s 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



volume range, then are amplified in a 2-stage feedback amplifier. Most of 

 the amplifier output is fed through the control circuit where it is rectified 

 in a germanium bridge circuit. A condenser-resistance filter in the control 



VOICE AMPLIFIER 



TRANSMITTING 



LOW-PASS 



FILTER 



2W0R 

 4W IN 

 2W0UT 



VOICE FREQUENCY 



TO MODULATOR 



INPUT VIA EXPANDOR 



SIGNALING 



SUBASSEMBLY 



lu^-wiKt ^ \AA/— 



OUTPUT ^G vvv--^ 



CONTROL 

 AMPLIFIER 



Fig. 3 (a) — Channel compressor circuit, 

 (b) — Channel expander circuit. 



circuit output passes only the rectified syllabic envelope of the speech 

 frequencies. The control circuit filter output current is fed to the midpoints 

 of the germanium variolosser bridge arrangement. The time constants of 

 the control circuit filter are chosen for fast attack (3-5 milliseconds)* 



* 90% of final value reached in time indicated. 



