252 



VISUAL COMMUNICATION 



for the message which is to be transmitted. 

 The telephone bell is an effective warning 

 signal, and in general, sound systems more 

 readily lend themselves to such signals, since 

 they do not require the receiver to be physi- 

 cally oriented towards a particular spot, as 

 does a purely visual communication display. 

 In exterior communications, the watch is 

 responsible for picking up the signals of en- 

 suing messages. It is doubtful if sufficient 

 work has been done in investigating the use- 

 fulness of bright flashing lights for this pur- 

 pose in interior communications. 



Dependability 



It is obvious that any system of communi- 

 cation should be ready for efficient use at 

 any time. Lack of dependability is one of 

 several factors which have led to the general 

 distaste for direct exterior visual communi- 

 cation, so that it has become but rarely em- 

 ployed by submarines. It is this factor also, 

 which is at the basis of criticism of certain of 

 the auditory systems. 



Security 



Although security is not an important fac- 

 tor in interior communication, it is a prime 

 problem in exterior communication. Direct 

 visual communication is easily intercepted, 

 and even though the exact content of a mes- 

 sage may not be intelligible to an intercep- 

 tor, nevertheless, the detection of such a 

 message reveals not only the presence of the 

 sender, and of the receiver, but it also may 

 lead to accurate inferences with respect to 

 their intentions. Many and ingenious de- 

 vices, as well as codes (e.g., the infra-red or 

 ultra-violet light telephone of the Japanese 

 and Germans and our infra-red blinker (8)), 

 have been developed to provide secure ex- 

 ternal communication systems. The need 

 for security is another of the factors which 

 have almost eliminated exterior visual com- 

 munication from the submarine service. 



Freedom from Hostile Jamming 



The activities of the enemy should not 

 make communication over the system impos- 



sible. The use of the radio transmission of 

 noise by the enemy to make radio voice or 

 code unintelligible, and the use of smoke 

 screens to make visual communication im- 

 possible, can be most effective in reducing 

 combat efficiency when it is dependent on 

 exterior communications. An optimal ex- 

 terior communication system is not easily 

 subject to such interruption. 



Freedom from Interruption 



A communication system should be de- 

 signed so that it will transmit only those mes- 

 sages which it is supposed to transmit. The 

 system should be easily limited to use for its 

 specified purposes, so that no problem of cir- 

 cuit discipline need arise. In practice, a 

 serious objection to many types of auditory 

 communication systems is the ease with 

 which they lend themselves to casual, un- 

 necessary, and trivial communications 

 which may render them temporarily inacces- 

 sible for urgent messages. 



Planning 



A communication system must be care- 

 fully planned, after analysis of its intended 

 functions, with respect both to the locations 

 of the sender and receiver who communicate 

 over it, and to the number and variety of 

 messages which must be transmitted over it. 

 Careful planning of the communication sys- 

 tem on the basis of its operational function 

 is essential for efficiency and freedom from 

 trouble. Such analysis not only makes it 

 possible to reduce the number of individuals 

 who must make use of the system (and so to 

 free it from an undue amount of traffic, with 

 consequent jamming at times of stress), but 

 also to reduce the number and complexity 

 of the messages which it must carry. When 

 the analysis of operational demand is com- 

 plete, the communication system may be 

 designed to be just as versatile in the variety 

 of messages it will carry as it need be, and 

 no more. The fewer messages and the 

 greater the simplicity of content which it 

 must carry, the easier, more precise, and 

 more rapid can be the receiver's response. 



