6 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



The stronger volumes need less increase in proportion to the volume. When 

 the circuit is idle 28 db gain is introduced by the compressor and 28 db 

 loss by the expander. Any disturbance in the transmission medium in the 

 absence of speech receives 28 db attenuation in the expandor. Loss is 

 removed from the expandor as the speech volume increases and the noise 

 increases correspondingly. In a well designed compandor with proper time 

 constants the increased noise will tend to be continuously masked by the 

 increased speech volume. Interferences to the listener during silent speech 

 periods, such as intelligible crosstalk or audible tones, receive the full 28 db 

 of noise suppression in the expandor. Interference in the presence of speech 

 receives less than full suppression in the expandor the stronger the speech. 

 Table II shows test results of noise advantage of the N-1 compandor at 

 several noise values and speech volumes. 



In Fig. 2(a) a level diagram shows the gain and loss introduced by the 

 compressor and expandor for signals of different strengths. A signal of 5 db 



Table II 

 Compandor Noise Advantage 



above 1 milliwatt (-1-5 dbm) is shown as unmodified by compressor and 

 expandor. A signal input to the compressor of —50 dbm receives 27.5 db 

 gain and the resulting —22.5 dbm signal input to the expandor receives 

 27.5 db loss. For each signal input to the compressor weaker than -|-5 dbm 

 by 2 db, the compressor introduces 1 db more of gain and the expandor 1 

 db more of loss to a maximum of 28 db gain and loss respectively at —51 

 dbm input to the compressor or —23 dbm input to the expandor. In Fig. 

 2(b) input vs output is plotted for compressor, expandor and the combina- 

 tion. The slopes of these input-output plots are 1/2 for compressor and 2/1 

 for expandor. 



In Fig. 3, (a) and (b), compressor and expandor circuit schematics are 

 shown for the N-1 Carrier System. Compressor input and expandor output 

 are connected to the resistance hybrid circuit at the left of the compressor 

 schematic for conversion to the 2-wire voice circuit input. Alternative 

 connections for 4-wire operation and an adjustable gain control to establish 

 over-all circuit net loss also are shown. Input voice signals to the compressor 

 pass through the germanium variolosser, are compressed to half the input 



