2()0 BEU. SYSTEM TECHNICAL JOURNAL 



portion of their characteristics. This is accomplished by appro- 

 priate biasing voltages {Eg), as shown. As a consequence of this 

 method of utilizing the straight line portions of the tube character- 

 istics, the current received at the far end of the line does not vary 

 between zero and finite value, but between two finite values. This 

 electrical bias is exactly matched in the light valve by a mechanical 

 bias of the jaws of the valve opening. 



Fig. 12 shows diagrammaticall>' the form of the band pass filters 

 used for separating the picture and synchronizing channels, together 

 with the transmission characteristics of the filters. The synchroniz- 

 ing channel filter transmits a narrow band in the neighborhood of 

 400 c. p. s., the picture channel filter a band between 600 and 

 2,500 c. p. s. 



In addition to the main circuits which have been discussed, arrange- 

 ments are made for starting the two ends simultaneously and for the 

 transmission of signals. These functions are performed by the inter- 

 ruption of the picture current working through appropriate detectors 

 and relays. Testing circuits are also provided for adjusting the 

 various elements without the use of the actual transmission line. 



Tnii Tr.\nsmission Line 



In \'iew of the fact already emphasized, that the currents used in 

 picture transmission are caused to be similar both as to frequency and 

 amplitude to those used in speech transmission, it follows that no im- 

 portant changes in the transmission characteristics of the telephone line 

 are called for. With regard to the frequency range of the alternating 

 currents which must be transmitted and also the permissible line 

 attenuation, the transmission of pictures is less exacting on the tele- 

 l>hf)ne line than is speech transmission. In certain other respects, 

 however, the requirements for picture transmission are more severe. 

 For speech, the fundamental requirement is the intelligibility of the 

 result, which ma\' be preserved e\-en though the transinission varies 

 somewhat during a conversation. In the case of picture transmission, 

 variations in the transmission loss of the line, or noise appearing e\'cn 

 for a brief instant during the several minutes required for trans- 

 mission are all recorded and presented to view as blemishes in the 

 finished picture. Picture transmission circuits must, therefore, be 

 carefully designed and operated so as to reduce the possibility of such 

 liisturbances. In transmitting pictures o\er telephone lines, it is also 

 necessary to guard against ccrt.iiii other elTects, iiuluding transient 



