3-19
varying either modulation rate or the individual character interval (length). For this reason, the
modulation rate (baud) method of reference rather than words per minute is used.
Formula for baud rate and words per minute are as follows
BIT is an acronym for the words binary digit. In binary signals, a bit is equivalent to a signal
element. Because of the influence of computer and data processing upon our language, modulation rate is
sometimes expressed in bits per second. When you understand all signal elements being transmitted are of
equal length, then the modulation rate expressed in bits per second is the same as the modulation rate
expressed in baud.
Dc Circuits
You were told the two conditions mark and space may be represented by any convenient means. The
two most common are NEUTRAL and POLAR operation. In neutral, current flow represents the mark,
and no current flow represents the space; in polar operation, current impulses of one polarity represent
mark, and impulses of the opposite polarity of equal magnitude represent the space.
NEUTRAL.—Neutral circuits make use of the presence or absence of current flow to convey
information. A neutral teletypewriter circuit is composed of a transmitting device, a battery source to
supply current, a variable resistor to control the amount of current, a receiving device, and a line for the
transmission medium.
POLAR.—Polar operation differs from neutral operation in two ways. Current is always present in
the polar system, and it is either positive or negative. A polar teletypewriter circuit contains the same
items as a neutral circuit plus an additional "battery" source. The battery referred to here is not an actual
battery but is a solid-state dc power supply. It provides variable current to the teletypewriters. The reason
for having an extra battery source is because polar circuits use positive battery for marks and negative
battery for spaces.
You will find in polar operation that the distortion of a signal is almost impossible through low line
currents, high reactance, or random patching of signal circuits or equipment. In polar signaling when you
experience a complete loss of current (a reading of zero on a milliammeter), you know you have line or
equipment trouble; whereas the same condition with neutral signaling may indicate a steady space is
being transmitted. This gives us a condition called RUNNING OPEN. Under this condition, the
teletypewriter appears to be running because the machines is decoding the constant space as the Baudot
character blank and the type hammer continually strikes the type box but there is no printing or type box
movement across the page.
Q15. What are the two teletypewriter modes of operation?
Q16. Define baud.
Q17. Define bit.
Q18. What are the two types of dc operations used to represent mark and space conditions?
