372 BELL SYSTEM TECHNICAL JOURNAL 



maintaining the received volume at a substantially constant value. 

 The gain control is operated by the incoming carrier. When the 

 fading is of the type in which the different frequencies in the transmitted 

 band do not fade simultaneously, the automatic gain control is not so 

 effective and considerable variations in volume out of the receiver 

 may occur in a short time. 



In the long-wave circuit, the variations are too slow to be classed as 

 fading, and occasional manual adjustments of receiver gain result in 

 keeping the volume at the receiving end within about ±5 db. 



Because of the gain adjustments to reduce noise, combined with 

 changes in radio receiver gain to compensate for fading or for variations 

 in radio attenuation, "singing" would occur if the hybrid coils and 

 echo suppressor were not augmented by additional means of singing 

 prevention. One way of preventing singing would be to reduce gain in 

 the receiving leg whenever gain was introduced in the transmitting leg 

 of the circuit. Volume penalties to the listener as great as 25 db would 

 frequently be encountered if this were done, and, in addition, con- 

 siderable agility would be required on the part of the technical opera- 

 tors to keep the circuit adjusted. However, this method would not 

 compensate for gain changes in the radio receivers, so that singing 

 might still occur under unfavorable conditions. 



Also, in the case of a long-wave transatlantic circuit, singing could 

 occur over transmission paths between the local radio transmitter and 

 receiver. The volume received from the local transmitter may occa- 

 sionally be as much as 40 db stronger than that from the distant sta- 

 tion if the transmitter and receiver are about 90 miles apart, even 

 though antenna directivity were used at both the transmitters and the 

 receivers. In general, if the receiver gain is adjusted to give the proper 

 volume on the distant station, the amplification in the local radio path 

 is entirely out of reason. 



It is therefore necessary to provide other means of preventing 

 singing to maintain optimum transmission conditions. 



\'ODAS 



There has been developed for meeting these difficulties an anti- 

 singing voice-operated device known as a "vodas." ^ Fig. 6 shows a 

 radio telephone circuit arranged with a vodas in its simplest form at 

 each end of the circuit. The vodas consists of a transmitting delay 

 circuit, detector, and certain relays, and a receiving delay circuit, 

 detector, and relay. These devices are operated by the voice currents 

 in the circuit so as to keep all singing paths blocked at all times. 



'Taken from initials of the words "Voice-Operated Device Anti-Singing." 



