Digital modes are really exciting, especially for QRP and portable activities, being more accessible for using on mobile devices, like Android tablets and smartphones.
In digital modes, a dedicated software application encodes the desired message using the specific alphabet of the digital mode (PSK31/63, RTTY, JT65, FT8 etc.) on an audio sub-carrier which will be takeover by the transceiver to modulate (SSB or FM) the main radio carrier. In reverse, the demodulated audio signal from the transceiver must be sent to the software application in order to decode the message.
In conclusion, digital modes are handling in their native structure only at the application level. Between the computing device and transceiver are handled only analog audio signals and that make very simple the start of working with these modes.
In the simplest way, the audio signals between computer and transceiver can be exchanged via direct cables (cross connections between the output of one device to the input of the other), but it is strongly recommended to have a good galvanic separation between these devices, in order to avoid issues generated by the DC components or different grounding.
The interface between computer and transceiver should provide, also, another feature: the PTT (Push-To-Talk) control, which puts the transceiver in TX mode when a message waiting to be sent on the software application.
Although there are a multitude of complex interfaces dedicated to working in digital modes, good results can be obtained with the below passive configuration. It was used, initially, to interface an Android device (a Samsung Tab3 tablet) with the FT-817ND transceiver.
The PTT control is ensured in the simplest possibleway: manually, using just a switch. This minimal approach is acceptable while your digital communication don't requires automatic data exchange, which is the case for PSK, RTTY or JT65, available on Android devices. For FT8/FT4, while these modes are available only on computer platforms with USB ports, the PTT can be handled via the serial cable available for your transceiver.
