Kantronics KPC-4 - AMTOR Theory; AMTOR Mode

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 error, 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, enabling the receiving station to request retransmission of any characters
received which contain errors. 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 station, and no handshaking is
performed. Mode B is the mode used 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 stations 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 displayed 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 it selectively. starts the receiver.
AMTOR stations are each identified with a SELCAL — the selective callsign used to
identify this station from all others. In most other 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 reasons for this are contained in the Consultive Committee
on International Radio document "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 characters. The second option in the KAM is to enter a 9 digit
men ap The KAM will then calculate the corresponding SELCAL in accordance
with 625.
AMTOR 47
© Copyright 1989, 1990, Kantronics, Inc. All Rights Reserved. l A
Duplication of this manual or the firmware without Aug. 13, 1990 : Version 3.0
permission of Kantronics, Inc. is prohibited. Operations Manual