202 BELL SYSTEM TECHSICAL JOURNAL 



effects and "echoes" caused hy reflections from impedance irregular- 

 ities. A high degree of l)alance between the lines and their balancing 

 networks at repeater points is also required. These conditions can be 

 satisfactorily- met on wire telephone lines. Radio communication 

 channels are inherently less stable and less free from interference, 

 and special means lo overcome their defects are required in order to 

 secure high-grade pictures. 



("llARACTICRISTICS OK Rli( ICIVED PICTURES 



\\\ elect rically transmitted i)iclures Ikuc, as a result ot iJu- processes 

 of scanning at the sending and recei\ing ends, a certain amount of 

 structure, on the fineness and character of which depends the detail 

 rendering of the result. 



The origin and nature of the microscopic structure characteristic 

 of pictures transmitted b>- the present process is illustrated by the 

 diagrammatic presentation of Fig. 13, which may serve at the same 

 time to give a re\'iew of the whole process. We will assiune that the 

 original picture consists of a test object of alternating oi)aciue and 

 transparent lines. Such a set of lines is shown at .4. The lines are 

 assumed to be moving from left to right across the spot of light fall- 

 ing on the film. The width of the sjxit of light (corresi)onding to the 

 pitch of the screw) is represented b>' the pair of ilashed lines. If 

 the spot of light were infinitely narrow in \\w diiection of motion 

 of the picture film, the photoelectric current would l)e represented 

 in magnitude in the manner shown at B. AclualK- the spot must 

 have a finite length, so that the transitions lietween the maximum 

 and minimum \alues of current are represented by diagonal lines as 

 shown at C. Due to the una\'oidable reactances in the amplifying 

 system, there is introduced a certain rounding off of the signal so that 

 the \'ariation of [lotential impressed on the modulator tube follows 

 somewhat the course shown at D. The alternating current intro- 

 duced by the vacuum tube oscillator is, then, given the character- 

 istics shown at K, the envelope being a close cop\' <if D. i^assing 

 out to the transmission line, the fact that the band nf frequi'ncies 

 transmitted by a telephone line is limited in extent results in a certain 

 further rounding olT of the i-iuelope of the |)icture current as shown 

 in /•'. The ribbon of the light \alvc when traversed 1)\- the alter- 

 nating current from the line performs oscillations to either side of the 

 center of the ajx-rture, consef|uently opening first one side of the 

 aperture and then the other. The two cur\cs of sketch (i repre- 

 sent the exctirsions of the light \al\e ribbon, ■rtith time, jiast the 



