character.   There   is   a   penalty   for   this   advantage. Character   length   is   increased   to   include   the   start (space)  and  stop  (mark)  signals.  These  start  and  stop signals  are  part  of  the  five  unit  code  (BAUDOT CODE) shown in figure 2-14. Synchronous Mode Synchronous   systems   have   an   advantage   over asynchronous systems. The start and stop elements are not   used.   This   allows   more   room   for   information transmission.  Time  is  not  wasted  on  start  and  stop units. Additionally, this mode has a higher capacity to accept distorted signals because it does not depend on the start and stop signals for timing. MODULATION  RATE The terms used in referring to tty modulation rates or signaling speeds are baud, words per minute (wpm), and bits per second (bps). Baud is the only term that is technically   accurate.   The   others   are   either   ap- proximations or require explanation. Baud By   definition,   the   word  baud  is   a   unit   of modulation  rate.  To  find  the  modulation  rate  of  a  signal in bauds, divide 1 by the time interval of the shortest unit in the signal. For example, 22 milliseconds (.022 see) is the time interval of the shortest unit in the five- unit code at 60 wpm. To find the number of bauds equal to 60 wpm, divide 1 by .022. Rounding off the results provides the figure 45.5, which is the baud equivalent of 60 wpm. You can see that increasing the wpm means the unit time interval has decreased. The most common baudot data speeds range from 45.45 to 100 bauds, or about 60 to 133 wpm. Higher speeds   are   obtained   using   the   American   National Standard   Code   for   Information   Exchange   (ASCII). This   is   a   seven-unit   digital   code   used   for   the transmission of teleprinter information. ASCII is used primarily with computer systems, but it is also used in some   teletypewriter   applications.   The   Defense Communications  System  standard  speed  for  teletype operation is 100 wpm or 75 baud. 2-16 Words per minute  is used only when speaking in general  terms  about  an  approximation  of  speed.  At  a speed of 100 wpm, 100 five-letter words with a space between   them   can   be   transmitted   in   a   60-second period. But, you can also get this wpm rate by varying the   modulation   rate   or   the   length   of   individual characters.   Because   of   this,   the   baud   method   of reference rather than wpm is used. Bit In binary signals, the term bit is equivalent to one place in a binary number. Because of the influence of computer   language,   modulation   rate   is   sometimes expressed as  bits per second (bps).  When it is expressly understood that each unit in the baudot character is the same  length,  the  modulation  rate  expressed  in  bps  is the same as the modulation rate expressed in baud. DC  CIRCUITS The  two  most  common  methods  of  creating  the mark   and   space   conditions   are   neutral  and  polar operation. In neutral operation, current flow represents the mark and no current flow represents the space. In polar  operation,  current  impulses  of  one  polarity represent the mark and impulses of the opposite polarity represent the space. Neutral  circuits  use  the  presence  or  absence  of current flow to convey information. These circuits use high  level  (60/20  milliamperes)  as  the  line  current value. (Low level operation uses (±6 vdc at 20 micro- amps  and  can  be  balanced  or  unbalanced).  A  neutral teletypewriter circuit consists of a transmitting device, a  current  source,  a  variable  resistor  to  control  current,  a receiving device, and a transmission line. Polar operation differs from neutral operation. In a polar system information is always present in either a positive  or  negative  condition.  The  circuit  composition is the same, but polar operation requires an additional current source. This current source is usually a solid- state dc power supply that provides variable current to the teletypewriters. The extra current source normally provides current from the positive side for marks and current from the negative side for spaces. There are some advantages to using polar circuits rather  than  neutral  circuits.  In  a  polar  circuit,  it  is almost impossible to distort a signal by high reactance, low-line  currents,  or  random  patching  of  circuits  or equipment. A big advantage for the ET is that a com- plete loss of current (a zero reading on the milliamme- Figure 2-14.—Mark and space signals in the teletypewriter- character  R.