A C Booth, Picture Telegraphy
A. C. Booth. Picture Telegraphy
From time to time during the last twenty years we have had various suggestions for providing means to telegraph pictures, that is to say, to produce at the distant end of a telegraph line a more or less exact reproduction of a picture, sketch or whatever else it may be at the sending end of the line.
Workers in this particular field include such well known names as Korn, Thorne-Baker, Belin, etc.
Generally speaking, all the methods adopt the device of traversing the picture with a contact or pencil for a large number of times, approximating 80 traverses per inch width of picture, and utilising the picture itself directly or indirectly to vary the strength of current in the line; while the receiving apparatus at the distant end utilises the varying strength of received current directly or indirectly to produce a siilar number of lines side by side in their proper position to constitute the picture more or less exactly, frequently less exactly.
In effect it is the equivalent of cutting up the picture into a large number of narrow strips and arranging for the varying gradations of intensity to be telegraphed over the line to the distant station, where they are received by suitable means, still in one long length. It may then be considered that the received slip is cut up into exact lengths which are placed side by side to form the picture.
The chief difficulty experienced has been in suitably telegraphing the varying gradations of density of the picture.
In some cases the peculiar property of Selenium cells is used to vary the line current according to the amount of light that could be passed through the picture or its equivalent; whilst in other the picture is converted by suitable means into a relief picture, which in turn acts on a lever controlling a microphone, thus varying the strength of the current.
At the receiving end the varying strengths of current act on apparatus which varies the amount of light that is allowed to pass to a given position, thus producing a varying intensity suitably affecting a photographic film and giving the required density.
All these arrangements require that there should be correct "unison" between the transmitting and receiving instruments.
Fairly good results have been obtained by some of these schemes, but all of them need the fitting of special apparatus at each end of the telegraph line.
Recently there has been developed by the proprietors of the Daily Mirror another method which requires no special instruments on the telegraph line, other than the ordinary telegraph apparatus; although other mechanism is needed at the offices where the pictures are prepared before being handed in to the telegraph office, as well as at the office where the received telegram is delivered for reproduction of the picture.
Briefly, the method is as follows:—The picture to be telegraphed is photographed in the ordinary way to produce a negative, from which a number of positive prints of different intensities are taken on flexible metal plates suitably coated. These prints after treatment are wrapped round a cylinder which is capable of being rotated and also of being moved in an endwise directly on a finely cut screw. A number of contact points are then placed on exactly similar positions on each print. When the cylinder is revolved the contact points make electrical connection with the metal plates, except where the developed picture provides an insulator. This insulator is obtained form the light which has passed through the negative and affected the film on the metal plates. Where the light has been sufficiently strong the film becomes insoluble, whereas the unaffected portion can be washed off leaving the metal plates clean. Hence, there are obtained a number of plates having different sized areas of insulating material.
The circuits of the contact points are taken through the magnets of a telegraph perforator, such for instance as a 5-unit perforator as is employed for reception on Duplex Multiplex circuits using the 5-unit code, i.e., Baudot, Murray, Western Electric, etc. A cam on a shaft, geared to the cylinder shaft in the ratio of 200 to 1, opens and closes the main circuit to the perforator magnets. The picture is thus translated from five prints of varying intensity into five more or less complete rows of holes in the telegraph tape. This tape can be sent to a telegraph office for despatch as a certain number of feet of perforated tape. An exact reproduction of the perforated tape is obtained at the receiving end of the telegraph line and is sent to the addressee, who passes it through a machine in which light is arranged to pass through the holes in the tape to affect a photographic film. The number of holes in the tape consequently varies the amount of light which reaches the photographic film and thus determines the amount of density or tone of the various parts of the picture.
Two examples of reproduced pictures are shown. The first one was sent on the Siemens' 5-unit High-speed telegraph apparatus from London to Berlin, where the perforated tape was re-transmitted to London and used to develop the picture. The second picture was translated into a number of ten-letter code words and sent over the Imperial cable from London to Halifax, Nova Scotia.
It was re-transmitted to London and the picture was produced. The reason for using code words was because of the fact that Cable code Morse is the system of working on this cable at present.
Five tones seems to be sufficient for all practical purposes in telegraphing pictures, but a larger number could be provided if considered to be necessary.
The promising feature of this new scheme is the practicability of applying it to any telegraph line without the slightest alteration of any of the existing telegraph apparatus, where the 5-unit code is employed, and it can be used for Morse circuits by a translation into code words. With the 5-unit code there is no checking of words, but merely the reception of so many feet of perforated tape.
The system therefore opens up a new field for Telegraphy for those cases where the cost is justified.