[SMCARA] RTTY Macros and Operating basics

[Tom Shelton]

Starting at 1:00PM (EST) today brings us the ARRL RTTY Roundup.  This post
isn't about the contest, but about RTTY in general.  If you are interested
in doing the contest, information and rules can be found on the ARRL
web-site at: http://www.arrl.org/rtty-roundup

 

An RTTY transmitter sends out a continuous carrier that shifts frequency
back and forth between two distinct frequencies. There is no amplitude
modulation, only a pure carrier similar to CW with the addition of a
frequency shift. The lower RF frequency is known as the SPACE frequency and
the upper RF frequency is known as the MARK frequency. The difference
between the two is known as the SHIFT. For amateur radio, the SHIFT has been
standardized at 170 Hz. This allows sharp 250-500 Hz audio filtering.  

 

The recommended audio frequencies are 2125 Hz for the MARK audio frequency
and 2295 Hz for the SPACE audio frequency. Note that the SPACE audio
frequency is higher than the MARK audio frequency - just the opposite of the
RF frequency of the actual transmission. This happens because amateur radio
RTTY uses lower sideband (LSB) transmission. If you happen to forget and set
your transmitter to USB instead of LSB, two things will happen. Because your
MARK and SPACE are now reversed in your receiver, any RTTY signals you hear
will not print correctly. All you will see is random characters that make no
sense at all. The other thing is that transmissions will also be reversed.
Modern RTTY software has simple controls to reverse received signals. Note
that USB is used in some parts of the world, esp. in Europe.

 

RTTY uses the Baudot code, invented before radio even existed, and still
widely used throughout the world. The Baudot code uses data bits to
represent letters, numbers and punctuation, much like a computer does.
Unlike a computer, which uses one byte (eight bits) for each character, the
Baudot code uses only five bit, plus a start bit and stop bit. Using fewer
bits is good because it speeds up transmission and reduces the chance of
errors, but there is a complication. Five data bits can only represent 32
different characters. Since there are 26 letters in the latin alphabet plus
ten numbers, plus some punctuation, 32 different characters is not enough,
even if you only use capital letters, as does the Baudot code. Rather than
using more bits, Mr. Baudot came up with an other solution. He reasoned that
most of what would be sent would be letters rather than numbers or
punctuation, so he assigned all the letters to the basic 32 characters. He
then had six possible characters left over and he did a very clever thing
with two of them. He made one of them a Figures Shift and another a Letters
Shift. When sending one of the basic 32 characters, nothing special happens.
But when a number or punctuation is to be sent, the Figures Shift  character
is sent first (it's a non-printing character). Whatever follows will still
be one of the basic 32 characters (bit patterns), but the receiver will
interpret it differently. For example the letter Q uses the same five data
bits as the number 1, but when the receiver gets a Figures Shift first, it
prints the next character as a 1, not a Q. This continues until a Letters
Shift character is received, at which time the receiver goes back to
"normal" printing. Characters may get corrupted due to interference from
lightning, static, man-made noise, channel fading, etc., causing inadvertent
letter-figure shifts.

 

Given this - when setting up your macros, try to minimize the shift between
numbers and letters.  For example, when sending a signal report, send it as
599 instead of 5NN.  As a side note - you may also see INN as a signal
report or TOO (early TTY terminals didn't have numbers and you shifted the T
& O keys to give you the 5 & 9 - operators using these tools didn't bother
with the shift and just sent TOO to increase their typing speed).

 

If you are using Ham Radio Deluxe (DM-780), which originated in Europe, the
default set-up for RTTY is designed for using USB in RTTY mode.  In the rest
of the world, LSB is the standard (regardless of band) - In order for your
log to match the receiving station, you'll want to use LSB and set DM-780 to
reverse the mark and space (there's a button for that on your screen).

 

For contesting, keep your macros short.  When calling another station, just
send your call.  For your report, send <his call> 599 MD <your call>.  Have
a couple of macros set-up to ask for repeats (in the RTTY-RU, you might need
to ask for a repeat on a QTH or serial number (for DX).  Also have a macro
set-up to repeat your QTH (mine reads MD MD MD).

 

Generally running a frequency will give you the best rates, but it's
difficult and if you get a pile-up, it can be very frustrating.  In this
particular contest, search and pounce can easily keep your Q-Rate at 40-60
QSOs per hour and is much less infuriating.  

 

A couple of good resources for RTTY and RTTY Contesting are:

http://www.nonstopsystems.com/radio/frank_radio_rtty.htm (almost all of the
above was stolen from this page)

http://www.rttycontesting.com/

 

Hope to hear you on the air today and tomorrow.

 

73

 

Tom Shelton, ND3N

Ham Radio, Motorcycling, and a Loving Wife who lets me play with both

What more could a man need