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 signal.
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 simultaneously!)

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 Keying).

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 clever).

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!!)

AFSK

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 experience of!!

Hardware

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 is required.
This is for two reasons.

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 makers exist).

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)