A SINGLE SIDEBAND MUSA RECEIVING SYSTEM 327 



Automatic Branch Selector 



As an alternative to the combination of the outputs of the diversity 

 branches after delay equalization it is possible to use a system which 

 discretely chooses one branch for connection to the line. Equipment 

 to do this has been provided. 



The common automatic volume control which is used with both 

 systems operates on the carrier and if the carrier and sideband fade 

 alike the output volume is held substantially constant. To the 

 extent that the received noise on the various branches is the same, 

 the branch having the highest output volume will be the most desirable 

 to use. In the equipment used part of the audio output of each of the 

 three diversity branches is rectified and applied differentially to three 

 polar relays in such a manner that the relay corresponding to the 

 branch having the highest amplitude is operated. This relay reduces 

 the bias of an amplifier which connects between that branch and the 

 output line from a high to a normal value, and thus permits voice 

 frequency signals to flow through that branch to the output. The 

 noticeable effects of switching are eliminated by several expedients. 

 Push-pull amplifiers with feedback are used so as to balance out the 

 low-frequency thump. The variation of biasing takes place with a 

 time-constant of 0.01 seconds in order to aid in this matter as well 

 as to render unnoticeable the instantaneous differences in the two 

 channels. A biasing winding on each relay insures that once a contact 

 is broken the relay moves to the opposite contact in a fixed time, 

 which permits the selection of time-constants for the suppression and 

 build-up which are as nearly complementary as possible, and so keeps 

 the volume constant. 



A difference in volume of 1 to 2 db is required to cause a switch. 

 During the switching period, which lasts about 20 milliseconds, the 

 output varies about ± 1 db. When no speech is being transmitted 

 the relays remain inoperative and consequently the line may not be 

 connected to the branch which at the next instant may deliver the 

 highest volume. It might be expected that clipping of the succeeding 

 initial syllable would be intolerable. To reduce selective fading to 

 an unnoticeable amount it is only necessary to suppress the unwanted 

 branches by 12 to 15 db. With the equipment adjusted to give this 

 suppression it is found that there is always sufificient signal transmitted 

 through one or more of the branches to practically eliminate noticeable 

 clipping. 



The use of the automatic branch selector has the disadvantage 

 that the effect of having more than one diversity branch contribute 



