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THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



Voice Frequency Units 



Two designs of voice frequency low-pass filters (one unit each) are used 

 in the transmitter modulator input and the receiver demodulator output 

 in each channel. The modulator filter limits the range of voice frequencies 



Fig. 9 — (a) Operator inserting test filter in jig. Traces aj)pt'aring on tlie cathode ray 

 tube are the characteristic of a reference filter and two reference discrimination levels. 

 The lower trace represents transmission in the test path, which circuit has not j^et been 

 completed, (b) Unadjusted filter characteristic appears on lower trace, (c) Adjustment 

 completed, mid-band loss level being checked for limits, (d) Close-up of cathode ray tube 

 which shows characteristic of filter under test coinciding with the characteristic of the 

 reference filter. 



to be modulated and provides suppression against 3700 cycle voice fre- 

 quency interference into the signaling circuit. The receiving low-pass filter 

 supplements the suppression provided by the channel filters to prevent 

 inter-channel crosstalk and has an attenuation peak at 3700 cycles to pre- 

 vent the signal tone from interfering with the message circuit. The pass- 



