TELEVISION IVES 299 



In our original demonstrations of 1-way television, scanning disks 

 were used which had 50 holes arranged in a spiral. With this num- 

 ber of holes it is possible to secure a definitely recognizable repre- 

 sentation of the human face. It was decided, however, that for the 

 2-way system a degree of definition should be provided such that 

 faces were rendered in an entirely recognizable and satisfactory man- 

 ner. Accordingly the number of scanning holes has been increased 

 to 72, which provides just twice the number of image elements. 

 The transmission band is, of course, doubled by this change, requir- 

 ing wire connections of considerably higher quality than heretofore. 

 "When a 72-hole scanning disk is used, the component frequencies of 

 the image signal encompass a range of from 10 cycles to 40,000 

 cycles per second, whereas intelligible speech ma}^ be reproduced by 

 a signal wave whose component frequencies cover a range of 2,500 

 cycles per second. This comparison indicates roughly how much 

 more difficult it is to transmit high-quality television images than 

 it is to transmit ordinar}^ speech. In general the electrical features 

 of the apparatus are similar to those previously used, although in the 

 interval improvements and refinements have been made in many 

 directions. 



Light reflected into the photo-electric cells gives rise to an alter- 

 nating electric current whose effective value is of the order of a 

 ten-thousand-millionth ampere. The neon glow lamp on which the 

 image is received at the distant station reproduces the image satis- 

 factorily when the effective value of the alternating current is of 

 the order of one-tenth ampere. This thousand-millionfold increase 

 in current variation, considerably greater than required for the 

 earlier 1-way system, is effected by amplifiers in which the vacuum 

 tubes are coupled by condensers and resistances. The tubes, which 

 operate at low energy levels, are shielded against electrical, mechani- 

 cal, and acoustical interference. 



For the transmission of images between 463 West Street and 195 

 Broadway, the appropriate stages of the amplifier systems are cou- 

 pled by special transformers to telephone cable circuits equipped 

 with special distortion corre'cting networks which are capable of 

 transmitting the extremely complex current variations without dis- 

 tortion. The amounts of distortion inherent in other parts of the 

 system are either kept small by design or annulled by means of cor- 

 recting networks. 



An indispensable part of a television system is the means for hold- 

 ing several scanning disks accurately at the same speed. For the 

 2-way television system, a simplified and improved synchronizing 

 arrangement is used. The disks at the receiving and transmitting 

 ends, which rotate at a speed of 18 revolutions per second, are 



