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USER'S GUIDE
If both appearances of the same character are mutilated and fail the polarity-ratio test, no
data are printed or displayed. Because the second appearance of the character occurs 280
milliseconds after the first appearance, the mathematical probabilities are that noise bursts
will not destroy both appearances of the character.
At the beginning of an FEC transmission, the message is preceded by ten Idle characters.
After 28 characters of the message, a sequence of five Idle characters Is inserted in each
message space, giving the receiving stations a reference timing interval to which they can
lock again if necessary.
The code sequence for the repeated idle string is different from the first idle string so that
they are identified correctly. The sequence looks like this:
Idle 1 - Idle 2 - Idle 1 - Idle 2 - Idle 1, etc.
for ten occurrences. Then each character is transmitted interleaved with four sequential
characters and its own repetition.
Unlike ARQ (Mode A), in FEC (Mode B), the transmitter operates at 100% duty cycle. Con-
sult the operating manual and duty-cycle specifications of your radio equipment for the cor-
rect operating conditions suitable for RTTY and FEC operation. As a general rule, many re-
cent transmitters must be operated at significantly reduced power levels to prevent exces-
sive dissipation in the amplifier stages. A 50% reduction in operating power is frequently re-
quired.
In FEC and ARQ, accurate synchronization or phasing is necessary. Each station sends phas-
ing bursts at the beginning of each contact, with a crystal clock maintaining the timing rela-
tionship during the contact. If the stations lose synchronization lock during the contact, the
program provides automatic re-phase procedures.
PK232UG Rev. E 9/87
E-3
AMTOR THEORY
187
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