fUllKI. I l<.l.\sMlsMi'\ ('//./? TF.I.r.rilONE I. IMS 



IW 



till' ti'lt'phone line is, then, the carrier wave ' inodulatet! by the photo- 

 eierlric wave s«) that the currents, in fre(iiiency range and in ampli- 

 tude, are similar to the currents corresponding to ordinary speech. 



When the carrier current, modulated according to the lights and 

 sJKules of the picture at the sending end, traverses the ribbon of the 

 light vahe at the receiving end, the aperture is o|iened and closed 

 with each pulse of alternating current. Tlu- envelope of these pulses 

 follows the light and shade of the pii tiirc. Imt the actual course of 



LU 



PICTURE CHANNEL 



SYNCHRONIZING CHANNEL 



Kig. 6 — Diagrammatic representation of the picture and synchronizing currents. 



(P) photoelectric cell; (.AM) amplifier modulator; (A) amplifier; (V) light valve; 



(M) phonic wheel motors; (T) tuning forks; {.\K) amplifier rectifier 



the illumination with time shows a fine structure, of the pcrio(licit\' 

 of the carrier. This is shown by the enlarged section of a picture. 

 Fig. 5; in this the black lines are traces of the image of the light valve 

 aperture. Superposed on the larger variations of width, which are 

 proportional to the light and shade of the picture, small steps will 

 be noted (particularly where the line width varies rapidly); these are 

 caused by the carrier pulses. 



Syn'chroniz.ation 



In oriler that the light and shade traced out on the receiving c\lin:ler 

 shall produce an accurate copy of the original picture, it is necess.iry 

 that the two cylinders rotate at the same uniform rate. This, in 

 general, demands the use of accurate timing devices. The means 

 employed in the present apparatus consist of phonic wheels or impulse 

 motors controlled by electrically operated tuning forks.'' Were it 



'A description of electrical communication by means of carrier currents will be 

 found in "Carrier Current Telephony and Telegraphy," Colpitis and Blackwell, 

 Trans. A. I. E. E., 1921, Vol. 40, pp. 205-300. A discussion of the relations between 

 the several components of the signal wave employed in carrier is given in "Carrier 

 and Sidebands in Radio Transmission," Hartley, Proc. I. R. E., Feb., 1923, Vol. 11, 

 No. 1, pp. 34-55. 



* .A detailed description of the construction and operation of the impulse motor 

 and its driving fork is given in "Printing Telegraph Svstems," Bell Trans, .\. I. E. E., 

 1920, Vol. 39, Part 1, pp. 167-230. 



