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A Unique Film Scanner for Testing Television Transmission Images
JUST as program broadcasting requires extensive networks of cable circuits to connect studios and radio transmitters, so television broadcasting will need similar cable connections for its most effective growth. To a limited extent the Bell System has already supplied such facilities on an experimental basis.
The development and testing of these facilities is complicated botb by the very wide band of frequencies that must be transmitted and by the varied requirements for television transmission. In some respects these are much more severe than those placed on multi-channel telephone circuits, even when the total band width is the same.
Scanning the Film Frames
A circuit entirely satisfactory for highquality telephone transmission might not be satisfactory for television transmission because the response of the eye is not at all like that of the ear to certain types of distortion. The most satisfactory way to test television circuits, therefore, is to transmit television signals over them, and to judge the results visually.
In making such tests it is desirable to transmit the same scene or series of scenes over the cable again and again. For this reason a motion-picture film is
By W. A. KTSOOP
MEMBER OF THE TECHNICAL STAFF BELL TELEPHONE LABORATORIES
the best source of the transmitted material.
With this method the picture frames on the films are scanned successively by some form of television scanner, and the resulting television signals, suitably amplified, are transmitted over the circuit. Each frame is scanned in a series of lines one above another, and thus there is a vertical as well as a horizontal component of the scanning.
In using film, it seemed desirable to let the motion of the film provide the vertical component, and thus to simplify the scanning equipment. Since the ordinary motion-picture projector moves the film intermittently, a suitable transmitter for these tests was developed by modifying a Western Electric film recorder to secure steady motion of the film and the other features that were required.
The complete machine is shown in Fig. 1. At the extreme left is the scanning equipment, and the rectangular case next to the right carries the projection lamp. Light from this lamp passes through a lens in one side of the case and then through an opening in the film case ad
FIGURE 1
The complete film scanning machine. At the extreme left is the scanning equipment, and the rectangular case next to the right carries the projection lamp.
jacent to it, where it is refracted by a right-angle prism to pass through the film and into the scanning equipment.
Just above the left end of the film cabinet is the equipment for sound pickup. It includes a photo-electric cell, lenses, and certain miscellaneous equipment. The film supply reel is at the upper center, and the film take-up reel is below it and just beneath the film compartment. At the right is the motor that drives the film. The lamp housing is mounted on a hinged bracket, and may be swung out to give access to the film cabinet.
A close-up of the apparatus with the various doors open is shown in Fig. 3. The film passes down from the film magazine, over the top of a film sprocket that pulls the film from the magazine, thence
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INTERNATIONAL PROJECTIONIST