448 BELL SYSTEM TECHNICAL JOURNAL 



plates is suddenly changed, and the beam is rapidly moved back to 

 begin the next line. Due to a black mask down the far side of the 

 film being scanned, there is no signal during this very short period 

 while the voltage on the plates is changed, and thus the electron beam 

 is deflected from the aperture and is not visible on the front of the 

 tube during its return. 



The potential on the other pair of plates is controlled at a frequency 

 of 24 cycles per second, which is the rate of scanning successive frames. 

 The efifect of the potential on these plates is to deflect the electron 

 beam downward in synchronism with the motion of the film at the 

 sending end. This results in the passage of the electron beam across 

 the front of the tube in successive rows, one below another. After 

 the last row has been scanned, the voltage on the plates is changed and 

 returns to the value that causes the beam to appear at the top line 

 of the tube. A properly synchronized blanking-out pulse is introduced 

 between successive frames of the film, so that no signal is received 

 during this interval, and thus the passage of the electron beam from 

 the bottom to the top of the frame is not visible. 



Figure 10 is a photograph of one of these cathode ray tubes. Due 

 to its superior design, the image is very sharp over the entire field and 

 a wide range of brightness is secured. The chief factors in its success 

 are the sharp focusing by the electron lenses, the linear deflection of 

 the beam at the aperture, and the great length of the tube, which 

 makes it necessary to deflect the electron beam over only a narrow 

 angle to cover the 7X8 inch field. Since this trial was a test to 

 determine the capabilities of the system, such matters as size and 

 cost, which would be important with commercial receivers, were not 

 controlling. 



Modulation System 



The frequency band which was generated at the sending end as 

 noted above was to 806 kc. The coaxial cable system used could 

 not transmit this band, because repeaters were not designed to pass 

 frequencies below about 60 kc. This limitation was incorporated in 

 the original design because the cable offers insufficient shielding to 

 various disturbances at low frequencies. For television transmission 

 it was necessary, therefore, to raise the television signal band to a 

 higher frequency position before attempting transmission over the line. 

 A number of considerations led to the decision to raise the entire 

 frequency band 144 kc. for transmission over the coaxial cable. 



Where such a wide frequency band is to be raised by an amount 

 less than the width of the band itself, a single modulation is not 

 generally satisfactory. The products of modulation include the 



