14 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



band. The carrier channel bandwith is made wide enough to carry both the 

 speech band and signal tone. Use of 1600-cycle tone of the national toll 

 dialing plan would impair the noise advantage of the compandor unless ar- 

 rangements were used to by-pass the signal tone around the compandor. 

 Because of the complexity and cost of these arrangements, 1600-cycle 

 signaling is used over the N system only in special cases. The N signal tone 

 is injected after the compressor in the transmitting terminal and removed 

 before the expandor at the receiving end of the system. Thus, the com- 

 pandor is left free to operate on speech signals of various levels without 

 interference and consequent impairment of its noise advantage by the 

 signal tone. Use of the compandor permits an unusually high level of signal 

 tone (0 dbm at level in contrast to —20 dbm at level for 1600-cycle 

 signaling) without interference into the message circuits. In the compressor 

 circuit [Fig. 3(a)] the compressor low-pass filter cuts the speech band off at 

 about 3100 cycles, preventing speech interference to the signal channel. 

 In the expandor circuit [Fig. 3(b)] the expandor low-pass filter passing the 

 voice band blocks the signal tone from the expandor. 



In Fig. 6, (a), (b) and (c), the three parts of the signal circuit are shown — 

 the signal oscillator circuit, the keyer circuit and the signaling receiver. 

 The 3700-cycle oscillator is a resistance condenser Wien bridge type using a 

 thermistor for stabilization of the output. One oscillator serves for the 

 signaling supply of 12 channels. It is housed in the low-group subassembly 

 of the group terminal unit. A low-impedance output circuit makes it possible 

 to key on one channel or remove channels without disturbing others. 



In Fig. 6(b) the signal keyer circuit is shown. On-hook or off-hook signals 

 are received over the M lead from the trunk as ground and battery, re- 

 spectively. With ground on the M lead the bias on germanium varistors in 

 the keyer bridge becomes positive, which connects the 3700-cycle oscillator 

 to the channel modulator through the keyer transformer. With —48 volt 

 battery off-hook signal on the M lead, the varistors receive negative bias 

 and 3700-cycle transmission to the modulator is blocked. 



The signaling receiver [Fig. 6(c)] is connected to the output of the carrier 

 channel demodulator in multiple with the expandor low-pass filter input 

 [Fig. 3(b)]. It consists of a receiving band filter about 150 cycles wide, an 

 amplifier stage, a limiter, a cathode follower impedance converting stage, a 

 germanium varistor rectifier, a delay circuit, a d-c. amplifier and an output 

 relay. The band filter is made as narrow as practicable to reject noise and 

 still pass sidebands of the dial pulses without perceptible distortion,, with 

 allowances for shift in the signal oscillator frequency and manufacturing 

 variation of the band filter. The multivibrator limiter gives constant ampli- 

 tude square wave 3700-cycle output for any 3700-cycle signal input to the 

 receiver up to about 7 db below normal level and well above normal level. 



