Figure  2-16.Basic  tone-modulated  (AFTS)  system.

system.  You  will  find  that  the  keyer  is  part  of  the transmitter  in  modern  systems.  The  keyer  shifts  the signal   below   or   above   the   assigned   frequency, corresponding  to  the  mark  or  space  required  for  tty transmission. Usually, the keyer is adjusted for 850 Hz (425   Hz   above   and   425   Hz   below   the   assigned frequency). A space will be 425 Hz above and a mark will be 425 Hz below the operating frequency. AFTS   systems   use   amplitude   modulation   to change dc mark and space pulses into audio. A basic tone-modulated  system  is  shown  in  figure  2-16.  The audio conversion is done by an audio oscillator in the tone   convertor.   Varying   the   tone   according   to   the characters   transmitted   from   the   tty   equipment amplitude-modulates the transmitter carrier wave. The receiver demodulates this signal, separating the audio signal from the carrier. In   both   the   RFCS   and   AFTS   systems,   the   tty signals  pass  through  the  panel  that  controls  looping current. Looping current is the current supplied by the tty  solid-state  power  supply.  The  tty  panel  integrates the   tone-modulated   and   carrier-frequency   shift systems. By allowing the tty equipment to be set up in any  configuration  desired,  this  panel  gives  the operators  maximum  operational  flexibility  with  the least amount of circuitry and equipment. Figure  2-16.—Basic  tone-modulated  (AFTS)  system. 2-18


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