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The case for continuing 30-line transmissions.
The author of this article, who is an engineer of repute with considerable experience in the field of television, puts forward some interesting views on the question of television development. Some of the opinions expressed are definitely controversial and may lead to discussion on this extremely topical subject.
This picture of Jean Harlow gives an approximate idea of the definition which may be expected from 30-line television where the size of the subject is suitable. Definition improves, of course, with movement. Compare this with the impression below.
High definition transmission which is known to have reached an advanced stage of technical development is on the point of being given a public trial by the BBC. Behind this effort are the hopes and best wishes for its success of all those who have the true interests of television at heart.
Low-definition television has already been given a trial on a medium wave-length by the BBC on the Baird system extending over a considerable period, but the demand for television receivers has not been large, and the conclusion has been reached that the public shows little interest in the continuance of the service. It has even been suggested that low-definition television has had its day and should now be dropped. The writer, however, believes that there is an active future for both high- and low-definition systems, and if certain misunderstandings of the technical position are removed, and it can be shown that the limited public support accorded to the BBC 30-line transmissions implies no reflection on low definition as such, then the situation would be altered; as a case is thus made out for mission on more favourable lines that are likely to meet with a better response from the public.
Such is the purpose of this article, which concludes with a statement of the conditions which it is recommended should apply to any further service of this nature.
First, let us clear up some of these misunderstandings. For instance, the terms 'low definition' and 'high definition' are misleading. Low-definition television can show better definition of a single head than high-definition television if there are many figures in the high-definition picture. 'Narrow frequency band' or 'narrow side band' shortened to 'narrow band', and 'wide frequency band' or 'wide side band' shortened to 'wide band' are more informative in their implications, and will now be used throughout this article.
It is often argued that for successful exploitation television must be developed to a stage at which the detail can compare with that given on a cinematograph film. The conditions, in their correct order, however, which are required to ensure successful exploitation are:-
- Interest value.
- Public support.
- Technical development.
If the interest is there, the public will grow, even although the technical development is not of the highest order. Broadcasting as a movement was made a success in this country not by catering primarily for the users of multi-valve receivers, but for the users of crystal receivers. The crystal user later became the owner of the valve receiver. In a similar way, if the broadcasting authority caters first for the narrow-band enthusiast, a public will be formed which later will take interest in the wide-band programmes.
The interests of narrow-band and wide-band television are not opposed. The systems are actually complementary to each other. Thus, the narrow-band system is suitable for medium-wave transmissions, and therefore can be transmitted over a considerable distance. The choice of suitable subjects is necessarily limited, and should preferably be confined to the head and shoulders type, but a fairly large home screen can be used giving a bright picture, to obtain which a variety of scanning methods are available. As there are many manufacturers able to make such apparatus, the market price should be comparatively low, and within the reach of the man of average means. As the subject must be a simple one, the interest value should already exist in the known personality of the subject or in the message he is able to deliver.
Problems of Wide-band
Compare this now with the wide-band system, which can only find a space in the ether on ultra-short-wave channels, and therefore must be confined to transmission over moderate distances. The subjects, however, may contain several figures, but although the size and bright- ness may be adequate the pictures are smaller than can be obtained on the narrow band, and the cost of the equipment, which at present is restricted to cathode-ray apparatus, must limit it, certainly for some years, to the fortunate few. The main interest value is likely to be given by its entertainment qualities, the human reactions between full-length figures, or groups with background effects, or outside scenes such as may be relied upon to create their own interest value.
A national broadcasting authority would naturally desire, however, if at all possible, to provide a television service of some kind for all its licence-holders. The expense of providing special ultra-short-wave stations out of the licence fees for wide-band television enthusiasts would necessarily have to be offset by an endeavour to provide narrow-band transmissions on a medium wave for those of smaller means.
Before commenting on the BBC 30-line transmissions, I should like to emphasise how valuable they have been to all those interested in the development of the art, indicating, as they have done, the lines on which further progress should be made; and it is fitting that the excellent studio technique for obtaining the best photo-cell response to the scanning spot impulses, which has been evolved at Broadcasting House for these transmissions, should also be acknowledged.
The period given for transmission, from 11 to 11;30 pm, has been the means of discouraging 'looking-in', even by many of those interested in television, except on infrequent occasions. If television is to become popular, provision must be made for it in the broadcast programme.
Technically, the pictures have been inferior to what could be obtained with a different picture ratio on the radio frequency band width employed.
The theoretical maximum fundamental frequency response required by the 30-line picture circuit is 13 kHz. The cut-off of the studio line circuit is about 20 kHz, but the frequency response of the radio transmitter is known to fall off at about 9 kHz.
Although there is some loss of detail due to uncorrected aperture effect, the correction would not be worth applying having regard to the radio cut-off. The monitor picture in the studio in the case of limited make-up is better than any picture received by radio under most favourable conditions. As the smaller detail of the picture is not-transmitted, it follows that to make most use of the frequency band which is available make-up can be employed to eliminate fine detail and increase the detail corresponding to 9 kHz and less. Thus, in the head and shoulders pictures, the eyes have been given depth by heavy shadows, and more make-up is employed than would be necessary for stage effect. In so doing, naturalness has disappeared, the face has become more static, and the picture has been robbed of much of its interest value. It has been stated that heavy make-up has been used to obtain better results from poor receivers. If true, the principle is a bad one.
It has been a mistake to put over 'weak' pictures such as are produced by televising two or even one full-length figure, with no special make-up, by the 30-line scan. There is so much change of detail per picture element that the mean light intensity is not very different from that of the background, with the result that the image appears faint, like a print from an under-exposed film.
As the finer detail is lost, the viewer's attention is attracted to the outline, and, in order to maintain an interest value, this outline must change, and the subject is therefore compelled to move, dance, or distort its shape. This type of picture is too ambitious for 30 lines, and should be cut out of the programme.
When the contrast is obtained by large areas of black and white make-up and there is no fine detail, the human interest value disappears and the picture, therefore, becomes unsuitable.
Making the Most of 30-line
What should be the conditions for successful narrow-band television? First, the picture requires to be put over so that every single picture element can be reproduced faithfully at the receiver, and therefore the frequency band used must have the width necessary to do this.
Further, if the frequency band cannot for any reason be adjusted to the full width required by the picture, then the number of picture elements must be reduced to correspond to the width of the maximum frequency band available.
If, for the reasons already given, the televising of a full-length subject need no longer be considered, then the picture ratio of 7 to 3 is no longer necessary, and an improved head and shoulders picture can be obtained within the present band width of 9 kHz.
Thus, a 30-line picture 40 elements long transmitted 12.5 times per second requires a band width of only 7.5 kHz. We can increase the picture frequency to 15 per second when a 30 × 40 picture will require the full band width of 9 kHz.
Alternatively, we can transmit a picture with more lines, such as 34, and if this is 42 elements long, repeated 12.5 times per second, the band width is 8.9 kHz.
It is convenient to have one dimension greater than the other to accommodate the synchronising signal; also, if the frame is turned so that scanning is horizontal instead of vertical, two heads can be scanned instead of one.
If the subjects are then chosen so that the minimum area of high light required for definition is given by not less than three or four picture elements, then make-up can be dispensed with, or it can be reduced to the minimum necessary for artistic needs.
As regards programmes. Having decided that the transmission will be confined to head-and-shoulders subjects, we have now to decide on those which are most likely to have the maximum interest value; they must be of the type which will enable a strong picture to be put over that is, one in which it is possible to bring out the high lights and dark shadows. Should anyone doubt that head-and-shoulders television can have sufficient interest value to become popular, they may be reminded that the first film that brought success to the talkies in the United States and this country was the 'Singing Fool', a film which put over a catchy song with a large proportion of close-ups showing a single face. It must be remembered; however, that the only appeal that television can claim over other highly developed methods of conveying pictorial intelligence is the natural and life-like character which can be given to the picture, and this appeal vanishes with heavy make-up, which, apart from the opening or closing of the eyes and mouth, removes all expression and vitality from the face.
Subject Matter and Times
The point from which narrow-band broadcast television should preferably start is a head-and-shoulders picture with a personality already publicly known. The announcer would make an excellent subject a man whose voice is known to every listener, chosen because he has not only the right kind of voice, but, in most cases, the right kind of manner, one, therefore, to whom the viewer is already favourably disposed. Nor is it at all necessary or advisable for all subjects to make up. All talks may thus be considered as subjects for head-and-shoulders television. The authors are usually men and women of note. The listener is interested beforehand in the personality of the broadcaster and would like to view him.
Next, as regards the times of showing. It follows from the type of picture recommended for televising, that the television sessions should take place well in the body of the programme; actually, when the greatest number of people are listening in. It is suggested that sound would be broadcast on the long-wave National, and vision on all the Regionals. Vision in this type of programme would be an accessory to sound and not the principal feature of the broadcast. The removal from the vision service of the responsibility for providing interest value should smooth the way for television, particularly while the service is in its early stages.
So far as the BBC is concerned, there would be a great saving in expense, as no special programmes need be considered. There would be a saving in programme time, as the sound channel would have been in use for the transmission of news or talks in any case. If it be considered necessary, however, to interpolate occasional variety broadcasts in the television programme, these would at best be of a very simple character.
The general public, as a result of these arrangements, would obtain a satisfactory narrow-band picture with as much detail as the method would allow. The cost of the equipment, through competition, should be comparatively low. Listeners would soon become both listeners and viewers until they became television minded, and then the limitations of the picture subject-matter would act as a direct incentive to them to support wide-band television with its more complex pictures when it finally enters the commercial field.
In this illustration an idea is given of the effect when too ambitious a subject is put over from the studio, i.e., a subject where there is a large amount of detail to which the 30-line system is unable to do justice. It is necessary to view this illustration from some distance to appreciate detail at all.
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