1060 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



ing centers. The equipment units which make up the monitoring facilities 

 are an auxihary IF ampHfier, an FM receiver, a video ampHfier and a video 

 monitor. A combination of these units is assembled in a bay to fit the needs 

 of each monitoring location. 



IV. FM Terminal Equipment 



A. General 



The TD-2 System will transmit a standard RMA black and white tele- 

 vision signal or a band of message channels built up on a frequency division 

 basis as provided by the coaxial cable message terminals. The FM terminal 

 transmitter converts either of these signals to a frequency-modulated signal 

 centered at 70 megacycles for application to the radio transmitter. The FM 

 terminal receiver recovers the television or carrier signal from a frequency- 

 modulated 70-megacycle signal. Thus the FM terminal equipment provides 

 the connecting links between the TD-2 radio equipment and other facilities. 



In a long system it may be necessary to bring the radio signal down to 

 voice and back up to radio frequency many times in order to add and drop 

 message groups. Each such process will require FM receiving and transmit- 

 ting terminal equipment which consequently estabHshes severe linearity 

 requirements for this equipment. An objective in the development of the 

 terminals was to meet long haul systems performance requirements with 

 sixteen pairs of terminals in tandem. 



B. FM Transmitter 



A functional diagram of the FM terminal transmitter is shown in Fig. 16. 

 It accepts a signal from an unbalanced 75-ohm line and delivers an FM signal 

 centered at 70 megacycles to the radio transmitter. The input level may be 

 adjusted from 0.2 volt to 2.5 volts peak-to-peak with an output level of 13 

 dbm at an impedance of 75 ohms. For television transmission with a ±4 

 megacycle swing the tips of the synchronizing pulses are at 74 megacycles 

 and the picture white at 66 megacycles. For message service the nominal 

 deviation is centered about 70 megacycles. For television transmission the 

 output is automatically clamped to a predetermined frequency during each 

 synchronizing pulse. These differences in operation are described in more 

 detail below. 



1. Description 



The input signal to the FM transmitter is applied through an adjustable 

 attenuator to a video amplifier consisting of two similar three-stage feed- 

 back amplifiers in tandem which have a combined gain of 42 db. The video 



