Committee's Report, R W Hallows, Wireless World, May, I945, pp. 130-132. See the link in the extras menu.} It is generally recognised that the suggested 1,000 line system proposed as the next advance in British television technique, is unlikely to be technically or economically practicable for many years to come.

It is extremely interesting, however, to consider whether a standard of 405 lines actual definition is inadequate. There is a strange dearth of information upon the actual resolution of current high definition systems. This. is all the more puzzling when it is recalled that the theory for calculation of resolving power of television systems using accurately defined apertures has been exhaustively studied. In actual practice, however, the effective spot size of modern electronic television systems is a rather ill-defined patch approximately circular in shape. Current trends in America are expressed by the adoption of 525 line transmission standards, which necessitate vestigial sideband transmission in order to accommodate the necessary bandwidth at the lower carrier frequencies. It has been proposed that British standards should be amended to the higher number of lines, in order to utilise the radio bandwidth available most effectively. The bandwidth required for 525 line transmission would be 3.2 MHz, as compared with the 1.9 MHz video bandwidth required for the Alexandra Palace signals.

Comparison With Cinema Standards

A consideration of the necessary number of lines required for a satisfactory television image may lead us to some startling conclusions. While no definite resolution figures have been published upon the performance of high definition systems, it seems reasonably certain that nothing like the theoretical resolution has ever been attained. Naturally, resolution of television images is referred to the cinema screen as standard, which for all practical purposes, may be taken as perfection. Some figures are available for the resolution of cinema film emulsion,  ^{[2] Optical and Mechanical Characteristics of 16 mm. Motion Picture Projectors. Nat. Bureau of Standards Circular C. 437.} but the actual resolution attainable in a normal projector is less than this. Factors such as the projection lens aberrations, inevitable mechanical imperfections in the projector, coupled with shrinkages and distortions of the film itself, reduce the definition appreciably. The resolution of a projected image from a 16 mm film under the best conditions corresponds to about a 700 line image in the centre of the field, but only to 300 lines or so at the edge of the field. The resolution of the cinema under ideal conditions, may attain to a standard of 1,000 lines, but it is doubtful if the average cinema definition really exceeds perfect 600 line standards at the centre of the picture. The estimates made by various workers upon the required minimum definition for cinema standard definition make rather interesting reading. Thus an early article by J H Owen Harries  ^{[3] Some Developments in Tele-Analysis, by J H Owen V Harries, Journal Television Society, Vol. 1, No. 1, pp. 1-I2.} based upon a figure of 0.00065 radian for the resolving acuity of the human eye, and assuming the best cinema seats with a viewing angle of 30 degrees, places the definition as that of a perfect 308 lines. When the London high- definition service was opened, surprise was expressed at the use of 405 line definition, which was tacitly assumed to be more than adequate. The opinion has been expressed that 405 line images were capable of results little short of perfection. Thus E Wikkenhauser  ^{[4] High Definition Television Service in England. Journal Television Society, Vol. 2, pp. 34-43.} ' . . . according to many theoretical investigations, a 240 line image gives ample definition for home use, and the 405 line picture would have a definition nearly equal to a large cinema screen, which definition is really not necessary for the home (my italics).

Furthermore, we may quote E H Traub  ^{[4] High Definition Television Service in England. Journal Television Society, Vol. 2, pp. 34-43.} as showing that a definition of the order of a perfect 240 lines should be entirely adequate: 'It has been shown again both on paper and in practice, that 180 lines represents a highly satisfactory definition sufficient for a public service, and that 240 lines gives a picture of such excellence as to leave no room for criticism whatsoever. To go beyond this figure would be to go beyond the capabilities of the resolution of the human eye, which can be regarded as a minute of arc under the worst conditions'. In the same symposium of representative opinion, E L Gardiner  ^{[4] High Definition Television Service in England. Journal Television Society, Vol. 2, pp. 34-43.} after pointing out that in some instances mechanical systems (in which the spot size is accurately defined) gave better pictures than some cathode ray systems using many more lines, comes to remark . . . 'Opinion has during the last year been unanimous that 240 lines represents a definition which is quite satisfactory for the majority of purposes', and further goes on to say 'I would advocate the use of a 180 line standard . . . at 50 frames as quite adequate for entertainment purposes'.

While it is clear therefore, that a definition standard of 405 lines is adequate when fully exploited, it is also clear that the quality of the images as received on normal commercial receivers was nowhere near the maximum theoretical resolution. It is questionable whether the actual home receiver definition was even equivalent to that theoretically attainable from a 200 line image. The absence of any definite figures upon the performance of television receivers is thus more understandable. It appears fairly certain that a perfect 405 line image would be adequate for all purposes and would even approach the maximum necessary for home use. It may therefore be a little premature to abandon all interest in the present standards, as Mr Hallows would have us do.

Spot Size

One of the main factors reducing the effective resolution of electronic television receivers is the increase in size with brightness of the cathode ray spot. Further, the cathode ray spot itself is not sharply defined, and it seems possible that some improvement in the production of a more accurately defined spot may be expected. It seems rather pointless to increase the number of scanning lines when the spot size is already too large. It is as well to remember that American 441 line receivers employing 6 inch tubes deliberately sacrificed bandwidth as the performance of the cathode ray tube was unable to exploit the full bandwidth.

It is perhaps as well to remark that this 'modulation increase of spot size' is a problem which so far I has defied solution by vacuum tube engineers. To quote E L Gardiner again,  ^{[4] High Definition Television Service in England. Journal Television Society, Vol. 2, pp. 34-43.} 'Unfortunately the public, and a great many engineers of limited television experience, still imagine that the number of scanning lines employed is the deciding factor in the clarity and excellence of a received image. While this is largely true of image analysis at the transmitter, it does not hold very closely by the time the viewing screen is reached, owing to the loss in image quality arising from distortion and high frequency losses through the system'. This remark seems true enough to repeat today, as a number of factors are often overlooked, which are of vital importance in deciding the quality of the television image. It must be remembered, for example, that the iconoscope type of camera does not provide a true DC level, and has no clearly defined contrast characteristic analogous to a photographic 'gamma'. The recent American orthicon camera apparently does have a stable gamma characteristic, and provides a definite DC level. It is probable that some of the improved performance of the new American systems may be due to this new camera, rather than to the increase in the number of scanning lines.

From the above it will be fairly clear that no undue concern need be felt at the prospect of British television standards remaining at the present definition level of 405 lines for a considerable period. At the same time it is to be noted that a great deal of experimental work needs to be done before the performance obtained with the present system reaches the maximum practicable. Some of the points which need investigation have been touched on in this article.

Interlacing

There is a further point that might be investigated is the desirability of retaining interlacing, or of employing a sequentially scanned picture. It appears certain that interlacing does offer a number of difficulties, and some improvement in the resolution of images might be achieved by employing vestigial sideband transmission to accommodate a 405 line image sequentially scanned at 50 frames per second. Such a transmission could be received upon present receivers with the minimum of alteration, and for a further slight cost the old receivers could be modified to utilise fully the extended bandwidth.

It should perhaps be pointed out that technical considerations are not the only considerations necessary for the provision of a public television service, and that aesthetic and psychological factors may be of considerable importance.  ^{[5] Objectives for Post-War Television, J Worthington, Miner, Journal Television Society, Vol. 4, No. 2.  [6] Television After the War, O J Russell, Journal Television Society, Vol. 4, No. 3, pp. 60-64.}

References

1. Television Committee's Report, R W Hallows, Wireless World, May, I945, pp. 130-132.
2. Optical and Mechanical Characteristics of 16 mm. Motion Picture Projectors. Nat. Bureau of Standards Circular C. 437.
3. Some Developments in Tele-Analysis, by J H Owen V Harries, Journal Television Society, Vol. 1, No. 1, pp. 1-I2.
4. High Definition Television Service in England. Journal Television Society, Vol. 2, pp. 34-43.
5. Objectives for Post-War Television, J Worthington, Miner, Journal Television Society, Vol. 4, No. 2.
6. Television After the War, O J Russell, Journal Television Society, Vol. 4, No. 3, pp. 60-64.