CARRIER SYSTEM FOR PROGRAM TRAXSM ISSfO.X 171 



more difficult. Consequently, the delay correction for that portion of the 

 band below 1000 cycles is actually done at sideband frequency, using quartz 

 crystal elements. The design of these delay equalizers is described in 

 another paper. ^* Transmission through the resulting modem unit is essen- 

 tially constant in both attenuation and delay over the usable frequency 

 range. 



The demodulator amplifier is a conventional two-stage resistance-coupled 

 amplitier. It is stabilized by 25 decibels of feedback to a nominal gain of 

 38 decibels, variable over a 12-decibel range by a potentiometer in the feed- 

 back circuit. The transmission characteristic is flat within 0.3 decibel 

 over the 35 to 15,000-cycle frequency range. The output impedance is 

 stabilized by the use of an output bridge for obtaining the feedback voltage. 

 This amplifier feeds a — 10 vu point in the circuit and can deliver up to +18 

 decibels above one milliwatt of output. Noise is kept to a minimum by 

 operating the input stage vacuum tube at reduced voltages, mounting it and 

 the magnetically shielded input transformer on a vibration-reducing sus- 

 pension, and providing heavy iiltering for the A and B batter>' circuits. 



The limiting source of noise in any communication system is usually the 

 transmission medium. In the carrier program system, the transmission 

 medium is a carrier system which introduces noise energy equally dis- 

 tributed over the program band. The program energy being transmitted, 

 however, is heavily concentrated at the lower frequencies. In order to 

 increase the signal-to-noise ratio without an increase in total transmitted 

 power, a predistorting network is introduced ahead of the modem, which 

 attenuates the lower frequencies relative to the higher. The total dis- 

 crimination is about 18 decibels, distributed symmetrically on a logarith- 

 mic frequency scale above and below 1500 cycles. A restoring network 

 having an inverse characteristic is inserted in the receiving program path to 

 return the program energy distribution to normal. The noise improvement 

 thus obtained is about 7 decibels. 



The reversing circuit consists of a set of five relays and a 78-kilocycle 

 amplifier-oscillator. Two of these relays, as shown in Fig. 4, set up the 

 transmission circuits for transmitting or receiving. The transmitting relay 

 connects the predistorter in the audio-frequency circuit and connects the 

 modem output to the transmitting high-frequency line. The receiving 

 relay connects the modem to the receiving high-frequency line and inserts 

 the restorer and demodulator amplifier in place of the predistorter. These 

 relays are interlocked so that only one at a time can be operated. Their 

 operation is supervised by two other relays, one transmitting and one 

 receiving, which respond to the transmitting and receiving control signals 

 respectively. The supervisory relays are similarly interlocked so that the 

 control signal from only one direction at a time can be eflfective. They are 



