DIGITAL MODES: the new
generation of HF HAM Radio !
AMTOR |
AMTOR -
BY TONY LONSDALE VK2DHU
AMTOR is a specialised form of RTTY. The
term is an acronym for AMateur Teleprinting
Over Radio and is derived from the
commercial SITOR system (Simplex Telex Over
radio) developed primarily for Maritime use
in the 1970s.
In the early 1980's, Peter Martinez, G3PLX,
made several minor changes to the SITOR
protocol and called it AMTOR.
AMTOR improves on RTTY by incorporating a
simple Error Detection technique. The system
remains relatively uncomplicated but AMTOR
performs well even in poor HF conditions.
While there can still be many errors in
AMTOR data, the Error Detection helps a lot
and the result is quite tolerable for normal
text mode conversations because of the high
redundancy in plain language text. Certainly
much better than RTTY. But for more critical
data such as program code, or even some
technical information messages, NO errors
can be tolerated.
There are two modes used in Amtor: ARQ and
FEC.
ARQ
------
This mode is a little different in that it
is a Synchronous protocol, which means both
stations are synchronised to each other's
signals.
In ARQ mode (Automatic Repeat Query),
sometimes called Mode A, data is sent in
groups of 3 characters. Although each
character is only 5 bits (same as for RTTY),
two additional control bits make it up to 7
bits per "character" and they are set so
there are always 4 marks and three spaces in
every transmitted character. If the
receiving station gets some other
combination it knows an error has occurred.
The 40 percent overhead is considered
worthwhile to get some error detection. This
technique can identify a lot of errors that
might occur but is not as thorough as the
methods used in PACTOR and Packet which we
look at later.
The receiver responds to each 3 character
group by sending either an ACK (ACKnowledge)
code (if OK) or a NAK (Negative AcKnowledge).
Each time the transmitting station gets a
NAK, that 3 character group is sent again.
If you listen around on the HF bands in the
recognised Data Segments of the bands, you
might hear a chirp-chirp sound that
identifies an ARQ transmission. Even when
there is no data actually being transmitted,
the transmitting station continues to send
idle "chirps" to maintainthe link.
Your AMTOR equipment probably supports a
Listen Mode too and that allows you to
monitor another ARQ session even though you
are not participating in the session with
the usual acknowledgements. Of course that
means you don't get the opportunity to say "NAK"
if you don't copy something properly!
FEC
------
In FEC mode (Forward Error Correcting),
sometimes called Mode B, the sending station
sends each character twice so this mode
provides a means of transmitting to several
stations at once. The receiving station does
not acknowledge the data received. If a
receiving station matches both instances of
a character, that character will be printed,
otherwise some error symbol is printed. This
mode does not provide for the receiver to
ask for the missing data to be
retransmitted. An FEC transmission sounds
more like a Baudot RTTY signal.
The two stations need to keep in phase with
each other so each FEC transmission is
started with several sets of "phasing pairs"
and these are sent at regular intervals even
while there is no data being transmitted.
FEC Mode is still better than ordinary RTTY
but its error detection is not as reliable
as that in the ARQ Mode.
AMTOR systems are still limited to the
technology of the 60s with limitations such
as the character set and the maximum
transmission rate (100 baud) geared to the
mechanical teleprinter. The Error Detection
technique provides improved accuracy over
the "vanilla" RTTY mode, but is still not
entirely reliable. It is perhaps better
termed Error Reduction than Error Detection
and has limited application for critical
data. |
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