Foundations of Amateur Radio

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How does RTTY work?

Posted: 12 Aug 2017 10:00 AM PDT


Foundations of Amateur Radio

The continued discussion in our community about Digital Modes got me
thinking about what a Digital Mode actually is. At the most fundamental
level, it's about encoding information into discrete chunks to exchange
information. Morse Code is an example of a Digital Mode, made up from
combinations of dits and dahs.

If you change frequency whilst sending dits and dah's you invented RTTY or
Radio-teletype. There are two frequencies involved, 170 Hz apart, where the
lower frequency is the SPACE frequency and the upper frequency is the MARK
frequency. If someone gives you a RTTY frequency, they're talking about the
upper frequency.

Instead of using Morse Code to send messages, RTTY uses 32 different codes,
5 bits, to exchange information. This isn't enough for the entire alphabet,
with digits and punctuation, so two of the codes are used to swap between
Letters and Numbers.

Some radios can change frequency between the lower SPACE and upper MARK
frequencies in a single transmission. This way of transmitting is called
FSK, or Frequency Shift Keying. It's a lot like moving the VFO around
whilst keying a Morse-key. Not something you'd do manually, since in
Amateur Radio, this is generally happening 45 times a second.

If your radio can't do the frequency shifting, then another way is to use
Audio Frequency Shift Keying of AFSK, where instead of changing the
frequency, you change an audio tone by 170 Hz. Without getting technical
about how this works, if you've ever listened to Morse Code with a radio,
you'll have noticed that as you change frequency, the sound changes. If you
were to change the frequency of your radio by 170 Hz, the sound would also
change by 170 Hz.

So with that in mind, if you were to change the sound by 170 Hz, the
receiver wouldn't care if you were changing the transmit frequency or the
audio frequency, since it both sounds identical at the other end.

Most of the time a computer is generating two tones, a tone for the SPACE,
or lower frequency and a tone for the MARK or the upper frequency. It comes
out of the speaker of the computer, which you feed into the microphone of
the radio and your radio then generates a normal SSB signal that is
experienced by the listener at the other end as a Radio-teletype.

Pretty nifty and if you understand this, then most of the other Digital
Modes in use today use similar methods.

I'm Onno VK6FLAB
This posting includes a media file:
http://podcasts.itmaze.com.au/founda...teur-radio.mp3