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about to start. In this manner any difference in speed between the transmitting and receiving devices can
accumulate only during the duration of one character. However, you should note that a penalty must be
paid for this advantage. The length of each character must be increased to include a unit (element) to start
the receiving device and another to stop it.
The start unit precedes the first intelligence unit and is always a space signal. Its purpose is to start
the receiving machine. The stop unit follows the last code unit and is always a mark signal. Its purpose is
to stop the receiving machine in preparation for receiving the next character. The start unit must be equal
to at least one unit of the code. The standard mode uses a stop unit that is 1.42 times the length of one
intelligence unit. It is common practice to refer to a code unit as an element and to use the terms
interchangeably. You will also hear duration of a unit referred to as the unit interval.
The length of time required to transmit the entire character is called the CHARACTER INTERVAL.
Character interval becomes very important in some transmissions because certain items of equipment are
character length conscious or code conscious. Stop unit intervals of various lengths are used or produced
by various equipment (1.0, 1.27, 1.5, 1.96, 2.0, and so forth). Basically, the only difference between them
is the length of time required to transmit one character.
SYNCHRONOUS.—Synchronous teletypewriter operation does not in all cases have to rely upon
elements of the transmitted character to maintain proper position in relation to the receiving device.
External timing signals may be used that allow the start and stop elements to be discarded. You will then
see only the elements necessary to convey a character.
Synchronous systems have certain advantages over asynchronous systems. The amount of time taken
to transmit stop and start elements is made available for information transmission rather than for
synchronizing purposes. Only the intelligence elements are transmitted. In start-stop signaling, the ability
of the receiving device to select the proper line signal condition is dependent upon signal quality. For
example, suppose the stop-to-start transition arrives before it should; then, because of atmospherics, all
subsequent selection positions in that character will appear earlier in time in each code element. A
synchronous system has a higher capability for accepting distorted signals because it does not depend on a
start-stop system for synchronization.
Modulation Rate
Several terms are used to refer to teletypewriter modulation rates or signaling speeds. These include
BAUD RATE, BITS PER SECOND, and WORDS PER MINUTE. Baud is the only term that is
technically accurate. The other terms are either approximations or require explanation.
The word baud by definition is a unit of modulation rate. You will sometimes see it used to refer to a
signal element, but this reference is technically incorrect. Baud rate is the reciprocal of the time in
seconds of the shortest signal element. To find the modulation rate of a signal in bauds, you must divide
the number 1 by the time duration of the shortest unit interval present in the signal. For example, 22
milliseconds (.022 seconds) is the time interval of the shortest unit in the five-unit code at 60 words per
minute. To find the number of bauds corresponding to 60 words per minute, divide 1 by .022. Rounding
off the result of the division gives us the number 45.5, which is the baud equivalent of 60 words per
minute. Each increase in words per minute will correspondingly decrease the signal unit time interval.
(The defense communications system standard speed for teletypewriter operation is 100 words per minute
or 75 baud.)
Words per minute is used only when speaking in general terms for an approximation of speed. The
term 100 words per minute means 100 five letter words with a space between them can be transmitted in a
60-second period. However, you can obtain this nominal words-per-minute rate in several systems by
