USER'S GUIDE
AMTOR OPERATING SUGGESTIONS FROM G3PLX
PK232UG Rev. E 9/87
D-5
183
er QSO, or both. It can only be the master station that leads in a 'hot' QSY. If the slave
station were to attempt to lead a QSY, then, in the event that it was not successfully
completed before the contact timed-out into a re-phase operation, the slave would no
longer be transmitting, and there would be no way for the slave to establish a new fre-
quency without restarting as a master and thus lose some traffic.
3.4.3.
Fixed-channel working
Since it is possible to leave an AMTOR station in 'standby' mode on a channel, and for any
other station to make a specific call to that station, a common practice in AMTOR is to
monitor such a specific frequency. The question arises as to what exactly is the 'fre-
quency' of an AMTOR emission. There are, unfortunately, two different conventions in
use, one more commonly used in amateur circles, and the other used universally by all
non-amateur users.
3.4.3.1.
The 'amateur' convention says that an AMTOR signal (and indeed any FSK signal) is
specified with reference to the frequency of the higher-frequency tone in the pair.
Thus, if a sked, for example, is prearranged to occur on 14075 kHz, this is taken to
mean that the two transmitted tones are on 14075.00 and 14074.83 kHz.
3.4.3.2.
The 'commercial' convention, also increasingly by amateurs, says that the signal is
specified with respect to the frequency of the imaginary centre-channel. This a signal
said to be on 21100 kHz will have one tone on 21099.915 kHz and the other on
21100.085 kHz, that is, 85 Hz either side of the nominal, rather than with one tone on,
and the other tone 170 Hz below, the nominal. Note that it is assumed that the fre-
quency-shift is universally accepted to be 170 Hz.
Thus, in any specification of a 'frequency' in connection with AMTOR working, due re-
gard must be taken of the convention intended, at least until such time as one of these
conventions is dropped in favour of the other.
Another factor is significant in respect of setting frequencies on the dials of SSB trans-
ceivers used on AMTOR with audio-frequency-shift tones used. Since the tones will re-
sult in transmissions offset from the suppressed carrier frequency, the dial, which nor-
mally indicates the suppressed carrier frequency, will not read correct. It will be neces-
sary to add or subtract a fixed amount to the dial frequency in order to establish the
actual frequency in use. For example, if the transceiver is in use or lower sideband,
with audio tone frequencies of 2125 and 2295 Hz, then, to operate on an 'amateur' fre-
quency of 14075, the transceiver dial must be set to 14077.125, that is, 2.125 kHz
higher than the desired frequency. The two radiated tones will then be on 14077.125-
2.125 (14075) and 14077.125-2.295 (14074.83). Other offsets must be used if a 'com-
mercial' channel is to be set up (2.210 Hz), and the offset will be in the other direction
if upper sideband is to be used in the transceiver. The offset must be recalculated if the
tone frequencies are different from those quoted.
Users of transceivers with an FSK connection, must consult the transceiver handbook or
supplier to establish if an offset has to be applied to the dial frequency. Even if the
transceiver supplier indicates that no offset is needed, it will be necessary to establish if
the 'amateur' or 'commercial' convention is implied, or some other convention.
3.4.4.
Use of the 'over' or 'break-in' facility
In ARQ mode, it is possible, by use of the 'over' or 'break-in' facility, to interrupt the send-
ing of the other station. This facility should be used with care, and only in situations
