128 BELL SYSTEM TECHNICAL JOURNAL 



As this instrument has a practically flat frequency characteristic 

 no collecting horn or mouth piece is used with it as resonance is intro- 

 duced by such chambers, with accompanying distortion. To insure 

 the insulation of the transmitter from building vibrations, a simple 

 spring suspension has been provided. To protect the transmitter 

 from injury, two types of transmitter mountings have been used, 

 both arranged for the suspension of the transmitter in a screen- 

 enclosed space — the first adapted to take a single transmitter for 

 indoor use only, while the second for outdoor use, mounts two trans- 

 mitters within a double screen enclosure to prevent any noise effects 

 due to winds. This second type is arranged to attach to a simple 

 pedestal-type of reading desk, which it has been found desirable to 

 provide as there is a slight tendency for the speaker to remain fairly 

 close to the desk. In this connection it is interesting to note that we 

 have found a small rug, so placed as to cover the area which the 

 speaker should occupy during the delivery of his speech, is of great 

 assistance in this regard, as he unconsciously confines himself to the 

 area of this rug. Both of these measures to insure the speaker re- 

 maining in proper relation to the transmitter, are supplemented, 

 wherever possible, by an explanation of the system to all the speakers 

 previous to the actual performance. 



Transmitter Switching Panel 



Resuming the path of the speech currents through the system, 

 the output from the transmitters is taken to a panel designed to enable 

 the operator to switch quickly from one transmitter to another, as 

 with some public functions, the speeches are made at different points 

 during the ceremonies. This switch is arranged to short-circuit the 

 output of the power amplifier when passing from one transmitter to 

 another, to prevent clicks in the projectors. In certain cases, the 

 equipment is arranged to permit two or more transmitters to be con- 

 nected to the amplifiers at one time, as is desirable when solo singers 

 and an orchestra are to be picked up in a theatre, with proper ad- 

 justment of their respective volumes. 



The amplifier equipment has been built in four units which may be 

 grouped as necessary under the various conditions encountered in 

 commercial installations. The proper amplifiers are determined, 

 first, by the source of the voice frequency current to be amplified, 

 that is whether a distance talking or a close talking transmitter is 

 to be used, or whether the speeches are brought in over a telephone 

 line, and secondly, the size of the space in which the amplified sounds 



