TRANSOCEANIC TELEPHONE SERVICE 261 



capable of taking many forms and is, of course, subject to change as 

 improvements are developed. The diagram illustrates how one of 

 these forms might be set up. This form employs electro-mechanical 

 relays. The functioning of the apparatus illustrated is briefly as 

 follows: the relay TES is normally open so that received signals pass 

 through to the subscriber. The relay SS is normally closed to short 

 circuit the transmitting line. When the United States subscriber 

 speaks his voice currents go into both the Transmitting Detector and 

 the Transmitting Delay circuit. The Transmitting Detector is a 

 device which amplifies and rectifies the voice currents to produce 

 currents suitable for operating the relays TES and SS which thereupon 

 short circuit the receiving line and clear the short circuit from the 

 transmitting line, respectively. The delay circuit is an artificial line 

 through which the voice currents require a few hundredths of a second 

 to pass so that when they emerge the path ahead of them has been 

 cleared by the relay SS. When the subscriber has ceased speaking the 

 relays drop back to normal. 



The function of the Receiving Delay circuit, the Receiving Detector, 

 and the relay RES is to protect the Transmitting Detector and relays 

 against operation by echoes of received speech currents. Such echoes 

 arise at irregularities in the two-wire portion of the connection and are 

 reflected back to the input of the Transmitting Detector, where they 

 are blocked by the relay RES which has closed and which hangs on for a 

 brief interval to allow for echoes which may be considerably delayed. 

 The gain control potentiometers shown just preceding the transmitting 

 and receiving amplifiers are provided for the purpose of adjusting the 

 amplification applied to outgoing and incoming signals. 



The relief from severe requirements on stability of radio transmission 

 and from varying speech load on the radio transmitters which this 

 system provides permits much greater freedom in the design of the 

 two radio channels than would otherwise be possible. 



The Radio Channels 



One of the first questions which comes up in considering the design 

 of a radio system is the power which can be sent out by the transmitter. 

 The word "can" is used advisedly, rather than "should," since in the 

 present art the desideratum usually is the greatest amount of power 

 that is technically possible and economically justifiable. There are 

 few radio systems so dependable that increased power would not 

 improve transmission results. At very high frequencies the generation 

 of large powers is attended by many technical difficulties but fortu- 

 nately the radiation of power can be carried out with much greater 



