628 BELL SYSTEM TECHNICAL JOURNAL 



energy. The operation therefore involves absorbing part of the energy 

 in the louder signals and a lesser amount from the weaker signals so 

 that the output contains speech which has been distorted in such a 

 fashion that the variations in energy may be only one-tenth as much 

 as they originally were. These two absorbing vacuum tubes are 

 controlled by potential built up across a circuit containing capacitance 

 and resistance. This circuit has a potential Eq produced on it by a 

 linear rectifier which secures its energy from the output circuit. The 

 strong signals appearing in the output produce a larger voltage on the 

 resistance-condenser combination, thereby causing the grids of the two 

 vacuum tubes to be more positive than with weak signals or no signals, 

 and the two tubes, then acting as conductive resistances, reduce the 

 intensity of the loud signals. The resistance-condenser combination 

 is so proportioned that the charge on the condenser rises and falls 

 with syllabic frequency. It must not have such a short time constant 

 as to wipe out individual cycles. It is to operate upon groups of 

 cycles only. With this Compressor between the telephone line and 

 the transmitter, and the amplifiers properly adjusted, the transmitter 

 can still be fully modulated with the louder sounds in the voice but it 

 will be modulated very much more than it would normally be by the 

 weaker sounds in the voice. 



At the receiving end the signal delivered by the receiver and trans- 

 mitted towards the telephone line will be the same distorted signal 

 which modulated the transmitter. Such a distorted signal, although 

 scarcely discernible from the original, is not in all situations the de- 

 sirable one to put upon a telephone line, so there are reasons for 

 restoring this distorted signal to its original form. This distorted 

 signal contains the weaker parts of speech amplified many times with 

 respect to what would occur without the Compandor and therefore 

 these weaker parts of speech will be many times above the noise which 

 would have interfered with reception under ordinary conditions. 

 This distorted speech now goes into the part of the Compandor in 

 the receiving branch which is called the "Expandor," as shown in 

 Fig. 11. The Expandor contains many elements similar to those in 

 the Compressor but they are arranged in a slightly different form. 

 Two vacuum tubes instead of absorbing energy are now used as 

 amplifiers. The signal comes in on the left and goes out on the right 

 but the output is not a true amplified picture of the input because as the 

 signal goes through the amplifier the amplification of the two tubes is 

 varied so as to restore the original signal. To do this use is made of 

 the remaining amplitude variations within the signal to operate a 

 linear rectifier and put a variable voltage Ex upon similar condenser 



