624 BELL SYSTEM TECHNICAL JOURNAL 



Rl and R2 causing the gain integrator to increase the gain of the main 

 ampHfier when the speech input levels decrease. However, this 

 increase in gain must continue only until sufficient speech volume is 

 going out to modulate the transmitter in an approximately satis- 

 factory manner. At this instant another part of the circuit called 

 the "Gain Increaser Disabler" operated by the output signal comes 

 into play and disables the gain increaser. If the input signal and, 

 therefore, the output signal become louder, then a fourth element, 

 called the "Gain Decreaser," comes into play and begins to reduce the 

 gain of the amplifier. The combination of these control circuits with 

 the main amplifier therefore causes the volume of output signal to be 

 reasonably constant with wide variations in the volume of the input 

 signal and at the same time to hold the gain substantially constant as 

 long as speech signals of the same volume are coming or while no 

 speech signals are coming. 



Returning to Fig. 5, note a situation which may be troublesome. 

 Anyone who has operated the earliest broadcast receivers with auto- 

 matic gain control will remember that when the incoming signal became 

 weak or disappeared the gain of the receiver climbed to such a point 

 as to produce disconcerting noise in the loud speaker. The receivers 

 in all of the systems indicated in this figure contain automatic volume 

 control and if the transmitter from the remote station stops momen- 

 tarily or if the signals fade out this automatic volume control boosts 

 the gain to such an extent that noise is delivered by the receiver to 

 the telephone line. Such noise may be sufficient to operate the Vodas 

 and in doing so will seize control of the circuit and not allow the signals 

 from the subscriber at this end to reach the transmitter. This would 

 lock up the circuit and put an end to the conversation. 



Such a contingency is avoided by the use of the device indicated in 

 the block marked "Codan."^ The word Codan comes from the 

 initials of the words "carrier operated device anti-noise." The Codan 

 is a device which is operated by the carrier picked up by the receiver 

 and connects the receiver to the telephone line only while a carrier is 

 present. Under these conditions the volume control will go up and 

 down as the carrier goes down and up but if the carrier disappears the 

 Codan disconnects the receiver so that noise will not operate the Vodas 

 and prevent the speech from the subscriber at the near end from being 

 transmitted. 



Specifically, circuits operating with ships at sea must have the 

 Codan or its equivalent because the ships usually employ a system in 

 which the carrier is cut off when the ship stops speaking so that the 

 disappearance of the carrier at the receiving station on shore is the 



