AN IMPROVED TELEPHONE SET 245 



not to exceed about 5 db, due primarily to noise and crosstalk problems in- 

 troduced with larger values. Along with this volume gain, improvements in 

 quality were desirable. 



Any such volume gains over present levels would of course be intolerably 

 loud on short loops, so if limitations in the variety of sets and the attendant 

 administrative, production, and merchandising benefits were to be retained, 

 it meant designing a set with transmission performance suitably adjusted for 

 short and long loops. Inasmuch as on cu to vers and on P.B.X. extensions and 

 the like, the same set would be at times on long and at others on effectively 

 short loops, it also meant that this change in performance should auto- 

 matically take place with change in connection rather than require manual 

 reconnection or adjustment. 



Fig. 4 — View of equalizer. 



This has been achieved in the present design by including an automatic 

 transmission equalizer. Fig. 4, which is adjusted in its inserted loss charac- 

 teristics by the magnitude of the d-c. line current through the set. One 

 element of the initial design (other preferable methods may develop in the 

 future) provides a tungsten ballast filament in series with the transmitter so 

 proportioned that the effect on transmitting on long loops is small, but on 

 short loops with high values of d-c, the combined battery supply and a-c. 

 circuit loss inserted is about 5 db. 



A corresponding graduated receiving loss is obtained by including a 

 thermistor bead thermally coupled to the tungsten filament in the same 

 structure. This bead, in series with a loss limiting resistance, is bridged 

 across the receiver. 



The filament is protected against abnormal voltages by a bridged silicon 

 carbide varistor. The resistance current characteristics of the elements of 

 this equalizer are shown in Fig. 5. 



The required gains in transmission called for completely new transmitter 



