548 BELL SYSTEM TECHNICAL JOURNAL 



Variants to the Devices Described 



In addition to these there are various devices which are essentially 

 modifications of those described. For example, a half-vogad, Fig. 2, 

 may have the same time functions as a vogad, Fig. 1, but the gain 

 changes in the transmission circuit are half as great for the same range 

 of input volumes. Thus in a vogad the range of gain changes in the 

 transmission circuit is equal to the range of input volumes, so that the 

 output volume is the same for all input volumes. In the case of the 

 half vogad the range of gain changes in the transmission path is one- 

 half the range of input volumes, so the output volume range is one-half 

 that of the input. It is also possible to construct a vogad whose output 

 volume range is any desired fraction of the input range. As another 

 example of modification of the devices described, for special appli- 

 cations it may be desirable to incorporate a certain amount of syllabic 

 compression in a vogad. 



Communication circuits which have separate paths for oppositely 

 directed transmission between the two terminals are usually operated 

 at such an overall loss that with ordinary terminations there will be 

 little tendency for circulating currents to build up to a "singing" 

 condition. Sometimes there may not be a great deal of margin, 

 however, so that volume controlled devices added to such circuits must 

 add loss at some point to counterbalance whatever gain is put in at 

 some other point. Thus a vogad inserted at the transmitting side of 

 one terminal of such a circuit to amplify speech energy from weak 

 talkers must be supplemented by a "reverse vogad" in the receiving 

 side of the circuit. The reverse vogad is simply another vario-losser 

 which is operated upon by the vogad control circuit in such a way that 

 it always has a loss numerically equal to the gain of the vogad. Any 

 vogad gain will be compensated by the reverse vogad loss, so no 

 greater tendency to sing will be effected by the addition of the combi- 

 nation to the circuit. In like manner half vogads must be used with 

 compensating reverse half vogads. 



Combinations of some of the devices also have interesting charac- 

 teristics. For example, a combined radio noise reducer and peak 

 limiter at the receiving end of a circuit would suppress noise and would 

 also reduce the amplitude of excessively high amplitude signals. 

 Likewise, a vogad, compressor, and peak chopper in tandem in the 

 order named could be made to reduce the range of input signals by a 

 very large amount for transmission over a medium having only a small 

 range between noise and maximum permissible signal. In this case it 

 would be practically impossible to recover the original signal range at 



