AMTOR
44
AMTOR Mode
KAM
only
AMTOR Theory
Information transmitted by AMTOR is specially encoded to provide a scheme of error detection.
This coding takes the form of each character containing 4 SPACE signals and 3 MARK signals. The
receiving station can use this 4/3 ratio to determine if the character received is "probably" correct.
If the received signal were to arrive with 4 MARK and 3 SPACE signals, the signal is obviously in er-
ror and the receiving station could take the appropriate action. The information is transmitted in
blocks of 3 characters, that is, the sending station will transmit 3 characters and then pause for an
answer from the receiving station. The answer will consist of a single character, indicating that the
block just sent was either received OK or that the block should be retransmitted.
AMTOR operation is possible in two basic modes. Mode A operation is a one-on-one mode, en-
abling the receiving station to request retransmission of any characters received which contain er-
rors. This mode provides a high degree of error immunity due to the handshaking between the
two stations involved in the communication. This mode is commonly referred to as ARQ. Mode B
operation is similar to RTTY operation, in that the signals are not error-checked by a specific sta-
tion and no handshaking is performed. Mode B is the mode for calling CQ or other operations
where more than one station is intended to receive the communication. Mode B operation is called
FEC – Forward Error Correction. In this mode, each character is sent twice, and the receiving sta-
tions will check each character for the proper 4/3 ratio. If the first character received is correct, it
is printed and the next one ignored. Otherwise, the second character is checked and if it is correct,
then it is printed. If neither character was received correctly, then a special character will be dis-
played on the terminal to replace the invalid character. On the
KAM
, the default character is a
space, but you may choose any character you wish with the MISSCHAR command.
A variation of the Mode B operation is called SELFEC – Selective FEC. In this mode, the sending
station would cause a specific receiving station to automatically enter the receive mode and copy
the signal – thus is selectively starts the receiver.
AMTOR stations are each identified with a SELCAL – the selective callsign used to identify this sta-
tion from all others. In most amateur communication, we think of the callsign being used for this
purpose, but in AMTOR, the SELCAL is normally composed of the first letter of your call and the
last three letters of your call. Thus the SELCAL for KA5ZTX would be KZTX. If your callsign does
not contain four letters, the standard practice is to duplicate the first letter of the SELCAL. Thus
WK5M would have a SELCAL of WWKM. The SELCAL is used in both the SELFEC and ARQ modes
of operation.
With the implementation of the CCIR 625 recommendation for AMTOR operation, the SELCAL may
now contain 7 characters. This SELCAL cannot include the characters G, H, J, L, N and W. The rea-
sons for this are contained in the 'Comité Consultatif International des Radiocommunications'
"CCIR Recommendation 491-1" which describes a TOR station identity.
The KAM allows you to enter this 7 character SELCAL in two ways. First, you may enter the actual
7 characters you wish to use. Currently, there is no defined standard for selecting your 7 charac-
ters. The second option in the
KAM
is to enter a 9 digit number. The
KAM
will then calculate the
corresponding SELCAL in accordance with 625.
Once you have entered your MYSEL7 and MYSEL4, the KAM will automatically respond to either
SELCAL for a mode A (ARQ) QSO, or (if AUTOSTRT is ON) for a SELFEC mode B (FEC) transmis-
sion.