Getting Started on RTTY - Introduction
Everyday more and more Amateur Radio operators are
operating on the HF digital modes, in particular, RTTY. In each RTTY
contest I operate, I find about 8-10% new calls that I've not seen before.
There are several reasons for wanting to be a RTTY op. No matter
what that reason might be, it is the purpose of these pages to assist getting
you started on RTTY.
Even though I've been active on RTTY for over 20
years, have won several RTTY contests and have achieved RTTY DXCC Honor Roll, I
don't claim to have all the answers. I do have a technical background and
am familiar with many of the technical aspects of operating RTTY. RTTY is
the most fun I've had in over 30 years of Amateur radio. It can be both
complicated and simple. So I'll try to keep things simple.
Anyone
can operate RTTY. You don't have to know how to touch type to run
RTTY. Hunt & peck works fine. Every program I know for RTTY
includes special "buffers" which hold pre-typed messages that can be sent by
pressing a function key or clicking a button on your screen with a mouse.
The late Junior, previously N5JR (this call has since been re-issued to Joel
Rubenstein who is an active RTTY operator too), was a paraplegic and earned RTTY
DXCC before he died. He operated RTTY with a stick in his mouth. He
should be an inspiration to all of us and shows that even those with
disabilities can enjoy this very fun and exciting mode. There's really no
excuse not to try.
Whether you desire to operate RTTY for DX'ing,
contesting or ragchewing, you have to start somewhere. It's my hope that I
can point you in the right direction. From there you will be able to make
your own decisions on how you want to operate RTTY from your
station.
RTTY is Baudot code (see http://chem.ch.huji.ac.il/~eugeniik/history/baudot.html for
excellent information on Mr. Baudot and his code). More specifically, on
the Amateur HF bands, it's 5 bit baudot meaning that every character consists of
five bits, either mark or space (in actuality Baudot is 8 bits because a start
bit and two stop bits are added for synchronization see http://www.aa5au.com/gettingstarted/rtty_diddles_technical.htm).
In general, a baud rate of 45.45 baud is used on HF. 45.45 baud is the
equivalent of 60 wpm. Even though 45.45 is standard, you will occasionally
come across a RTTY signal at a different speed. Ed, P5/4L4FN, preferred to
run 50 baud rate (66 wpm) when he was active on RTTY from North Korea. 75
baud (100 wpm) can also be used on the Amateur HF bands.
The standard
mark and space tones are 2125 hz and 2295 hz respectively. These frequency
tones are also referred to as “high” tones. Although these standard tones
are used by most Amateurs, it's possible to operate RTTY using other frequency
tones. This is fine as long as you maintain the standard 170 hz shift (2295-2125
= 170 hz). Some commercial TNC's such as the KAM and PK232 use a 200 hz
shift when running AFSK. Although 200 hz shift will work OK,
170 hz is standard on HF. Sound card programs will sometimes change the
frequency of the mark and space tones when transmitting AFSK while using the NET
feature to line up your transmit signal with that of a received station, but
they maintain the 170 hz shift. Although that's important to know, I
suggest not using the NET feature just yet.
For the purpose of getting
you started, MMTTY will be used here as the example program to use for running
RTTY. Why? For several reasons. MMTTY is a simple program to
use. It's easy to set up and in all the years I've operated RTTY, MMTTY is
the best decoder I've ever used. The best part of MMTTY is that it's
free! The MMTTY help file has an abundance of information about RTTY in
general.
AFSK vs. FSK (Audio Frequency Shift Keying vs. Frequency
Shift Keying)
The biggest decision you will make when you begin to
set up for RTTY, is whether you want to use AFSK or FSK to transmit RTTY.
Either way is acceptable.
There is an excellent explanation of AFSK and
FSK in the MMTTY help file so I won't go into a lot of technical detail
here. For simplicity sakes, AFSK and FSK are terms to describe how RTTY is
transmitted. AFSK is when you send audio from a TNC or Sound Card to the
audio input of your transmitter either via the mic input or accessory
jack. FSK is when you send on/off keying from a TNC or Serial COM port
(with MMTTY you can also use a parallel LPT port to transmit FSK) to the FSK
input of your transmitter. Most modern transceivers today have an FSK
input. By using the FSK input to your transceiver, you can then operate
the radio in the RTTY or FSK position and make use of filters available for
receiving RTTY, such as a narrow 250hz or 500hz IF filter. In most cases,
when using AFSK, your radio will be placed in the LSB position (although some,
especially in Europe, prefer using USB).
I am biased in my opinion of
AFSK vs. FSK. I prefer FSK over AFSK all of the time. There are
advantages and disadvantages to both. When operating AFSK, you must make
sure the audio coming from your TNC or Sound Card is at the correct level and
maintain this level. If the audio is too high it will overdrive your
transmitter and more than likely result in a distorted RF signal or cause your
radio to put out "image" signals across the band. You also must make sure
you do not have the speech processor turned on when transmitting AFSK
RTTY. This will also cause problems on your RF signal. The only real
advantages to running AFSK are that you can get started rather quickly using
this method because it's simple audio-to-a-soundcard input for receive and
audio-from-a-soundcard output to your transceiver for transmit and you can also
make use of the NET features of sound card programs. But personally, I
think NET is a nice feature, but it can cause problems when not used
correctly. Another disadvantage of using AFSK is that most transceivers
will not allow you to use the narrow IF filters in your radio when operating in
the LSB or USB positions. Some recently manufactured radios allow you to
change the characteristics of your IF filters so you can effectively receive
RTTY in the SSB position. Check your manual to see if this is the case for
your radio. And if you wonder how your radio works on RTTY, go back to my
RTTY
Page and find your radio listed under my "RTTY Radios"
section.
I prefer FSK because it's straight on/off keying into the FSK
input of my radio. I don't have to worry about the audio level or whether
I forgot to turn the speech processor off. I can then use the FSK mode on
my radio along with the narrow 250 and 500 hz filters. When I switch from
SSB to RTTY, I only have to change the mode on my radio. There is a little
more work involved when using FSK. If you use a sound card, you must have
a spare serial COM port (or parallel LPT port) available on your PC in order to
key both FSK and PTT, whereas if you use AFSK, you don't need this extra COM or
LPT port and you can use VOX to key the radio. However, if you use MMTTY
or any other sound card program to generate AFSK and use VOX to key your radio,
any other sound generated by Windows could possibly key your radio and send that
sound out on the air. That would not be good. There are ways to keep
this from happening and I will explain that as well.
There is more detailed information covering AFSK and FSK
on pages 5, 6 and 7 which will help you make your decision on whether to set up
for AFSK or FSK RTTY transmission.
Let's get started. The first
thing we will do is to download MMTTY from the Web.
