The comparator section of the convertor compares the signals received in the diversity mode and allows only   the   stronger   signal   to   be   passed   on   to   the communication  patch  panel. Communication Patch Panel The  communication  patch  panel  performs  the same  function  on  the  receive  side  as  it  does  on  the transmit side. Crypto  Equipment The crypto equipment decodes the received signal so it can be printed out in its original form. Teletype The tty equipment converts the dc pulses from the patch panel to a printed copy of the original message. The tty shown is a page printer only and does not have transmit  capability. AFTS  SYSTEM Figure 2-20 is a block diagram of a half-duplex uhf system.   Half-duplex   communication   can   be   in either  direction,  but  cannot  happen  at  the  same  time. The term half-duplex is qualified by adding send only, receive only, or send or receive. AFTS Transmit Dc   signals   from   the   tty   are   sent   to   the   com- munications patch panel where they are fed to the tone terminal  set.  The  tone  terminal  set  converts  the  dc signals  into  audio  tone-shift  signals.  The  audio  tone- shift  signals  are  patched  to  the  transmit  side  of  the transceiver   through   the   transmitter   transfer switchboard.  The  audio  tone-shift  signals  are  then  used to modulate the rf carrier for transmission. AFTS  Receive The rf tone-modulated signals are received at the antenna and then patched, via the multicoupler, to the receive side of the transceiver, where demodulation to audio  tone-shift  signals  takes  place.  The  signals  are then  patched  to  the  tone  terminal  set  through  the  re- ceiver transfer switchboard, where they are converted back  to  dc  signals.  The  dc  signals  are  then  patched through the communication patch panel to the tty. FACSIMILE Facsimile (FAX) is a method of transmitting still images  over  a  communications  system.  The  images may   be   weather   maps,   photographs,   sketches, typewritten  or  printed  text,  or  handwriting.  Military use  of  FAS  is  primarily  limited  to  transmission  and reception   of   weather   maps.   Because   of   long transmission times, its tactical uses are limited. TEMPEST Compromising   emanations,   generally   called TEMPEST,   are   unintentional   data-related   or intelligence-bearing  signals  which,  if  intercepted  or analyzed,  can  disclose  the  classified  information transmitted,   received,   handled,   or   otherwise processed   by   electrical   information   processing equipment or systems. The   Navy   uses   MIL-STD-1680   (SHIPS), Installation  Criteria  for  Shipboard  Secure  Electrical Processing   Systems,   as   the   guide   for   TEMPEST. Within communications centers, ETs must understand and  be  familiar  with  both  RED  and  B L A CK designated   systems   and   equipment.   The   RED   and BLACK  designations  are  explained  in  the  following paragraphs. RED  CRITERIA The   RED   designation   applies   to   all   crypto equipment,   subscriber   terminal   equipment,   and interconnecting   conductors   involved   in   processing classified  plain  language  information.  It  also  applies Figure  2-20.—Half-dup1ex  AFTS  teletype  system. 2-23