The main interest in my operations has
been "Digimodes" (Digital Modes).
Digital radio covers a vast area of radio, and some modes are not
true digital modes, but semi-digital or "Fuzzy" combinations
of digital and analogue signals.
True digital transmissions are very rare,
and seldom found on amateur radio frequencies..
Amateur radio digital transmissions are generally modes that are
encoded, or decoded by a computer, but transmitted as an analogue
Such modes are RTTY, PSK, JT65, MFSK and arguably CW (Morse Code).
(CW doesnt need a computer to transmit or recieve, but is a form
of "Fuzzy" digital mode as uses digital and analogue tequniques
There are generally two ways of transmitting
a signal, Keying the transmitted directly, CW (Morse) and FSK (Frequency
Shift Keying), or modulating a carrier AFSK (Audio Frequency Shift
CW and FSK
This generally requires a transmitter
designed to take a digital input directly from a computer.
With CW the transmitted a carrier is
switched either ON or OFF depending upon the output from the computer
(or operators Morse key). With RTTY the radio transmits a continuous
carrier, and the output from the computer tells the transmitter
to change frequency and back to send different characters (all very
To decode the signals, generally the
received audio is fed from the receiver into the computer sound-card,
and converted from sound (analogue) to digital using Analogue to
digital converters (clever people computer geeks). This is then
converted to text (if everything is working well) by software usually
written by radio hams who are also computer Geeks (and very clever!!)
This decodes the signal in exactly the
same way as FSK, but transmission is a little different.
The digital signal from the software
(written by clever radio amateur/computer geeks) is converted from
computer language (Digital) to audio (Analogue) by very clever Digital
to Analogue converters in the sound card of the computer. This audio
signal (now analogue) is sent to the microphone input of the transmitter,
and transmitted as a modulated signal. This modulated signal appears
(in the case as CW and RTTY) identical to the FSK modes, but has
the advantage that it can now support a wider variety of "Digital"
modes such as MFSK, PSK, SSTV, JT65.
There are numerouse other digital modes
used by amateurs, but I shall only refare to ones I have direct
The clever geeks who have written software
also include ways for the software to control the transmitter, and
can tell it when to transmit, when to receive, and sometimes more!
Also in order to use either CW (computer
generated or recieved), FSK, or AFSK it is necessary to connect
the output from the computer to the input of the radio, and the
output from the radio to the computer. In order to do this an interface
This is for two reasons.
- Convert the output from the computer to a signal
that can control the radio.
- Isolate the computer from the radio to prevent
interference and other problems.
The first is usually achieved using an
electronic isolating switch such as an "Opto-isolator and the
second by using audio isolator transformers.
Commercial interfaces are manufactured, but can be quite expensive
(especially for "brand name" models, but cheaper independent
However a digital interface must be one
of the simplest projects that can be undertaken by a ham ( even
I made one, so it must be easy) and it sound very impressive when
you tell all your (none ham) friends that you have constructed a
"Digital Computer to Radio Interface for AFSK, and Remote TX/RX
control", and they will start to think you are now one of those
clever computer Geeks!!!
My first interface was a commercial one
made by G0TSG. It was very cheap, and introduced me to digimodes
very successfully. It basically was only an audio isolator, and
as my radio at the time did not have VOX control, transmitting/receiving
control was handled by a switch on the unit. This was fine, but
I occasionally forgot to operate, or release the switch, which became
very tiresome (and a little embarassing!!!). This was therefore
given away to a friend to try digimodes.
I replaced it with another commercial
unit from Watson, a WM-03. This now had automatic TX/RX control
by using one of the computers comports and software control.
However this also had to go, as it did not isolate the ground between
the radio and computer, and I was getting significant RF-Feedback
in the audio. Also I had two radios I wanted to use with the interface,
and both had completely different audio input and output requirements,
which meant that I had to constantly change the sound-card settings
when changing the radios over.
So I wanted an interface that could be
switched between the two radios, with a switch, and that could be
set up to work with the computer sound-card without changing sound-card
level settings. Only one commercial solution seemed to exist (though
not sur it did exactly what I wanted), but it was too expensive,
hence the need to make my own, which has been quite successful,
but with one big disaster (more about that later!!!)
Below is a schematic and some photos
of the one I made. (Sorry to follow shortly!!!......But it does
work as using it now)