WIDE-BAND TRANSMISSION OVER COAXIAL LINES 673 



speech band to be transmitted and (2), the sharpness of available 

 selecting networks. 



As to the width of speech band, the present requirement for commer- 

 cial telephone circuits is an effective transmission band width of at 

 least 2500 cycles, extending from 250 to 2750 cycles. It has been found 

 that a band of this width or more may be obtained with channels 

 spaced at 4000-cycle intervals. Band filters using ordinary electrical 

 elements are available,^ for selecting such channels in the range from 

 zero to about 50 kilocycles. Channel selecting filters using quartz 

 crystal elements ^^' ^^ have been developed in the range from about 30 

 to 500 kilocycles. The selectivity of a typical filter employing quartz 

 crystal elements is shown on Fig. 15. 



V) 70 



O 40 



70 71 72 73 74 75 76 77 78 79 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 15 — Frequency characteristic of quartz crystal channel band filter. 



Initial Step of Modulation 

 The initial modulation (from the voice range) may be carried out in 

 an ordinary vacuum tube modulator or one of a number of other non- 

 linear devices. The method chosen for the present experimental work 



