How does RTTY work?

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