APPLICATION OF PRINTING TELEGRAPH 605 



been selected at the zero indicated on this figure. The tolerance in 

 received current level variations usually obtained is about ± 3 db in the 

 case of single-tone signaling as compared to about ± 7 db for two-tone 

 signaling. On the transatlantic long-wave radio circuits variations 

 greater than those tolerated by the two-tone transmission method 

 seldom occur with sufficient rapidity to escape manual correction. 



With the two-tone system the amount of noise entering the receiv- 

 ing mechanism comes in through double the band width used in the 

 single-tone system, ^ and the intelligence transmitted is completely 

 contained in both the marking and the spacing signals. It is, therefore, 

 logical to expect that there will not be much difference between the 

 two-tone and single-tone systems from the noise interference stand- 

 point. If there were no received signal the noise through the marking 

 and spacing filters probably would balance out to some extent but dur- 

 ing operation either the mark or space signal is always present. The 

 noise may effectively annul either signal by being approximately of 

 equal intensity and opposite phase, but the noise through the other 

 filter is received with the full intensity and, therefore, may operate the 

 relay falsely. 



Employing printing telegraph equipment on radio circuits is not 

 new.^ .9 ,10 .11 There has, however, been comparatively little commercial 

 use of such systems and there have been very few quantitative data 

 published. Such practical information and quantitative data as have 

 been obtained by the Bell System regarding the application of printing 

 telegraph to radio circuits relate to long-distance overseas point-to- 

 point communication and a short-distance point-to-point overland cir- 

 cuit. Both of these circuits were operated on long waves (about 60 

 kilocycles) . 



For the past three years printing telegraph has been employed on 

 the long-wave radio telephone circuit i- between New York and London 

 to exchange information pertaining to the operation of this telephone 

 service. The printer is admirably suited to this kind of service since 

 the information exchanged frequently consists of foreign names of 

 places and people not familiar to the switchboard operators. By the 

 use of the printer, these can be spelled out with speed and accuracy 

 without the necessity of attempted pronunciation. 



The printing telegraph arrangements provided at New York for use 

 of the telephone traffic department on the transatlantic circuits are 

 shown in Fig. 2. The instruments are installed on the table in the 

 foreground. This table is located just behind the switchboard opera- 

 tors. As a large majority of the business transacted is of a question 

 and answer nature, there are special arrangements in the printer to 



