IMAGE TRANSMISSION SYSTEM 449 



Physical Arrangement and Operation 



The detailed description of the optical and electrical elements of 

 the two-way television system will be more readily grasped if it is 

 preceded by an account of the general arrangement of the parts and 

 of the method of operation of the system from the standpoint of the 

 user. 



The physical arrangement of the two-way television system is shown 

 by the pictorial sketch Fig. 1, and in the photographs Fig. 2 and 

 Fig. 3. The terminal apparatus is largely concentrated into a booth, 

 — the television booth — similar in many respects to the familiar 

 telephone booth, and a pair of cabinets, which contain the scanning 

 discs and light sources. As in the 1927 demonstration, scanning 

 is performed by the beam method, the scanning beam being derived 

 from an arc lamp whose light passes through a disc furnished with a 

 spiral of holes and thence through a lens on the level of the eyes of the 

 person being scanned. The light reflected from the person's face is 

 picked up by a group of photoelectric cells for subsequent ampli- 

 fication and transmission to the distant point. The signals received 

 from the distant point are translated into an image by means of a 

 neon glow lamp directly behind a second disc driven by a second motor 

 placed below the first and inclined at a slight angle to it. The two 

 discs, which are shown in the center cabinet of Fig. 2, are of slightly 

 different sizes; the upper one 21" in diameter and the lower one 30". 

 They difl'er from the discs used in the earlier demonstration in that in 

 place of the 50 spirally arranged holes formerly used, they carry 72 

 holes whereby the amount of image detail is doubled. While with the 

 earlier "50 line" picture recognizable images of a face were obtainable, 

 the aim in this new development was to reproduce the face so clearly 

 that there would never be any doubt of recognizability, and so that in- 

 dividual traits and expressions would be unmistakably transmitted. 

 This doubled number of image elements necessarily requires, for the 

 same image repetition frequency (18 per second) twice the transmission 

 band, or approximately 40,000 cycles as against 20,000 for the 1927 

 image. 



The only part of the television apparatus visible to the user is the 

 array of photoelectric cells which are in the television booth behind 

 plates of diffusing glass. In addition to the photoelectric cells and 

 their immediately associated amplifiers, the booth contains a con- 

 cealed microphone and loud speaker. By means of these, the voice 

 is transmitted to the distant station and received therefrom without 

 the interposition of any visible telephone instrument which could 

 obscure the face. 



