Some of the finest achievements in the field of electrical engineering are incorporated in the picture telegraphy service which is in regular operation between London and the principal European countries
THE PICTURE TRANSMISSION ROOM at the Central Telegraph Office, London. On the left is the power switchboard, and on the right the control panel for the transmitter and the receiver. The small black cabinet to the left of the control panel is the transmitter, and that on the right of the control panel is the receiver.
PICTURE telegraphy, sometimes known as facsimile telegraphy, is unique. All other systems of telegraphy - from beacon fires to teleprinters - depend on a human agent or on a machine which writes, in its own way, the message which has been transmitted from a distant point. A telegram is handed across the counter in the sender’s handwriting; as received at the far end, however, it is printed or, in some instances, written in the handwriting of some other person.
When facsimile telegraphy is used, the message or picture which is being transmitted is “translated” into signals which may be sent over a wire, or through the ether, but they are retranslated into a replica of the original message.
Line drawings, photographs, halftone illustrations, written matter or printed tables can thus be reproduced at a distance. The process is not slow: the transmission of a picture measuring about seven by five inches takes roughly twelve and a half minutes.
Facsimile telegraphy was first used as long ago as 1843. Handwriting and simple line drawings were then reproduced by a chemical process involving the action of an electric current on an impregnated tape. Several other processes superseded this, and some were used by newspapers early in the twentieth century. Most of these processes, however, were crude and cumbersome by comparison with the modern systems, of which one of the best known is the Siemens-Karolus.
This system is used by the British Post Office, and pictures are regularly transmitted from the Central Telegraph Office, in London, to the capitals of most of the countries in Europe and to many other large cities on the Continent. On July 16, 1929, the announcement was made that a public service was to be opened, and the installation was formally inaugurated on January 7, 1930, by the opening of a regular service with Berlin. The inauguration took the form of an exchange of photographs and greetings between the Postmaster General and the German Minister of Posts.
Many technical and practical improvements have been introduced since then, and the British Post Office is now able to offer the finest possible service, the reliability of which is remarkable in view of the fact that it has been in operation for only a few years.
The Press and the newspicture agencies make extensive use of the system, but its scope is not limited to such work. Documents, plans and specifications can be transmitted, and the system offers facilities to fashion experts, picture dealers, artists, architects and designers.
It might reasonably be expected that the apparatus for handling this service would be cumbersome and complicated. Picture telegraphy has certain points in common with television, and the complexity of television apparatus is well known. All the apparatus at the Central Telegraph Office, however, for handling this important service is housed in one room. The transmitter and the receiver are compact units. Between them stands their control panel; a power switchboard occupies another wall of the room. Apart from the batteries and the photographic apparatus, which are situated in another room, the units shown in the illustration at the top of this page represent the entire installation.
In the Siemens-Karolus system used by the General Post Office the picture or message to be transmitted is clipped round a drum. This drum is lowered into a cylindrical casing on the transmitter, where it engages with a rotating spindle. The apparatus keeps the drum revolving at a uniform speed - one revolution a second.
A spot of light is thrown on to the picture, and as the drum revolves this light-spot “scans” the picture. It travels upwards at a uniform rate until it reaches the top of the picture. In other words, it is covering the picture in a spiral, in a manner somewhat resembling the cutting of a cylindrical phonograph record.
Electric Tuning Fork
There are five and a third lines to a millimetre; this spacing gives exceedingly good definition and has been adopted as a suitable compromise between high speed and perfection of reproduction.
The beam of light which falls on the picture is focused by optical means into an extremely minute point, and the beam is interrupted by a toothed disk 1,300 times a second. Thus, in effect, a picture to be transmitted is divided into elemental areas or dots of equal size, 1,300 to a line and five and a third lines to a millimetre - about 135 lines to the inch - according to the light-value represented by the particular part of the picture on which the light-beam falls.
The light reflected from the surface of the picture falls on a photoelectric cell. This cell converts the varying train of light-values into a pulsating electric current which is exactly proportional to the varying tones of the picture. The lighter the picture, the more intense is the light reflected from its surface, and the greater is the current passed by the photoelectric cell. This current is extremely small, and is passed on to a valve amplifier, after which it is transmitted over the telephone line to the receiving station. At the receiving end it is amplified again, and the receiving process may be regarded as the converse of that which takes place at the transmitter.
To outward appearances the transmitting unit is surprisingly simple and compact, consisting merely of the apparatus for rotating and elevating the drum, the motor for rotating the toothed wheel which interrupts the light-beam, the light-source and its optical system, and the photoelectric cell. The internal parts, however, contain gear-wheels which have been accurately cut.
Any small imperfection in the main gearing gives rise to disturbances which show themselves as lines or bands of varying density across the picture. Another complex part of the apparatus is the control board which houses the synchronizing tuning fork and the valve amplifiers.
SENT BY WIRE - an untouched reproduction of a photograph, exactly as it was received from the G.P.O. after it had been transmitted over a short length of line between the transmitter and the receiver. The original was an ordinary halfplate photograph of the George Washington Bridge over the Hudson River, New York. The clearness with which details are reproduced is evident from this striking example of picture telegraphy.
The interrupter speed of 1,300 cycles a second must be maintained within extremely fine limits, and the motor driving it is controlled by a centrifugal governor. Synchronism is maintained by an electrically driven tuning fork, the output of which is amplified by valves before going to the synchronizing control device.
At the receiving end the impulses sent along the wire from the transmitter are amplified and passed on to an oscillograph, which translates the pulsating electric current into mechanical movements of a small mirror. A beam of light is focused on to this mirror; the mirror reflects it through a further optical system on to the surface of a sensitized film which is wrapped round a drum similar to that on which the picture was mounted at the transmitting end. The specially designed aperture through which the light passes ensures that the steady beam of light does not reach the film. The vibrations of the oscillograph mirror deflect the light-beam to and fro across the aperture, so that the intensity o f the focused light-spot varies in proportion with the pulsating current flowing along the telephone wire. This current, in turn, is an electrical equivalent to the varying intensity of the light-beam reflected from the picture in the transmitter, and the film at the receiving end thus reproduces the picture faithfully.
It is the general practice to make a negative at the receiving station, but the apparatus may be so adjusted that a positive is received.
The drum on which the sensitized film is mounted is charged in a dark room and placed in a metal cylinder. An ingenious method of construction ensures that it is sealed against light, until it is placed in the receiver. As it goes into position, a catch on the top of the cylinder engages with a notch on the receiver. The safety shutter of the cylinder can now be opened, exposing the film on the drum to the beam of light. The cylinder cannot be removed from the receiver until this shutter is closed. These precautions are of great importance, since the accidental exposure of a film to light would necessitate a retransmission.
THE CONTROL PANEL (right) contains a large amount of intricate apparatus, some of which is common to the transmitter and the receiver. The tuning fork for the synchronizing apparatus and its valve amplifier are housed behind this board. The engineer in the illustration is speaking to the distant operator before beginning a transmission.
THE RECEIVER MACHINE. The operator is removing a negative, completely enclosed in its light-proof case, before taking it to the dark room for developing and printing. In the centre of the machine, above the nameplate, is the disk which indicates synchronism between the receiver and the transmitter.
When once the whole picture has been received and the drum removed, the remainder of the process is pure photography. The drum is opened in a dark room, and the film is developed, printed and fixed in the normal way. Films and prints are dried mechanically, and the whole process takes only fifteen minutes.
The public service is carried on over the ordinary telephone lines, which, in general, do not respond satisfactorily to frequencies higher than 2,500 per second. Television - the instantaneous transmission of pictures - involves frequencies as high as two million. Picture telegraphy, however, spreads the same number of impulses out over a much longer period and can be handled satisfactorily over ordinary lines.
When the characteristics of the telephone lines to the Continent have been improved it will be possible to speed up the service, as the lines are the chief limiting factor. There are no other technical difficulties in the way of increasing the interruption speed to 5,200 cycles a second instead of the 1,300 cycles a second now used, and this would be one means of quadrupling the speed of transmission.
One of the most important technical aspects of picture transmission is synchronism. The drum at the receiving end must be rotating at precisely the same speed as that at the transmitter, or the picture will be distorted. If the receiving drum “catches up” on the transmitting drum, however slowly it may do so, the lines on the film will slope. A transmitted rectangle might be reproduced in these circumstances as a parallelogram.
Correct synchronism is obtained by using the stroboscopic disk method of observing periodic motion. To the shaft of the driving motors is fixed a, disk with a number of radial lines, alternately black and white. If such a disk is illuminated by a neon lamp flashing at the same rate as that at which these lines pass any fixed point, then the disk appears stationary.
In the picture apparatus the disk rotates at the speed of the receiving motor, and the neon lamp is flashed according to the speed of the distant sending motor. Any difference in speed between the two motors is seen by a drift on the disk, and is thus adjusted at the receiving end.
Another point is the correct “phasing” of the receiver and the transmitter. A wave-train of electrical energy travels from one point to another at a rate dependent upon the medium over which it is transmitted. In telephone engineering this time of travel is called the propagation time. To compensate for this variation, the drum at the receiving end must be orientated or pushed back through an angle equivalent to the time of travel along the line from the transmitter. Another neon lamp fixed under a slot in a rotating disk indicates when the receiver is correctly phased with the transmitter.
The operators at either end speak to each other by telephones connected to the picture apparatus. All the preliminaries to a transmission are carefully arranged, n such a way that the risk of failure, or even of a partly unsatisfactory transmission, is reduced to a minimum.
The present standard of the service is amazingly high, and some of the received pictures are almost indistinguishable from the originals. When a radio link is used, atmospheric disturbances sometimes cause blemishes on the picture, but most of the transmissions over wires are perfectly clean.
Transmission of Fingerprints
For long-distance work, pictures are exchanged in London between the Post Office and Cable and Wireless, Ltd., who use their radio channel to New York and also maintain a service to Australia.
Pictures may be transmitted from any part of Europe to New York and San Francisco. If they are sent from, or intended for, towns with no facilities for picture transmission, they are sent by air mail or by the most rapid means available to or from the nearest picture station.
Facsimile telegraphy is not limited to the use of roman characters. Chinese, Russian, shorthand characters and hieroglyphics of all kinds may be transmitted. Fingerprints are not beyond the capabilities of the system, and the service has been used by British and foreign police authorities. Greeting cards and Christmas cards, bearing the signature of the sender, are reproduced exactly in the form in which they are handed over the counter.
The illustration shown above was sent over a short line in the Central Telegraph Office from the transmitter to the receiver. The original was a halfplate photograph of the George Washington Bridge over the Hudson River, New York, and the print received from the General Post Office was sent to the printers without retouching the photograph. There was no point in showing, side by side with the transmitted picture, a reproduction of the original photograph, as it would have been almost impossible to distinguish between the two - a fact which testifies to the excellence of the system more clearly than any verbal description of it.
INSERTING A PICTURE in the transmitting apparatus. The photograph, diagram or telegram to be transmitted is clipped round a drum which engages with a rotating spindle. The drum is kept revolving at a uniform speed, and a light-spot “scans” the complete picture in about twelve and a half minutes.