Berlin’s Scanning Equipment 

16 October 2023 tbs.pm/78937

Details of the television scanning apparatus and light-spot scanner for the ultra-short-wave transmitter in Berlin-Witzleben

 

 

Cover of World-Radio magazine

From World-Radio magazine for 30 August 1935

FOR the regular television transmissions of the German Reichspost two scanning devices, designed by Fernseh A.-G., Berlin, are used. One of these is employed for the transmission of films and the other for direct transmissions of persons, especially of speakers. A description of the technical constructional features and details of the scanning devices installed in the television house in Berlin-Witzleben, Rognitzstrasse, will, no doubt, be of interest to World-Radio readers.

The Fernseh A.-G., during the last six years, has built a large number of television scanning devices. The new types embody all the latest improvements, a special feature being the scanning discs, which run at 6,000 r.p.m. in a vacuum. The driving power required is very small, only 40 watts being necessary to drive a disc 2½ft. [76cm] in diameter, or about one-thousandth of the power necessary to drive the same disc in air. The form of construction avoids all mechanical vibration that might disturb the “ head ” amplifier or the microphone. The greatest advantage is that the material of which the disc is made can be very thin (0.1 mm.), which ensures that the disc rotates in an absolutely true plane.

Arc lamps are employed in both scanners, so that the light flux that falls on the photocell (film scanner) is high (1/20 lumen), which provides an output of 6 mV with a working load of 3,000 ohms. The measured noise-level of the amplifier lies near 10-20 µV, and, assuming that the light amplitude must be thirty times larger than this value in order to avoid distortion of the picture, these mechanical scanners can be used for high definition systems, the limit being of the order of 360 lines.

The greatest care has been taken in the construction of the apparatus to ensure reliability in operation and uninterrupted performance throughout several hours of regular daily transmissions.

 

A bakelite cabinet with dials and gauges

Fig 1 – General view of the film scanner

 

The Film Scanner

Fig. 1 gives an idea of the arrangement of the scanner and of its associated equipment. The whole apparatus is mounted on a very heavy cast-iron housing. On the left is the arc lamp; in the centre, the film-driving machine with the lenses, and, on the right, the vacuum housing with the 6,000 r.p.m. motor and disc. The film window of the film-driving apparatus is water-cooled and is in the form of a small slot.

There is also a sound head to deal with the recorded sound on film. Between the film itself and the scanning disc are mounted two lenses of very high light efficiency (aperture 1.9).

The vacuum housing includes a vacuum pump mounted in the cast-iron housing; the motor has no commutator, and is driven by a special 100-cycle supply synchronised with the main supply of 50 cycles per second.

The disc has forty-five apertures for 180-line scanning, giving twenty-five pictures per second, and there are also forty-five slots for the line-synchronising impulses.

A special slot is provided for the picture synchronising impulses cooperating with a shutter, driven by a special synchronous motor. At every fourth revolution of the disc, a synchronising impulse is produced.

The small synchronous motor, as well as the main motor, is connected with phase – shifting transformers, mounted in the cast-iron housing. The disc can, therefore, be brought into synchronism with another transmitter already in operation. The knob for turning the phase-shifting transformer can be seen in the photograph on the control panel on the left side of the cast-iron housing. The panel contains a large number of controls and instruments for the motors, the arc lamp, the synchronising lamp, and so on; also instruments for measuring the vacuum and the pressure of the cooling water. In both cases, if the allowed limits are exceeded, an alarm signal is brought into action. This film scanner has been operating daily for about a year, and is working very satisfactorily.

 

A large wooden and metal cabinet with a chimney, gauges and dials

Fig. 2 – The light-spot scanner

 

Light-Spot Scanner

The construction of the light-spot scanner (see Fig. 2) is similar to that of the film scanner, but the light efficiency of the former is higher. This device, too, has a cast-iron housing, with special arc lamps on the left, and, on the right, a vacuum housing.

A spiral disc of 75 cm. diameter is employed, and this runs at 6,000 r.p.m. The slots are relatively large. The size of the picture on the disc is about 45 by 45 mm. This disc consists of particularly strong material, as the speed of rotation at the periphery is 240 metres per sec. In spite of this high velocity, the disc runs noiselessly.

In this apparatus the shutter disc runs quite near the main disc, and is driven by another synchronous motor at 1,500 r.p.m. The slot in the shutter disc has the form of a spiral, arranged near the periphery. The material of this disc is glass, which, except at the spots where the light passes through, is covered with an opaque layer. This layer serves at the same time as a mirror, and reflects the light so that it prevents undue heating of the glass disc. This is of great importance because, in the vacuum, the heat can be dissipated only by radiation. Quartz glass is employed for the window between the arc lamp and disc, as, owing to the intense heat, glass would crack. The arc lamp, which is mounted on a support, is of recent design and gives a light flux of 25,000 lumen when consuming 150 amps.

The shutter disc also produces the synchronising impulses. It may be of interest to note that there are several objectives in the vacuum housing, with different focal lengths, so that it is a simple matter to transmit pictures of varying size without changing the distance by a simple turn of the objectives (e.g. to change from a close-up of the head of a speaker to a distant picture of full length, or more, and so on).

The use of a so-called white studio is preferred, the extremely high reflecting power of the walls preventing disturbances by shade effects, etc., and the light efficiency is also increased. The light current falling into the photocell amounts to 1/300 lumen. This studio, w’hich measures about 5ft. square, lies behind the partition on the right of Fig. 2, and can accommodate two persons. The front wall of the studio has the necessary aperture for the light pencil.

All the experiments gave good results in the control receivers and, even without full use of the 40,000 picture elements, the reproduction of the images has been completely satisfactory.

 

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