A NEW TELEPHOTOGRAPH SYSTEM 557 



It is ob\ious that, since tlu- illumination incident upon the picture 

 is pulsating at the carrier frecjuency, the currents present in the output 

 of the photoelectric cell will consist of the picture signal currents and 

 the carrier frequency niotlulated by these currents, the picture itself 

 acting as a simple direct product modulator. 



Fillers and Delay Equalizer 

 The application of single-side-band transmission methods to the 

 present telephotograph equipment has resulted in the design of 

 electrical filters of rather unusual phase shift and attenuation-frequency 

 characteristics. It has previously been pointed out in connection with 

 a discussion of telegraph transmission theory ^ that three conditions 

 should be fulfilled for single-side-band transmission: 



1. The system should have a linear phase shift-frequency characteristic. 



2. The sluggish in-phase component of the signal resulting from a displacement of 

 the carrier from the middle of the transmitted band should be eliminated. 



3. The received quadrature component resulting from the loss of the component 

 of the side band suppressed, equal in magnitude but opposite in sign, should also be 

 eliminated. 



The first two conditions are met by the careful design of a delay 

 equalizer network and a special filter giving a suitably shaped ad- 

 mittance characteristic for the system. This characteristic exhibits a 

 type of symmetry about the carrier frequency which would result in a 

 superposition of the regions adjacent to the carrier if rotated about 

 this point. Consequently the attenuation of the filters and associated 

 delay equalizer should be 6 decibels greater at the carrier frequency 

 than at the middle of the band of the transmitted frequencies, in 

 addition to meeting the requirement of a linear phase shift-frequency 

 characteristic. Over-all attenuation and phase shift-frequency char- 

 acteristics of the filters and equalizer of the sending and receiving 

 equipment of the present design are shown in Fig. 6. 



In regard to the third condition for single-side-band transmission, 

 experiments have shown that the effect in received pictures of the 

 quadrature component is not of practical importance in the present 

 equipment. The quadrature component is determined essentially by 

 the slope of the signal envelope which is in turn restricted by the 

 equivalent transfer admittance characteristic ^ of the scanning aper- 

 ture, and by the slope of the filter characteristic to meet condition 2. 



Receiving Optical System 

 The receiving optical system of the new telephotograph equipment 

 is similar in its general aspects to that employed in the earlier Bell 

 System equipment. Illumination from an incandescent lamp is 



