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USER'S GUIDE
A third control code is used to command the direction of sending to be reversed, allowing
the receiving station to reply. If there is no data waiting to be sent, Station A fills the three-
character blocks with Idle characters.
Station A can send special blocks to indicate the following:
o
The SELCAL of the station being called.
o
A QRT request to put Station B back into the standby mode.
o
Acknowledgement of Station B's request to transmit.
o
A request to repeat a badly received control code.
The receiving station controls the changeover in communication direction. Station A uses a
software command, '+?,' to indicate the end of his transmission. Station B recognizes the
command and initiates the turnaround routine. Station B can also interrupt Station A's trans-
mission with a break-in control code.
The 'chirp-chirp' you hear on the air is the characteristic frequency-shift keying in bursts.
One of the stations is transmitting a longer burst, allowing a time gap between transmis-
sions during which the other station sends its reply. If both stations have equally loud sig-
nals at your location, the 'chirp' will sound more or less continuous. If one station appears
weaker than the other, your receiver AGC may not be fast enough to copy both sides. Try
turning the AGC off.
In the ARQ mode, signal fading simply slows the text received on your screen as the sta-
tions transmit a higher proportion of retries. The overall accuracy of the data is maintained.
In the ARQ mode, your station must know the identity of the other station. Therefore, FEC
(see APPENDIX D) is normally used to call CQ and start a contact. Then stations switch to
ARQ. The ARQ mode also allows you to access and activate selectively any other stations
monitoring on the frequency.
Your station can usually monitor a contact between two linked stations by using the ARQ
Listen mode (also called Mode L). This mode may need a few moments to phase or sync
with the other stations. When the linked stations you are monitoring are experiencing ARQ
retries your screen displays all the retries.
1.4.
Mode B – FEC
The second AMTOR mode is called FEC (Forward Error Correction), or Mode B. FEC provides
the means of transmitting to several stations simultaneously. This is the proper mode to use
for sending QST or CQ.
FEC is a synchronous system that transmits an uninterrupted stream of characters. FEC
does not operate with a 'handshake' as does ARQ.
In FEC, the sending station transmits each character twice. The first transmission of a spe-
cific character is followed by the transmission of four other characters, after which the first
character is retransmitted. This allows for time diversity reception at 280 milliseconds time
space.
The receiving station's computer searches the inbound data characters to verify and validate
characters having the required 4:3 mark-to-space tone polarity ratio.
PK232UG Rev. E 9/87
E-2
AMTOR THEORY
186
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