RADIO EXTENSION LINKS TO TELEPHONE SYSTEM 621 



cally eliminate the clipping and allow of relays being adjusted so as 

 not to be operated by small noises from the telephone line. 



This circuit also includes arrangements to prevent other difficulties. 

 It will be observed there are two sets of relays, one operated by the 

 transmitting branch rectifier and one by a receiving branch rectifier. 

 These are so arranged that when speech signals operate the trans- 

 mitting branch rectifier, the receiving line is short-circuited to prevent 

 any signals, such as noise, from the radio receiver reaching the talker 

 or from going out on the transmitting branch to interfere with the 

 transmitted speech. When no talking is occurring at this end of 

 the circuit signals coming in on the radio receiver operate a receiving 

 relay which short-circuits the transmitter circuit so that the received 

 speech will not be retransmitted by the transmitter and so set up a 

 singing condition. This particular diagram indicates the hybrid coil 

 and balancing network which are used to assist in operation but not 

 to provide the main means for preventing the received speech from 

 reaching the transmitter. 



This circuit as indicated is about as simple as a satisfactory circuit 

 can be made. Figure 7 indicates a more complex circuit which has a 

 number of advantages, among which is that of connecting in privacy 

 equipment. This circuit allows of using one piece of privacy equip- 

 ment which is used in the transmitter branch for outgoing signals and 

 is switched to the receiver branch for incoming signals. 



Returning now to Fig. 5, attention is called to another device in the 

 first diagram labeled "Vogad." ^ This word comes from the initial 

 letters of the words "voice operated gain adjusting device." ^ This is 

 a type of device which is very useful in telephone practice but is seldom, 

 if ever, used with a broadcasting transmitter. Every telephone user 

 is cognizant of the fact that different people with whom he speaks 

 over the telephone use different intensities of voice; also different 

 lengths of telephone line introduce different amounts of attenuation. 

 If the incoming telephone signals are to operate the radio transmitter 

 to its full modulation capacity some means must be provided to 

 equalize these signals of various levels. It is therefore desirable to 

 have a device which will maintain the output level nearly constant 

 regardless of the variation in the input level due to different speakers 

 and different lengths of telephone lines. This operation is provided 

 by the Vogad indicated in Fig. 8. The channel across the top is 

 the direct path of the speech signals. Within the dotted lines marked 

 "vario repeater" are some elements including the amplifier whose 

 gain is varied to make up for variation in intensities at the input. 

 It is not sufficient to construct an amplifier which will give a large gain 



