Foresight

K. F. Teodorchik, Moscow

The issue of the journal of the American Telephone and Telegraph Company, The Bell System Technical Journal, which appeared in October 1927, contains a series of articles devoted to the description of a television system developed by the company’s scientific staff. The results obtained in these experiments show that the problem of seeing at a distance must in general be regarded as solved. Transmission during these experiments was carried out with equal success both by wire and by radio. By wire, transmission was conducted from Washington to New York over a distance of 250 miles, and by radio—from the company’s experimental radio station in New Jersey to New York over a distance of 22 miles. The transmitted images were received with the aid of two types of receivers.

The first type reproduces the transmitted object on a small ($2 \times 2 \tfrac{1}{2}$ inch) screen and, being connected to a telephone, makes it possible during a telephone conversation to see one’s interlocutor.

The second type of receiver reproduces the transmitted image on a screen visible to an entire audience, measuring $2 \times 2 \tfrac{1}{2}$ feet, and, in connection with a loudspeaker, is intended for broadcasting purposes. Below we give a brief description of the idea underlying this method of television, as well as of the instruments developed for its implementation.

To carry out television by electrical means, it is necessary at the transmitting station to convert the light

signal into an electrical one, transmit it to the receiving station, and there carry out the reverse conversion of the electrical signal into an optical one. The first part of the problem, as is known, is solved by a photoelectric cell; for the latter, in the system described, a neon lamp is used.

In principle, with the aid of these two devices, tele-vision can be carried out in the following way. Let us imagine, at the transmitting station, a screen formed by a large number of photoelectric cells arranged in the form of a grid. Onto this screen is projected the image of the object being transmitted. The current produced in each photoelectric cell (after amplification) is transmitted by wires to the correspondingly located neon lamp on the receiving screen. Such an apparatus obviously gives satisfactory transmission under the conditions: 1) proportionality of the photocurrent to the brightness of the illumination, 2) proportionality of the brightness of the neon lamp to the current supplied to it, and 3) a sufficient number of elements receiving and reproducing the image. The photoelectric cell and the neon lamp satisfy the first two requirements; the third, however, makes this system impracticably complicated, since for satisfactory transmission even of a single human face it is necessary to transmit no fewer than 2,500 elements of the image.

For tele-vision, however, it is not necessary to transmit all the elements of the image simultaneously.

As is known, a light source flashing more than 15 times per second is perceived by the eye as continuously luminous. Therefore, if on the receiving screen the separate points of the transmitted image flash successively with such speed that in one second each point will be transmitted no fewer than 15 times, then the eye perceives the whole transmitted picture in its entirety.

Thus tele-vision can be carried out by the ordinary method of transmitting images over a distance, provided the transmission speed is sufficient. This method is the basis of the tele-vision system described here. The number of image elements has been chosen equal to 2,500; the number of transmissions of each element is 177 per second. This gives a transmission speed of...

gave about 40,000 signals per second. For transmission at such a speed without distortion, a resonance-free transmission to the receiving station of a frequency interval from 0 to 40,000 periods per second is theoretically necessary.

Fig. 1.

Amplification of low frequencies, as is known, presents very great difficulties; in the present case the difficulties are still considerably greater because of the width of the frequency interval required for transmission. In practice it proved possible to obtain satisfactory results when transmitting a smaller interval, from 10 to 20,000 periods.

Fig. 2.

With the above-mentioned transmission speed, the usually employed method of projecting the image onto a photoelectric cell...

...of the transmitted object does not give sufficient amplification of the photocurrent.

Therefore, at the transmitting station the path of the light rays was reversed (see Fig. 1). The object to be transmitted was illuminated by a bright light beam sweeping over the entire field of view being transmitted. The light scattered by the illuminated part of the object was collected by three photoelements, whose photosensitive surface was equal to 120 sq. inches (see Fig. 1).

The movement of the beam over the field of view being transmitted is produced, as may be seen from Fig. 1, by a rotating disk, around the circumference of which 50 apertures are arranged along a spiral line.

Each of the apertures, passing by the lens, traces with the light beam a horizontal strip on the object being transmitted. Thus the entire field of view is divided into 50 strips, transmitted successively.

Thanks to such a system, photocurrents sufficient for amplification are obtained with a not overly bright average illumination of the object being transmitted.

Since only the alternating components of the photocurrents were amplified and transmitted, the constant additional illumination of the object being transmitted, as changing only the direct component of the photocurrent, did not interfere with transmission. After appropriate amplification, the alternating components of the photocurrent were transmitted to the receiving station. As already mentioned, transmission was carried out both by wire and without wires. In the latter case, the image-carrying currents were used to modulate a high radio frequency. In view of the need to transmit a very wide band of frequencies, the carrier frequency had to be chosen relatively very high, namely 1575 kilocycles (wavelength 190 m).

At the receiving station, after secondary amplification, the image-carrying currents were applied to a neon lamp, which served to reproduce the television signals. In the receiver, which reproduced the image in a small size, the receiving neon lamp had two flat electrodes, placed opposite one another at a distance of one millimeter, the electrodes being \(2 \times 2^{1/2}\) inches in size (see Fig. 3).

The pressure inside this lamp is such that the glow covers, in a uniform layer, only the outer surfaces of the electrodes. The brightness of this glow is proportional to the current.

To reproduce the transmitted image, the receiving lamp was placed behind a perforated rotating disk, exactly similar to the disk of the transmitting apparatus and rotating synchronously with it (see Fig. 4).

With synchronous rotation of both disks, the transmitting and the receiving one, an observer situated in front of the receiving apparatus will see, through the aperture of the rotating receiving disk, a portion of the luminous electrode of the neon lamp in the same relative position and of the same brightness as the portion of the transmitted object illuminated by the moving spot. Owing to the rapidity of transmission, the separate currents merge for the observer into a complete image. For receiving the transmitted image on a large scale, a screen was used, formed by a neon-filled tube bent fifty times so that its bends fill the entire screen. Inside this tube there is arranged a spiral

Fig. 3.

Fig. 4.

electrode running along its entire length. On the rear side of the tube, 2,500 electrodes are fastened to the glass, each with a separate lead-in wire. One lead of the current carrying the image is connected directly to the inner spiral electrode; the other, through a special synchronous distributor, is connected successively to all the interrupted electrodes. Fig. 5 gives details of the construction of the screen and a general view of the synchronous distributor.

Fig. 5a.

Fig. 5b.

For synchronization, two synchronous motors coupled on a common shaft were used: one two-pole motor and a second with 120 pairs of poles. The synchronous frequencies of these motors, 17.7 and 2124 periods per second, were transmitted together with the image-carrying current.

In conclusion, let us note a curious fact discovered in the transmission of foresight signals by radio. At night, when fading was observed, in addition to the main image, an additional image appeared on the receiving screen, sometimes positive and sometimes negative, shifted relative to the main one (see Fig. 6).

Fig. 6.

These additional images are evidently due to the fact that the receiving antenna picks up not only the ordinary wave, but also one reflected from the Heaviside layer. The formation of a positive or negative additional image depends on the phase difference between the direct and reflected waves. The height of the reflecting layer, calculated from the relative displacement of the two images, was found to be 100 km, in agreement with other observations of the layer height.