604 



BELL SYSTEM TECHNICAL JOURNAL 



circuit which can be tolerated by employing this two-tone method of 

 transmission instead of the single-tone method with a fixed bias is 

 shown by Fig. 1. 



100 



80 



60 



40 



20 



-10 -5 5 10 15 20 25 30 35 40 



RECEIVED CURRENT (DECIBELS ABOVE REFERENCE) 



Fig. 1— Relation between received current and printer operating margin for the 

 two-tone and single interrupted tone methods of signaling. 



These curves show the relation between operating current and the 

 limits of printer margin ^ within which correct operation is secured for 

 both the single interrupted tone method of signaling and for the two- 

 tone method. The upper and lower limits of the printer margin are 

 shown to meet at the lower levels of received current, indicating that 

 the printer fails completely at these levels. As might be expected, in- 

 creasing the received current level a few db does not affect the orienta- 

 tion range as seriously as a corresponding decrease. In each condition, 

 however, the margin is less affected when signaling by the two-tone 

 method. Were it not for the presence of noise on the radio circuit, it 

 would be possible to establish the normal operating current at a higher 

 value. The reason why this is not feasible is that the detectors in the 

 voice-frequency telegraph receiving equipment are operated near the 

 upper bend of their characteristics. Under this condition increasing 

 the gain causes a relatively small increase in current from the rectifier 

 that is receiving both noise and signal inputs while the current from the 

 rectifier that is receiving noise only is increased. 



Thus because of the desirability of operating through high noise 

 levels on long-wave radio circuits, it is not advantageous to utilize all 

 of the available protection against signal level changes. Rather, a 

 compromise is sought which will afford satisfactory protection against 

 reasonable signal level variations without making the receiving equip- 

 ment unduly vulnerable to noise. This practical operating point has 



